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ATF Photoinjector

The ATF photoinjector is an S-band (2.586 GHz), 1.6 cell, water cooled, elliptical, copper, cavity maintained at a base pressure of ~ 1x10-10 Torr. Two ports symmetrically placed in the first 0.6 cell are used for laser irradiation and imaging of the cathode. One port in the full cell is used for coupling fundamental power and a second port is placed to preserve the symmetry. RF coupling into the half-cell is through the iris.

The gun is powered by SLAC XK-5 klystron delivering a maximum power of 10 MW, typically run at 2.5 MW in 2.5 µs pulse. The design accelerating field is 100 MV/m and corresponding peak accelerating and surface fields are 130 MV/m and 160 MV/m respectively. A combination of two solenoids, one at the exit of the gun and another at the back of the gun provide emittance compensation to the electron beam while maintaining zero longitudinal magnetic field at the cathode. The center of the removable back flange serves as the cathode. Quantum efficiency of the laser cleaned copper cathode is typically in the range of 0.3-0.4%. 

Parameter Typical Value/Range Best Values
Charge 0.1 - 1 nC 3 nC
Repetition rate 1.5 Hz 10 Hz
Electron sport size on cathode 2.86 mm 0.2 - 4 mm
Bunch length 1 - 8 ps 100 fs w/compression
Avg. current during pulse 100 A 1.5 kA w/100 fs bunch
Peak current density 3kA/mm2  
Emittance 1.3 mm mrad 0.8 mm mrad@0.5 nC
Image of photoinjector gun