RF / Laser Timing for UED@ASTA
5/20/14
Frisch
Requirements, Jitter and Drift
• Looking for 100fs Pk-Pk measurements
– 30fs RMS. (state of the art)
• Jitter:
– Short term (< few seconds), dominated by noise
– Relatively easy to measure / predict
• Drift
– Long term (minutes), dominated by thermal length changes
– Very difficult to predict
– 30 femtoseconds / deg C / Meter!
• Cables, Optical table, fiber-optics, vacuum pipe
– Need excellent temperature control.
• Real world systems see ~1ps/deg C.
Timing “Ring”
Timing Errors
• RF gun compresses beam, so experiment time error is
approximately 50% contribution from laser vs gun RF
jitter.
• RF gun amplitude also changes beam time: .01%->30fs
• Drift is corrected by finding “zero time” in the
experiment
– Continuous monitoring allows re-ordering of data, used
with time-tool at LCLS
– Frequency of measurement determines drift timescales.
• A good e-beam vs laser measurement is more
important than anything else for timing!
476MHz Reference
• Input 119MHz from fiber very high noise
• Common mode, but different subsystem
bandwidths will convert to relative timing
jitter.
• Ron Akre designed PLL to clean up phase
noise
– Unknown performance but probably OK
• Need to measure existing reference
• Can build a new reference if needed
– Not fundamentally difficult
– Takes a skilled RF engineer.
• Good RF sources have integrated noise < few
femtoseconds in out bandwidth.
Gun / RF Chain
• X6 multiplier, LLRF PAC,
SSSB, Klystron, Modulator,
Gun all similar to LCLS
• LCLS performance 35fs
RMS, 0.01% amplitude on a
typical measurement
• Should be OK
• This is the result of a large
amount of tuning work at
LCLS. Not all RF systems
are this good. 100fs RMS is
more typical.
Laser Locker
Note: most of the hardware / firmware complexity is “boring” stuff not related to
precision locking and not shown here. (bucket jump reset etc).
Locking System (XPP)
Performance
• 25fs integrated noise 100Hz to
10KHz
• Above 10KHz, measurement noise
dominates
• Below 100Hz, reference noise
dominates
• Locking banwidth is ~3KHz
• This is an Out Of Loop
measurement
• Drift relative to LBNL system
• 500fs in 3 hours
• Includes >200M stabilized
cable
Status of Locking Systems
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Installed Systems
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Operation
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High level automation
Common design / interface for all systems
Good reliability
Performance
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Early versions running LCLS Injector lasers (X2) for > 1 year
Current version running at XPP, MEC, FACET
Being installed for AMO, SXR, CXI, RLL
Depends on the unlocked noise of the laser!
Schedule
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Parts being fabricated / ordered
Few weeks
Lots of control system infrastructure needs to be ready
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Motor control, A-D, D-A, Epics panels, Python support etc etc.
This is not difficult, but is a BIG job
Support:
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Femtosecond timing group isn’t actually a group!
Joe Frisch, Steve Smith ½ time, Justin May ~full time, Karl Gumerlock, Dave Nelson, Jing Yin, Alex Wallace –
part time, engineering, installation for experiments.
10 Systems being installed
Can support locking systems, but NOT LLRF, Controls, Laser.
Expected Performance
• If the RF and laser locker systems both operate as well as our best
systems (LCLS Gun and XPP laser) expect 30fs RMS!
• Drift: 30fs/DegC/M
– Need good temperature stabilization
• Acoustic noise
– Normal conversation levels (for Joe), will double the laser phase noise!
– Need sound absorbing tiles. Move noisy crates out of the room etc.
• Laser locker itself is low risk, but overall performance depends on a
lot of systems
• 100fs RMS is a more comfortable target than 30fs RMS, but still not
certain.