Time-Resolved Images of Coherent Synchrotron Radiation Effects P. Emma et al, SLAC Presented at FEL-2012

Motivation & Idea CSR is one of the limiting effects for FELs “…would like to see the time-resolved effects of CSR” - Joe Bisognano, 2008 LBNL FEL Workshop “Can we use a skew quad?” - Oleg Shevchenko (BINP), 2008 LBNL FEL Workshop Yes… Skew quad is a poor man’s transverse RF deflector, with a few differences Skew quad installed at LCLS BC1 in 2009 (PAC’09) Creates vertical dispersion after chicane revealing x vs t space (CSR effects) Measurements & simulations discussed here…

Add a CSR-Diagnostic Skew Quad in BC1 screen BC1 Skew quad maps the time coordinate of the pre-BC1 bunch onto the vertical axis of the screen

How it Works (first measurements Dec. 15, 2009) OTR11 screen in BC1 ~1% chirped energy spread at 220 MeV and large dispersion at chicane center Skew quad generates y' from x: Dy' = x/f OTR12 OTR11 skew Dy' = x/f x ~ t ~ E OTR12 screen after BC1 (skew quad OFF) 0.05 mm rms OTR12 (skew quad ON at -0.86 kG) 4 mm (~1 ps) Ipk = 250 A (after BC1) y ~ time

Simulation of skew quad effect at OTR screen with and without CSR OFF CSR Skew quad ON CSR OFF spot streaked on screen x-y spot on screen Simulation of skew quad effect at OTR screen with and without CSR Skew quad OFF CSR ON Skew quad ON CSR CSR blows up x spot size CSR is time resolved

BC1 Area in LCLS (S-band & X-band RF) (L1X) 135 Me13V OTR12 SQ11 OTR11 BPM

measured temporal profile is used in simulations Simulated pre-BC1 temporal profile Measured pre-BC1 temporal profile sz  0.5 mm sz  0.5 mm ignore red Super-Gaussian fit Temporal profile measured at 135 MeV using RF deflector

X-band RF (L1X) varied: LiTrack simulations too weak (16 MV), too strong (22 MV), and just right (19 MV) HEAD L1X = 16 MV (too weak) LiTrack simulations L1X = 22 MV (too strong) TAIL L1X = 19 MV (just right) – linearizes phase space, but not shown here.

L1X Voltage = 16 MV (too weak) Scan L1S phase holding BC1 energy constant L1X Voltage = 16 MV (too weak) -29.0° Measured on OTR12 -31.0° HEAD -31.5° BPM after BC1 -32.0° -32.5° -33.0° -33.5° -34.0° -37.0° TAIL skew OTR12 E X OTR11 BPM Energy loss in the final BC1 bend kicks to the right here HEAD t

L1X Voltage = 22 MV (too strong) Scan L1S phase holding BC1 energy constant L1X Voltage = 22 MV (too strong) -26.0° Measured on OTR12 -28.5° -29.0° HEAD -29.5° -30.0° BPM -30.5° -31.0° -31.5° -32.0° -32.5° -33.0° -33.5° -34.0° TAIL skew OTR12 E X OTR11 Energy loss in the final BC1 bend kicks to the right here HEAD t

L1X Voltage = 19 MV (just right) Scan L1S phase holding BC1 energy constant L1X Voltage = 19 MV (just right) -27.0° Measured on OTR12 -30.0° HEAD -30.5° BPM -31.0° -31.5° -32.0° -32.5° -33.0° -35.0° TAIL skew OTR12 E X OTR11 HEAD Energy loss in the final BC1 bend kicks to the right here t

Now Switch OFF L1X RF Measurements (left) and simulations (right) with X-band RF switched off. L1S phase settings differs from simulation by 2°. acceleration measured simulation

Summary Skew quad is simple, inexpensive, and adds time-resolved measurements of bunch Note this resolves the pre-compressed bunch length coordinate, not the compressed one Reasonable agreement with simulations, but preliminary results Should repeat and run more careful simulations as well

At BC1 Extreme Compression with Laser Heater OFF e/e0 > 10

Emittance Growth with CSR Measured in 2009… (good agreement with Elegant and CSRtrack) Phys. Rev. ST Accel. Beams 12, 030704 (2009)

Blow-up reasonably well modeled at extreme compression -26.0° -22.0° -22.0° -26.5° Vertical (time) is pre-BC1 bunch-length coordinate, not post-BC1, so blow-up is not so familiar wrt usual simulations Elegant Measurement (L1X = 19 MV)