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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 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).
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 • 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.
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.