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LCLS 20 pC SASE FEL studies

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

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  1. LCLS 20 pC SASE FEL studies Z. Huang et al., Feb. 25, 2011 HXRSS collaboration meeting

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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)

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