1. Multi-bunch Operation for LCLS, LCLS_II, LCLS_2025
Josef Frisch Oct

2. Multi-Bunch
2-bunch operation demonstrated at LCLS (8.4ns spacing,2 KeV)
Wakefields and other accelerator physics issues probably do not limit operation at 10 bunches (2.5nC total)
E-158 used 100nC (!!!) in a 270ns bunch train with 0.1% energy spread (emittance was of course much worse than LCLS)
Multi-bunch “flamethrower” is probably not very difficult – but only interesting for a limited set of experiments.
Independent control and measurement of bunches would allow a single accelerator to drive multiple FELs. Ideally could have completely independent parameters for each bunch

3. Multi-bunch operation – Source Laser
Gun Laser: Laser design will depend on details of the required pulse structure.
Split / delay is straight forward but requires a very powerful laser for multiple-pulses.
Can use UV (BBO) electro-optical switches to increase efficiency
Other schemes possible for large numbers of bunches
Long delays are more difficult due to long optical delay lines.
LCLS changes the spot size on the cathode for different operating currents.
For small numbers of bunches, use multiple apertures and split / delay.
LCLS changes the pulse timing (~15ps) between high and low charge modes.
Easy with split / delay
Difficult otherwise
LCLS operates with the same laser wavelength and pulse width for all operating modes.
It may be very useful to have a different pulse length for high and low charge – implies 2 bunch compressors / harmonic generators.

4. Multi-bunch Operation - Gun
Normally run gun with short pulse, never reach equilibrium
Minimize dark current, heating.
2007 data
Pulse trains require a flat-top on the RF pulse
Possible with RF shaping (need to be careful, easy to destroy the gun with excess power!)
Flat top at full gradient will increase dark current (probably OK).
Gun fill time is ~1us.
Phase shifts ~0.05 degree/ns are possible with normal RF power.
Normally move gun phase 8 degrees between high and low charge operation: Would require 160ns.
Sudden phase shifts risk exciting other modes in the gun.
Probably need to operate with fixed laser to RF phase.

5. Multi-bunch operation L1S
L1S I and Q amplitudes adjusted by ~5% for different operating modes.
Structure fill time is ~900ns, so need ~50ns to change RF

6. Multi-bunch operation L2
L2 I/Q amplitude adjusted by ~750MeV (out of 5 GeV) for different operating modes. (~8 degrees).
Would take about 135ns for non-sled operation
SLED makes this more difficult. (~X2)
PROBLEM!

7. What to do about L2 ? Move L2 Phase? Move injector time?
With SLED, can probably manage ~3 degrees in 50ns
Move injector time?
Limited by Gun to about 2.5 degrees
Laser timing shift from 350pC to 20pC is in the wrong direction (I think!).
Move beam in BC1 to change delay?
Total delay is 54 degrees.
Maximum range is probably limited to a few degrees before running into nonlinearities in magnet fields.
Changing BC1 energy will use some of the range of L1S adjustment.
Maybe can use L1X
Combining all 3 above, may give enough range in 50ns.
Option: add 350MeV of fast fill (~50ns) Structures
~3 X-band stations with short structures - (~$10M)

8. Other RF issues
In traveling wave structures, when you make a change in RF that is less than a fill time long, the distribution of gradient in the structure will change -> mismatch problems.
Less severe for multiple structures.
Phase changes will introduce additional frequencies in the RF – may lead to breakdown or processing issues
Existing (and planned LCLS_II) LLRF system is not flexible enough to implement fast RF changes in a feedback-controlled fashion.

9. Other Issues
Need to investigate other operating modes (eg. Over compression in BC1) to try to find conditions where we can change from pC with minimal RF changes
No obvious solution, but a big space to explore
Scheduling – how much flexibility is really needed for multi-bunch.
Energy flexibility has not been investigated yet. What range of energy beams can be transported through the linac / LTU (before DL2’) without wrecking the match.