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 (1  ~300ps) gasPlasma Electron bunch (1  ~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: /- 1 MHz – and must be locked to the laser frequency (frequency multiple) SPS RF frequency at extraction = / 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 f rev LHC f rev SPS = 7/27 For “coarse” rephasing, we use the common frequency f c = f rev LHC /7 = f rev SPS /27 as a reference. There is one f c pulse every 27 SPS turns. By manipulating the SPS RF frequency, we can align the SPS beam with the f c 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” f c = f rev SPS / n (n integer) – RF frequency reference: / MHz We can then do as for the LHC case: – coarse rephasing to the f c – fine rephasing to the RF frequency reference Also needed in order to synchronize with the laser pulse: – f rep laser pulse repetition frequency (~10 Hz)

Synchronization signals

Frequency choices Turns = number of turns between fc pulses Laser_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

SPS extraction timing Start flat-top, launch rephasing, wait 500ms for rephasing to be finished Wait for next f rep pulse, arm counters etc. for next pulse Next f rep 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: InjectionRamp 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 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