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Multi-bunch Operation for LCLS, LCLS_II, LCLS_2025

Multi-bunch Operation for LCLS, LCLS_II, LCLS_2025. Josef Frisch Oct 18 2011. 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)

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Multi-bunch Operation for LCLS, LCLS_II, LCLS_2025

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  1. Multi-bunch Operation for LCLS, LCLS_II, LCLS_2025 Josef Frisch Oct 18 2011

  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 • 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. Normally run gun with short pulse, never reach equilibrium Minimize dark current, heating. 2007 data

  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? • 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 20-250pC 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.

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