The ELETTRA Streak Camera: System Set
Contents
1. SL BI internal Note 1995 19 ECLinPS IC family www motorola com 20 P Buoncompagno Progetto e realizzazione della scheda Ausiliaria per il sistema di controllo della Streak Camera degree thesis under preparation Univ di Trieste 21 SIEMENS C166 u Controller www siemens com 22 R P Walker European Project to develop a UV VUV Free Electron Laser Facility on the ELETTRA Storage Ring NIM A to be published Fig 7 synchroscan image of four bunch mode beam eee ae DopE Mie Nene Scales HOR 44 1ps VERT 34us 7 CONCLUSIONS The Streak Camera system recently installed at ELETTRA has been presented in this paper A description of the system has been given together with the 76 CT07 Contributed Talks2. 5ms 3 THE OPTICAL PATH The optical path partial view shown in fig 1 performs the following operations on the synchrotron light 1 deflection and focusing onto the SC input pin hole with 50 100 or 200Um diameter 2 band pass A 500nm filtering and attenuation 3 shutter for interlock purposes 4 optical power reduction The reduction of the optical power on the time scales typical of a SC is not a straightforward task Contributed Talks Proceedings DIPAC 1999 Chester UK Fig 1 A partial view of the Optical Path to the SC showing the Pockels Cell system The timings needed to achieve an effective reduction are reported in Table 1 Furthermore the shutter operation has to be synchronous with the light pulses ON OFF ratio GATE y Da a feos 1 KAZ Table 1 fast shutter parameter list To meet the requirements a Pockels Cell opto electronic shutter 10 has been adopted 3 1 Pockels Cell operation A Pockels Cell 11 introduces a rotation typically by 90 on the polarisation axis of the incident wave upon the application of an external electric field E By placing two polarisers rotated by 90 respectively up stream and down stream the Pockels Cell such a system acts as a fast shutter driven by E To operate the system a careful alignment of its elements is required Furthermore two pinholes are necessary to remove unwanted spots which generate inside the system due to spurious r
3. Proceedings DIPAC 1999 Chester UK THE ELETTRA STREAK CAMERA SYSTEM SET UP AND FIRST RESULTS M Ferianis Sincrotrone Trieste Trieste Italy Abstract At ELETTRA a Streak Camera system has been installed and tested The bunch length is a significant machine parameter to measure as it allows a direct derivation of fundamental machine characteristics like its broadband impedance At ELETTRA the Light from a Storage Ring Dipole is delivered through an optical system to an Optical Laboratory where it can be observed and analysed The Streak Camera is equipped with different time bases allowing both single sweep and dual sweep operation modes including the Synchroscan mode The Synchroscan frequency equal to 250 MHz which is half of the ELETTRA RF frequency allows the acquisition of consecutive bunches 2ns apart To fully exploit the performances of the Streak Camera an optical path has been arranged which includes a fast opto electronic shutter By doing so the optical power deposited on the photo cathode is reduced in the different ELETTRA fillings 1 INTRODUCTION The bunch length measurement on Storage Rings has to be non destructive therefore a classical approach to the problem is to measure the length of the Synchrotron Light Pulses generated by the transversely deflected electrons as it happens in the Bending magnets Dipoles This is correct as the light pulses propagating from the source point to the measureme
4. Streak Camera shows up It has to be taken into account for accurate beam stability observations The final check on the shutter operation has been performed live with the SC sweeping in fig 3 the Pockels Cell effect is shown on a Synchroscan acquisition Fig 3 Pockels Cell effect on a Synchroscan acquisition HOR 44 1ps VERT 3 46es full screen The Pockels Cell shutter is now currently used to improve the photo cathode lifetime typical operating conditions are f pep lt 20Hz 200ns lt GATE lt 20us For stability studies on long time scales it may be removed 4 THE SYNCHROSCAN OPERATION The main advantage of Synchroscan deflection associated to a dual sweep streak tube is to provide picosecond resolution at a very high repetition rate typically less than 150MHz To fully exploit this technique both positive and negative slopes of the sinusoid are used For ELETTRA a dedicated streak tube has been developed by Photek 14 this unit provides stable Synchroscan operation at 250MHz with 3 2ps resolution For the f for ELETTRA is 499 654MHz and this feature has led to the unique possibility of observing CT07 73 Proceedings DIPAC 1999 Chester UK consecutive bunches of a multi bunch beam Another outstanding feature of the ELETTRA SC is that the fastest sweep on the orthogonal slow axis is equal to 9 14ns This feature allows the observation of a small number of consecutive bunches fi
5. age Ring SR Free Electron Laser FEL 22 The vs current was measured 0 to 30mA in single bunch 0 to 100mA in 4 bunch filling In fig 6 single bunch profiles at different currents are shown These profiles show that the electron distribution is close to a Gaussian distribution as predicted by the theory Time Fig 6 single bunch profiles at 1 7 15 and 30mA The charge bunch being equal for both fillings a direct comparison of the acquired data has been possible The data show a very good agreement except at higher currents where a small difference of 2 to 3ps is visible This small increase in the four bunch mode is not at present understood It is not thought to be due to bunch oscillations since other streak camera images as well as other measurements indicated an absence of bunch motion 6 3 1 Stability investigations For these measurements the SC has been used in Syncroscan mode with the fast axis set to 441ps and the slow axis ranging from lus to 35ms For FEL operation the SC confirmed the instability free settings see fig 7 first measurements Future work will provide a Local Control Panel which will enable non expert Users to safely perform measurements with this powerful diagnostic tool 8 ACKNOWLEDGEMENTS The author thanks the colleagues and students of the ELETTRA Instrumentation Group who have helped him in setting up the Streak Camera system C J Bocchetta and R P Walker for their s
6. eflections By carefully aligning all these elements the ON OFF ratio of the light can be maximised while keeping a good transmission through the shutter The low frequency trigger pulses synchronous to the synchrotron light pulses for the Pockels Cell high voltage V 2 5kV driver 12 are generated by the SC timing system The duration of the trigger pulse GATE can be varied according to the SC operation mode and to the synchrotron light time structure 3 2 Pockels Cell characterisation The correct operation of the Pockels Cell shutter has been checked at low repetition rates 1 Hz with long opening times Sms for direct eye observation while at higher repetition rates up to 2kHz with the nominal opening time 100ns to 10uUs a wide band 1GHz photodiode has been used Contributed Talks At the highest repetition rates gt 2KHz the closing of the Cell begins to show a ringing fig 2 which is believed to be due to an opto acoustic phenomenon induced by the piezoelectric effect A similar effect has been observed by other authors 13 2 50MS S Tek Run PK Detect Pts F s a Bot aah sed PER iai E orate con 44 baka 1 iei k Teden rh Lott tend Ve omve drs OOS MOLONS CMA J 2 25 Fig 2 Pockels Cell ringing hor scale 10 uUs div With long Gates gt 50 Us an amplitude modulation effect of the transmitted light observed with a laser on the photodiode and with the synchrotron light on the
7. ing system used to generate the appropriate trigger pulses and Synchroscan signal e control on the safety and interlock functions generation of the video signal for the Control Room 74 CT07 The main reason for using two PCs is the cutting of development times which has been achieved thanks to the full integration of the desktop PC in the Local Control System leaving the only internal developments to be the timing and interlock boards 5 2 The timing interlock control computer The timing and interlock functions have been implemented on VME VXI custom boards on a VXI crate with an Intel based VME CPU This solution provides the typical VXI hardware environment EMI EMC ease of integrating custom boards power supplies with the Win95 OS software environment The safety critical interlock functions are hardware implemented on a custom board and use the software only for signalling its internal status 5 2 1 The CPU of the VXI PC Different CPU boards and operating systems have been tested A PCVXI 745 486 uP 66MHz board from National Instruments running LabView under Win95 has been tested first 16 A Eurocom 128 Pentium 100 from Eltec running Linux has been used due to the temporary unavailability of the PCV XI 745 The final solution will adopt a VME 7591 941 board Pentium 233 from VMIC running either Win95 or Linux This was the only board with the PMC 17 connector used for the PMC frame grabber Con
8. irect verification of the Streak Camera time resolution lt 3 2PS wu 6 1 2 Single Shot measurements In a single shot measurement the light pulse is acquired with a single trigger event without any possible averaging effect In a Synchrotron light source the worse case for single shot acquisition is with MB beams The bunch charge may be some 10 100 time less than SB beam Single Shot acquisitions were compared to averaged acquisitions and the negligible differences confirmed the jitter free operation of the whole Streak Camera system 6 2 Multi bunch MB measurements At ELETTRA MB mode is the standard User Mode 345 buckets filled out of a total of 432 80 The injection current is 300mA 2GeV The coupled bunch longitudinal instabilities are cured by tuning the RF cavity temperatures and by checking the beam spectrum A direct observation of these longitudinal oscillations is now possible as shown inf fig 5 At 2 4GeV a more stable beam is observed compared to 2GeV Fig 5 comparison of beams at 2 0GeV upper half and 2 4GeV lower half with two Synchroscan acquisitions 6 3 Single bunch SB measurements In SB mode two different fillings were measured e asingle bunch Tey 846ns 79 Proceedings DIPAC 1999 Chester UK e 4 bunches evenly spaced T 216ns New long time scale information was obtained on beam stability These studies are an important factor in understanding operation of the Stor
9. nt point preserve in their longitudinal profile the electron longitudinal distribution that is the Bunch Length o In third generation light sources this measurement is critical due both to the very short duration of the synchrotron light pulses which lies in the range of Pico seconds and to the low energy per pulse available Streak Cameras are routinely used as powerful diagnostics tools in both linear usually observing Transition Radiation rather than Synchrotron Radiation and circular accelerators The main features of a Streak Camera are Pico second resolution even in Single Shot e very high 100s of MHz repetition rate of the fast sweeps with dual sweep Synchroscan mode e high sensitivity thanks to built in photo multiplier 2 SYSTEM SET UP At ELETTRA the Light from a Storage Ring Dipole 1 is delivered through an optical system to an Optical Laboratory the first vacuum mirror allows only the Visible and near UV part of the Synchrotron Light 72 CT07 Spectrum to be used Beside a Transverse Profile Monitor system 1 other instruments 2 3 and 4 have been installed and tested Bunch Length measurements have been already performed 5 both with a Streak Camera and an Ultra Fast Photodiode At ELETTRA a Streak Camera SC specifically manufactured by Photonetics 6 has been recently installed and successfully tested 7 8 The basic operating principle of a SC is a time to space conversion of ul
10. panel will also enable the non expert user to safely use the SC 6 MEASUREMENT RESULTS The SC was delivered to Sincrotrone at the end of November last year Since then in a two month period full performance has been achieved The goals for these commissioning tests were e to acquire single shot images both with single bunch SB and multi bunch MB beams e to operate the Synchroscan at 250 MHz e to test the fast opto electronic shutter with the SC The SC has been also operated without the Pockels Cell mainly on slow sweeps 10 50 ms where the effect of optical power reduction introduced by the fast shutter is minimum Since February during dedicated shifts the first measurements of the ELETTRA beam were taken under Contributed Talks CT07 different machine conditions multi bunch and single bunch 6 1 Preliminary tests of the SC system The operation of the peripheral devices and the SC compliance to the specifications has been verified The jitter of the Trigger pulse to the SC has been measured to be less than 3ps with the repetition frequency varying from 2Hz up to 2kHz The spectral purity and the level OdBm of the 250 MHz Synchroscan signal have been checked The synchronous opening of the Pockels Cell has been checked by directly cutting the light of the beam with a T T time down to few nanoseconds Synchroscan has been operated at 250 MHz although ELETTRA o gt 10ps doesn t allow a d
11. pment and will perform e further programmable divisions and level conditioning on the TCK to obtain the required low frequency Trigger signal e filtering and amplification are applied to the fren to obtain the sinusoid for the Synchroscan signal Pockels Cell GATE generation e intensity loop control on board 16 bit uP 21 e interlock function 5 2 3 The interlock system The interlock system checks the following conditions e light intensity level trespass detected by means of a photodiode with hardware threshold and comparator e absence of the f and Trigger signals These alarm conditions are OR ed together so that as soon as any of these become true the shutter to the SC is closed and a message sent to the Local Control Panel 5 3 The Local Control Panel At present no remote control will be delivered to the Control Room The reasons for this are e precautions in using the photo cathode and MCP e the Storage Ring FEL 22 control will be close to the SC Optical Laboratory e development cost and time to safely remotise the SC control a non negligible effort is required A Control Panel will be developed on the desktop PC using the CVI National Instruments development environment 23 This panel will integrate the functions available on both PCs using links both to the Optodll dll image analysis library provided by Photonetics and to the VXI PC by using CVI built in TCP client server library functions This Control
12. tra fast optical events The incoming photons are converted into electrons by a photo cathode The emitted electrons are accelerated and deflected by a high voltage fast ramp applied to deflection electrodes As a result electrons are streaked out on the back end phosphor screen creating a strip whose length is proportional to the duration of the photon bunch The image formed on the phosphor screen is amplified with a Micro Channel Plate MCP image intensifier and acquired by a CCD camera The ELETTRA SC is equipped with different sweep Units 9 which allow the following operation modes e single sweep providing lt 2ps resolution e dual sweep with Synchroscan Unit at 250MHz Synchroscan is a deflection technique widely used in streak cameras where the high voltage deflection is driven by a sinusoid rather than a saw tooth ramp Thanks to the narrow band deflection signal the linearity of the high voltage deflection amplifier is more easily achieved than in a wide band amplifier which is needed for a saw tooth linear deflection The Single sweep unit FITSU 1 provides the following full screen deflections 176ps 441ps 882ps 1 7ns 4 4ns 8 8ns and 17 6ns with lt 2ps resolution The Synchroscan unit FSSU 1 operates at a frequency of 250MHz res 3 2ps The Secondary sweep units FTSU 2 and STSU 2 used for vertically displacing successive Synchroscan traces can cover the range from 9 15ns to 69 3
13. trol Room video signal generation The Control Room monitor will provide the operator with a live image of the SC output together with the relevant data time axis computed 6 peak to peak amplitude of longitudinal oscillations etc 5 2 2 The timing system The timing system generates the e Trigger signal for FTSU 1 FTSU 2 and STSU 2 e Synchroscan signal for FSSU 1 e 6Pockels Cell GATE These signals have to be synchronous to the light pulses with a minimum jitter lt 3ps To have the light pulse exactly in the middle of a fast sweep the trigger to FTSU 1 has to be adjustable in 2ns steps for one revolution period 864ns at ELETTRA with a fine adjustment in 10ps steps The repetition frequency ranges from 2 5kHz FTSU 1 average acquisition down to 1Hz slowest sweep of STSU 2 The Synchroscan signal is a pure sinusoid OdBm derived directly from the RF accelerating voltage Thanks to a loan from E Rossa at CERN we are presently using the CERN Pico timing 18 VME board to obtain the divide by 2 f version of the RF and the Turn Clock signal f divided by 432x20 57 83kHz At the time of writing a new RF programmable divider board has been developed using the same Contributed Talks Proceedings DIPAC 1999 Chester UK ECLinPS Motorola family 19 Measured jitter is lt 2ps it could replace in the future the Picotiming module A second auxiliary board 20 is presently under develo
14. upport R Roux for the active participation in measurements and for data post processing 9 REFERENCES 1 M Ferianis et al The Optics of the ELETTRA Transverse Profile Monitor EPAC 94 London 2 M Ferianis et al An Optical System for the observation of Transverse Beam Motion EPAC 96 Barcelona 3 M Ferianis M Pros A New Frequency domain method for Bunch Length measurement PAC 97 Chicago 4 M Ferianis et al Operation and Control of the Slow Beam Motion Monitor at ELETTRA DIPAC 97 Frascati 5 M Ferianis et al Bunch Length measurements at ELETTRA DIPAC 97 Frascati 6 PHOTONETICS Gmbh D 77694 KEHL 7 Elettra News issue n 29 http www elettra trieste it 8 Elettra News issue n 33 http www elettra trieste it 9 Optoscope user s manual Photonetics 10 S Parker Optics Source Book McGraw Hill 1987 11 Optics Guide CASIX inc www casix com 12 HVP SLP User s Manual Quantum Technology FL 32746 6212 USA 13 E Fulkerson et al Driving Pockels Cell Using Avalanche Transistor Pulsers 11 IEEE Int Pulse Power Conf Baltimore 1997 14 Photek UK 15 Oculus F 64 Frame Grabber CORECO inc Canada 16 S Bassanese Studio e realizzazione del sistema di misura della Streak Camera ad ELETTRA degree thesis Dec 1998 Univ di Trieste 17 PCI Mezzanine Connector 18 P Joudrier E Rossa Picosecond Bunch Train Module CERN
15. ve are shown in fig 4 Filling MB acquisition in average mode 80 J 220mA E 1GeV Fig 4 Synchroscan acquisition at 250 MHz Scales full screen HOR 441ps VERT 9 14ns Due to the longitudinal coupled bunch instabilities present at the time of measurement and also because of the average acquisition mode each bunch image is much longer than its natural value The operation of Synchroscan sweeping is made evident by the slope of the even and odd bunches 5 THE STREAK CAMERA LOCAL CONTROL SYSTEM The ELETTRA SC has been delivered with a user friendly interface Optoscope 9 running on a desktop Personal Computer PC under Win95 Through this graphical interface it is possible to control e The sweep units and MCP intensifier gain e The status of the shutter e The image acquisition and analysis processes e The storage and export of the images The SC and the PC are linked via a standard Serial line The image acquisition and analysis functions are implemented on a powerful Image Analysis board 15 with a TMS 320C40 DSP chip on board A push button keypad is also provided for direct control of the SC 5 1 The Local Control system It is based on two Personal Computers 1 the desktop PC delivered with the Streak Camera 2 a VXI PC developed in house The following functions are integrated e control over SC sweep units MCP and shutter e acquisition and analysis of the SC output images e control on the tim
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