Measured Properties of the DUVFEL High Brightness, Ultrashort Electron Beam

The DUVFEL electron linac is designed to produce sub-picosecond, high brighness electron bunches to drive an ultraviolet FEL. The accelerator consists of a 1.6 cell S-band photoinjector, variable pulse length Ti:Sapp laser, 4 SLAC-type S-band accelerating sections, and 4-dipole chicane bunch compressor. In preparation for FEL operation, the compressed electron beam has been fully characterized. Measurement of the beam parameters and simulation of the beam are presented. 1 PHOTOINJECTOR Table 1: DUVFEL photoinjector properties Laser system Type Titanium:Sapphire IR laser energy 30 mJ at 800 nm UV laser energy 300 uJ at 266 nm on cathode Laser pulse length Adjustable 1-5 ps FWHM RF Cavity Type Modified 1.6 cell BNL/SLAC/UCLA Gun IV Cathode material Copper Maximum energy 5.1 MeV Loaded Q 7100 Unloaded Q 13700 RF pulse length 3.5 us Quantum efficiency ~1.E-5 Vacuum ~8.E-9 torr Timing jitter < 1ps The properties of the laser and photoinjector are summarized in Table 1. In typical running, 10 mJ of IR light is produced by the Spectraphyics Tsunami Ti:Sapphire oscillator and TSA50 amplifier, which is frequency tripled to produce 450 uJ of UV light. After spatial filtering and aperturing of the gaussian mode to produce a nearly uniform laser spot, about 200-300 uJ is delivered to the cathode. This produces 300 pC of charge at the accelerating phase of 30 degrees. The RF cavity is a Gun IV [1] with copper cathode that has been modified for better performance [2]. In principle, the laser pulse length may be adjusted from 100 fs to 10 ps, however there are practical limitations on the range of adjustment due to dispersion characteristics and efficiency of the BBO crystals. The thickness of the harmonic crystals is optimized for pulse lengths from 1-5 ps. Within this range of pulse lengths there is evidence [3] of variations in the time profile of the UV light that are sensitive to the phase-matching angle of the crystal. 2 LINAC The linac consists of 4 SLAC S-band traveling wave sections with a maximum energy of 210 MeV. There is a four-magnet chicane [4] between linac tanks 2 and 3 to compress the electron beam to sub-ps bunch lengths. The photoinjector cavity and linac tanks 1 and 2 are powered by a single Thomson 45 MW klystron. Each of these sections is independently phased with high power phase shifters. Tanks 3 and 4 are each powered by independent 45 MW klystrons. A single klystron for the photoinjector and linac tanks before the chicane has the advantage of eliminating relative phase jitter and thus variations in bunch length exiting the chicane. The fast phase jitter of these sections relative to the laser arrival time is estimated to be approximately 0.5 ps. Following the fourth linac section are a number of quadrupoles for matching into the NISUS undulator and two 70 degree dipoles that are used as an energy spectrometer and for bunch length measurement using the RF zero phasing method.