rf / laser timing for ued@asta
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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RF / Laser Timing for UED@ASTA
5/20/14Frisch
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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.
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Timing “Ring”
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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!
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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.
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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.
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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).
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Locking System (XPP)
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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
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Status of Locking Systems• Installed Systems
– Early versions running LCLS Injector lasers (X2) for > 1 year– Current version running at XPP, MEC, FACET– Being installed for AMO, SXR, CXI, RLL
• Operation– High level automation– Common design / interface for all systems– Good reliability
• Performance– Depends on the unlocked noise of the laser!
• Schedule– Parts being fabricated / ordered– Few weeks– Lots of control system infrastructure needs to be ready
• Motor control, A-D, D-A, Epics panels, Python support etc etc. • This is not difficult, but is a BIG job
• Support: – 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.
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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.