High brightness photoinjector development at the SLAC/SSRL Gun Test Facility

-John Schmerge, SSRL

Proposed single pass FELs using Self Amplified Spontaneous Emission (SASE) to generate high energy photons (0.2-1 keV) all require a high brightness electron source. Typical requirements for the source are 1 mm-mrad normalized emittance with 1 nC of charge. This emittance is approximately the diffraction limit for 4 Å (3 keV) photons assuming an electron energy of 15 GeV and can also be used to drive a SASE FEL at wavelengths down to about 1.5 Å. A RF gun test facility is now in operation at SSRL as part of the R&D on the proposed Linac Coherent Light Source (LCLS) dedicated to the measurement of the full 6-D phase space distribution from a photocathode RF gun generated electron beam. This Gun Test Facility (GTF) will allow optimization of the beam brightness by independently adjusting parameters such as accelerating field, laser pulse shape (both longitudinal and transverse) 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 gun under test is the BNL/SLAC/UCLA 1.6 cell symmetrized cavityvgun with a copper cathode. PARMELA simulations predict normalized emittances of < 1 =BC mm-mrad using solenoidal emittance compensation with 1 nC of charge after acceleration in a single SLAC linac to about 30 MeV and field gradients in the gun as high as 150 MV/m. Emittance measurements as a function of the laser pulse width and shape in both the longitudinal and transverse dimensions will be made downstream of the linac at an electron beam energy of approximately 30 MeV. A current transformer and a faraday cup are used to measure the charge while the energy spread is measured as a function of linac phase to determine the longitudinal emittance. The first measurements made at the GTF in July will be discussed as well as expected future measurements.

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