Electron beam measurements on the first stage of the HPMC high brightness gun

Abstract The TRW High Power Modular Components (HPMC) program is developing the technology for a high average current ( I avg = 100 mA) superconducting linac for FEL applications. To test the concepts a high brightness injector system is under construction. The injector incorporates a CW 500 kV high voltage electron gun and a pre-accelerator to raise the energy to 1 MeV where the beam may be finally bunched with minimal emittance growth in the face of the large space charge in the bunch. The injector gun cathode is grid modulated to produce beam pulses with 2 A peak current, 500 ps flat-top at 100 MHz repetition rate. The gun was modeled using both steady state (E-GUN) and time dependent (MASK) codes. The modelling using the MASK code showed that because the space charge in the beam is not in equilibrium with the gun focussing elements during the rise and fall of the pulse large emittance growth can occur. We have therefore designed for the rise and fall periods to be as short as possible with a flat-top peak current. We have constructed a test stand to simulate the first stage of the gun, which operates at 100 kV. On the test stand we have demonstrated 500 ps wide flat-topped beam pulses with rise and fall times of approximately 300 ps. The pulser is capable of repetition rates of up to 100 MHz. Measurements have begun of the beam emittance, which has been yielding values for normalized emittance, thus far, of approximately 15π mm mrad. Work is underway to further characterize the gun.