CTF3-Note-101 Comprehensive user manual for the
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1. electron beamsize sigmay electron beamsize sigmax 4 m 3 P T 25 E S 7 e gt i5 gt gt PR 4 0 5 0 10 20 30 40 50 0 10 20 30 40 50 Phase deg Phase deg phase scan for bunchlength 1 8 16 4 1 4 a y g 12 y z 1 BO B08 S 0 6 0 4 0 2 0 0 10 20 30 40 50 Phase deg Fig 5 2 1 7 Electron beam size as function of the phase measured on the OTR screen top charge as a function of the phase bottom Simulations were done by O Mete to see whether the behaviour is as expected Fig 5 2 1 8 The input parameters for the PARMELA code were taken from the measurements of the laser pulse length with the streak camera and the laser spot size on the virtual cathode camera The measured data is not consistent with the simulations even after correction for the transport line and the beam spot size 0 pwu PS Simulation version2 Simulatiom version4 longer initial bunch length Measurement E 4 85 MeV Q 1 57 nC V2 Pulse length 3 57 ps V4 Pulse length 12 07 ps RF phase Version 2 gt initial bunch length laser pulse length 3 57 1sigma ps Version 4 gt initial bunch length laser pulse length 12 07 1sigma ps Laser spot 0 35 mm 1sigma ps FWHM Z o Z Gg U O Simulation version5 O Measurement 10 0 10 20 RF phase Fig 5 2 1 8 Simulations with different initial pulse2. do the modification without sending all the parts including phase coding and booster amplifier to them which was not an option given the complexity and the operational constraints We could do the modification at our own risk Possible damage to the amplifier crystal would result in 4 weeks delay we would have to send it to them for repair and 18000 CHF repair cost 1 5 GHz Synched oscillator 320m BW 1 2m in 1 2 p05 Fig 4 3 1 Original setup including pre amplification to 10W Tests were done on the AMP1 amp 2 stages to find out how much energy per pulse could we expect if we reduce the input to AMP1 and AMP2 from the usual 7W to 250mW including losses between the fibre output and the APM 1 input Close to nominal 350nJ pulse was still achievable with these parameters Reduced power into AMP1 o 3 8 S Z expected energy on cathode S C cs Q E pej co w co 9 L K aad Yu E ss 2 4 6 Power into AMP1 W Fig 4 3 2 Expected energy pulse nJ on the cathode with reduced input power to the amplifier chain Therefore the following setup was chosen Fig 4 3 3 adoa tox Boaster AMP1 AMP2 zew P whining coding amp 2 9kW 8 1kW F5mJin 1 2us 5 oscillator 10mW 340mW 320mW Fig 4 3 3 Amplification scheme chosen for the final setup O 89kV imJin 1 2us Beam size matching of the fibre booster output to the amplifier chain was still necessary T
3. 14 6 mvp Ay 7 62 mV pay 27 mvp Ay 2 3 mY X2 69 9998 us 1 AX 5 000 kHz Fig 4 3 7 Amplification curves showing build up of steady state measured with DET10A Thorlabs photodiodes at the exit of each amplifier Red AMP1 yellow AMP2 With no additional adjustment made to the lens systems in the harmonics stages 710 nJ pulse was obtained in the UV which after transport losses gave a maximum of 450nJ pulse on the cathode well above the 370nJ specification Conversion efficiencies of 38 and 27 were measured to green and to UV respectively 4 4 Timing signals for the phase coding Once amplification and phase coding takes place at the same time timing signal settings become crucial Switching has to take place when PC is ON and timing signals are available for streak measurements We have also found that the timing quality of the switching signal degrades when it is used ms s after the START and so it is important to amplify and use the phase coded signal lt 1ms after the START is given to the modulator drivers CNT6 START phase coding Thales CH2 Start AMP1 Thales CH3 Start AMP2 CNT2 START streak Thales CH4 Start of PC Thales Ch5 Stop of PC CNT7 STOP phase coding Red signals come from Thales clock and have 4ns jitter performance Blue signals come from ECL counters and have 5ps Jitter performance Fig 4 4 1 Triggers for phase coding 5 Measurements with phase coding 5 1 Measurements on the laser 5 1 1 Timing and
4. 3 2 L dl LA 7 eu c B t dd 0 50 100 150 200 250 300 350 400 450 Gating ns Gated Solenoid Scan 18 02 2011 Focusing Magnet Current 171 67 A 4 T T T T T T T y lt 0 gt 2 71 20 16483 mm LPC ON wa lt a gt 2 77 20 15977 mm LPC Unbalanced Intensity 36 lt o gt 2 79 20 21766 mm LPC OFF 3 4 eo t E 3 gt 28 T 2 6 F 2 4 22 2 L L i i L i i 4 0 50 100 150 200 250 300 350 400 450 Gating ns Fig 5 2 4 1 Emittance measurements over three consecutive sub pulses using 100ns gating time 6 Future improvements On the laser side a fibre booster amplifier to bring the laser power up to the 10W level after phase coding would ensure more stable operation KV 1 2mJin 1 2 US Fig 6 1 Scheme with fiber booster at 10W On the phase coding system side To increase transmission through the phase coding system all FC APC connector losses could be suppressed by fusing the fiber components together which would give a 20 increase in the output power Active control of the amplitude balance between the two arms via a waveplate and polarizer based remotely controlled attenuator could compensate for long term temperature drifts and would provide a finer and more reliable attenuation The modulators themselves could be temperature stabilized to 40 C to ensure that any drift is minimized The bias controller for Mod1 should be replaced with a newer model with finer bias voltage
5. A p p PETE TE PRITE TEST AEE SAAE TEE steady state 7 steady state 4 700 800 900 1000 1100 1200 1300 500 800 900 1000 1100 1200 1300 time nsec time nsec Dipole scan Penra A Mevic CT MTV0265 averaged over 10 shots 15 10 y iir c 0 A pip SS OIG 10 lt 6 D 5 10 18 x mimi PHIN Single shot vs dipalse scan CT_MTV0265 horizontal projection integrated over te 1100 1300 ns i0 dis nnan B 1 o X ann BE mm oal SPiwhm 2 25 Dipole scan Z gl Son Piwnm 1 g HE S f C e e t al L z j K 0 2 kanann Aaaa gecosooos A o too l i I _ 20 10 0 10 50 F 0 5 10 A pip 95 x mm Fig 5 2 3 1 Dipole scan top left single shot measurement top right no phase coding and smaller beam bottom Beam instability and beamsize was limiting our measurement accuracy and although a 140 ns initial step can be seen at the end of the pulse we are not sure that this corresponds to the switching of the phase coding as one would expect the same behaviour at the beginning of the pulse too To see more dramatic effect we have introduced an amplitude and consequently charge imbalance between the sub trains sub pulses Again a time structure similar to that of the phase coding can be seen on the time resolved energy scan but the difference between balanced and unbalanced cas
6. This means that 4 5 points are recorded during one pulse The interval accuracy is 3 3ps The trigger jitter is specified to be 2 5ps RMS As mentioned earlier the jitter of the laser is about 0 5 ps RMS to the reference of 1 5GHz This means that the delay cannot be directly measured accurately between two consecutive pulses at the switching This can be solved by saving a trace and post processing using the following algorithm The time of each pulse is recorded a phase for each pulse is calculated using the oscilloscopes internal clock as reference The average phase of the pulses from the attenuator arm is then subtracted from the average phase of the pulses from the delay arm With a large number of pulses the phase jitter of the laser and the phase jitter of the sampling will decrease proportional to sqrt n where n is the number of samples Another advantage is that the method can be used when the modulators are on A sin x x interleaving is applied directly on the oscilloscope The trace is copied from the oscilloscope and processed on a PC with MATLAB A trigger level is set The first sample point above this level and the previous point are used with a linear fit to determine the time when the signal crosses the trigger level Fig 3 1 2 2 From this time the pulses expected time according to the oscilloscopes time base is subtracted The expected time of a pulse in this sense is the number of pulses so far times the nominal time between tw
7. amplitude accuracy Long term amplitude stability measurements were carried out to check performance in the phase coded and non phase coded cases As with the reduced input to the amplifiers we have less saturation and the overall stability was expected to be worse then during the previous run A DET10A small area photo diode sensitive down to 220 nm wavelength was used The detector is sensitive to beam movement and previous noise floor measurement show 700UV RMS noise The measured RMS values are expected to be combination of pointing and amplitude error No difference between the phase coded PC and non phase coded NPC cases were observed Fig 5 1 1 1 Energy integrated over the whole pulse train in the two cases was also measured with charge stability measurements running parallel Fig 5 1 1 2 In the phase coded case the measurement data was not useable for the BPR E r Z rdesktop labscope x File Vertical Timebase Trigger Display Cursors Measure Math Analysis Utilities Help Zoom Undo Stability from UV photo diode with no phase coding B gt gt 1 tel L l 7 b L f 4 Trigger ENED 0 50 0 mid 200 mwidiv 200 mV idiv 500 mvidiv 100 ns div Normal 97 G 84 0 mY 550 0 mv 770 0 mY 0 0 mV ofst 00kS 5 0GS s Edge Posi 3 3 mV Ay 4 4 mV N 1 8mV ay 70 m i Re F Signal level V i NT 1 rdesktop labscope File Vertical Timebase
8. and bias control mainly due to high input powers of 160mW This power was above the safe 100mW limit specified by Photline to avoid losses and heat dissipation related instabilities As the bias controller is using the signal from a photo diode placed by inside the modulator housing to feed back on the drive voltage heat effects will cause incorrect bias settings e Flatness of the used DR GA 10 driver was not satisfactory Fig 2 3 2 Later discussions with Photline suggested that the incorrect impedance matching to the driver input could have caused profiles measured in a however sloping risetimes measured during delivery test were unavoidable Long delay 140 74 ns bag Variable delay 333 ps E Variable attenuator gt 35 mW EO fibre modulator Fig 2 3 1 Single modulator setup M he u Seran measured using Agilent oscilloscope model 86100B B Fig 2 3 2 Output response of the DR GA 10 driver amplifier with flat input voltage a measured at INFN using input from pulse generator bottom curve shows input pulse and top shows output response b test report by Photline upon delivery e Fibre launch system was not accurate enough to achieve good coupling efficiencies into the fiber and required regular realignment e Fibre core sizes were not matched and connectors were not compatible some with European and some with US standard sizes The
9. becomes more significant compared to the peaks average value as the peak gets smaller the peak linearly increases with amplitude balance error if the timing error is small The limiting factors are e Fluctuation of the measured peak due to noise for the laser oscillator which limits the accuracy e Modulators have to be off this can cause drifts of the working point and so for amplitude error measurement between the arms time domain method is preferred The drift of the in fibre attenuator was a major problem As the measured peak has drifted accuracy for setting the timing goes down This was solved by replacing it with a variable neutral density attenuator wheel 3 1 2 The time domain method As previously mentioned the delay can also be measured by oscilloscope the best oscilloscope available in the lab was the LeCroy Serial Data Analyzer SDA18000 This oscilloscope has a bandwidth of 18 GHz and New Focus 1024 photodiode an impulse response of 12ps FWHM 0 025 0 02 0 015 l 0 01 Amplitude 0 005 0 005 L C lt l 0 01 i r r E i r r r 501 3 501 4 501 5 501 6 501 7 501 8 501 9 502 502 1 502 2 502 3 Time ns 3 1 2 1 Figure An oscilloscope trace and four times sin x x interpolation This results in our 5 5ps pulse appearing as a roughly 35ps wide pulse FWHM with some post oscillations With the 60 Gsamples s rate a point is taken every 16 7 ps
10. lengths and measurement left relative bunch lengthening right 5 2 2 FCT measurements A fast current transformer FCT installed directly after the gun is used to monitor the extracted charge This provides a good tool to check phase and amplitude errors between sub pulses The phase error and amplitude imbalance between the laser s sub pulses appear as a current imbalance between the sub pulses of the electron pulse A 10 ps error was introduced in the delay arm of the phase coding which means that one sub pulse arrives off phase As shown in Fig 5 2 2 1 this resulted in a reduction of extracted charge in that sub pulse which appears as a DC change with the 30MHz bw filter on the oscilloscope An amplitude error was also introduced between sub pulses to check the sensitivity of the FCT which was found to be lt 1 An example with 20 amplitude error is also shown below Current A 100 Current A Current A 100 Correct timing and amplitude 500ns train with 3 sub pulses 2nC bunch 50 100 150 200 2150 300 350 400 450 500 Time ns Correct amplitude and 10ps delay error 500ns train with 3 sub pulses 1 5nC bunch 100 150 200 250 300 350 400 450 500 Time ns Correct delay and 20 amplitude error 500ns train with 3 sub pulses 1 6nC bunch so 100 150 200 250 300 350 400 450 500 Time ns 550 600 550 550 600 Fig 5 2 2 1 FCT transformer measurements with intentional phase and amp
11. to capture the whole beam and it is also desirable to have the smallest beam in the emitter plate With 2mm sigma measured at the focus in PHIN one would expect 11ps time resolution from the plate Electron beam Fig 5 2 1 2 Geometry of the photon emission from the Sapphire plate The number of photons produced will depend on the number of electrons their energy the thickness of the plate the spectral range observed and the transverse and angular acceptance of the streak camera We are expecting 5 10 3 photons electron to arrive at the streak camera which with the 2 3nC nominal charge should be plenty However we have found that to have enough light intensity on the streak camera for single bunch measurements the smallest bandwidth filter which could be used to improve resolution was one with 200nm bandwidth Fig 5 2 1 3 filter used at streak measurements 355 550nm c 2 a d D c 450 500 Wavelength nm Fig 5 2 1 3 Bandpass filter used for streak measurements Dispersion calculations through the transport optics using the spectral width of the filter were done by A Rabiller For single bunch measurements including screen beam size streak camera slit acceptance and camera resolution a total resolution of 24 ps was estimated for this set up In summary the single bunch measurement is limited due to e Transport optics total thickness of optics from ZEMAX is 223mm e e beamsize on
12. two arms together Delay setup First assemble the system without the modulators to get familiar with the delay setup Install variable fibre attenuator on the arm1 of the fibre splitter and the manual delay line on the arm2 of the fibre splitter Use Ozoptics splitter combiner 32775 FUSED 12 1053 6 125 50 50 3A3A3A 1 1 PM to recombine the signals Connect the output signal to SDX18000 LeCroy scope for tiling delay analysis No modulation 0 25 0 2 0 1 Amplitude 0 05 0 05 3 1 3 2 3 3 34 3 5 3 6 37 3 8 3 9 Time s x 10 Fig 3 2 2 1 Delay error between the two arms The amplitude balance is restored by adjusting the fibre optic attenuator later this was left at fixed position as damage was observed after repeated changes and was later replaced by a variable attenuator in the free space delay line Delay measured on 18GHz scope 1 18ps turn delay measured L on 18GHz AN Ni LCU E assured on 18G 48GHz HE spate Tan 3nansured Zon de GI BIR Sa ape 3 S803 Baen COLE 0 ZN X teasured delay easured A 2 L SQ e E Lops PRMGHErec JERR AIA Tw Z SRAN IGE aF ed EL 330 4 QD number of turns on delay R 0 9898 9P Delay in ps Fig 3 2 2 2 Delay calibration with oscilloscope Once delay is set for equal time between the two trains on the oscilloscope by turning the delay line connect signal to the spectrum analyser Fig 3 2
13. varying delay is shown on Fig 3 1 1 1 The effect of time delay on the spectrum is greater if we look at higher multiples of 1 5 GHz unlike the effect of the amplitude imbalance between the arms Measured accuracy was limited by the finest adjustment possible manually in the delay and attenuator arm and shows that with stable input source phase coding scheme can provide accurate settings well within specification Changing the amplitude of the delayed sub pulse train pulse changes the size of the spectral lines at 1 5GHz or any of its multiples in the same way the amplitude increases linearly with the imbalance regardless of which arm is more intense The effect of time delay on the spectrum is greater if we look at higher multiples of 1 5GHz unlike the effect of the amplitude imbalance between the arms The frequency domain method is capable of setting the delay and amplitude balance to accuracy significantly better than 0 1 ps and 0 1 The great accuracy can be achieved due to the high dynamic range of the spectrum analyzer A great advantage of this measurement method is that noise such as detector noise generally increases the measured peak and thus makes our calculations of maximum amplitude balance and delay error conservative The fluctuation is greater when the amplitude of the peak measured at harmonics of 1 5GHz is smaller This partly due to the nonlinearity of the decibel scale and partly due to the fact that the fluctuation in one arm
14. voltage across the crystal The two arms are then recombined and depending on their relative phase either interfer constructively or deconstructively giving an output signal which is sin dependent on the applied voltage 6 The high bandwidth of 10GHz available at the lasers 1047nm wavelength provides the fast risetime necessary for the application In order to adjust the operating point of Mach Zehnder modulators independently from the high frequency modulation signal applied they can be designed with two sets of electrodes one set of electrodes the RF Electrodes is used to apply the RF signal The second set of electrodes the DC Bias Electrodes is generally used to adjust with a fixed voltage the working point of the modulator The required system above was called to tender and Photline Technologies was offering the most promising solution with the following notes 7 Laser wavelength 1047 nm Photline Technologies commercially introduced a range of modulators specially designed to operate in the 1000 nm wavelength region in 2003 The so called NIR LN family includes intensity and phase modulators that operate in the 980 nm to 1100 nm window Polarization electro optic modulators are single polarization devices The reason is the electro optic effect only appears in the electric field direction Consequently one should use a linearly polarized source possibly terminated with a preserving polarization fibre Maximum average power Th
15. 0 mW 11 20 aeia 1 Modulator ae Splitter Variable attenuator 1 Combiner 5 95 splitter amplifier 340mW 1 5 transmission 4 8mW possible Fig 3 2 2 1 Total losses in the system 3 3 Assembly 3 3 1 Fiber input set up to the Thorlabs Nanomax stage e Set input power to maximum 1W in case of oscillator max 320mW available e f oscillator is used install Faraday isolator between nanomax stage and fiber input as disturbed mode locking was observed due to backreflections e Make sure light is entering the stage at normal incidence both horizontally and vertically a pair of input mirrors are the best suited for this purpose e Check with card that light exiting through the fibre mount is centred on the pinhole e Install yellow Thorlabs single mode fibre for initial setup onto Nanomax stage 3 3 2 Put other end of the fibre in front of the CCD camera Optimize the throughput with camera first by optimizing objective position and then iterating with X Y Z of the fibre mount Once light intensity is high continue the optimization with a power meter in place of the camera Expected transmission coupling is 50 Exchange fibre to Thorlabs blue single mode polarization maintaining one and reoptimize with X Y and Z using the power meter As the modesize is smaller then 40 is the typical value achieved here Install fibre splitter from Photline and check transmission A total transmission of 50 is expected from the
16. 1 f1 D gt 1 L N qx z d1 fl DN Prop z 41 f1 g 907 eee 1 qy z d1 fl prop z dl fp guy prop z dl D 1 1 r d1 fl lT E Raa i qx z dl fl A fl 2290 focal length d1 2300 distance from output of booster amp 3 qx 200 400 890 200 1 117ix 10 prop 20 d1 fl y g 1 4sigma beamdiameter 4 K wxprop z dl f1 2 2 1 0 210 4x10 6x10 810 Z Fig 4 3 5 Calculated beam propagation to the end of AMP1 An additional 4 2 plate and a polarizer were installed to set the correct polarization input to AMP1 All alignment references were kept from the previous setup With this configuration the following power levels were measured through the system Table 4 3 1 Position Transmission respect to booster output AMP1 in 96 5 AMP1 out 80 7 Before PC 46 4 Table 4 3 1 Transmission through the system The values corresponded well with previous measurements of the preamp input confirming that there was no clipping in the amplifiers The beam was amplified to 1 7kW mean power after AMP1 at 90A pump current corresponding to 1 1uJ per pulse However strong over pumping was observed and so pumping time of AMP1 was reduced to 250us from the original 400us The mean power in this case was 1 6kW Conversion was checked to see the ASE content of the signal 30 conversion efficiency to green was measured and confirmed clean amplification For AMP2 the pumping time ac
17. 2 1 Observe the peak at 4 5 GHZ The fibre optic attenuator is adjusted until the peak is minimized Then the time delay is adjusted to decrease the peak at 4 5 GHz Fig 3 2 2 2 The two steps are repeated until noise floor is reached 3 3 3 Modulator transmission setup Add modulators after the attenuator and delay line Make sure that the input power at the input of the fibre stage is no more than 650mW to avoid damage to the modulators As the new modulator MOD1 has better transmission it should be installed in the attenuated arm The signals are then recombined and observed the same way as above Each arm is observed independently at the start The basic operation of the modulator and the bias controller can be found in the users manual Here only the practical tips are mentioned for setting the QUAD working points Bias point drifts with average power load on the modulator For fast setup use the built in scan of the bias controller to find MAX and MIN operating points Take the mean value of the two for QUAD setting Alternatively scan with MANUAL option the bias voltage and monitor the fibre output from the modulator on the oscilloscope A full scan is shown below Fig 3 3 3 1 Offset 2 1 V Relative transmission L ka gt Modulati n 4 7 V Modulator voltage V Fig 3 3 3 1 Modulator transmission versus bias voltage measurement for setting QUAD operating point 3 3 4 Driver setup Please note that driv
18. CERN EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CTF3 Note 101 Comprehensive user manual for the phase coding system setup and operation with PHIN including the measurements performed with the beam By Marta Csatari Divall A Andersson B Bolzon E Bravin E Chevallay S Dobert A Drozdy V Fedosseev C Hessler T Lefevre S Livesley Mete M Olvegaard A N Rabiller Contents 1 Introduction 1 1 Advantage of laser based phase coding 1 2 Requirements specification precision Design choice 2 1 Pockels cell option at high power 2 2 Fiber optic polarization switch 2 3 Fiber modulator solution 2 3 1 Initial tests one modulator scheme 2 3 2 Two modulator scheme System setup 3 1 Online amplitude and phase adjustment 3 1 1 The frequency domain method 3 1 2 The time domain method 3 1 3 Comparison of accuracy between the two methods 3 2 Components 3 2 1 Coupling into the fiber 3 2 2 The 50 50 splitter combiner from Photline 3 2 3 Modulators 3 2 4 Delay line 3 2 5 Attenuator 3 3 Assembly 3 3 1 Fiber input set up to the Thorlabs Nanomax stage 3 3 2 Delay setup 3 3 3 Modulator transmission setup 3 3 4 Driver setup 3 3 5 Final adjustment Integration into the laser chain 4 1 Fiber amplifier 4 2 Beam profile and stability 4 3 Full amplification Harmonics 4 4 Timing signals for the phase coding Measurements with phase coding 5 1 Measurements on the laser 5 1 1 Timing and ampl
19. The coding system will be inserted between the existing oscillator and preamplifier Losses in the coding system will have to be compensated for in the preamplifier and subsequent power amplifiers These losses should therefore be minimized Reliability The laser system including the coding system needs to operate for long periods days weeks with very high reliability Laser input parameters Wavelength 1047 nm ee Pulse length Gaussian FWHM Pulse repetition rate cw mode locked 1 5 GHz Time between pulses 666 ps Table 1 1 Laser parameters at the input of the phase coding The coding required Switching duration 140 us at up to 50 Hz Length of sub pulse 140 5 ns 212 pulses Switching frequency 7 1 MHz Delay between generated sub 333 ps pulses Pulse timing stability lt 0 2 ps ees S Table 1 2 H i e WII V mM AN AE UW UW n i Delay of 4 rf period amp recombination Phase coded Laser pulse train AA 11 S G67 ps Pma N j i _i_ i Fig 1 1 Time structure requirement 2 Design choice 2 1 Pockels cell option at high power In the past systems based on fast switching Pockels cells at the end of the laser chain were considered 3 Fig 2 1 1 However for good transmission at high laser power levels these cells require large apertures to avoid optical damage a 7kV of pulsed drive voltage together with the fast rise and fall times lt 500ps This
20. Trigger Display Cursors Measure Math Analysis Utilities Help 1 Stability from UV photo diode with phase coding 150 k i 100 Fani 1 50 0 5 0 55 0 6 0 65 0 7 0 75 l Timebase 398 nsf Trigger GGL 0 50 0 mvidiv 200 mvidiv 200 mid S00 mV div 100 nsidiv Stop 97 mv G 84 0 mV 550 0 mV T70 0mV 0 0 mVofst 5 00kS 0GS s Edge Posi 61 my dy 24 4mV dy 14 6mV ay 52 9 mV E ais l Signal level V 11 4 03 14 PM AT 1 N Fig 5 1 1 1 Micropulse stability at fixed point of the train measured with PD in the UV before beam leaves the laser room Phase coding energy train No phase coding energy train 400 H stdev 1 073mV 1 7 RMS 300 Bat oO a iste 200 100 55 histe hi ste Signal mV Signal measured mV Background BPR no phase coding o stdev 0 99 lL mV n stdev 6 25mV 1 3 RMS histe DL 0 38 40 42 44 46 48 0 45 0 46 0 47 0 48 0 49 0 5 histe histe d Signal giang ag mV Signal meas est V Fig 5 1 1 2 Macropulse energy stability measurements and parallel charge stability measurement The laser energy measurement in CTF2 showed the same stability in both the PC and NPC cases and gives an upper limit to the energy stability as the background noise of the detection system is relatively high 1mV with measured values being in the region of 60mV in this case BPR measurements show good consistency with the measured 1 9 RMS at the laser in the NPC ca
21. ailed knowledge of the beam emittance and divergence we can only make a rough estimate of the effect of the intrinsic beam size More detailed measurements with better matched beamsize and solenoid setup would improve these measurements It would also be useful to carry out the same measurements under the same conditions without phase coding 5 2 4 Emittance and beam size measurements Similar time resolved measurements were also carried out at the first OTR to see the effect of phase coding if any on the beamsize and transverse emittance Details on the methods used for this measurement can be found in Ref 13 Here we integrated over the central 100ns of each sub pulse In parallel laser beamsize measurements were taken over the 3 consecutive sub pulses in the 500ns long pulse Fig 5 2 4 1 Variations were within the measurement errors and have shown no change between balanced un balanced and non phase coded beam cases Gated Emittance Scan 18 02 2011 Focusing Magnet Current 171 67 A Laser beamsize variation in X along the train 12 c c c c c c c c lt e gt 6 3 21 6888 mm mrad LPCON N aa 0 67 T lt gt 7 38 1 0964 mm mrad LPC Unbalanced Intensity 104 lt e gt 7 4 21 3157 mm mrad LPC OFF 0 66 z 0 65 G i 0 64 S 8 i Unbalanced i f 0 63 E 7 r Gair as E Balanced E P3 0 62 c 6 i a 0 61 5 0 100 200 300 400 500 m Time ns
22. al amplitude due to the QUAD 50 working point of the modulators Detailed explanation on the mathematics of the signal evolution at different multiples of the 1 5 GHz can be found in ref 10 As we approach 180 phase shift odd multiples of the 1 5GHz the lines will disappear from the spectrum while the even multiples of the 1 5GHz the spectral lines will corresponding to a clean 3GHz periodic signal spectrum increase At different amplitudes of the two signals however the odd spectral lines will remain with some non zero amplitude even at perfect time delay on A N 0 6 0 54 a OLY P Pa Amplitude m 1 m 2 m 3 F 60 120 240 300 360 Phase in degrees 0 2 0 18 0 16 b 0 14 0 12 0 08 0 06 Amplitude compared to 3 GHz peak 170 175 180 185 190 Delay in degrees Fig 3 1 1 1 a Expected variation of the spectral line relative to the peak at different frequencies with perfect amplitude balance b Amplitude of the 4 5GHz peak respect to the 3GHz main peak with fit including amplitude error By minimizing the 4 5GHz peak through iteration of time and amplitude adjustment peak suppression 40 67 dB below the 3 GHz reference was achieved depending on the noise arriving from the oscillator noise on each day This corresponds to a maximum amplitude error of 1 76 0 044 with maximum timing error of 0 1 0 016 ps The change of the 4 5 GHz peak by
23. cording to the change on AMP1 was reduced to 200us 9 4kW mean power was measured this way with excellent beam profile after AMP2 Fig 4 3 6 This corresponds to 6 3uJ pulse at the exit of the PC Fig 4 3 6 Beam profiles after AMP2 with booster amp input at 140mW at AMP1 left and with previously used HighQ 7 4W input right with same attenuators in front of the camera Although mean power levels at the time of the PC switching are comparable with the higher seed input to AMP1 and AMP2 reduced pumping time results in slower build up of the steady state Although amplitude stability of the booster seed is higher than of that of the HighQ preamp we benefit less from the stabilizing effect of the steady state saturated amplification in this case 0 6 RMS stability was measured in the IR which is only slightly above the 7W seeded case where 0 35 was the typical value x U 4 P2 ampl C2 P3 ampl C3 P4 ampl C1 P5 ampl C4 PB pkpk C2 Vv 77 91 mY 950 mY amv 30 mY 31 mY 30 08 mY 76 8176 mv 905 4 mY 170 601 mv 30 39 m 30 43 mV 5 mv m mY Measure Pl ampl c 30 m Y mi min max sd 35 mV 78 80 mv 972 mY 176 5 mY 35 mY 34 mY 1 95 mY 753 2 UV 23 2 mV 2 216 mV 1 99 mY 1 24 m 104 num 104 status K a R L histo E EEE er T PEE OS eee eee rigger EDEN 50 0 mid 20 0 mVidiv 500 mWdiv 200 mWdiv 0 psidivi Normal 164 0 mv 33 00 mv 1 050 V ofst 230 0 mv 1 00MS 5 0GS s Edge Positive X1 130 0000ps Ars 200 00 US Ay
24. dB below the 9 GHz reference This corresponds to a maximum of 0 6 amplitude error or a maximum delay error of about 0 19 ps These measurements seen on 3 1 3 1 where done with 4 times interpolation and calculated from almost 300 000 pulses for each point Delay error Amplitude error 1 trigger level 0 027 ps 1 31 2 trigger levels 0 136 ps 1 31 Table 3 1 3 1 From the delay error and the amplitude error determined in the time domain the expected magnitude of the 10 5 GHz peak can be calculated Turns Magnitude of 10 5 GHz peak N 96 N AO x 5 4 3 2 1 0 l 2 3 4 5 Spectrum analy zer measurement Calculated from time domain measurement Fig 3 1 3 2 Comparison of time delay with the two methods As seen on Fig 3 1 3 2 the calculated and measured values fit each other very well From this we can conclude that amplitude error measured in the time domain is larger than in reality As mentioned before the accuracy of the amplitude error measurement can be greatly improved by applying this method to the phase coded train 3 2 Components 3 2 1 Coupling into the fibre In order to couple light into an w diameter fibre with a D 1 e2 diameter beam the ideal focal length is D TW 4 where is the wavelength In our case the fibre is w 6um and the oscillator output at this point is 2 2mm which suggests an optimum focal length of 11mm New objectives were purchased to improve coupling efficiency but as th
25. e NIR LN modulators have been characterized with an input of up to 100 mW icontinuous optical power Operations at higher power have been experimented but the devices have not been tested for reliability at such levels 320 mW is above the 100 mW CW recommended maximum optical input power Secondly all performed tests have been conducted with continuous input optical power For the phase coding pulsed application Photline has no data in quasi continuous conditions Later discussions with a Photline engineer in 2010 suggested that 4W peak power is the maximum allowed input for pulsed operation in the ps range Drifts and stabilization Lithium Niobate is subject to the photo refractive effect That effect increases when the optical wavelength is decreasing and it is noticeable at 1060 nm above a few tens of mW of input power It translates into drift of the operating point of the modulator and consequently in variations of the output optical power To compensate for the drift of the operating point several feed back loops techniques can be implemented Photline proposed the MBC 1000 Modulation Bias Controller that locks the operating point of the modulator at a user selected position The principle relies on applying a low frequency tone signal to the DC electrodes of the modulator and analyzing the modulated optical output power The frequency and amplitude of this modulation has to be set right for the operating point QUAD and will depend on the
26. e are not as significant as expected Fig 5 2 3 2 RF pulses were unfortunately not recorded this time to be able to tell whether these variations really came from the phase coding or RF variation along the pulse as it was observed before 12 Unbalanced Balanced RMS momentum spread o p p p gt gt gt 9 9 x So000 4 828Mevitc Uncorrected P Centra 4 937Mevic balanced Uncorrected P gt unbalanced 700 800 900 1000 1100 1200 1300 time ns Centra Phase coding with 500ns Central momentum 5 o p 2o71 balanced 4 85 gt gt unbalanced gt time ns time ns 4 83 o 10t 102 103 9 2 b oo o tot 4 82 K ia os APIP T aa rel APIP 700 800 900 1000 1100 1200 1300 time ns PHIM sepdump ime resoked momania 5b 5 lt 700 4800 Bd imo 1100 100 12300 jadi time nsec Fig 5 2 3 2 Energy measurement using dipole scan with balanced and unbalanced phase coding M Olvegaard carried out the measurements and analysis and concluded that for absolute values regarding energy and energy spread the following needs to be taken into account e We measured the beam size on the screen before the dipole but this is not sufficient for the prediction of the beam size at the segmented dump e The horizontal width was kept constant within lt 5 e The horizontal position showed a 15 variation from shot to shot e Without a det
27. e coding is set up 4 Integration into the laser chain Several modifications were necessary to the laser system to accommodate the phase coding setup Fig 4 1 e interlocks had to be modified to make sure AMP1 and 2 turn off in case of failure to the booster amplifier e Output of the booster amplifier had to be matched to the rest of the laser chain both in power and in size e Amplification of the phase coding had to be confirmed at several stages of the system so additional beam paths had to be created to send the beam to the streak camera station e As power of the oscillator was not sufficient to drive the phase coding without the booster amplifier part of the HighQ preamlified beam had to be coupled into the fiber Legend Mirror Periscope R Lens Sb Camera 2 Halfwave plate Eh Photodiode Faraday isolator or part N Shutter Polarized beam splitter f Filter k Pockels Cell Optional iris ew Power meter Optional mirror Fig 4 1 Setup of the laser system including optional beam passes for streak measurements 4 1 Fibre amplifier The fibre booster amplifier was designed to run with a minimum of 10mW input to ensure that final output power of 320mW is reached with a high level of saturation for stability There is an interlock built into the system which turns the amplifier off when the seed is lost to avoid damage due to ASE This is triggered when the input signal level drops below 5mW Due to th
28. e low damage threshold level of the fibre modulators and the number of connectors in the system as well as the 75 inherent loss due to the switching and non polarizing recombination as mentioned in paragraph 3 2 the maximum achievable output from the system is lt 5mW After discussions with the company the seed error limit was dropped to 4mW and the maximum output power revised down to 250mW Despite this we have found that the system was repeatedly producing output error when run at these extreme parameters and lowered the output to 200mW for operation N B the factory password needs to be acquired each time a modification to the system is done Amplification with the fibre booster amplifier was measured and switching was confirmed with streak camera measurements and with the freespace fast photodiode AlohaLas UPD 40 on the SDA18000 oscilloscope Fig 4 1 1 Please note that Pockels cell was necessary to gate out the cw mode locked signal from the booster amplifier to make the streak measurement possible Booster output at 50mW Amplitude 210 215 220 225 230 Time ns Fig 4 1 1 Streak camera measurement at second harmonic of the booster output left and PD measurement at the fundamental right showing the 999ps switching 4 2 Beam profile and stability There any many advantages in using the fibre based amplifier to power levels below the threshold where non linear effects appear and where bandwidth broadening does not make la
29. ers should not run when they are not correctly terminated so signals should either be connected to a 50Q terminated scope or to the modulators As modulator halfwave voltages are known from the previous measurement the signals can be connected first to an oscilloscope to check the voltage levels are set correctly in the driver amplifier In our case typical values for Vx where 4 93V for Mod1 and 4 87V for Mod2 Connect timing signal to the driver box to trigger the switching One STOP and one START signal is required Counters normally used for CALIFES pulse pickers were connected for this purpose There is no power switch so once the cables are connected plug the device into the mains Observe the signals on the scope and adjust the voltage to the measured level for each driver output Two potentiometers crews are on the front of the driver for this purpose Unplug the driver box and connect the signals to the modulators Observer recombined signals on the oscilloscope with driver box on and modulators biased to QUAD and QUAD The timing of the driver switch determines the envelope of the sub trains It is necessary to set the cable length from the driver to the modulator to make sure that switching takes place in between pulses and not cutting through them Observe the start and end of pulsetrain over serveral shots as the jitter is 140ps 3 3 5 Final adjustment Set timing between odd and even sub trains to 333ps and check 140ns later
30. etween two consecutive pulses can be easily ensured by choosing the right cable length between the output of the driver box and the modulators Fast trigger signals to start and stop the counters also synchronized to the RF frequency were provided by CNT6 and CNT7 e Driver amplifier with improved square pulse response was purchased to improve flatness of the sub trains As within the 666ps cycle time the pulses are 5 5 ps long both the in house switching system and the driver amplifier provide the flatness in the 140ns switching period a File Control Setup Measure Calibrate Utilities Help 22 Oct 2010 17 17 ia Oscilloscope Mode Amplitude y Measure 54 r I V apt d 2 SS V ta B5 V SIC TT Tm rainl P Setup width 2 104 0 ns 104 01 ns 22 ps 103 9 ns 104 1 ns amp Info l Frequency 2 6 668 MHz 6 668 MHz 641 9 Hz 6 667 MHz 6 672 MHz Duty cycle 2 69 4 69 4 0 ae 69 4 1 445 udi 1 19 Vd 3 4 Precision Timebase Time 50 00 ns div Trig Normal Patten 1225 3 uv 1 200 V E 3 Reference 12 00000 GHz i Delay 163 000 ns l 531 mV l 24 Lock Fig 2 1 2 2 DR PL 10 test performed at Photline 3 System setup 3 1 Online amplitude and phase adjustment A diagnostic was needed to provide easy accurate and preferably online measurement of the timing difference and the amplitude balance between sub trains Photo diodes provide fast and accurate measurement for both timing and amplitude Our highest bandwidth pho
31. ey arrived after the experiment they have not been tested for improved coupling efficiency 3 2 2 The 50 50 splitter combiner from Photline Due to either connector fibre incompatibility the output measured though arm 1 was 25 4 and through arm 2 24 5 this is for the splitter SN 7317 In the case of splitter SN 7504 arm 1 only had 7 5 transmission These splitters can also be used for the combination of the two beams to ensure the same polarization from the two arms although the overall transmission cannot exceed 50 Polarization based combiners have been purchased from Ozoptics with high transmission but the two combined arms carried opposite polarizations where only one arm was amplified by the booster amplifier This is due to the fact that a Faraday isolator is placed inside the booster amplifier Although booster amplifier could amplify both polarizations the Nd YLF amplifiers could only amplify one due to the birefringence of YLF 3 2 3 Modulators Care should be taken not to have more than 100mW average and 4W peak power input into the modulators By scanning the bias voltage either manually or automatically one can find the maximum transmission operational point Here we have measured a transmission of 24 3 for modulator 1 and 27 4 for modulator 2 including losses on the connections One should note that both halfwave voltage and operating point bias are drifting with temperature Photline suggested temperature stabilization
32. hase coded case The indicated 29ps full width half maximum is not corrected for the time resolution of the measurement system 21 02 2011 21 02 2011 Data Streak Camera Data Streak Camera Fit Skew Gaussian Fit Skew Gaussian P length FWHM 29 8379 4 0 91087ps intensity a u 0 100 200 300 400 0 400 200 300 400 time ps time ps Without phase coding With phase coding Fig 5 2 1 5 Single bunch length with and without phase coding As Cherenkov line was available for the first time a phase scan was also performed to measure the dependence of the bunch length on phase Fig 5 2 1 6 The phase is relative to maximum charge phase moving towards operational phase Measurements below are corrected for time resolution of the measurement system taking into account the effect of the beam spot size on the crystal and the dispersion of the optical transport However as it can be seen from the beam size measurement Fig 5 2 1 7 there was asymmetry in the beam size variation as a function of the phase in X and Y which makes it difficult to take these variations into account in the calculations 24 02 2011 25 ps mm Measurements versus phase of 44 9807 1 4655 bunch length FWHM and average ps 10 0 10 30 40 50 20 Phase deg Fig 5 2 1 6 Bunch length with correction of beam size and lens thickness as a function of the phase Sigma y mm
33. he output of the fibre collimator was measured with UEye digital USB camera to obtain the right lens system T main 2 O NW UND Beam size measurement after the booster A ita ene Pr output collimator dm df 0 45 fitx O 0 401 E 04 pas Fit for Y X L 6 T N o 0 2 b xX 4 wm 0 15 ym 2 g o1 3 Y 3 005 poa E o 1 2 0 500 1000 1500 2000 0 Distance from fiber outcoupler mm 0 2 10 410 610 dm df ProPX o 0 401 as 0 274 theoretical waist size qOx i propx q0y ipropy complex beam parameter Fig 4 3 4 Beam size as a function of distance from the exit of the fibre booster output coupler and beam size prediction at the end of the AMP1 By applying Gaussian beam propagation fit here a complex beam parameter of i 0 401 and i 0 374 was determined for X and Y respectively To be able to propagate the beam through the 9 3 meters of total distance from the fibre booster to the 3rd exit pass of AMP1 without clipping or diffraction the beam has to be kept at lt 3 5mm 4sigma beamsize For simplicity a single lens configuration was found using a f 2300mm focal length placed 2300mm from the exit of the booster amplifier Fig 4 3 5 single lens arrangement to get through AMP1 Is z f s z 1 f prop z d1 f1 m if z lt dl is z d1 fl s d1 otherwise prop z d1 1 gee FPR dL O 1 qx z d1 fl prop z dl fl P prop z dl f1 1 f wxprop z d
34. ing fiber optic modulators on the drive laser NIMA53834 Nucl Instr and Meth Phys Res A 11 S Battisti Measurement of the short bunch length in the CLIC Test Facility CTF CERN PS 93 40 BD 12 D Egger et al Design and Results of a Time Resolved Spectrometer for the 5MeV Photo injector PHIN IPAC2010 13 O Mete E Chevallay A Dabrowski M Divall S Dobert D Egger K Elsener V Fedosseev T Lefevre M Petrarca CLIC Note 809 2010 http cdsweb cern ch record 1262856 files CERN OPEN 2010 013 pdf version 1
35. itude accuracy 5 1 2 Satellite measurements 5 2 Measurements on PHIN 5 2 1 Cherenkov line measurements 5 2 2 FCT measurements 5 2 3 Energy measurements Future improvements Conclusion 1 Introduction The following document is intended to document the set up and measurements performed during the phase coding demonstration experiments Following this document an expert laser operator should be able to reproduce and hopefully improve on the results documented here It is not meant to be a complete and reviewed scientific publication One of the main challenges on CTF3 is multiplying the electron bunch rate from 1 5 GHz to 12 GHz This is carried out using a delay loop and a combiner ring with high frequency transverse deflectors to separate and combine the electron bunches For this to work the initial electron bunch train must be phase coded i e some of the bunches must be delayed by half of a 1 5 GHz period with respect to the others 1 With the current thermionic gun this is achieved by subharmonic bunching which is done over 8 consecutive pulses and causes considerable amount of unwanted satellites in the system 2 With the laser based photo injector the phase coding can be done on the laser pulses before they are sent to the photocathode The time structure produced by the laser directly correlates with the electron bunch produced The source of the laser pulses is a 1 5 GHz mode locked ps laser see Table 1 1 The required codi
36. l and circular polarization states in between 4 The range and degree of the change in the SOP can be varied by adjusting the magnitude of the DC bias and RF drive voltage A possible setup is shown on Fig 2 2 1 in O Losses In Yo Polarizing combiner 20 55 333ps waveplate 55 waveplate 11 Peay 11 Oscillator Polarization mN T 10 320mW 2 Fiber Polarizer outcoupler 20 WEVateleli Fiber Polarizing splitter coupler Fig 2 2 1 Phase coding system based on fiber optic polarization switches Because of the insertion loss of the device even two modulators with the second one switching the polarization back after recombination would not help With this scheme we would have 30 mW output As the modulator is made using GaAs the optical wavelength is currently limited to a wavelength range from 1520 nm to 1570 nm With some redesign it may be possible to develop a device to work at 1310 nm but operation at 1047 nm is not possible without further R amp D on suitable materials 2 3 Fiber modulator solution Lithium Niobate LINbDO3 fiber optic modulators have been developed at 1um wavelength for amplitude modulation and offer high extinction ratio of 40dB 5 The waveguide based Mach Zehnder interferometer consists of a Y split where the polarized optical input signal is split into two equal components Fig 2 3 1 One arm contains the electro optic crystal LINbDO3 where phase shift can be introduced by applying
37. litude error introduced to the system Fig 5 2 2 2 Phase coding with nominal parameters measured on PD s and FCT 5 2 3 Energy measurements Energy measurements were done with the phase coded beam to see the difference between sub sequent sub pulses In all cases a 500ns long train was measured covering two full switches As during this run one of the aims was to produce high charge relatively large laser beamsizes were used As there are no additional optics after the first OTR for transverse measurements this was not in favour of the energy measurements The beamsize on the OTR dedicated for energy measurements was too big and part of the beam was clipped some parts supposedly even passed the frame before the segmented dump Some channels corresponding to these outer parts of the beam were disclosed from the data analysis Single shot measurements carried out with segmented dump therefore were not so successful Dipole scans were performed measuring momentum evolution along the pulse each step is integrated over a 20 ns window Fig 5 2 3 1 Note that the momentum spread also includes the intrinsic size of the beam which with our case was quite significant PHIN beam spectrum p 4 84 MeV c Ap _ 8 02 PHIN segdump single shot 9 23 A 1300 1 11 b 3 2 S S Beam instability PHIN segdump average momentum along pulse p MeV c KR N o A p p 4 55 central momentum fo fwhm
38. mized for shorter wavelength In INFN they reported 15 loss in the fibre and 10 loss at each connection Variable attenuator was not implemented on the system 2 3 2 Two modulator scheme In light of the previous test several improvements were made to the system The design was changed to a two modulator scheme shown in Fig 2 3 2 1 The main modifications are listed below Reflected Lossin 2 100 20 11 Oscillator 333ps delay 320 mW 20 5 95 splitter Booster amplifier 340mW Modulator Transmitted Waveplate Splitter Variable attenuator 1 Combiner and PBS Fig 2 3 2 1 Two modulator based scheme with measured losses e Two modulator scheme is safer against power damage Lossy elements such as the fiber splitter attenuator and the delay line can be placed before the modulators Furthermore the power is split in between the two modulators This way full power of the 320mW oscillator can be utilized without the risk of exceding the safe 100mW power limit for the modulators e As the total delay introduced is only 333ps between the two arms sensitivity to temperature changes is very small well below the requirement e A fibre oscillator purchased from Fianium provides amplification back to the oscillator power level of 320mW and includes an active feedback stabilization system with 100uUs response time to ensure constant output power by adjusting the pump current The amplitude stability meas
39. ng system will delay or not so called even and odd trains of laser micro pulses which subsequently will result in phase coded sub pulses of electron bunches in the accelerator The coding does not need to be continuous It will be sufficient to apply the coding over periods of 140 us at up to 50 Hz repetition rate to match the amplification window of the amplifier chain Each sub pulse will contain 212 laser pulses The delay applied to the even sub pulses has to be equivalent to a 180 deg phase shift at 1 5 GHz i e 333 ps See Table 1 2 and Figure 1 1 The specifications for the phase coding are summarized in Table 1 2 The important performance parameters of the coding system include Switching time The system must completely switch from one sub pulse delay to the next between two consecutive 1 5 GHz oscillator pulses i e within 666 ps Together with the jitter of the switching electronics a rise time of lt 200 ps on the optical modulator or switch is required Stability There are very tight requirements on the pulse to pulse energy stability and the timing Stability from the overall laser system The coding system must not increase the timing jitter of the laser pulses by more than 0 2 ps rms Variations in the pulse energy must be minimized and ideally they should be kept below 0 1 rms although higher values than this may be acceptable as the laser system will include both passive and active amplitude stabilization Losses
40. o non linear processes the energy in the UV will be only 5 of the main pulses To be within the specified 4 for CLIC one would have to make sure that bias drift are kept below 1V which given the full switching voltage of 5V is reasonable under normal laboratory conditions 1 5V bias error 20 LV 12 bias error mtroduced la Transmission e 0 1V bias error e 0 3Vbias error lt 1V bias error L 1 o9 0 9902 05 0 9 9 0990 Se Z 0 8 6 3929 07 0 100 200 300 400 Time ns Transmission Amplitude normalized Qo _ 0011 a 1 The two non linear conversion stages 05 T L a suppress even 20 of satellites from Modulator voltage V ein ne Time ns the phase coding Fig 5 1 2 1 Bias error propagation measurement 5 2 Measurements on PHIN 5 2 1 Cherenkov line measurements The Chereknov line was designed in collaboration with BI group with the aim to resolve bunch separation and was based on the CTF2 design Fig 5 2 1 1 Solenoid magnets a steering magnet and a beam positioning monitor are used to optimize the beam transport to the position of the beam profile monitor For bunch length measurements a 300um aluminium coated sapphire Cherenkov target is used which generates a flux of visible photons compatible with the sensitivity of the streak camera OTR used for transverse profile measurement would have provided better resolution but with the 5MeV beam then light intensity would n
41. o pulses The result of the subtraction is phase of the pulse It is important to note that this is implemented in a way to avoid underflow even with a large number of pulses 0 03 0 025 0 02 2 E 0 015 D L I Trigger Level 9 0 01 a x rigger inactive 0 005 gt Che l S K A J J 5 a T A AS C3 0 g Q dX A PE T l a Y J M s d UAR s 2 L 0 005 M lim Phase nT S J 0 01 N Al L L L L L L L S L E 501 3 501 4 501 5 501 6 501 7 501 8 501 9 502 502 1 5022 502 3 Time Fig 3 1 2 2 Trigger definition for post processing Because the pulse repetition rate is not exactly 1 5 GHz and because the oscilloscope time base is not perfect the measured phase is constantly drifting in one direction This can be corrected by applying a linear fit to the phase and then subtracting values of the fitted curve The resulting corrected phases for the pulses from the delay arm and the pulses from the attenuator arm both follow more or less Gaussian distributions The difference between the mean of these phases gives us the delay delay error When the method is used with the modulation on it is important to use an equal and integer number of odd and even sub trains as input 3 1 3 Comparison of accuracy between the two methods The 10 5 GHz peak was used for the spectrum analyzer measurements to demonstrate the increased delay sensitivity In initial setup the 10 5 GHz peak was 44 1
42. ot have been sufficient for detection with a streak camera Detailed design and information can be found dfs Workspaces p PHIN_photoinjector Cherenkov line Optical line to transport line from CTF2 to the laser lab v Design ofthe optic line done with Zemax software in December 2010 v Construction and alignment of the optical line done in December January Focusing eo iers Streak z T EFL 160mm 8 EFL 1100mm EFL 1189mm EFL 1189mm laser lab a ee m L CTF2 gt Transmisiongiven by Zemax 70 Fig 5 2 1 1 Cherenkov line layout from CTF2 PHIN to laser room a Time resolution The least time principle will assure that the photons generated by electrons arriving with the same speed at the same time in the same place will also appear at the same time on the exit of the plate However there is a delay between photons which were produced by electrons at different speeds and on a different part of the plate The electrons which propagate through the plate will produce photons both at the entry and the exit surface with a time delay which is proportional to the relative speed of the electrons and the photons and also to the thickness of the plate To calculate the time resolution achievable from the plate only one needs to take into account the path difference between AB and DEFC fig 5 2 1 2 The captured area as a consequence will also affect the time resolution It is the best however
43. overall performance for transmission is shown in Fig 2 3 3 misure con fascio laser in continua non mode locked Beam Splitter fiber 30 m fiber lt collimator fiber in out a EO 200 man S49 Modulator U np CCLRC 1 4 5 05 05 0 5 0 5 0 5 loss dB assesments 0 8 0 35 09 OF 09 09 0 9 EIB Milano ok 73 ok gt 0 5 ok ok gt 0 5 measurenents 0 18 lt 0 9 lt 0 9 Fig 2 3 3 Transmission measured at RAL and at INFN e INFN reported a decrease of output from the modulator over time 18 to 7 There was also earlier data from RAL which shows 36 transmission The modulator was sent back to Photline and the input fiber was found to have been damaged due to high input power levels e It was apparent that without further amplification the system would not provide enough power throughputs e Furthermore a 140 7 ns fiber delay line is necessary with less than 500fs timing accuracy Resistors were installed to heat the plate holding the fibre bundle but no power supply and monitoring device for the temperature stabilization was implemented After accurate cutting of the fibre temperature can be used for fine alignment and has to be kept at 0 2 C accuracy to achieve the required timing stability From the temperature induced refractive index variation 1 92ps C can be calculated for the 30m delay fibre e The fibre used Cicorel UK polarization maintaining singlemode fibre patchcord FC PC to FC PC on jacketed fibercore HB980T was opti
44. places a high average power demand on the driver due to the 50 duty cycle operation for the phase coding switch and is too challenging for the currently existing solid state voltage drivers In addition the relatively long switching time would cause birefringence to build up in the crystal which is against the EO effect and would cause instabilities or degradation to the flatness of the pulse The fast switching might also introduce ripples on the top of the pulse train Further advances in MOSFET drivers and small aperture low voltage Pockels cells used directly after the laser oscillator could provide a solution in the future Losses In 70 waveplate 2 2 2 55 Leme 5 Polarizer Polarizer waveplate Oscillator ana E S T 38 320mW 0 2 Lens system Polarizer Fig 2 1 1 Possible Pockels cell based phase coding setup in the infrared 2 2 Fiber optic polarization switch Elecro optic polarization modulators based on polarization switching dependent on applied voltage could provide a similar solution to Pockels cell based systems but with much smaller driving voltages For example the Versawave 40 Gb s Electro Optic Polarization Modulator is capable of changing the state of polarization SOP of light at ultra high speeds Functioning as a high speed electrically variable wave plate the modulator is able to change the SOP of linearly polarized laser light to an orthogonal linear polarization passing through elliptica
45. rate the required electron bunch structure for frequency multiplication The photo injector with the fibre optic modulator based switching system would significantly improve radiation and efficiency losses for the 1 5 12GHz beam combination in CTF3 and would also provide a solution for the future CLIC machine References 1 C P Welsch et al Longitudinal beam profile measurements at CTF3 using a streak camera 2006 JINST 1 PO9002 2 P Urschutz et al Beam dynamics and first operation of the sub harmonic bunching system in the CTF3 injector Proceedings of EPAC Edinburgh Scotland 2006 pp 795 3 A Masi Report on the visit to the 2005 Pulsed Power Conference CERN AB 2004 001 GGG 4 www versawave com 5 Ed L Wooten et al A Review of Lithium Niobate Modulators for Fiber Optic Communication Systems IEEE Journal of Selected Topics in Quantum Electronics Vol 6 No 1 Jan 2000 p69 6 F J Leonberger High speed operation of LINbDO3 electro optic interferometric waveguide modulators Opt Lett Vol 5 No 7 1980 pp 312 7 www photline com Herve Gouraud 8 G Kurdi et al Development of the CTF3 photo injector laser system STFC CLF annual report 2007 pp 225 9 Boscolo S Cialdi D Cipriani and F Castelli CERN CTF3 Laser Phase Coding System and Tests October 26 2007 EDMS number 10 M Csatari Divall et al Fast phase switching within the bunch train of the PHIN photo injector at CERN us
46. se Streak camera measurements were carried out directly on the laser to confirm accurate time delay settings Fig 5 1 1 3 18 02 2011 12000 10000 8000 6000 intensity a u 4000 2000 0 200 400 600 800 1000 20005 pixels 200 400 600 800 1000 pixels 999ps switch 333ps switch Fig 5 1 1 3 Measurements performed on the 523nm on the laser system to confirm switching The pulse separation was within specification and is not expected to drift by more than few 100fs 5 1 2 Satellite measurements When a bias drift occurs in the modulators the extinction ratio between the ON and OFF states gets worse The ON state signal will drop and in OFF state there will be pulses leaking though as shown in figure 5 1 2 1 resulting in satellites in between the wanted electron bunches As there was no active stabilization used for the bias this time we were interested to see how these bias errors affected the amplification and later the conversion and electron production A bias error larger than expected from temperature drifts at 1 5V level was introduced causing satellites at 20 level For illustration the other modulator was kept in a perfect switching state The satellites on the relative slow detectors will appear as a DC error in the other sub train These pulses are conserved and amplified with the same gain as the main pulses However as these pulses contain very small pulse energy relative to the main pulses during the tw
47. settings For ease of measurement and operation dedicated timing signals counters for start and stop of phase coding would be essential Purchase of fast UV detectors would allow us to characterise the laser without performing lengthy and complicated streak measurements 7 Conclusion Fast phase switching within the 1 5GHz electron trains was demonstrated on the PHIN photo injector The switching is achieved by direct manipulation of the laser pulse structure The phase switch can be set with 0 1ps accuracy and introduces no additional jitter to the laser oscillator which is locked to the 1 5GHz RF frequency with accuracy lt 500fs The amplitude balance between sub trains can be set to the required accuracy of 0 1 rms and is currently limited by fast noise of the laser oscillator Emittance energy and stability measurements show no measureable beam degradation due to the phase coding Some long term drift measurements could be performed in the future with improved drift controls on the laser side Energy measurements with full recording of the RF and beam parameters and with optimized beam on the spectrometer screen would compliment the measurements so far Improved resolution on the Cherenkov line could provide a useful tool for single bunch measurement studies in the future The PHIN photo injector is the alternative to the thermionic source for CLIC test facility 3 where sub harmonic bunching and fast RF phase switching are used to gene
48. target e Thickness of the Sapphire plate e Light intensity bandwidth If there are enough photons produced one could think of using a thinner plate and only taking part of the emitted light too to improve time resolution Even with an infinitely small electron beam there will always be a residual dispersion due to the difference of speed between the photons and the electrons The ideal vertical acceptance for transport to keep resolution below 2ps is 1 5mm Dispersion will also play a role in the time delay but with narrow pass filters or reflective optics this could be minimized However most of these cause loss of photons at the streak camera b Bunch separation measurement Taking into account the time resolution of the streak at 250ps mm sweep speed the observed bunch separation was within the specification Amplitude variations over the sub trains came directly from the laser oscillator Fig 5 2 1 4 21 02 2011 338 594 666 9523 333ps switch intensity a u ar 500 1000 1500 2000 2500 3000 3500 time ps 18 02 2011 992 5361 999ps switch intensity a u 0 500 1000 1500 2000 2500 3000 3500 time ps Fig 5 2 1 4 Bunch separation when switching takes place measured on the streak camera with Cherenkov light from PHIN c Single bunch measurements Phase scan The first aim was to show that phase coding does not have an effect on the bunchlength Figures below show no difference between phase coded and non p
49. ter stages of amplification inefficient After discussions with Fianium it was concluded that 10W out power can be delivered at 1047nm wavelength with lt 1nm spectral bandwidth and lt 10ps pulse length The spectrum is within the gain bandwidth of Nd YLF and the pulselength after harmonic stages would still provide the specified lt 10ps pulses on the cathode However the purchase of the 10W amplifier was not possible at this date We made the following remarks on the laser Fibre booster amplifier was found to provide exceptionally good beam quality As the delivery is through a fibre the pointing stability of the front end of the system is determined only by the stability of the optical mounts and not the stability of the laser such as the solid state HighQ pre amlifier These two together results in less daily alignment of the system Amplitude stability is regulated by an internal feedback and was measured at 0 08 RMS Currently with the 200mW input to the amplifiers we reach higher UV at the end of the system due to the improved beam quality before we had 7 5W from the HighQ preamp as an input 4 3 Full amplification Harmonics The scheme above was planned where the oscillator output provides the input for the phase coding after the losses are recovered by the recently purchased fibre booster amplifier The output is then amplified with HighQ preamplifier and the high power amplifiers as in the original system HighQ was not willing to
50. that 999ps switch is also present Block the delay arm and check that there are no background pulses Adjust QUAD setting NO adjustment of the drive voltage should be needed Modulator 1 only MOD1 with bias error 0 2 g o 2 0 15 _ 0 05 B 0 1 lt BO 0 05 4 N s ea 7 a ast ssf 7 0 05 H 30E 07 2 25E 07 4 2pEp0F T 2ASE Q 4 10E 07 0 4 0 1 L L L 1400 1600 1800 2000 L 1 1 600 800 1000 1200 4 EL Time s Time ns Fig 3 3 5 1 Bias erroron MOD1 manifest is small background pulses in the OFF state left which disappear into the noise floor when bias setting is correct right e Disconnect attenuator arm and repeat with the unblocked delay arm e With both arms connected set delay and amplitude balance as well as possible between the two arms e Switch off driver and leave on bias controllers e Connect the signal to the spectrum analyser e Optimize as described in 3 3 2 e Turn the drivers back on a let the system stabilize for 15 minutes e Check again on spectrum analyser and optimize if necessary e The system is now ready to be connected to the laser chain balanced Both arms balanced 03 bat ZO 0 2 0 15 Q a Ln 0 1 Amplitude Amplitude 0 05 2 f a 0 al 0 05 l li L 400 600 800 1000 1200 1400 1600 746 747 748 749 750 751 752 753 754 Time ns Time ns Fig 3 3 5 2 Typical oscilloscope traces when phas
51. time structure of the produced signals The Photline engineer also suggested temperature stabilization to 40deg which has produced good results at the factory In addition special attention must be given to the driving electronics to reach the required power stability of the resulting optical signal Length of sub pulses 140 5 ns This is compatible with the rise and fall time of the proposed modulator as long it is correctly driven Special attention must be given to the drive electronics to optimize the rise and fall time of the resulting optical signal An AC coupled driver DR PL 10 MO LR was purchased from Photline in 2010 to achieve flat pulses over the required period with adjustable gain Pulse timing stability 0 2 ps The modulator is not known to bring additional jitter to an electronic driving signal However special attention must be given to the drive electronics to reach the high required timing stability of the resulting optical signal An additional point here is that as the driving voltage only provides the window for the pulse train it does not inherently affect the jitter of the individual pulses as long as the rise time is well below the 666ps separation of the pulses 2 3 1 Initial tests one modulator scheme The original setup implemented at STFC CLF 8 and later further studied by INFN 9 is shown on Fig 2 3 3 Oscillator These studies have revealed several problems e Instabilities were observed from the modulator
52. to 40 C as the active bias control is not optimized for our 50 duty cycle transmitted signal case Settings can vary from day to day depending on the laboratory conditions 3 2 4 Delay line The Ozoptics 35856 ODL 100 11 1053 6 125 P 40 3A3A 1 0 5 variable delay is based on a manual translation stage between two collimator lenses and is a free space device which must be used in a clean environment The total length of the stage only allows 300 ps delay and so additional large delay to one arm might be necessary to get into the required 333ps delay range In our case a 95 5 tao purchased from Photline was used to compensate for the mission delay The 5 output from the delay arm can be also useful for monitoring bias drifts in the future 3 2 5 Attenuator The Ozoptics 33686 BB 700 11 1053 6 125 P 50 3A3A 1 1 LL variable attenuator is based on a small screw interrupting the beam between two collimator lenses Although we have not exceeded the average power specification it is possible that backreflections from the screw can cause damage to the collimators with a pulsed laser Over time we have observed decreasing throughput with the screw removed from the beam Waveplate and polarizer based attenuators with remote options exist from Ozoptics and would provide a more robust solution for the future also with the possibility to use it as part of an amplitude feedback 60 50 Loss in 2 50 20 11 Booster Oscillator 333ps delay 32
53. todiodes are fiber coupled the New Focus 1024 with an impulse response of 12ps FWHM and the New Focus 1014 with a 3dB bandwidth of 45GHz The New focus 1024 is optimized for time domain measurements and the 1014 is optimized for frequency domain measurements however they were both found to be well suited for both applications in this case although the 5 5ps laser pulses cannot be fully resolved by these detectors While the delay can be roughly set using the digital oscilloscope LeCroy Serial Data Analyzer SDA18000 18GHz 60 Gsamples s this does not provide an accurate measurement because of the timing jitter of the trigger signal and the digitization itself Furthermore the oscilloscope cannot be triggered on a balanced train as the sub trains are not distinguishable so a single trace has to be recorded and post processed which can be very tedious Post analysis provides accurate evaluation of the timing switch but not a fast setup with visual aid It is possible to check the output signal in the frequency domain The Fourier transform of a train of Gaussian pulses consists of spectral lines at multiples of 1 5GHz and of course at DC If a part of this signal is phase shifted other frequency components also appear depending on the sub train length 3 1 1 The frequency domain method When no drive voltage is applied to the modulators a continuous 3GHz signal will be created after recombination of the two 1 5 GHz arms with half of the nomin
54. urement was carried out by monitoring the mean value over the 500 microsecond amplification window for the laser chain and over 215 shots using Thorlabs DET10A Fig 2 3 2 2 Histogram 14 215 2 215 abs vp mean 353 487mV S T min 352 7mV x ao o max 354 mV 6 STD0 312V 0 088 E 4 HL a IDI il 0 Lil lul i uh II md WG m edm 06 m e AN Pw NA Pw ew Dee T MM OM MM OM MM MM MOM ST St S amp F Mean over 500us mV Fig 2 3 2 2 Stability measurement of the laser booster amplifier e The required RF modulating signal to drive the modulators was produced by a custom made circuit designed at CERN Stephen Livesley A Andersson A low phase noise 1 5GHz beam synchronized signal was provided and capacitively coupled to an 8 bit programmable ECL counter MC100EP016F The counter was followed by a D flip flop MC100EP31DG These circuits together allow the selection of a pulse length within the required range 140ns and the terminal count output connected to the D flip flop creates the required square wave signal The signal is only delivered for the amplification window of the burst mode amplifiers AMP1 AMP2 to allow resynchronization between each burst The electronics box also includes AC amplifiers DR PL 10 MO LR which provide the fast rise times necessary and will also dampen variations on the driving voltage With the 300ps accuracy of the switching cutting b
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