Short pulse, low charge LCLS operation ARD R&D meeting Zhirong Huang on behalf of the LCLS commissioning team Nov. 10, 2009 LCLS

Outline Introduction Low charge operation Outlook and summary General considerations Machine setup Compression studies Soft x-rays results Hard x-rays results Outlook and summary Sub-fs possibility Bunch length diagnostics Summary

Promise of LCLS Ultra-bright Ultra-fast LCLS

Single Molecule Imaging with Intense fs X-ray R. Neutze et al. Nature, 2000

LCLS gain and saturation at 1.5 Å (250 pC) gex,y  0.4 mm (slice) Ipk  3.0 kA sE/E  0.01% (slice) (25 of 33 undulators installed) Lg  3.3 m

Typical Measured LCLS Parameters P. Emma

Outline Introduction Low charge operation Outlook and summary General considerations Machine setup Compression studies Soft x-rays results Hard x-rays results Outlook and summary Sub-fs possibility Bunch length diagnostics Summary

Lower charge LCLS operation* 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 pulses Less charge, smaller laser spot size on cathode smaller thermal emittance Less peak current in the accelerator (until the last step of compression) Less linac wakefield and CSR Less microbunching Longitudinal space charge of a very short bunch should be considered and minimized if possible (compression prior to the entrance of undulator?) * Suggested by J. Frisch; Also by J. Rosenzweig et al., NIMA2008

Bunch Compression & CSR Measured after BC2 (0.25 nC) sz < 5 mm nominal sz  2 mm old screen used sz > 25 mm BC2 (4.3 GeV) BSY (14 GeV) TCAV (5.0 GeV) 550 m L2 4 wires PRSTAB, 12, 030704 (2009)

Time-sliced x-Emittance at Very Low Charge TAIL 0.14 µm (not same data) 20 pC, 135 MeV, 0.6-mm spot diameter, 400 µm rms bunch length (5 A)

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 -34.5 deg. weaker CSR emittance blowup L2 at -33.5 deg (under-compress) L2 at -35 deg (over-compress) Horizontal projected emittance measured at 10 GeV, after BC2, using 4 wire-scanners.

Simulated 20-pC LCLS FEL performance 1.5 Å 15 Å z = 25 m @ 25 m, 15 Å, 2.41011 photons, Ipk = 2.6 kA, ge  0.4 µm 1.5 Å, 31011 photons Ipk = 4.8 kA ge  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)

Resistive wall wake in undulator Transient effect ~10 m 2a vz c wake catch-up distance ~ a2/2sz ~ 10 m for a=2.5 mm, sz=0.5 mm Steady-state wake for very short bunch (K. Bane) For 20 pC, 5 GeV beam, take 0.7 correction factor for flat vacuum chamber  sE/E = 5x10-4 at z = 60 m < FEL bandwidth Undulator wake appears relatively weak at low charge

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 mm (x/y) injector bunch length 220~250 mm (rms) Laser heater off L1S&X are the same as 250 pC configuration Vary L2 chirp to find maximum compression

Bunch length monitor signal J. Frisch H. Loos LiTrack simulation assumes 20 pC bunch charge 3 keV initial rms slice energy spread 0.23-mm initial rms bunch length

FEL gas detector signal photon energy @ 840 eV Half of undulators inserted (to prevent pulse lengthening due to slippage after FEL saturation) BL signal -1 deg +1deg LiTrack simulation assumes 20 pC bunch charge 3 keV initial rms slice energy spread 0.23-mm initial rms bunch length FWHM bunch length (LiTrack) X-ray pulse duration should be <10 fs, but no direct measurement yet possible 4-fs

Charge dependence Can easily go to 40 pC (laser iris diameter 0.8 mm) or 10 pC (laser iris diameter 0.5 mm) FEL energy is approximately proportional to charge X-ray pulse length probably also increases with charge (no direct measurement yet)

Measured Energy spread (@ 4.5 GeV) Energy spread measured on the vertical dump OTR screen (FEL suppressed) full-compression under-compression over-compression Difference in under/over-compression shows up in FEL bandwidth

20pC, 4.3 GeV, L2 = -33 deg (Elegant simulation by Y. Ding) What happens at full compression? 20pC, 4.3 GeV, L2 = -33 deg (Elegant simulation by Y. Ding) BC2 END Undulator entrance σδ = 0.16% σδ = 0.4% Increase E spread due to longitudinal space charge DL2 compression At full compression, energy spread is too large, and bunch is too short. X-ray slips out of the beam before significant gain occurs.

Peak of compression shifted by DL2 gun 4 wire scanners + 4 collimators TCAV0 L1X old screen L0 3 wires 2 OTR 4 wire scanners 3 OTR vert. dump heater L1S m wall 3 wires 3 OTR sz1 L2-linac sz2 L3-linac DL1 135 MeV BC1 250 MeV stopper BC2 4.3 GeV TCAV3 5.0 GeV BSY DL2 4.3-14 GeV undulator R56-DL2=130 m L1S = -17.5 degS DE/E(DL2) = 0 BC2 Ipk OTR33 20 MeV = 0.2 degS shift due to additional chirp from LSC & Linac wake

From P. Emma

From P. Emma

Compression study vs L1S phase 20 pC studies confirm this physical picture Bunch length signal FEL signal at 4.5 GeV J. Wu Hor. shifts of peaks removed in this plot 100 MeV L2 chirp voltage = 1 degS L2 phase

20 pC hard x-ray results Results shown here from June, recent results not as good Full 33 undulator with fixed und. taper (13.6 GeV, 1.5 Å) YAGXRAY Full compression? BL signal Full compression rms jitter ~25% Over compression rms jitter ~11% Under compression rms jitter ~13%

Gain length and FEL energy at Full compression Unlike soft x-rays, FEL can perform well @ full-compression for the hard x-ray case (~140 mJ with 25% fluctuation) Undulator K taper Gain length =2.74 m 140 mJ FEL energy

L1S = -22 deg, BC2 full compression, 13.6 GeV ~150 mJ 1 fs Elegant to Genesis simulations (Y. Ding)

Laser heater effects at full compression Low charge, low current beam yields less microbunching Not necessary to heat up E-spread to damp the instability At full compression, initial E-spread becomes bunch length, reduces the final peak current and decreases FEL power sd sz =sdR56 Indicate sub-mm bunch length w/o heater Changed by laser heater, measured on injector spectrometer

Outline Introduction Low charge operation Outlook and summary General considerations Machine setup Compression studies Soft x-rays results Hard x-rays results Outlook and summary Sub-fs possibility Bunch length diagnostics Summary

Sub-fs hard x-rays? L1S = -17 deg, BC2 full compression, 13.6 GeV simulations (Y. Ding) @ 70 m ~10 mJ 300 as

J. Frisch, FEL2009

AMO indicates 5-10 fs x-rays 20/40 pC short pulse mode accounts for ~1/3 AMO run time so far AMO experiments yield valuable FEL information (data consistent with FWHM x-ray duration 5-10 fs) Ne charge state distribution vs x-ray pulse duration (2 keV) “Mismatched intensity of 2+ and 3+ is because the model leaves out shake off and Double Auger processes.” Courtesy of Linda Young and Simulations by Robin Santra (ANL)

FEL bandwidth AMO also saw significant difference in bandwidth from under to over compression @ 40 pC L. Young et al. Multi-shot measurements include energy jitters 7eV or 0.8 % fwhm 17eV or 2 % fwhm

Summary LCLS low charge beams deliver short x-ray pulses (<10 fs) to soft x-ray users (hard x-rays also available) These studies illustrate an interesting mode of running SASE FELs Future x-ray FEL designs may benefit from low charge configurations Smaller emittance  lower beam energy for the same FEL Less charge  less wake, more compact accelerators (x-band?) and more bunches Much diagnostic challenge, especially the need for reliable bunch length measurements with fs resolution