Measurements and Simulations of Ultralow Emittance and Ultrashort Electron Beams in the Linac Coherent Light Source

The Linac Coherent Light Source (LCLS) is an x-ray free-electron laser project presently in a commissioning phase at the SLAC National Accelerator Laboratory. We report here on very low-emittance measurements made at low bunch charge, and a few femtosecond bunch length produced by the LCLS bunch compressors. Start-to-end simulations associated with these beam parameters show the possibilities of generating hundreds of GW at 1.5 Å x-ray wavelength and nearly a single longitudinally coherent spike at 1.5 nm with 2-fs duration.

Figure 1

LCLS 1-km accelerator layout showing gun, off-axis injector, BC1 and BC2 compressors, and various linac segments. The undulator (not shown) is 400 m off scale at right.

Figure 2

Measured (blue or dark gray dots) and simulated (green or gray curve) time-sliced horizontal emittance along the length of the bunch at a charge of 20 pC and cathode UV spot diameter of 0.6 mm. Smooth curves aid the eye and $f(t)$ is the bunch temporal distribution on an arbitrary scale.

Figure 3

Measured core time-sliced emittance at 135 MeV versus rms UV laser spot size (diameter/4) on the cathode with a constant bunch charge of 20 pC (red points). The data are fitted to a single parameter: a linear slope with no offset (blue, solid line). The theoretical thermal emittance is shown in green (dashed) line.

Figure 4

Measured (red dots) and simulated (blue curve) projected emittance at 10 GeV versus L2-linac rf phase with a bunch charge of 20 pC.

Figure 5

Measured (red dots) and simulated (blue curve) photodiode OTR signals at OTR22, versus the L2-linac rf phase with a bunch charge of 20 pC. The green dashed curve (right axis) shows the simulated fwhm bunch length.

Figure 6

Longitudinal phase space at OTR22 after undercompression (top left) and overcompression (top right), and the corresponding current profiles below. The L2 rf phases for the two compression modes are at $-33.5°$ and $-35°$. The bunch head is to the left.

Figure 7

Genesis simulation at 1.5 Å for the average FEL power (solid line) and the rms fluctuation (dashed lines) along the undulator. The inset plot shows a snapshot of a typical 2-fs FEL pulse at 100 m.

Figure 8

Genesis simulation at 15 Å for the average FEL power (solid line) and the rms fluctuation (dashed lines) along the undulator. The inset plot shows a snapshot of a typical 2-fs FEL pulse at 25 m (inset).