RF / Laser Timing for UED@ASTA

5/20/14Frisch

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• 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.