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; PAC09, WE5RFP040 ( with C. Pellegrini ). 1 1 Yuantao Ding
Outline Introduction; Measurements and simulations of 20 pc beam at injector and main linac; Expected FEL from simulations; FEL performance on charge dependence. 2 2 Yuantao Ding
Introduction Original design parameters are based on 1 nc. 1 nc is typical for some other XFEL projects. 3 3 Yuantao Ding
Introduction 200 pc operation mode: Many of the most challenging issues, such as emittance generation, wake fields, CSR are associated with high charge. The resulting performance from simulations is more stable, with expected FEL x-ray pulse of 70 fs FWHM, 10^12 photons using optimized taper. 4 4 Yuantao Ding
Introduction 1 pc scheme: Full compression at compressors; Preserve the small emittance to undulator entrance; It is very interesting to operate XFEL at very low charge; We choose the charge of 20 pc in this low charge study, which is almost at the limit with the present LCLS diagnostics (BPMs, ) 5 5 Yuantao Ding
LCLS layout Impact-T Elegant Genesis (Slice) emittance and bunch length measurements at OTR2; Measure emittance using wire scanners after BC1 and BC2; Measure bunch length signal after BC1, BC2 and PR55; Start-End simulations to evaluate FEL performance. 6 6 Yuantao Ding
Experiments and simulations on electrons at LCLS: @ Injector; @ after BC2. 7 7 Yuantao Ding
LCLS Injector: diagnostics Quad-scan to measure emittance; Quad-scan + TCAV0 to measure slice emittance; TCAV0 + OTR2 to measure bunch length. OTR2 8 8 Yuantao Ding
Emittance at injector : 20 pc Transverse cavity and Quad-scan method are used to measure the slice emittance at OTR2 ( 1 um thick aluminum screen). emittance at the central slice time-sliced emittance Impact-T simulations are based on the measured thermal emittance. laser phase: 30 deg, laser spot 0.6 mm, Gaussian temporal shape of 4 ps (fwhm). 9 9 Yuantao Ding
Emittance at 10 GeV : 20 pc Laser phase at -15 deg to get velocity bunching, rms bunch length of 260 um at OTR2; No laser heater; R56: BC1 = -45.5 mm, BC2 =-24.7 mm. Four wire scanners to measure projected emittance; In simulations, thermal emittance is based on measurements, LSC and CSR are included; An rf phase shift of -1.5 deg to fit the data. OTR22 10 10 Yuantao Ding
Bunch length after BC2 : 20 pc Photodiode (1-2.5 µm) signal collected from OTR22 Elegant Simulations: L2 = - 33.5 deg L2 = - 35 deg Integrate the bunch form factor over 1-2.5 µm from simulated particles at OTR22. Different temporal shapes lead to the asymmetry of the photo diode signal. OTR22 11 11 Yuantao Ding
FEL studies Expected FEL @ 1.5 Å @ 1.5 nm One preliminary measurements at 1.5 Å with 20 pc FEL performance on charge dependence 12 12 Yuantao Ding
14 GeV, 20 pc : Elegant and Genesis simulations, Over-compression-1 (L2 = -34.5 deg) UNDBEG(14 GeV) Longitudinal phase space Current profile head Average FEL power along undulator X-ray FEL power profile @ 120 m 2 fs head 2.8e11 photons 13 13 Yuantao Ding
14 GeV 20 pc : Elegant and Genesis simulations, Over-compression-2 (L2 = -35 deg) At undulator entrance (14 GeV): Longitudinal phase space Current profile head Average FEL power along undulator FEL power profile @ 120 m 5 fs 14 3.1e11 photons 14 Yuantao Ding
4.3 GeV 20pC : Elegant simulations, Over-compression-2, L3 off laser heater to 5 kev energy spread; Linac-3 off, tracking down to undulator (not yet fully optimized for single longitudinal FEL spike); Slice emittance At undulator entrance (4.3 GeV): σ z = 1.99 µm head 15 15 Yuantao Ding
1.5 nm: Genesis simulations Average FEL power along undulator Radiation power profiles @25m head 16 16 Yuantao Ding
Power profile varies from shot to shot due to noisy startup Examples of 4 shots. Average photon number: 2.4x10 11, with 20% fluctuation. Estimated time-bandwidth product ~ 3 times Fourier-transform limit. 17 17 Yuantao Ding
Preliminary measurements at 1.5 Å, 20 pc Preliminary FEL measurements show an FEL gain over 5 orders, and the electron energy loss due to FEL is ~ 60 µj. Absolute bunch length and FEL power haven t measured yet, and more studies will be done. 18 18 Yuantao Ding
1.5 Å : FEL performance vs charge For different user requests different x-ray pulse length, changing the charge is an option; At some charge, it could provide x-ray for some special applications, like single molecule imaging with 100 pc. 19 19 Yuantao Ding
Summary Measured low emittance and short bunch with 20 pc; Expected ultrashot x-ray pulses with hundreds of GW; Such high-power, ultrashort x-ray pulses may open up new applications; The achieved beam may enable a more compact design of a future x-ray FEL; Thanks to SLAC engineering, controls, operations,and RF support groups; thanks to R. Fiorito, C. Pellegrini, G. Stupakov and D. Xiang for many discussions. Thanks Paul Emma for providing many slides. 20 20 Yuantao Ding