Abstract
A high brightness electron injector is a necessary component for x-ray FELs. A dedicated rf gun test facility is being developed at SLAC to measure the phase space distribution from a photocathode rf gun generated electron beam. This Gun Test Facility will allow optimization of the beam brightness by independently adjusting parameters such as accelerating field, laser pulse shape and total charge. The test facility is comprised of a single S-band klystron, 3 m SLAC linac section, analyzing magnet, diagnostic section, a cathode drive laser and the gun under test. The laser is comprised of a Nd:YLF oscillator and a Nd:glass regenerative amplifier. The light incident on the cathode is capable of both normal and near grazing incidence and is currently frequency quadrupled into the UV. In the near future a Nd:glass oscillator will be installed which will be capable of generating pulses as short as 200 fs. This oscillator will be used to make emittance measurements as a function of the laser pulse width and shape. Both oscillators will be phase-locked to the 24th sub-harmonic of the linac frequency. Emittance measurements will be made downstream of the linac at an electron beam energy of approximately 30 - 50 MeV using a quad scan with a beam profile screen and/or a wire scanner to measure the spot size. A current transformer and a Faraday cup will be used to measure the charge while a streak camera or a transition radiator can be used to measure the micropulse width. The first gun to undergo testing will be the BNL/SLAC/UCLA 1.6 cell symmetrized cavity gun with a copper cathode. With field gradients in the gun as high as 150 MV/m, using solenoidal emittance compensation and spatial and temporal flat top laser pulses, PARMELA simulations predict normalized emittances of less than 1.5 pi mm-mrad with 10 ps long pulses and 1 nC of charge after acceleration in a 3 meter linac section to about 30 MeV. Appropriate additional acceleration can further reduce the emittance below 1 pi mm-mrad.
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