Multi-Bunch Operation for LCLS

1 Franz-Josef [email protected]

Multi-Bunch Operation for LCLS

Multi-Bunch Operation for LCLSFranz-Josef Decker

March 17, 2010

1. Definitions and goalsmulti-bunch within a pulse (vs different pulses [beam code])a few bunches (vs >1000 bunches)

2. History of SLAC’s multi-bunch runningphase manipulationhigh chargelow charge

3. Multi-bunch scenarios for LCLSphase and amplitude manipulation for Gun, L0, L1, L2, L3

4. Transverse consideration: kickers …



Definitions and Goals

Multi-bunch: few (2, 3, …) to many bunches (1400) within a pulseSLC: 3 bunches, 60 ns apartE-158: >1000 bunches every RF period, 6.5E11 particles (>100 nC)

[100pC/bunch]E-155: >1400 bunches, 2.9E9 particles (effectively zero) (<500 pC)

Different pulses on different beam codese.g.: 60 Hz here and 60 Hz there separated by pulsed magnets, quads, …or: 59.5 Hz to experiment and 0.5 Hz to BYKICK dump

Goals for LCLS (-I and/or -II):Mainly a few bunches up to 8 (420ns/60ns +1) for different

undulators after separation by kicker magnets Shorter separation possible if RF deflection is used (TCAV 4, 5, 6, …)More bunches for one experiment (seeding, high power, target hit rate improved?) Two bunches 105 mm apart + delay one (like sail-boat chicane for FACET)



RF Amplitude and Phase ChangeSLC 1990: Energy spread for e-

scav was not compensated:

RF phase was changed fast (30 ns) by 90 in compressor cavity to achieve about 6 integrated phase change in 60 ns:



RF Phase ManipulationSLC 1990: 6.8 in 60 ns in NRTLLCLS 2009: 4.0 difference between

over- and under-compression in L2

with SLED: 1/2 at input (1/3 at output)and 800ns fill time (600 ns

NRTL):we need about 100 ns for 4

change



High power (600 kW) E-158 BeamEnergy of the different SLAC beams

The E-158 beam (blue, __) is at 45 (48) GeV at the end of the linac (3000 m) and has the highest energy along the linac. The scavenger beam (magenta, --) ends at 25.5 GeV, the PEPII beams for filling HER (red, -.) and LER (green, --) have an end-energy of 9.0 and 3.11 GeV.

Conclusion: Transverse betatron lattices can be designed for beams with different energies like a factor of 2 in sector 4 or 1.8 in sector 10.



Synchrotron Light on Gated Camera (60 ns)

<6.5E11 particlesE

= 5 m

16%



Start of Pulse like Electron in BPM, Tail like Positron

Charge jitter caused energy jitter

Linac phase offset reduced energy jitter



Transverse jitter of beam tail

Transverse (dispersive) jitter at end of pulse could be reduced from 300 m (shown)

to 25 m rms by launching oscillations early

in the linac

Jittery setup

Jitter of tail reduced, but “bowed” trajectory?



RF Tricks for Low Current (E-155)Instead of one 180 switch PSK to make a SLED RF pulse use three, or five:

RF energy gain

RF SLED pulse with notch

»



Double Notch in SLED Pulse

0.5% energy variation 0.1%



Multi-Bunch Scenarios for LCLS

Five different longitudinal regions for LCLS (same parameter):RF-Gun: effectively standing wave: (lower RF ampl. to keep peak, more power)L0: traveling wave section no SLED (keep RF flat at low charge)L1: traveling wave section with SLED (we have Ampl/Ph control for E and chirp)

L2: traveling wave section with old SLED (needs Ampl/Ph control, PSK for few bunches)L3: traveling wave section with old SLED (needs Ampl control, PSK mostly o.k)

TCAV3 is an indication what is possible with EPICS RF system: L1S TCAV3

PSK A/P change 180 to

compensate

temperature, [300 ns gives

90, TCAV3 separates multi

bunches]

100=1000ns



Energy Change after BC2

If different energies aredesired: L3 can give

E = -5 GeV in 350 ns.(needs big notch for all SBSTs)



Transverse EffectsWe saw mismatches with up to twice in beam size (Li01: 30% energy difference),so local compensation will be necessary for LCLS at least before BC2.

Separating the bunches to different beam lines (stability?)a) By kickers and septum/Lambertson magnetsb) RF separators (TCAV)

Fast quadrupoles (kicker type) might not be necessary (Lattice for 2 * E is o.k.)

…



Diagnostics and Controls

BPM at 140 MHz: 2856/140 = 20.4 times 5 give 102 buckets or 35.7 ns,at this bunch delay the BPMs will see the sum of two bunches,at about 400 ns EPICS might be able to separate two signals.

BLM ?

Wires: 60 ns gate was used during SLC

Screens and cameras: 60 ns gated camera (maybe faster these days)

Feedbacks

Laser “stacks” now already two pulse, just delay one.More ideas: “just” ask Bill White

…



Summary

Multi bunch operation is possible with the SLAC Linac even for LCLS

Rule of thumb: RF can change: 10% in 80 ns (Tf = 800 ns), with SLED half

Energy difference of up to 30% have been equalized to 0.1 % historically.

Transverse: One lattice can accommodate different energies

Kickers and multiple undulators are necessary for additional beam lines

Two bunches 105 mm apart and delay the first one in chicaneeliminates timing jitter difference from two linacs

Multi-Bunch Operation for LCLS

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