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LCLS 20 pC SASE FEL studies. Z. Huang et al., Feb. 25, 2011. HXRSS collaboration meeting. Gain length and FEL intensity at 20 pC (06/01/2009). Well tuned beam, LTU emittance ~0.9/0.5 um FEL intensity ~140 m J (linear taper)
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LCLS 20 pC SASE FEL studies Z. Huang et al., Feb. 25, 2011 HXRSS collaboration meeting
Gain length and FEL intensity at 20 pC (06/01/2009) • Well tuned beam, LTU emittance ~0.9/0.5 um • FEL intensity ~140 mJ (linear taper) • U28/U14 = 6 FEL intensity @ U15= 23 uJ, well above 1 GW seed power requirement YAGXray gain length measurement FEL intensity after U14 ~ 20 uJ 140 mJ FEL energy Gain length =2.74 m
Gain length and FEL intensity at 20 pC (02/24/2011) • Not well tuned beam (limited time), LTU emittance ~1.1/0.9 um • Poor FEL intensity ~ 60 mJ (> 5 m gain length!) • U28/U14 = 130 FEL intensity @ U15 ~ 0.5 uJ, well below 1 GW seed power requirement YAGXray gain length measurement Gain length = 5.35 m FEL intensity after U14 ~ 0.5 uJ 60 mJ FEL energy
Pulling out U15 • Missing undulator (U15) affects the FEL intensity (25% drop) • Tuning U14 and U16 K values has little effect Missing U15 between two data Power drops after U16 due to phase mismatch
FEL performance improved by machine tuning • Running LEM and tweaking undulator matching improves FEL performance in 10 minute! • U28/U14 = 50 FEL intensity @ U15 ~ 90 uJ, this ratio means U15 has 0.4 GW of seed power (assumes 5 fs FWHM pulse width) Gain length = 4.29 m FEL intensity after U14 ~ 1.8 uJ
FEL divergence after U14 • FEL divergence needs to be << Crystal mono Darwin width (~15 urad) • Measured rms beam size on direct imager (260 – 280 um). • Direct imager ~170 m from U14 rms divergence ~1.6 urad • 15% to 20 % FEL centroid stability after U14 FEL rms beam size on DI FEL beam position on DI
Summary • HXR self-seeding requires sufficient SASE power (~1 GW) to overcome shot noise in the second part of undulator. • This power level depends critically on FEL gain length, beam brightness and machine setup. • More tune-up time and studies are necessary to decide the location of self-seeding (U15 or U16). • The drop of 25% SASE power after pulling out U15 may be associated with phase mismatch in the extra drift space and poor FEL performance. • Self-seeding chicane can acts like a phase shifter to compensate part or all of power drop (under study)