LCLS_II High Rep Rate Operation and Femtosecond Timing

J. Frisch 7/22/15

High Repetition Rate Operation

• Baseline maximum rate is 928.6KHz (1300MHz/1400)– Set by various numerology in the gun, laser and RF systems

• Max beam power is ~1MW. • Some experiments would like higher rate at lower

charge. – LCLS_II should be able to lase down to a few pC (assuming

deferred diagnostic devices are installed).– Potential for ~10MHz opearation

• Consider an upgrade to 7.428MHz (8X baseline) What is needed to upgrade?

8X (7.5MHz) Operation• Beam power

– Linac limit: 1.2MW -> 40pC– Undulator limit 120KW -> 4pC

• Numerology– RF frequency / 175– Gun frequency / 25– Laser frequency / 5 (assuming 37.14MHz laser)

• Rate limiting issues– Kicker: The existing kicker design is limited to ~1MHz (see later)– Diagnostics: Significant changes needed (see later)– Timing System: Minor changes needed (see later)– Lasers: Laser operates at higher rate normally, minor changes to select higher rate

output. Laser heater may need an average power upgrade.– MPS / BCS: No significant changes needed– RF system: No significant changes needed

Kickers• LCLS_II baseline is a multiple element FET driven kicker• Kicker power is ~(field energy) X (rep-rate). Higher rate -> higher power.• Faster kickers possible:

– ATF2 has operated a kicker at 3.3MHz, 3ns rise / fall time, in burst mode (60 pulses at 1 Hz).

– Not clear if the technology can be extended to high average rate• RF spreader:

– Use off frequency RF to direct the beam to multiple targets– No rate limit– Less flexible than kicker– Requires beam timing to be adjusted to select different destinations. Needs

study to see if this is consistent with all of the other timing contraints.• Kicker system is the largest technical challenge for high rate, and will

likely also dominate the upgrade costs.

Diagnostics• Charge limits

– LCLS lases down to 10pC, limited by diagnostics.– LCLS_II operation limited to >100pC without deferred diagnostics. – Deferred diagnostics designed for 10pC, but will probably work at lower charge (needs study)

• Stripline BPMS: The stripline BPM hardware is expected to function at this beam rate. Additional algorithm / firmware development may be required to eliminated effects from reflections of previous bunches.

• Cavity BPMs: The cavity BPM hardware is expected to function at this beam rate. Algorithm and firmware work is required to improve algorithms for measuring independent bunches in a train.

• Bunch Length Monitors: The baseline bunch length monitors are already beyond the state of the art. This will substantially increase the difficulty. A new solution may need to be found (for example looking at CSR induced orbit changes in the bunch compressors).

• Wire Scanners: The baseline wire scanners should operate. The wire scanner detectors will need <100ns response but this should be possible

• Low rate diagnostics: The low rate diagnostics should operate without any changes (assuming the diagnostic kickers are upgraded.

Timing System• The planned LCLS_II timing system sends beam information (~2000

bits) at the 900KHz beam rate– Includes bunch charge, destination and “beam code” like information.

• No straightforward upgrade to 8MHz– In principal could use 40Gbit networks, but would need to replace a large

amount of the control system hardware• Instead limit the differences between the 8 bunches in a microsecond

– Use a few bits (~16) to give beam destination and charge data only. – Limits the multi-bunch flexibility, but probably OK for most applications.– No new hardware, spare bits already exist.– Firmware development required, but probably not too bad. – It may not be practical to ring-buffer all accelerator data at full rate – needs

study.

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Femtosecond Timing

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• This includes the systems required to control and measure the

relative timing of the optical lasers and the X-ray beam.

• The present assumption is that we will duplicate the LCLS_I

system, modified to operate at the higher beam rates of LCLS_II

• High repetition rate will result in better performance than LCLS_I due

to higher feedback bandwidths and more signal averaging• High repetition rate adds considerable complexity to the beam phase

cavities data acquisition / control• Technology improvements that are in development will probably

improve the overall system performance.

• Higher performance designs are possible but at considerable cost

and development effort.

• Not for initial turn-on• Need prioritization relative to other improvements.

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Baseline Timing System (copy of LCLS_I)

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The phase cavities (located in the undulator hall) correct drifts in timing signal from the accelerator.

A RF link transmits the timing information to the experiment lasers A laser locking system locks the mode-locked laser oscillators to the RF reference

signal Note that the timing drifts / jitter in the laser amplifier and transport chain are not

controlled An X-ray / photon cross correlator is used to measure the relative timing at the

experiment.

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Baseline Performance

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• Accelerator: The LCLS_I accelerator has 70-100fs RMS jitter relative to the RF

reference line. Drift is corrected by the arrival time monitor.

• LCLS_II accelerator timing jitter expected to be lower. • Power spectrum of the jitter is what really matters!

• Beam arrival time monitor: Copper RF cavities. LCLS_I has ~15fs RMS single shot

noise and ~100fs 1-day drift in normal operation. An experimental algorithm that will be

implemented in production soon has given 10fs RMS noise, 100fs 1-day drift.

• LCLS_II expects similar performance but high rep rate allows averaging.

• Timing Transport: The existing system uses a reflection stabilized coaxial cable. The

added noise is <25 fs (consistent with zero) however drifts are ~1ps 1-day. An

improved system is under development that is expected to reduce drift to ~100fs 1-day.

• Laser locker: A RF based laser locker provides 25s RMS locking, drift has not been

measured but is <1ps 1-day.

• Laser amplifier and transport: The laser transport is not stabilized. Jitter is < 100fs

RMS, consistent with zero, and drift is <1ps, consistent with zero.

• LCLS_II rate is high enough to allow feedback from the amplified experiment laser

pulses.

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So, what is the timing stability we expect?

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• There is no simple answer:

• Multiple feedbacks of different bandwidths are controlling errors from

different (unknown) noise sources• Performance will depend on the beam rate and the experiment time

between finding “zero” times

• Timing errors in the laser amplifier and transport systems are not

controlled by the present timing system

• Very difficult for LCLS_I due to the low repetition rate• Might be possible for LCLS_II depending on the beam rate• Expect mostly minutes->hours drifts at <1ps.

• The X-ray / Optical cross correlator will allow offline re-sorting of data

for some experiments.

• This will provide better performance than any RF or fiber based locking

system• Needs substantial development for high repetition rate use.

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Pulsed laser fiber system - Upgrade

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• A fiber based timing system will improve the performance of all of the

subsystems except the laser transport. Experiments at other labs

have demonstrated long distance laser locking with noise of a few

femtoseconds. Several sub-systems are required

• Optical bunch arrival time monitor• Optical timing transport• Optical Laser Amplifier and Transport systems

• For future laser seeded operation, using a fiber laser to lock the seed

laser to the experiment laser could provide good performance.

This is a complex system with multiple fiber lasers. In order to be useful,

this needs to include fiber stabilization links around the Laser

Amplifier and Transport Systems. Substantial engineering and

controls integration work is required.

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X-ray / Optical Cross Correlator

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• For experiments where it can be used, an X-ray / optical

cross correlator provides the best timing performance.

• If the cross correlator can be used at high rate, it can provide

direct laser feedback in addition to re-sorting experimental data.

• A high frequency general purpose cross correlator will provide

timing performance to the limit of the measurement resolution –

better than can be expected from any other technology

• There are a variety of possible approaches to high frequency

cross-correlators and substantial R&D is required.

• This is probably the best path to <10fs timing!

Timing Develoment

Ready for First Light

Possible within the first year or two of operation

It could happen – don’t laugh

Short term laser vs X-ray jitter

<100fs RMS* <50fs RMS 10fs RMS probably the limit

X-ray vs laser 1-day drift

1ps 100fs 10fs with pulsed fiber locking

X-ray / optical cross correlator**

Probably not ready

10-50fs <<10fs

LCLS_I to LCLS_II jitter

200fs RMS 100fs RMS 100fs RMS

*Assuming full performance LLRF system for the accelerator** if applicable for beam operating conditions