Diagnostic of Ultra High Brightness Electron Beams

1. Diagnostic of Ultra High Brightness Electron Beams Mikhail Krasilnikov (DESY) for the PITZ Team • Content: • High brightness electron beam diagnostic overview • longitudinal • transverse • slice • Photo Injector Test facility at DESY in Zeuthen (PITZ) • setup • electron beam diagnostic at PITZ • Emittance measurements at PITZ • single slit scan technique • recent results • Summary Ultra Bright Electron Sources Workshop, 29 June – 1 July 2011 The Cockcroft Institute, Daresbury

2. Diagnostic of High Brightness Electron Beams HB E-beam diagnostic • General: • bunch charge FC, ICT, Toroids • transverse position BPM: button, stripline, cavity • arrival time (phase) BAM, RF pickups Projected Slice Longitudinal (temporal) Transverse Energy distribution Current profile Transverse profile • Challenges • high charge / charge density • small beam dimensions (100…50um  nm!) • high repetition rate • all the intercepting devices are damaged by measured beams • beam halo measurements Slice energy spread Long. phase space Proj.emittance, transv. phase space Slice emittance

3. Longitudinal Parameters Measurements • Temporal beam profile (bunch length) r(t) Frequency Domain Optical and laser techniques Time Domain Coherent radiations + Interferometer Electro Optical Sampling = EOS RF deflector Radiator + Streak-camera • Non intercepting and not disturbing • Based on optical properties of a non-linear crystal interacting with Coulomb field of e-beam Transition = CTR Diffraction = CDR • Intercepting diagnostic • Radiators • OTR (Al foil) • Cherenkov (aerogel) • Streak camera • T-resolution limited to 200...300 fs • Rather expensive • Light collection \transport  dispersion free beam line is needed (reflective optic instead of lens) Synchrotron CSR Laser Wire Smith-Parcell • Non intercepting and not disturbing • Multi shot • Complicated setup Cherenkov • Interferometers (Michelson; Martin-Puplett) • Spectrum acceptance is restricted  can impact the reconstructed beam profile • Guess distribution is needed

4. Longitudinal Parameters Measurements: RFD (TD) Map time (longitudinal) axis onto transverse coordinate • RFD: • Self calibration (cavity phasing) • Single shot • Intercepting diagnostic • Resolution down to tens of fs (f, V, str) • small transverse beam size and large beta function • uncorrelated transverse energy spread • induced slice energy spread • phase jitter of the beam w.r.t. RFD OTR beam images in the LCLS injector at 135MeV for a 800um long bunch with the deflecting cavity off (left) and on at 1MV (right). H. Loos, “Longitudinal diagnostics for short electron beam bunches”, SLAC-PUB-14120

5. Longitudinal phase space measurements • Spectrometer dipole ┴ RFD (Energy scale from beam in spectrometer, time scale with transverse deflector) Imaging of longitudinal phase space with RF deflector SPARC: 145MeV LCLS: 135 MeV t E Fast single shot measurements

6. Transverse Parameters Measurements Usually the largest error in transverse parameters measurements is coming from transverse profile and rms size determination • OTR monitors (~um Al foil) • High energy (>tens of MeV) • No saturation • Resolution limit closed to optical diffraction limit (~10um) • COTR effects (especially for compressed bunches) • Scintillator screens (e.g. YAG:CE, 100um) • High photon yield • Resolution  grain dimensions (~50um?) (powder / thin crystal) • Saturation (0.04pC/um^2?) • Wire Scanner (good agreement with YAG measurements!) • Almost non-invasive • Higher beam power • Multi-shot measurements • 1D • Rather complicated setup, long measurement time • IPM (residual gas monitor) • CVD (diamond screen)  long pulse train • “Indirect” (SR – Optical Synchrotron Interferometry, Scattering, ODR, etc) • Read out: • Zoom optics • CCD camera • 12bit • l~ 1um..400nm • controllable iris • Insertable filters

7. Transverse Phase Space (Emittance) Measurements Emittance dominated Space charge dominated • Multi screen: • Based on linear beam matrix approach • 3 beam size measurements (at 3 positions) are needed for the known elements of the transport matrix (phase advance) • Quad(s) scan • Beam size measurements as a function of varied transport matrix • Tomography • Related to Radon theorem: N-dim object reconstruction from M projections in (N-1) dim space • Slit mask techniques: • based on conversion of a space charge dominated beam into emittance dominated beamlets • Systematic error from quad strength determination(e.g hysteresis) • Thin lens model is not adequate • Large energy spread chromatic effects • Phase space distribution assumed to be homogeneous CSR (undulator) based?

8. Slit technique - general Emittance dominated beamlets • Multi slit mask = single shot measurement, but: • Overlapping of beamlets when optimized for high resolution • Low sampling of the phase space Space charge dominated beam Transverse phase space reconstruction • Slit mask: • Opening: small enough (scem) • Thickness: thick enough to scatter beam, but alignment / angle acceptance • Distance L: • Long enough  divergence resolution • Not too long  signal-to-noise L SPARC e-meter = phase space measurements along the beam line

9. Position of reconstruction RFD 12.28 14.56 13.8 13.04 15.32 z Phase Space Tomography (e.g. at PITZ) • The most used technique  quadrupole(s) scan, but it yields only Twiss parameters and emittance, not the phase space. Thereforephase space tomography • Back projection • Filtered Back projection • Algebraic reconstruction technique (ART) • Maximum entropy (MENT) Courtesy G.Asova (PITZ)

10. Slice parameters measurements • Slice emittance: • RFD + Quad(s) scan • Acc off-crest + dipole • Slice energy spread • RFD + dipole P. Emma, A. Brachmann, D. Dowell, et al., SLAC, “Beam Brightness Measurements in the LCLS Injector”, Compact X-Ray FELs using High-Brightness Beams, 5-6.08.2010, LBNL 1 3 2

11. Photo Injector Test facility at DESY in Zeuthen • The Photo Injector Test facility at DESY in Zeuthen (PITZ) focuses on the development, test and optimization of high brightness electron sources for superconducting linac driven FELs: •  test-bed for FEL injectors: FLASH, the European XFEL •  small transverse emittance (~1 mm mrad @ 1 nC) •  stable production of short bunches with small energy spread •  further studies: dark current, QE,thermal emittance, … • + detailed comparison with simulations = benchmarking of photo injector physics • extensive R&D on photo injectors and test of new developments (laser, cathodes, beam diagnostics)

12. XFEL Photo Injector Key Parameters to be tested at PITZ 20ps Main efforts at PITZ towards XFEL photo injector

13. Beam diagnostics at PITZ Diagnostics for Transverse phase space andLongitudinal phase space <7 MeV <25 MeV

14. Slit scan technique at PITZ: evolution • ~2002-2003 rough divergence estimation using center beamlet, 8 bit camera • 2003-2005 sheared emittance estimation using 3 slit positions (0; +/-0.7*sigmaX), 8-bit camera • 2005-2008 – standard “manual” slit scan (~200um step)  phase space reconstruction, 12-bit camera • 2009-2011 – automated synchronized slit scan with adjustable scan speed  phase space “on-line”, 12-bit cameras, zoom option, scale procedure • The emittance measurement procedure at PITZ: • under permanent improvement in terms of resolution and sensitivity • as conservative as possible (100% rms emittance)! • !NB: measured emittance numbers are permanently reducing as a result of machine upgrades and extensive optimization of beam parameters “we are measuring more and more of less and less…”

15. 2.64 m Observation screen EMSY1 (z = 5.74 m) Transverse projected emittance measurements at PITZ beam at EMSY screen transverse phase space Single slit scan technique • Emittance Measurement SYstem (EMSY) consists of horizontal / vertical actuators with • YAG / OTR screens • 10 / 50 m slits • Beam size is measured @ slit position using screen • Beam local divergence is estimated from beamlet sizes @ observation screen • 12-bit camera, quality criteria: max bit>3000 (from 4095=2^12-1); adjustment = gain X NoP • Image filtering (3sigma, bkg, x-ray, MOI) 2D scaled normalized RMS emittance scale factor ( >1 ) introduced to correct for low intensity losses from beamlet measurements x - RMS beam size measured with YAG screen at slit location SQRT(<x2>)- RMS beam size at slit location estimated from slit positions and beamlet intensities  “100% RMS emittance” Statistics over all pixels in all beamlets pixel intensity

16. Slit scan technique at PITZ: how it works now • Setup the machine • laser temporal and transverse • laser BBA at the cathode • gun phase • bunch charge • booster phase and gradient – beam energy (longitudinal momentum) • For a chosen main solenoid current (bucking in compensation) • Beam transverse distribution (rms size) at EMSY  12-bit camera!; frames=50xSignal+50xBkg (laser shutter closed) with adjusted camera gain G and NoP • Beam transverse distribution at beamlet collection screen for MOI  12-bit camera! frames=50xSignal+50xBkg (laser shutter closed) with adjusted camera gain G and NoP • Slit scan (typical speed 0.1-0.5mm/sec) with simultaneous beamlet image taking. Synchronization of the slit position and the frame acquisition (10Hz!) with adjusted camera gain G and NoP • Slit scan with closed laser shutter for the average bkg calculation • Transverse phase space reconstruction and emittance calculations • Phase space linear shift to take the slit position into account • Scale procedure • Error analysis (systematic and statistics – e.g. 3x5)

17. Emittance measurements at PITZ (typical solenoid scan) Machine setup (07.05.2011M): BSA=1.2mm; gun=6deg off crest; 1nC; booster=on-crest preliminary Beam momentum after the gun Laser temporal profile Bunch charge tuning Beam momentum after the booster Laser transverse distribution

18. Emittance measurements at PITZ: typical solenoid scan 1) Solenoid scan for X and Y emittance 2) 3x3 statistics for the bes solenoid current preliminary beam at EMSY: 3x1 XPx: 3x3 YPy: 3x3 07.05.2011M: 3x3 stat: Xemit=(0.742±0.021stat) mm mradYemit=(0.782 ±0.028stat) mm mradXYemit=(0.761± 0.017stat) mm mrad 07.05.2011M: sol.scan: Xemit | Yemit | XYemit 0.778 | 0.701 | 0.738 Geometrical averaged emittance:

19. beam dump DISP3.Scr2 electron beam DISP3.Scr1 PITZ setup: now and upgrades during this year <7 MeV <25 MeV • HEDA2 • together with TDS: measure slice momentum spread down to 1 keV/c • Transverse Deflecting Structure (TDS) • time resolved measurements

20. Summary • High brightness electron beams require particular diagnostic for longitudinal and transverse phase space characterization: • Strong energy dependency (YAG for low energies, OTR – for high energies) • Non-invasive techniques are highly desirable • Slice parameters measurements are important • The main focus at PITZ  small emittance electron beams. To reach this: • high gun gradients • cathode laser transverse and temporal shaping • machine stability • extensive machine optimization • Emittance measurement procedure • nominal method  single slit scan • as conservative as possible  100% rms emittance • continuous improvement of the procedure • PITZ sets a benchmark for ultra high brightness electron sources: • specs for the European XFEL have been met (emittance <0.9 mm mrad at 1nC) and even surpassed • beam emittance has also been optimized for a wide range of bunch charges (20pc…2nC) • rather high duty factor (average power, long pulse trains) in stable operation

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