1. LCLS_II High Rep Rate Operation and Femtosecond Timing
J. Frisch 7/22/15

2. 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?

3. 8X (7.5MHz) Operation Beam power Numerology Rate limiting issues
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

4. 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.

5. 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.

6. 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.

7. Femtosecond Timing
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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8. Baseline Timing System (copy of LCLS_I)
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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9. Baseline Performance
Accelerator: The LCLS_I accelerator has fs 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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10. So, what is the timing stability we expect?
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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11. Pulsed laser fiber system - Upgrade
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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12. X-ray / Optical Cross Correlator
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!
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13. 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
For ranges of values consider providing
Ready on Day One -- These parameters will be available when the machine turns on.   I am not asking for the baseline KPP, but rather what is being planned for day one availability within the project scope.
Possible within the first Year of Operation -- This range of parameters represents areas where decision making within the project is still underway and could be made (possibly at some additional cost) to realize these parameters.  
It could happen--don’t laugh -- This range of parameters is within reach on paper when all stars are aligned.   Realization involves more risk than cost.   No one be laughed at for using these values, but it is also understood that there are no promises of delivery.  
*Assuming full performance LLRF system for the accelerator
** if applicable for beam operating conditions