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
LiTrack calculations. 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 pC operation
Compare for ~3KA peak current

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

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

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

6. Beam before BC2
150 pC
(no cuts)
250 pC cut 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 pC beam for good emittance
Measure emittance and FEL performance as compression is changed.
Compare with normal 150 pC running
Optional – measure beam profile using ALINE
Optional – look at THz and COTR from maximum compression beam.