AWAKE synchronization with SPS

Andy Butterworth, Thomas Bohl (BE/RF)

Thanks to: Urs Wehrle (BE/RF), Ioan Kozsar, Jean-Claude Bau (BE/CO)

Outline

• Requirements on synchronization• Proposed synchronization scheme

– cf. SPS/LHC synchro

• Timing signal distribution options• Precision and jitter: what’s possible?• Conclusions

Requirements

• Synchronization between e- gun and laser: few hundred femtoseconds– cf. the plasma period (~ 4 ps)– required for deterministic injection of the witness electron bunch into the

plasma wakefields.– Achieved by driving the RF gun with a laser pulse derived from same laser

system as used for ionization.

• Synchronization between proton beam and laser: 100ps desirable– cf. proton bunch length ( 300ps)

laser pulse (30fs)proton bunch (1s ~300ps)

gasPlasmaElectron bunch (1 s ~5ps)

Requirements

• SPS RF situated in Point 3 next to CCC

• Low-level RF electronics in Faraday Cage in BA3 surface building

• Synchronization signals to be exchanged on fibres between AWAKE and BA3

BA3

AWAKE

Frequency constraints

• Laser requires stable mode-lock frequency reference– between about 50 and 100 MHz– it cannot follow the changes in SPS frequency through the acceleration cycle SPS and AWAKE cannot be permanently locked together in frequency SPS must synchronize to AWAKE reference just before extraction of the p+

beam

• e- RF gun has limited bandwidth: 2998.5 +/- 1 MHz– and must be locked to the laser frequency (frequency multiple)

• SPS RF frequency at extraction = 200.394 +/- 0.001 MHz– adjustment limited by radial beam position

• Relationships between frequencies should be feasible to generate in hardware (fairly small integer division/multiplication ratios)

→ Need to find a frequency scheme which satisfies all these constraints

Example: SPS/LHC synchro

• Beams transferred from SPS to LHC must be injected at the correct azimuthal position in the LHC

• Precision required ~100ps (15° in 400MHz)• Rephasing of the beam is performed in SPS to align with LHC

SPS

LHC

frev LHC

frev SPS

= 7/27

For “coarse” rephasing, we use the common frequency

fc = frev LHC /7 = frev SPS /27

as a reference.

There is one fc pulse every 27 SPS turns.

By manipulating the SPS RF frequency, we can align the SPS beam with the fc pulse.

“Fine” rephasing is then done by locking the RF frequency to the LHC RF frequency with a PLL.

Rephasing for AWAKE

• Reference signals from AWAKE required by SPS for rephasing:– “common frequency” fc = frev SPS / n (n integer)– RF frequency reference: 200.394 +/- 0.001 MHz

• We can then do as for the LHC case:– coarse rephasing to the fc

– fine rephasing to the RF frequency reference

• Also needed in order to synchronize with the laser pulse:– frep laser pulse repetition frequency (~10 Hz)

Synchronization signals

Frequency choicesTurns = number of turns between fc pulsesLaser_h = number of laser periods between fc pulses = Turns * Laser_freq/frev Turns Laser_h Laser_freq Gun_harm Gun_freq div mult Divider_frq SPS freq 1 960 41.640312 72 2998.1024416 16 77 2.6025195 200.394000 1 1080 46.845351 64 2998.1024416 18 77 2.6025195 200.394000 1 1440 62.460468 48 2998.1024416 24 77 2.6025195 200.394000 1 1920 83.280623 36 2998.1024416 32 77 2.6025195 200.394000 1 2160 93.690701 32 2998.1024416 36 77 2.6025195 200.394000 3 3190 46.122429 65 2997.9578571 29 126 1.5904286 200.394000 5 5082 44.086680 68 2997.8942400 11 50 4.0078800 200.394000 5 5236 45.422640 66 2997.8942400 17 75 2.6719200 200.394000 5 6776 58.782240 51 2997.8942400 22 75 2.6719200 200.394000 5 7854 68.133960 44 2997.8942400 17 50 4.0078800 200.394000 5 10164 88.173360 34 2997.8942400 11 25 8.0157600 200.394000 5 10472 90.845280 33 2997.8942400 34 75 2.6719200 200.394000 13 13020 43.442056 69 2997.5018601 31 143 1.4013566 200.394000 17 22176 56.581835 53 2998.8372706 24 85 2.3575765 200.394000 23 26950 50.824565 59 2998.6493478 35 138 1.4521304 200.394000 23 31185 58.811283 51 2999.3754130 27 92 2.1781957 200.394000 29 28644 42.842855 70 2998.9998621 31 145 1.3820276 200.394000 29 32340 48.370966 62 2998.9998621 7 29 6.9101379 200.394000 29 35805 53.553569 56 2998.9998621 31 116 1.7275345 200.394000 29 47740 71.404759 42 2998.9998621 31 87 2.3033793 200.394000 29 64680 96.741931 31 2998.9998621 14 29 6.9101379 200.394000 29 71610 107.107138 28 2998.9998621 31 58 3.4550690 200.394000 31 33495 46.866339 64 2999.4456774 29 124 1.6160806 200.394000 31 36960 51.714581 58 2999.4456774 8 31 6.4643226 200.394000 31 40425 56.562823 53 2997.8295968 35 124 1.6160806 200.394000

SPS extraction timing• Start flat-top, launch rephasing, wait 500ms for rephasing to be finished• Wait for next frep pulse, arm counters etc. for next pulse

• Next frep pulse sends external event to central timing (for extraction forewarnings) and starts fine timing for bunch rotation and extraction extraction takes place at variable time in cycle

SPS cycle:

Injection Ramp Flat-top

Signal distribution

• Need low-jitter transmission of signals between AWAKE and BA3– distance ~2.1 km

• Most critical signal is the RF frequency reference used for fine rephasing– the others can have higher levels of jitter

• Several solutions exist:– Phase compensated fibres

propagation delay stabilized to < 5ps/km/°C– Phase feedback systems e.g. Libera Sync

jitter: 50 fs RMS drift: 500 fs for +/-2.5°C @ 300m fibre length

– White Rabbit (CERN BE/CO) < 8.5 ps jitter in first test

Tx

Rx

Uncertainty in bunch position

• Electronic noise from multiple sources:– AWAKE reference– frequency dividers– fibre optic transmission– locking of SPS RF to reference total expected to be > 10ps rms

• Beam position wrt. SPS RF:– Multi-bunch beam movement wrt. RF up to 200ps– Bunch rotation and extremely high single bunch intensity (wrt. LHC

beam) will add some more uncertainty

Measurements needed!

Conclusions

• SPS will rephase to the AWAKE reference before extraction– similar to technique used for filling LHC

• A timing scheme for this has been proposed– extraction at variable time on flat-top

• A workable schema can be found for the frequency generation chain to synchronize laser, RF gun and SPS RF

• Need to choose a technology for signal distribution between AWAKE and BA3

• Quantative measurements needed of jitter due to electronic noise and beam motion wrt. RF

• Resources (material and manpower) are under discussion