Generation and characterization of ultra-short electron and x-ray x pulses

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1 Generation and characterization of ultra-short electron and x-ray x pulses Zhirong Huang (SLAC) Compact XFEL workshop July 19-20, 2010, Shanghai, China

2 Ultra-bright Promise of XFELs Ultra-fast LCLS

3 Methods to generate ultra-short x-ray x pulses E-beam manipulation: selective emittance spoiling Current-enhanced enhanced SASE (ESASE) Chirped SASE (+ slicing or undulator taper) Seeding with ultrashort laser/hhg pulses Very low-charge, short-pulse mode of operation (I will discuss both generation and characterization aspects)

4 Motivation for lower charge FEL gain depends on peak current, not charge X-ray SASE has many random spikes, each spike ~1 fs Less charge, same peak current shorter x-ray x pulses Less charge, smaller laser spot size on cathode smaller (thermal) emittance Brighter beam more compact accelerator Less peak current in the accelerator (until the last step of compression) Less linac wakefield Weaker CSR Less microbunching * Low charge mode suggested by J. Frisch; Also by J. Rosenzweig et al., NIMA2008

5 Time-sliced x-emittance at 20 pc 20 pc, 135 MeV, 0.6-mm spot diameter, 400 µm m rms bunch length (5 A) 0.14 µm TAIL (not same data) Time-sliced x-emittancex (constant charge 20 pc) P. Emma, D. Dowell Theoretical limit = 0.5 μm/mm Thermal emittance greatly improved with smaller laser spot

6 LCLS low charge machine setup UV laser ~1 ps (rms), 0.6-mm spot diameter, 15 deg to gun rf injector projected norm. emit. ~0.2 μm m (x/y) injector bunch length 220~250 μm m (rms) Emittance Scan on OTRS:IN20: Nov :00:56 RMS cut area Normalized Phase Space TCAV bunch length on OTRS:IN20: Oct :37:33 Super Beam Size (μm) Beam Size (μm) E = GeV Q = 0.019± 0.00 nc γε x = 0.20± 0.00 ( 1.00) μm β x = 1.26± 0.01 ( 1.11) m α = 0.10± 0.01 ( 0.07) x ξ = 1.01± 0.00 ( 1.00) x χ 2 /NDF = QUAD:IN20:525:BDES (kg) E = GeV Q = 0.019± 0.00 nc γε = 0.19± 0.00 ( 1.00) μm y β y = 1.21± 0.02 ( 1.11) m α y = 0.10± 0.01 ( 0.07) ξ y = 1.00± 0.00 ( 1.00) χ 2 /NDF = QUAD:IN20:525:BDES (kg) Norm. Angle Norm. Angle Norm. Position Normalized Phase Space Norm. Position Beam Size (μm) σ y = 37.55± 0.21 μm σ z = ±1.341 μm cal = 1.043±0.001 μm/μm TCAV:IN20:490:TC0:AACT (norm) Laser heater off L1S&X are the same as 250 pc configuration Vary L2 chirp to find maximum compression

7 Measurements and Simulations for 20-pC Bunch at 14 GeV Y. Ding et. al, PRL 2009 Photo-diode signal on OTR screen after BC2 shows minimum compression at L2-linac phase of deg. weaker CSR emittance blowup Horizontal projected emittance measured at 10 GeV, after BC2, using 4 wire-scanners. L2 at deg (under-compress) L2 at -35 deg (over-compress)

5 Å, 3 10 11 photons I pk = 4.8 ka γε 0.

4 10 11 photons, I pk = 2.6 ka, γε 0.4 µm 1.

8 Simulated 20-pC LCLS FEL performance 1.5 Å 15 Å z = 25 m 1.5 Å, photons I pk = 4.8 ka γε m, 15 Å, photons, I pk = 2.6 ka, γε 0.4 µm 1.2 fs LCLS FEL simulation at based on measured injector beam and Elegant tracking, with CSR and LSC, at 20 pc. (power profile at z = 25 m varies from shot to shot due to noisy startup)

Curren (ka) 10 5 Pyro Detector Pyro Detector Pyro Detector H.

9 Bunch length monitor signal 15 BC2 Peak Current at 20 pc measurement (20 pc) LiTrack simulation J. Frisch Mesh Filter Mesh Filter Mesh Filter Paraboloid Paraboloid Paraboloid Beam Splitter Beam Splitter Beam Splitter BC2 Peak Curren (ka) 10 5 Pyro Detector Pyro Detector Pyro Detector H. Loos Beam Beam Beam Edge Radiation Edge Radiation Edge Radiation L2 Phase (degs) LiTrack simulation assumes 20 pc bunch charge 3 kev initial rms slice energy spread 0.23-mm initial rms bunch length

10 Gas Detector (mj) and I pk (a. u.) FEL energy vs. compression photon 840 ev BL signal Half of undulators inserted (to prevent pulse lengthening due to slippage after FEL saturation) FWHM bunch duration after BC2 (fs) deg+1deg L2 phase (degs) BC2 bunch duration at 20 pc FWHM bunch length (LiTrack) 4-fs L2 Phase (degs) LiTrack simulation assumes 20 pc bunch charge 3 kev initial rms slice energy spread 0.23-mm initial rms bunch length X-ray pulse duration should be <10 fs, but no direct measurement yet possible

Measured Energy spread (@ 4.5 GeV) Energy spread measured on the vertical dump OTR screen (FEL suppressed) RMS energy spread at undulator (%) 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.

11 Measured Energy spread 4.5 GeV) Energy spread measured on the vertical dump OTR screen (FEL suppressed) RMS energy spread at undulator (%) RMS energy spread vs L2 phase at 20 pc, 4.5 GeV over-compression full-compression L2 Phase (degs) measurement on OTRDMP under-compression Full compression may yield an extremely short bunch, LSC and CSR blow up E-spread E to stop lasing?

12 Bunch length diagnostics Transverse cavity S-band (10-20 fs resolution) X-band deflector (4X, under development) Optical deflector (?) Wide-band coherent radiation spectrometer (difficult to reconstruct if bunch shape is not smooth) Fluctuation-based method (indirect but cheap, later if time allows) Longitudinal transformation that maps time to energy (Single short method to measure fs bunch length and shape)

13 X-Band Deflector for Short Bunch Diagnostics Transverse S -band RF Deflector for Bunch Length Measurement P. Emma, J. Frisch, P. Krejcik, G. Loew, X.- J. W ang e x S -band 2.44 m V (t) RF streak σ x σ z Δ ψ 90 β c β p σ z ( σ y σ y 0 ) λ rf E s 2π ev sin Δ ψ cosϕ β β Structures built at SLAC in 1960 s now installed in linac for testing S-band deflectors built at SLAC in 1960 s now 2 installed P. P. Krejcik in et. et. al., LCLS al., WPAH116W X-band deflector improves temporal resolution by 4! Plans in place to install an x-band x deflector after LCLS undulator to monitor bunch length on shot-by by-shot basis (f = 11 GHz. V 0 = 33 MV, σ z ~ 5 fs) d s f f = 2856 M Hz Hz V M V σ z z μ m y -z streak generated by deflector

14 A single-shot shot method to measure fs bunch length* Slightly adjust BC2 R add a diagnostic chicane 56 R 56 To high-resolution energy spectrometer L2 (φ 2 ) BC2 4.3 GeV Run 10 GeV L3 at zero crossing (-90 deg) h 3 Over-compression δ Zero-crossing σ δ z σ z Final energy spread/profile corresponds to compressed bunch length/profile * Z. Huang, K. Bane, Y. Ding, P. Emma, SLAC-PUB PUB-14104, 2010; based on a technique by T. Smith (PRST, 2000)

LCLS example Run LiTrack with 20 pc setup (L2 phase at -31 deg, under-compression) Run L3 at -90 deg (10 GeV over 553 m leads to h 3 = 139 m -1 ) Increase BC2 R56 by R 56 = -1/ h 3 = -7.

15 LCLS example Run LiTrack with 20 pc setup (L2 phase at -31 deg, under-compression) Run L3 at -90 deg (10 GeV over 553 m leads to h 3 = 139 m -1 ) Increase BC2 R56 by R 56 = -1/ h 3 = mm Turn off Linac-3 3 wake (discussed in next slides) After nominal BC2 After adjusted BC2 and L3 Conventional RF zero-phasing has no chance here (induced( E-spread E << initial E-spreadE spread) The technique here is insensitive to initial E-spread E or chirp

16 Linac Wakefield L3 wake introduces an additional energy spread to the measurement For very short bunches (<10 μm), wake-induced energy spread (chirp) is independent of bunch length N: # of e - L: : L3 length a: : iris radius Over-compression δ δ More over-compression Zero-phasing z Zero-crossing with wake z With wake σ z Wakefield un-corrected σ z Wakefield corrected This simple wake-correction scheme works for almost arbitrary (short) bunch length we want to measure!

17 Numerical example w/ wake Run LiTrack with 20 pc (L2 phase at -31 deg, under-compression) Run L3 at -90 deg (10 GeV over 553 m leads to h 3 = 139 m -1 ) Turn on Linac-3 3 wake E-spread too large E-spread just right Wakefield un-corrected Increase BC2 R56 by R 56 = mm Wakefield corrected Increase BC2 R56 by R 56 +ΔR 56 = mm

Now Scan L2 phase to change post-bc2 bunch length True bunch length E-spread/chirp R 56 = -8.

18 mm L3 wake corrected after setting R 56 = -8.

18 Now Scan L2 phase to change post-bc2 bunch length True bunch length E-spread/chirp R 56 = mm True bunch length E-spread/chirp E-spread/chirp (shift φ 2 by 1 ) R 56 = mm L3 wake corrected after setting R 56 = mm Wake effect can be corrected empirically by identifying full compression phase Issues to be addressed: CSR/LSC must be taken into account in bends and linacs High-resolution energy spectrometer is necessary

19 A-line as a high-resolution spectrometer Spectrometer screen (PR18) η x = -6.4 m β x = 100 m

20 Elegant simulation (L2 at deg) Y. Ding BC2 END L3END A-line PR18 ~ 2 mm

21 RMS bunch length (Elegant simulations) Y. Ding Intrinsic temporal resolution this method is below 1 fs Linac wake adds some nonlinear energy chirp that distorts measurements ements

Before saturation, E: pulse energy M: bunch length/coherence length FLASH (Nature, 2007) LCLS

22 Fluctuation method to measure x-ray pulse length SASE chaotic temporal profile leads to statistical intensity fluctuation that depends on the bunch length. Before saturation, E: pulse energy M: bunch length/coherence length FLASH (Nature, 2007) LCLS preliminary fluctuation data (~10000), 40 pc, 800 ev photon energy, overcompression estimated x-ray length ~ 2 fs estimated x-ray length ~ 8 fs J. Wu

23 Summary LCLS low charge beams deliver short x-ray x pulses (<10 fs) to soft x-ray x users (hard x-rays x also available) These studies illustrate an interesting mode of running SASE FELs Future x-ray x FEL designs may benefit from low charge configurations Smaller emittance lower beam energy for the same λ FEL Less charge less wake, more compact accelerators and more bunches A single-short short method for measuring fs bunch length has been proposed and will be tested at SLAC The method requires no extra hardware (besides a high- resolution spectrometer) and can be applied to other facilities

Femto-second FEL Generation with Very Low Charge at LCLS

Femto-second FEL Generation with Very Low Charge at LCLS Yuantao Ding, For the LCLS commissioning team X-ray Science at the Femtosecond to Attosecond Frontier workshop May 18-20, 2009, UCLA SLAC-PUB-13525;