Normal-conducting RF Photo Injectors for Free Electron Lasers

Transcription

1 Normal-conducting RF Photo Injectors for Free Electron Lasers Frank Stephan (DESY, Zeuthen site) for the PITZ team Content: Introduction: FELs and their electron sources Different NC photo injectors at SLAC for LCLS (low average current) at Berkeley for NGLS (high average current) at PITZ for FLASH and European XFEL (medium average current) Outlook and Summary

2 European XFEL - a next generation light source The XFEL will deliver: > wavelength down to 0.1 nm atomic-scale resolution > ultra-short pulses ( 100 fs) ultra-fast dynamics, molecular movies > ultra-high peak brilliance investigations of matter under 21+ extreme conditions ( Xe ) > transverse spatial coherence imaging of single nanoscale objects, possibly down to individual macromolecules (no crystallisation needed!!) Why brilliance is ~10E+8 higher? Synchrotrons: P ~ N e² FELs (coherence): P ~ (N e)² = N² e², N ~ 10E+8 XFEL 8 10 Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 2

3 SASE FEL: How does it work? Coherent motion is all we need!! Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 3

4 radiation power on logarithmic scale SASE FEL: How does it work? undulator electron absorber laser beam SASE = self amplified spontaneous emission undulator period λ U λ photon λ = u K γ 2 2 upper magne tic poles of the undulator electron trajectory perpendicular to magnetic field N S N S bunch of electrons with increasing density modulation elektromagnetic wave lower magnetic poles of the undulator S λ em N S N direction of motion direction of motion λmin[nm] 4π 10 Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 4 ε [mm mrad] I n p [ka] L u [m]

5 One XFEL key component: the high brightness electron source Why electron injector is so important.??? Any linac based short wavelength, high brilliance light source (e.g. SASE-FELs) contains the following main components: - electron source - accelerating sections - in between: bunch compressor(s) - undulator to produce FEL radiation - electron beam dump - photon beamline(s) for the users e.g. wakefields, coupler kicks e.g. coherent synchrotron radiation (CSR) increase normalized emittance Example: FLASH RF stations Diagnostics Accelerating Structures sflash Soft X-ray Undulators Beam Dump RF Gun Laser 3 rd harmonics Bunch Compressors 5 MeV 150 MeV 450 MeV 1250 MeV 315 m LOLA Photon Diagnostics property of linacs: beam quality will DEGRADE during acceleration in linac electron source has to produce lowest possible emittance!! Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 5 FEL Experiments

6 What is Emittance? long.: ε z ~ (e - bunchlength) (energyspreadofe - bunch) trans.: ε x,y ~ (e - beam size) (e - beam angular divergence) 2 σ x 2 σ x' ε = 6 dimensional phase space volume occupied by given number of particles effect of acceleration on transverse emittance (adiabatic damping): βγ 2 σ x' transverse phase space p x ~ cov( x, x' ) θ 2 < θ 1 angular divergence is reduced normalized RMS transverse emittance: n v 1 ε x = ß γ σ x σ x' cov (x, x ) ; ß =, γ = c 2 1- ß, x = (ε n is conserved in general) β γ 2 σ x' Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page σ x dx ds x

7 Why electron injector is so important Why emittance must be small output peak power of 6.4 nm (GW) peak power Q = 1 nc FLASH length of the undulator εn = 1 mm mrad εn = 2 mm mrad εn = 4 mm mrad output peak power of the XFEL 0.1 nm (GW) absolute numbers XFEL peak current: 5 ka energy spread: 2.5 MeV εn = 1 mm mrad εn = 2 mm mrad εn = 3 mm mrad path length in the undulator (m) path length in the undulator (m) XFEL goal: 0.9 mm mrad@injector = 1.4 mm mrad@undulator if even smaller emittance new horizons: shorter wavelength, higher repetition rate Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 7

8 Situation in 1999 ICFA workshop on high brightness beams at UCLA in autumn 1999: Summary talk of P. O Shea (U Maryland, USA) on electron source developments: Goal for community in next years: Get transverse normalized emittance of 1 mm bunch charge of 1 nc!!! Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 8

9 Different NC photo injectors at SLAC for LCLS (low average current) at Berkeley for NGLS (high average current) at PITZ for FLASH and European XFEL (medium average current) Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 9

10 NC RF Gun Design for LCLS pulsed / CW single bunch charge single bunch rep rate average current norm. trans. emittance (rms, slice) rf frequency pulsed 1.0 / 0.2 nc 120 Hz 120 / 24 na 1.0 / 0.8 mm 135 MeV 2856 MHz modified UCLA/BNL/SLAC 1.6 cell S-band gun: - larger mode separation ( MHz) - larger iris radius, reduced iris surface field - dual rf feed, z coupling, racetrack shape - field probes in both cells - increased cooling channels - klystron pulse shaping reduced dissipated power improved emittance and stability In-Situ Hydrogen Beam Cleaning of Cathode Surface Initial 1H: 2H: 3H: 4H: Baked: 15 min, 0.7uA 60 min, 0.4uA 40 min, 0.4uA 60 min, 2uA 230 deg ~ 3mC H-ion charge sufficient! LCLS-TN-05-3 After H-beam cleaning 1.2x10-4 tests on unbaked copper sample Courtesy of D. Dowell LCLS QE Specification SLAC-PUB August 2005 Quadrupole (γβ) r (1/m) cylindrical cavity (lc=2.475cm) 8 racetrack cavity (lc=2.413cm) 6 with d=0.315cm 4 racetrack cavity (lc=2.413cm) with d=0.356cm rf phase (degree) Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 10 r d r

11 From Paul Emma, First Lasing -talk at FEL2009 in Liverpool: Injector Transverse Projected Emittance ~0.5 µm Exceptional beam quality from S-band Cu-cath. RF gun Time-sliced x-emittance: 0.4 µm γε x 0.43 µm 0.25 nc 135 MeV 35 A γε y 0.46 µm D. Dowell, et al.

12 From Paul Emma, First Lasing -talk at FEL2009 in Liverpool: Measurements and Simulations for 20-pC Bunch at 14 GeV MEASURED SLICE EMITTANCE 20 pc 135 MeV γεx = 0.14 µm SIMULATED FEL PULSES 1.5 Å, photons Ipk = 4.8 ka γε 0.4 µm Simulation at 1.5 Å based on measured injector & linac beam & Elegant tracking, with CSR, at 20 pc. 15 Å, photons, Ipk = 2.6 ka, γε 0.4 µm time-slicing at 20 pc Y. Ding PRL Y. Ding Z. Huang Y. Ding Z. Huang 1.2 fs Simulation at 15 Å based on measured injector & linac beam & Elegant tracking, with CSR & 20 pc.

13 Some more experimental results from the LCLS gun: > Phy.Rev. ST AB 11, (2008): 1 nc, 100A bunch, 135 MeV: ε n, x (95%) = 1.14 mm mrad, ε n, y (95%) = 1.06 mm mrad > P. Emma, "Beam Brightness Measurements in the LCLS Injector, Mini-WS on compact XFELs using HBB, LBNL, Berkeley, USA, nc, 35A, 135 MeV ε n, x (95%) = 0.32 mm mrad, ε n, y (95%) = 0.39 mm mrad > J. Frisch, "Operation and Upgrades of the LCLS", LINAC2010, Tsukuba, Japan: 0.02 nc, 135 MeV: ε n, x (95%) = 0.18 mm mrad, ε n, y (95%) = 0.19 mm mrad Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 13

14 Different NC photo injectors at SLAC for LCLS (low average current) at Berkeley for NGLS (high average current) at PITZ for FLASH and European XFEL (medium average current) Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 14

15 Berkeley approach: NC RF gun design for CW operation (1) Design requirements high duty cycle, high average beam current, low emittance: repetition rate: up to ~1 MHz, CW bunch charge: from ~1 pc to ~1 nc, normalized beam emittance: from sub 10-7 (low charge) to 10-6 m, beam energy at the gun exit: ~500 kev (space charge), electric field at the cathode: ~10 MV/m (space charge limit), bunch length control at cathode: from ~1ps to 10s of ps (to handle space charge effects, for allowing different modes of operation), compatibility with magnetic fields in the cathode and gun regions (mainly for emittance compensation) operational vacuum pressure: Torr (high QE photo-cathodes), easy installation and conditioning of different kind of cathodes, high reliability compatible with the operation of a user facility. low RF frequency (from Journal of Modern Optics, Vol. 58, No. 16, ) Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 15

16 Berkeley approach: NC RF gun design for CW operation (2) pulsed / CW single bunch charge single bunch rep rate average current norm. trans. emittance (rms, slice) rf frequency CW 0.01 to 1 nc up to 1 MHz up to 1 ma < 1.0 mm mrad (design) 187 MHz (pictures from F. Sannibale, Mini-WS on Compact X-Ray FELs Using High Brightness Beams, LBNL, August, 2010) Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 16

17 Berkeley approach: NC RF gun design for CW operation (3) > APEX phase 0 layout > Under preparation: further diagnostics for 6D phase space characterization and subsequently booster cavities for up to 30 MeV. The following design parameters have already been demonstrated at Berkeley: - full conditioning to max. RF power (100kW in CW mode, allow for 750keV beam energy) - max dark current 19.5MV/m should be OK for high rep rate FEL operation - first photoelectrons at 1 MHz from temporary molybdenum cathode - beam energy: 745 ± 41 kev - cathode field: >10 MV/m - pressure: ~5x10-11 Torr after baking (RF off, 1 of 20 NEG modules activated) (factor 3-4 higher with RF on) ( from PRST-AB 15, (2012) ) Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 17

18 Different NC photo injectors at SLAC for LCLS (low average current) at Berkeley for NGLS (high average current) at PITZ for FLASH and European XFEL (medium average current) Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 18

19 The PITZ gun (Photo Injector Test facility at DESY, Zeuthen site) Bucking solenoid Photo cathode (Cs 2 Te) QE~0.5-5% RF gun: L-band (1.3 GHz) NC (copper) standing wave 1.6-cell cavity Coaxial RF coupler Cathode laser 262 / 257 nm 20ps (FWHM) Mirror in vacuum Main scan parameter during measurements Main solenoid, Bz_peak~0.2T Main properties of PITZ gun: Electron bunch 1nC, ~6MeV 1.3 GHz cavity, coaxial RF coupler (flexible solenoid position) Capable of high average power long electron bunch trains (SC linac) Very low normalized transverse emittance Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 19

20 Some parameters of FLASH and European XFEL pulse trains: repetition rate train length micro pulses: bunch spacing bunch length acc. field in SC cavity (FLASH) ~ RF power 500 µs 900 µs beam time Parameters FLASH European XFEL final beam energy 1.2 GeV 17.5 GeV max. repetition rate 10 Hz 10 Hz max. train length 800 µs 650 µs bunch spacing 1 20 µs µs required injector emittance (1 nc) 2 mm mrad 0.9 mm mrad SASE output wavelength nm nm Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 20

21 current PITZ 2.0 setup

22 Improvement of the RF gun phase stability (gun has no field probe) 2009 (no FB) 2011 (FB is ON!) FPGA phase, reconstructed from virtual ADC probes based on 2x5MW directional couplers FPGA phase, measured by 10MW in-vacuum directional coupler Zero phase slope within the electron pulse train FPGA phase (chanel[40]) t, µs installed in 2010 phase, deg Phase of the 1 st bunch measurement # measurement# 150 mean=-94.3+/ deg counts Phase fluctuations: deg (p-p) 2..4 deg (rms) phase, deg Phase fluctuations: deg (p-p) deg (rms) Manufacturer: Mega Industries, USA Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 22

23 How we measure the transverse projected emittance Single slit scan technique > Emittance Measurement SYstem (EMSY) consists of horizontal / vertical actuators with YAG / OTR screens 10 / 50 µm slits > Beam size is slit position using screen > Beam local divergence is estimated from beamlet observation screen (12 bit camera) 2D corrected normalized RMS emittance ε σ x 2 2 n = βγ x x xx 2 x 2 Observation screen 2.64 m Statistics over all pixels in all beamlets EMSY1 (z = 5.74 m) correction factor ( >1 ) introduced to correct for low intensity losses from beamlet measurements σ x - RMS beam size measured with YAG screen at slit location SQRT(<x 2 >) - RMS beam size at slit location estimated from slit positions and beamlet intensities 100% RMS emittance (conservative estimate) Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 23 pixel intensity

24 High stability of the measurement I_main (A) Xrms, mm Yrms, mm EmitX_2D, mm mrad EmitX_2D, nonscaled XYrms, mm EMSY1 NoP EMSY1 Gain MOI NoP MOI, Gain XBL NoP XBL gain EmitY_2D, mm mrad EmitY_2D, nonscaled YBL NoP YBL gain EmitXY_2D, mm.mrad EmitXY_2D, nonscaled X-scale factor Y-scale factor Example, measurement on : 1nC, Emittance vs. Imain (gun SP phase=6deg) For all measurements f250 lenses and 2x2 binning were used ε x =(0.707±0.032) mm mrad ε y =(0.686±0.024) mm mrad ε xy =(0.697±0.026) mm mrad emittance, mm mrad Beam size and 2D emittance for BSA 1.2mm, 1nC, gun 6 deg w.r.t. MMMG, booster on crest EmitX EmitY EmitXY Xrms Yrms XYrms Imain, A beam size, mm 3x3 Statistics Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 24

25 Emittance vs. Laser Spot size for various charges in 2011 xy-emittance (mm mrad) Emittance optimization in nC, measured 1nC, meas.(0deg) 1nC, meas.(6deg) 0.25nC, meas. 0.1nC, measured 0.02nC, meas. LCLS, 1nC, 95% LCLS, 0.25nC, 95% LCLS, 0.02nC, 95% 100% RMS emittance data Slice emittance required at XFEL undulator for 1 nc rms laser spot size (mm) Charge, nc Minimum emittance PITZ, 100%, mm mrad LCLS, 95% mm mrad > PITZ is setting a new benchmark for minimum emittance at a given charge + is capable of operation at fairly high duty cycle!! LCLS data: P. Emma, "Beam Brightness Measurements in the LCLS Injector, Mini-WS on compact XFELs using HBB, LBNL, Berkeley, USA, J. Frisch, "Operation and Upgrades of the LCLS", LINAC2010, Tsukuba, Japan. PITZ data: M. Krasilnikov et. al., Experimentally Frank Stephan minimized NC RF photo beam injectors emittance for FELs from an LA3NET L-band Workshop, photoinjector CERN, PRST-AB , Page (2012). 25

26 Core Emittance for various bunch charges Idea: Cut low intensity region of MEASURED phase space (i.e. remove non-lasing part) Slice emittance required at XFEL undulator for 1 nc 2 nc (0 deg) 2 nc (+6deg) 1 nc (0 deg) 1 nc (+6 deg) An example for 1 nc: emittance (mm-mrad) nc (0 deg) 0.1 nc (0 deg) 0.02 nc (0 deg) LCLS, 1nC LCLS, 0.25nC LCLS, 0.02nC charge cut (%) Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 26

27 2011 PITZ 1.8: measurements vs. simulations xy-emittance (mm mrad) nC, measured 2nC, simulated 1nC, meas.(0deg) 1nC, meas.(6deg) 1nC, simulated 0.25nC, meas. 0.25nC, simul. 0.1nC, measured ASTRA simulations: cathode laser pulse with 21.5ps FWHM, 2ps rise/fall time 1.0 Optimum machine parameters 0.1nC, simulated (laser spot size, gun phase): nC, meas. experiment simulations nC, simul. (~on-crest) 0.4 Difference in the optimum laser spot size is bigger for higher 0.2 charges (good agreement for pC) RMS laser spot size (mm) For each bunch charge the photo injector has been optimized (both experiment and simulations), but keeping the laser temporal profile fixed. Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 27

28 Reasons of discrepancy for high Q? Emission from the cathode? bunch nc Measured and simulated Schottky scans (1nC) 1nC emittance measurements measured charge (XYrms=0.3mm, LT=62%) measured charge (XYrms=0.3mm, LT=100%) simulated charge (XYrms=0.4mm, Qb=1nC) simulated charge (XYrms=0.3mm, Qb=1nC) gun phase-mmmg, deg Direct plug-in machine settings into ASTRA does not produce 1nC at the gun operation phase (+6deg), whereas 1nC and even higher charge (~1.2nC) are experimentally detected Simulated (ASTRA) phase scans w/o Schottky effects (solid thick lines) have different shapes than the experimentally measured (thin lines with markers) bunch charge@low.ict1, nc Measured and simulated laser energy scan (1nC) Laser intensity (LT) scan for the MMMG phase (red curve with markers) shows higher saturation level, whereas the simulated charge even goes slightly down while the laser intensity (Qbunch) increases Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page measured charge (XYrms=0.3mm, 0deg) simulated charge (XYrms=0.3mm, 0deg) ~ laser intensity, nc Photo emission (bunch charge) needs more detailed modeling in simulations.

29 Key components for successful operation of NC RF photoinjectors > good RF stability > good cooling water stability (increasingly difficult for high average RF power) e.g. better than ±0.05 degrees are needed at 50 kw average power for the PITZ case > robust and high quantum efficiency (QE) photo cathodes (especially important for high average current) e.g. the PITZ photo cathode system was developed by INFN-LASA Milano see talk of Daniele Sertore > stable photo cathode laser system with high flexibility e.g. the PITZ photo cathode laser system was developed by the Max-Born-Institute in Berlin see talk of Ingo Will High flexibility of photo cathode laser opens new research possibilities excursion to plasma acceleration fancy shaping of the photo cathode laser pulses can further improve the beam quality Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 29

30 Photo cathode laser system installed at PITZ (for more see I. Will) Yb:YAG laser with integrated optical sampling system Highly-stable Yb:YAG oscillator E micro = µj pulse selector pulse shaper τ ~ 1.7 ps E micro = µj Nonlinear fiber amplifier Optical Sampling System (OSS) Resolution: τ < ps Yb:YAG power regen G ~ 10 6 E micro ~ 2 µj scanning amplifier (Yb:KGW) Two-stage Yb:YAG double-pass amplifier G ~ 40 Pulse selector E micro ~ 80 µj LBO BBO UV output pulses E micro ~ 10 µj Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 30

31 Photo cathode laser: temporal pulse shaping (for more see I. Will) Multicrystal birefringent pulse shaper containing 13 crystals Gaussian: Shaped ouput pulses motorized rotation stage temperature controlled birefringent crystal Gaussian input pulses FWHM ~ 2 ps FWHM ~ 11 ps FWHM ~ 20 ps FWHM ~7 ps FWHM ~ 17 ps Simulated pulse-stacker edge ~ 2.2 ps FWHM = 25 ps OSS signal (UV) edge ~ 2 ps Will, Klemz, Optics Express 16 (2008), FWHM ~ 24 ps FWHM ~ 24 ps birefringent shaper, 13 crystals important for good beam quality high flexibility new options! Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 31

32 Particle driven plasma wakefield acceleration (PDPWA) ( AWAKE ) e- Courtesy of Carl Schröder, LBNL Does this work? Dephasing? Hose instability? Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 32

33 PITZ: Studies for Particle Driven Plasma Acceleration 1) Self-modulation of electron beam (proof of principle for CERNs AWAKE exp.) - use high flexibility of photo cathode laser system: - Example: flat-top e-beam through plasma cell: Bunch 6.35m from cathode: Q = 100 pc L b = mm σ xy = µm ε xy = mm mrad Peak density e - /cm 3 Plasma density /cm 3 2) Study high transformer ratio - resonantly drive plasma wave with specially shaped electron bunch (5 bunchlets inside the bunch) high transformation ratio: V probe bunch to be sent to bunch compressor Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 33

34 New option for the photo cathode laser 3D ellipsoid BD simulations for 100 pc bunch charge: Transverse phase spaces at z=5.74m Collaboration IAP N.Novgorod / JINR Dubna / DESY for more see Ekaterina Gacheva x, mm Gaussian Flat-top Ellipsoid Longitudinal phase space (Z-Pz) at z=5.74m px, mrad almost no beam halo almost pure sine shape > Benefits from 3D ellipsoidal laser pulses for ALL linac driven light sources: 30-50% lower average slice emittance higher brilliance ~pure sinusoidal longitudinal phase space +3 rd harm. simplify/allow required compression ~no beam halo better signal/noise, reduced radiation damage less sensitive to machine settings higher stability Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 34

35 Summary > Normal conducting RF photo injectors are a mature and reliable technology to produce high brightness beams for FEL applications > From low to medium average currents (~30µA) a new benchmark in optimizing photo injector performance was demonstrated from M. Krasilnikov et al., PRST-AB 15, (2012). > For high duty cycle / CW applications the main challenges (cooling, vacuum properties) can be solved see Berkeley approach > High brightness photo injectors are also interesting beyond FEL applications, e.g. plasma acceleration, electron diffraction (REGAE), > Sincere Acknowledgements: to all international colleagues for discussions over the years, colleagues at Hamburg and Zeuthen, the PITZ team, Frank Stephan NC RF photo injectors for FELs LA3NET Workshop, CERN Page 35