ETA-II accelerator upgrades

Recent improvements to the ETA-II linear induction electron accelerator are discussed. The accelerator´s cells were carefully reconditioned to raise the maximum accelerating gap voltage from approximately 100 to 125 kV. Insulators of Rexolite plastic in a zero-gap arrangement replaced the alumina originals after several alternative materials were investigated. A multicable current feed system will be used to eliminate pulse reflection interactions encountered in earlier experiments. Improved alignment fixtures were installed to help minimize beam perturbation due to poorly aligned intercell magnets between 10-cell groups. A stretched wire alignment technique was utilized to enhance overall magnetic alignment and to characterize irreducible alignment errors. These changes are in conjunction with an expansion of the accelerator from a 20-cell to a 60-cell configuration. When completed. the upgraded accelerator is expected to deliver 2.5 kA of electron beam current at 7.5 MeV in bursts of up to 50 70-ns pulses at a 5-kHz repetition rate. A 5.5-m-long wiggler converts the energy into 3-GW microwave pulses at 140 GHz for plasma heating experiments in the Microwave Tokamak Experiment