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Cutting Beam Horns in BC1

Cutting Beam Horns in BC1. Paul Emma, Josef Frisch. Why and what. In LCLS compression system tails on the beam are folded over by longitudinal wakefields and produce high peak current spikes on the ends of the bunch The high current spikes cause CSR emittance growth

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Cutting Beam Horns in BC1

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  1. Cutting Beam Horns in BC1 Paul Emma, Josef Frisch

  2. Why and what • In LCLS compression system tails on the beam are folded over by longitudinal wakefields and produce high peak current spikes on the ends of the bunch • The high current spikes cause CSR emittance growth • Investigate cutting the ends off the bunch in BC1 • LiTrackcalculations. Need to do MD experiment • Note – might have issues with stability, transverse wakes etc. • Start at 250 pC, cut tails to get 150 pC. Compare with standard 150 pCoperation • Compare for ~3KA peak current

  3. Operating Conditions • 150 pC Case (no cuts) • 560 um bunch length from gun • L1S = -22.6 deg., L1X = 20MeV, L2 = -33.3 deg. • Set to get good energy spread in LiTrack • 250 pc Case (BC1 cuts to 150 pC) • 560 um bunch length from gun • L1S = -21.3 deg. L1X = 20MeV, L2 = -37 deg • Set for good energy spread in LiTrack

  4. Beam before BC1 150 pC (no cuts) 250 pCwill cut to 150 pC

  5. Beam after BC1 150 pC (no cuts) 250 pCcut to 150 pC -1.6%, +0.8% energy cut

  6. Beam before BC2 150 pC (no cuts) 250 pCcut to 150 pC

  7. Beam at Undulator 150pC (no cuts) 250pC cut to 150pC

  8. Set L2 phase for ~6KA 150pC (no cuts) 250pC cut to 150pC

  9. Set L2 Phase for max compression

  10. Max Compression • Wakefields, longitudinal space charge and CSR will probably make FEL operation at high current impossible. • Might be interesting for THz – Near IR generation. • For clipped beam, peak current limited by beam longitudinal phase space. • What if we turn off the heater……..

  11. Just for fun >300 kA! This is NOT a possible operating mode, but it does indicate that the nonlinear wakefields are well compensated.

  12. Practical Issues • Collimators – exist in BC1 • Radiation: cutting 100 pC, 120 Hz, 250 MeV= 3W (out of 7.5W total beam). • OK with Stan Mao for 2 shift test (up to 5 W) • Probably need to bypass MPS toroids • May need extra shielding for long term running • Similar load to TD11, so not impractical • Measurements • TCAV3, ALINE, and FEL performance

  13. Rough Experiment Plan • Start with good FEL operation at 150 pC • Set injector to 250 pC, Tune for emittance to WS12 • Clip tails with BC1 collimator to get 150 pC • Remeasure WS12 emittances (effect?) • Compare % cut from each collimator with LiTrak • Use OTR11 to look at collimators cutting beam (COTR not too bad here) • Measure profiles with TCAV3 (with cuts and without) • Maybe work at partial compression to set cuts. • Tune resulting 150 pCbeam for good emittance • Measure emittance and FEL performance as compression is changed. • Compare with normal 150 pCrunning • Optional – measure beam profile using ALINE • Optional – look at THz and COTR from maximum compression beam.

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