Accelerator R&D status at PITZ

Transcription

1 Accelerator R&D status at PITZ Frank Stephan for the PITZ team, Zeuthen, December 5 th 2017 Content: Refereed papers + theses New contributions to PITZ Collaboration Report on operation of Gun 4.6 Installation and test of gun quads at XFEL Status of Gun 5 development Developments towards ELLA 2.0 Slice emittance Recent THz Measurements First static electron diffraction test at PITZ News from plasma acceleration experiments: SMI, HTR, LabAstro Summary & Outlook (Gun 4.5)

2 G. Loisch co author of a paper from Frankfurt University about a new method to determine the hydrogen density in a lowly ionized hydrogen plasma Page 2

3 Paper about 3D laser shaping published in Physics Uspekhi Page 3

4 PITZ paper about emission studies published in NIM A Page 4

5 PITZ paper about longitudinal phase space tomography published in NIM A Page 5

6 Paper on Self Modulation Instability (SMI) Version submitted to Nature Communications: was turned down by 2:1 referee decision. Sticky point: have we shown self modulation instability? Direct measurement of self modulation amplitude growth along the plasma channel length will only be possible with new experiments next year. We currently have two indirect evidences for SMI: defocusing (needs growth of seed transverse fields) and phase slippage (washed out focusing region due to velocity difference between bunch and SMI wake). But indirect evidences are not convincing enough for referees New start to end simulation have been performed meanwhile. New version of the paper is under review at PRL: Page 6

7 Paper on improved emission modeling Page 7

8 Paper on laser based bunch shaping for high TR studies Page 8

9 Paper on triangular pulse shaping with SLMs > In last 6 months: 4 papers published, 4 drafts submitted, review ongoing good! let s continue common efforts on refereed publications! Page 9

10 Paul Weidemann: Spectroscopic Plasma Diagnostics on a Plasma Cell for Plasma Acceleration Bachelor Thesis, TH Wildau, defended on Calibration of the 2m, high resolution Fasty Ebert spectrometer Measurement of breakdown curves for gas discharges in different gases (top middle) Time and spatially resolved hydrogen plasma density measurement for PWFA (top right) Evaluation of plasma density jitter (top right) with old cell design (top left) Preparation for successful PWFA experiments in September Page 10

11 Annika Schütze: A program for gamma spectra analysis for the application in the radiation protection at DESY Bachelor Thesis, TH Wildau, to be defended on Problem: The internal analysis of the detector used for clearence is often not usable for the official procedure. Solution: A stand alone MATLAB program was written for more detailed data analysis of the measured radiation spectra. Issues: The energy resolution of the detector used for clearence is not good enough to find the exact nuclid for each peak. Nevertheless the program has improved the clearance procedure for components which are not activated (i.e. spectra without peaks). Page 11

12 Igor Isaev: Stability and performance studies of the PITZ photoelectron gun PhD Thesis, University Hamburg, defended on Conditioning process The PITZ RF photoelectron gun is able to provide the parameters required by modern superconducting linac based FELs (like FLASH and European XFEL): peak performance: the beam quality of each bunch average performance: the beam quality along a pulse train (that kind of operation is required for SC linacs) stability of beam parameters: pulse to pulse stability as well as stability within a pulse train reliability: low interlock rate Nevertheless, several items causing issues during gun characterization and continuous operation have been addressed: The conditioning process: careful conditioning procedure was established and tested. typical time of the gun conditioning process ~3 to 4 months (main phenomena: multipacting and field emission dark current). two RF windows setup can work reliably up to European XFEL specifications. Phase jitter reduction RF gun stability: achieved RF amplitude and phase stability: ~0.02% and ~0.07. further improvement of the RF gun stability seems limited by 20 khz modulations probably originating from the klystron modulator. Beam asymmetry compensation Electron beam asymmetry: RF coupler kick is an issue for the operation of the PITZ gun. Simulations show the presence of an asymmetry on the central axis of the coaxial RF coupler ( beam transverse kick and asymmetries of beam phase space). corrector coils designed + installed and work at PITZ, XFEL (and FLASH). Page 12

13 PITZ Collaboration Partners (formal contract signed) Founding partners of PITZ: DESY, HH & Z (leading institute) HZB (BESSY) (A. Jankowiak): magnets, vacuum MBI (S. Eisebitt): cathode laser TU Darmstadt (TEMF, T. Weiland, H. DeGersem): simulations Other national partners: Hamburg university: most PhD students; HGF Vernetzungsfond; generation of short pulses plasma experiments HZDR: BMBF PC laser project between MBI, DESY and HZDR, until ~2009; collaboration between HZB, HZDR, MBI and DESY in SC gun cluster International partners: IAP Nizhny Novgorod + JINR Dubna: 3D elliptical laser pulses, THz radiation INFN Frascati + Uni Roma II (L. Palumbo, M. Ferrario): TDS and E meter pre studies INFN Milano (C.Pagani): photocathodes INR Troitsk (L. Kravchuk): CDS, TDS, Gun5 INRNE Sofia (D. Tonev, G. Asova): EMSY + personnel LAL Orsay (A. Stocchi): HEDA1 + HEDA2 STFC Daresbury (S. Smith, B.Militsyn): phase space tomography Thailand Center of Excellence in Physics (T. Vilaithong, Ch. Thongbai): personnel YERPHI (V. Nikoghosyan) + CANDLE (V. Tsakanov, B. Grigoryan), Yerevan: personnel Page 13

14 Additional contributions to the PITZ collaboration XFEL participation in PITZ has started Part of personnel and operation costs are covered by XFEL funding (XFEL operation and R&D) INFN Milano (LASA) started working on the development of green cathodes on the LASA plug design (in operation at PITZ / XFEL / FLASH / REGAE / SINBAD / LBNL / FNAL) Existing instrumentation was checked and brought into operation, also data acquisition chain set up successfully. Sources were tested everything ready for sequential deposition on test samples First sequential deposition on test sample in week 47 ( proof of principle ) Sb 10 nm K until max QE Cs until max QE Lesson learnt Improve temperature control QE = 514 nm (2.4 ev) Not uniform cathode growth Improve experimental layout Optimize source layout in view of co evaporation in the near future View into the prep chamber Page 14

15 Operating experience with Gun 4.6: current setup at PITZ Features of the Gun 4.6 setup (reminder) dir.cpl. dir.cpl. Gun 4.6 with new type of cathode spring holder design (watchband reloaded) dir.cpl. dir.cpl. e det e det PMTs radius changed PMT air air PMT PMT IR IR PMT nose removed dir.cpl. two conditioned DESY type windows for the 2 window setup (as in 2011) T Combiner with optimized RF design for best window positioning for reflections gun conditioning started on PMT Photomultiplier tube e det Electron detector IR Infrared sensor IGP Ion getter pump (pressure reading) PG Pressure gauge IGP PMT e det IGP PG temp. IGP valves temp. temp. IGP temp. pump temp. Page 15

16 Operation history of gun 4.6 (from to >1.6 years) ~1 month to reach full us. Initial conditioning was a little slow, but everything was new: gun, directional coupler, T combiner. Slow increase of pressure in the cathode region: TSP has not been fired from May to November Problems with the cathode insertion on : Resonance temperature difference 7deg Strongly increased dark current Exchange of the Cathode Box, Cathode Spring, Cathode Spring Holder, Bellow, z Actuator (guilty). cath. TSP not fired Gun Restart. Nominal operational parameters recovered within 1 week. Stable operation reached in ~4 weeks. No problem observed afterwards. Dark current at end of Run: DC at the end of run. The data is measured as amplitude of the signal. PITZ: Max. Power=6.4MW, Imain=0A, FC@1379mm FLASH: DC= 6.5 Power=5MW, Imain=312A, FC@1540mm XFEL: DC= 25 Power=5.2MW, Imain=336A, DCM@2250mm DC from the PITZ 41 A 1.4 A 2 A Gun restart Problems with cathode insertion: exchange of cathode actuator and components in cathode vicinity Page 16

17 Conclusions from the gun opening in May 2017 Marks at gun backplane Some peculiarities have been observed when opening Gun4.6 Microscope view of spot at cathode spring holder (by S.L.) (Cs 2 Te) on (Cs 2 Te) on (by S.L.) No obvious reason for problem with inserting cathodes found. Most probable explanation: malfunctioning/damage and misalignment of z manipulator. Reason for damages on inserted cathodes also not fully clear. Possible explanation: misaligned z manipulator and marks on cathode spring holder. Operation of Gun4.6 restarted on without major problems; standard run parameters reached in ~1 week; stable operation reached in ~4 weeks. End of run: dark current became smaller than before problems showed up. Traces and spots at cathode spring holder (top: endoscope view, bottom: after dismount.) Page 17

18 Power and pulse length usage during operation of gun 4.6 (all data with >100kW in gun taken into account) Plasma experiments (SMI, HTR, limit load on cell and windows) The problem with cathode insertion appeared ELLA study with short PL 11 weeks spent to reach 6MW at 650 s at the beginning (similar to gun 4.2 which had 2 Thales vacuum windows) Restart 1/1/2017 At restart: after only 5 days reached 6.5MW at 650 s UED experiments Reduce dark current for emittance measurement at low charge with 2 bunches Restart 1/1/2017 Restart DLW experiment Problem with FB Before the cathode insertion problem: % of weekly run time above 6MW and 650 s since week 24 (weeks 41 and 53 excluded). After the cathode insertion problem and reconditioning (since week 69) the corresponding weekly run time was % (weeks excluded). THz experiments Experiments done with same beam momentum after gun 1/1/2017 When aiming for >6 MW and >600 s we had run weeks were for ~97% of the time (and more) these parameters were met (before and after the gun restart). Page 18

19 Evolution of interlocks for gun 4.6 (all data with >100kW in gun taken into account) Interlock rate has a clear decreasing trend (also after the restart of the gun)! In summer to winter 2016 there was a period where we got many PMT interlocks from vacuum windows (when we started to use higher peak and average power), in week 23 even from air side. Most ILs had been max. reflection ILs No problem from windows anymore! Remarks: interlock sensors more sensitive than in Hamburg, it was noticed that interlock rate is reduced when gun settings are fixed (~operation at FLASH/XFEL) Page 19 1/1/2017 1/1/2017 Disabled ILs from PMTs looking at vacuum and RF windows in WG2 air Problems with cathode insertion New IL counter feature (upper estimate) Reconditioning

20 Run time at given Power (all data with >100kW in gun taken into account) Average peak power: 5.46 MW Average RF pulse length: 329 s Total operation time: 6369 h Operation with s most of the time. 412 GJ deposited (based on average values) Page 20

21 Summary of Gun 4.6 operation No problems with watchband reloaded cathode spring design seems to work! No problem anymore with DESY type RF windows when operating with 2 vacuum windows at optimized position at 6.5 MW, 650 s, 10 Hz (>24 weeks of full operation mainly at these parameters) two RF vacuum window solution seems to work! In between (after 13 months successfull operation of Gun4.6 at PITZ) problems to correctly insert cathode have shown up on Most probable explanation: malfunctioning, damage and misalignment of z actuator of cathode system (~10 years old)! actuator was exchanged together with cathode spring, its holder, bellow, and cathode box. Operation of Gun4.6 restarted on without major problems; standard run parameters reached in ~1 week; stable operation reached in ~4 weeks. Next steps: Gun 4.6 was sent to XFEL on and a single Thales windows at optimum position will be mounted to have 1:1 replacement of current gun ( > hot spare) since increase of gun gradient currently not needed. Gun 4.4 (contact stripe cathode spring design) will be equipped with DESY type window at optimum position to have a 1:1 spare for FLASH in case current FLASH gun has a problem Gun 4.5 (contact stripe cathode spring design) will be equipped with 2 Thales windows at optimized position for conditioning and operation at PITZ Page 21

22 Installation (Aug 2017) & test (Oct 2017) of gun quads at XFEL Installed: normal/skew quads on 1 frame Injector settings for the tests: Gun Normal and Skew Quads: QLN.23.I1 QLS.23.I1 Gun power: 5.13MW (53MV/m) RF pulse length: 70 s Gun RF phase: 43deg (w.r.t. zero charge phase) BSA: 1.2 mm Beam momentum after the gun: unknown Beam momentum after AH1: 130 MeV/c Number of pulses: 1 Gun main solenoid current A Gun bucking solenoid current 17.7 A Bunch charge: 500 pc A1 and AH1 adjusted for MMMG phase Quad design parameters: Relative XY emittance Results of the tests: Transverse beam profiles at the screen OTRC59 (s=36,6 m) QLS current, A Combination of normal and skew quads Aluminum frame 0.56 mm copper cable 140 windings per coil Q_grad = A QLN current, A QLN = 0A and QLS = 0A: mm mrad QLN= 0.3A and QLS= 0.1A: mm mrad ~20% reduction (further improvement by solenoid tuning possible) Page 22

23 Status of Gun 5 development Gun 5 includes RF probe (stability, symmetric power coupler), improved geometry (reduced heating & surface field), better water cooling and reduced deformation (more reliable operation at high duty cycle) Experiments on test piece showed the need of an improved probe position second probe hole was machined on existing test piece, RF tests are pending Setup for RF tests first approach new design Technical design of the center part of Gun 5 is settled its production will start this year Gun 5: V. Paramonov et al., NIM A 854 (2017) Gun 5 center part highlighted Drawing of the center part Page 23

24 Ellipsoidal Laser System (ELLA 2.0) Simplified, linear, robust pulse shaping scheme with new PHAROS frontend 20 W, 1 MHz IR commercial laser system MHz, MHz, MHz etc IAP system dismounted, PHAROS frontend installed, utca integration and mounting of optical elements on laser table ongoing Basic RF synchronized photoelectrons already tested during last run period ( simplified backup for MBI laser) Pulse shaping experiments with SLMs (1 st ) & Volume Bragg Grating (2 nd ) under preparation Laser transport beamline concept to be reconsidered Initial laser table layout Page 24

25 Slice Emittance Measurements Used emittance measurement methods: Slit Scan and Quadrupole Scan Deflecting cavity: bunch length mapped to vertical axis Prototype for the E XFEL injector (2 MW, 0.5 m length) Design parameters not fully reached because of high reflection of waveguide matching pieces being tested now Designed & manufactured by Institute for Nuclear Research (INR, Troitsk, Russia) PITZ slice emittance measurement Page 25

26 Slit Scan and Quadrupole Scan Slit Scan method: No space charge impact due to charge reduction via slit Low signal due to streaking and charge reduction via slit LYSO screen to be mounted soon much higher SNR First results (YAG) look quite promising Quadrupole Scan method: Higher signal strength, since no slits reduce the charge Space charge at 20 MeV distorts beam transport Discrepancies between theoretical and experimental transfer matrix even at low charge Quadrupoles transfer function being probed with steerer kicks to obtain experimental M 12 values Analysis and refinement of both methods ongoing (PhD student) Magnet M 12,exp /m M 12,theo /m M 12,exp M 12, High1.Q6 x High1.Q6 y High1.Q7 x High1.Q7 y High1.Q8 x High1.Q8 y Page 26

27 Measurements of THz Coherent Transition Radiation (CTR) PST.Scr2 was modified to be a CTR station. The radiator is an Al sheet with dimensions of 27mm x 55 mm x 1 mm. The vacuum window is made of Z cut crystal quartz (polarization maintaining). The radiation measurement system was set up beside the CTR station to measured the backward transition radiation in normal room environment. A THz pyroelectric detector was used for radiation pulse energy measurement. The spectral distributions were measured by using a Michelson interferometer. PST.Scr2 Radiation measurement system The setup of a Michelson interferometer E beam direction Page 27

28 Examples of THz CTR Measurement Results photocathode laser shape Short Gaussian Comb with 4 uniform peaks Interferograms obtained from the Michelson interferometer Laser t FWHM ~ 2.5 ps? Bunch charge 1 nc 1 nc Bunch compression Average CTR pulse energy booster off crest at J J Beam current profiles measured by TDS Spectral distributions obtained from the Michelson interferometer resolution ~3.4 ps resolution ~5.8 ps Page 28

29 First static electron diffraction test at PITZ Collaboration between PITZ, Max Born Institute (MBI) and Fritz Haber Institute (FHI) DESY/PITZ: Installation, beam experiment, MBI: Sample substrate, Au sample, EMCCD, beam experiment, FHI: WS 2 sample, diffraction pattern analysis, EMCCD Q1/Q2 Q5/Q6 LYSO screen ~320 fc, ~100 nm.rad Au (polycrystal, 100 nm thick) WS 2 (single crystal, nm thick) Au WS 2 Page 29

30 First static electron diffraction test at PITZ 1st test results Some conclusions: Electron beam at sample 1st Test Unit Energy ~4 MeV Wavelength ~0.3 pm Pulse rate 10 ~ 100 pulse/s Electron per pulse ~2x10 6 e /pulse Bunch FWHM length ~2 ps Normalized emittance ~100 nm.rad Beam rms size ~250 um Transverse coherence length ~1.9 nm PITZ beam demonstrated good diffraction quality on solid state samples with ~ps time resolution and ~nm transverse coherence length. PITZ bunch train made signal accumlation time short for diffraction pattern with very good signal to noise ratio. With gun, booster and laser phase jitter improvements sub ps to 100 fs time resolution is expected. Collaborators are excited about 1st successful results, further tests on using high quality PITZ bunch train for UED are under planning. Single cyrstal WS 2 Polycrystal gold Page 30

31 Self modulation of Long Electron Bunches Motivation: AWAKE experiment at CERN Single stage electron acceleration with self modulated proton beam PITZ: Demonstration and characterization of self modulation with flexible electron machine (this time: experiments with Argon gas discharge plasma cell) Plasma off d = 1.11 d = 0.98 d = 0.82 Time resolved bunches at high plasma densities (d cm 3 ) Long electron bunch is broken up into sub bunches Page 31

32 Simulation Results: Self modulation Instability Start to end of plasma cell simulation: Beam transport to plasma cell with ASTRA Beam plasma interaction: PIC simulation with HiPACE Beam and plasma properties as in self modulation experiment with Li heat pipe oven (Oct. 2016) Simulated transverse wakefield shows strong growth (starting point depends on beam focusing) and saturation As expected if self modulation instability is causing this behavior (without instability the curve would be flat) Maximum transverse wakefields along the simulation box (solid lines) E Z field of first maximum (dash dotted lines) 3 focusing solenoid currents Page 32

33 Latest High Transformer Ratio (HTR) experiments Setup and principle Argon gas discharge plasma cell in PITZ in September 100mm discharge channel Densities of up to 5x10 16 cm 3 Achieved various bunch shapes for self modulation & high transformer ratio experiments E + E driver Head Tail witness High Transformer Ratio: E + / E > 2 Head Tail Page 33

34 Experimental Results Transformer Ratios Driver ~715 pc Witness ~15pC Driver and witness measured with different camera gain! Beam losses when plasma ON due to modified beam properties with fixed beam transport here: TR = 4.6 ± 0.4 TR defined by ratio of witness energy gain and driver energy loss, error represents systematic uncertainty Observed TRs up to 4.9 (even 5.4, without e.g. discharge jitter that previously compromised measurements) Simulations at measurement parameters ongoing: reached TR of 4.2 up to now total energy changes too high for assumed plasma length, plasma dynamics under investigation Publication on bunch shaping submitted; publications on plasma cell, density diagnostics and high transformer ratio PWFA in preparation Page 34

35 Laboratory Astrophysics Idea: find parameter scaling so that astrophysical phenomena can be investigated in the laboratory DESY Strategy Fund project since 2016: Investigate Bell s instability (Amplification of magnetic fields in a plasma, induced by cosmic rays) Initial estimation showed possibility of successful experiment at PITZ Results so far: Explicit analytical calculation of growth rates in cold electron beam approximation for parameter range accessible at PITZ Detailed plasma simulations with the PIC (particle in cell) code OSIRIS using modulated PITZ beam as input Bell s instability growth is slow, and there is a lot of competition from other instabilities Conclusions: Bell s mode is difficult to create at PITZ, but there are alternatives fitting better to the available experimental parameter space and which are easier to simulate Possible candidates which are evaluated: Electrostatic modes (acceleration of electrons) / Electromagnetic filamentation modes (relativistic shock waves) Growth rates for plasma instabilities at PITZ conditions Work is progressing, goal has to be adjusted Page 35

36 Summary Last 6 months: 4 papers published, 4 drafts submitted, 1 invited talk at EAAC 2017 good! 2 BSc and 1 PhD theses finished Collaboration: XFEL participation in PITZ has started INFN Milano (LASA) started activity on producing green cathodes Gun 4.6 was in operation at PITZ for >1.6 years: watchband reloaded cathode spring design and 2 DESY type RF vacuum windows operated at optimized position with 6.5 MW, 650 s, 10 Hz seem to work reliably! Gun quads installed and commissioned at XFEL and delivered to FLASH Gun 5 production is being started Installation of ELLA2.0 system ongoing: PHAROS front end with pulse shapers of SLM and VBG type Slice emittance methodic shows first promising results First THz measurements and first static electron diffraction tests at PITZ Plasma activities: SMI measurements with discharge cell, start to end simulations Very successful high transformer ratio measurements have been done (new WR?) LabAstro work ongoing, goal to be adjusted Page 36

37 Status of the next gun for PITZ (Gun4.5) New gun setup was moved to the tunnel on November 14 th, 2017 Gun4.5 with solenoids was pre mounted on the girder in the vacuum lab Further mounting will be done in the tunnel installation of double gun quads (normal/skew) before installation of the coax coupler delivery of new T Combiner (optimized & compact location of 2 Thales windows) from U.S. on November 28th Gun 4.6 on the way to the European XFEL: at the crane and on the lorry quality control, vacuum check (mass spectrometer), dry ice cleaning (in HH) needed before its installation at the gun setup mounting of directional coupler, T Combiner and Thales windows follow installation to the PITZ beamline and baking Time line T Combiner defines the time line: in the best case (i.e. if the T Combiner is back from dry ice cleaning in December) the gun setup can be completed and installed at the PITZ beamline in January 2018; after baking conditioning can start beginning of February 2018 Page 37