Improvements to the cathode-follower RF amplifier system for the LANSCE Proton Storage Ring buncher

Los Alamos Neutron Science Center (LANSCE) is upgrading the Proton Storage Ring (PSR) for the Short Pulse Spallation Source, to deliver 200 microamperes (μA) of beam current to a new target assembly. Previous operation had been limited to 70 μA, partly as a consequence of the peak radiofrequency (RF) voltage available from the h=1 buncher. After an extensive series of performance tests in 1996, an upgrade was proposed to the 2.8 Megahertz (MHz) cathode-follower RF amplifier system and the ferrite-loaded gap assembly. A new intermediate power amplifier (IPA) was designed and installed at the ring, to eliminate a troublesome oscillation which occurred when the final power amplifier (FPA) was operated in class A. This also increased the drive to the FPA to raise the gap voltage. The cathode-follower FPA was modified to accept the direct connection of a pair of short transmission line feeders from the IPA. The beam gap was replaced with one having larger ceramic insulating sleeves in a simplified mechanical configuration. A new RF coupling loop/ferrite bias conductor was designed to tightly couple the two halves of the ferrite-loaded resonant circuit connected to the gap, to improve waveform symmetry and to reduce the effects of gain differences in the amplifiers. Improvements were made to the amplitude feedback controller and a phase controller was designed and installed. Changes to the utility systems, the controls, and the power supplies completed this upgrade in 1998. A description of the circuit modeling and the hardware changes in the equipment are presented. As demonstrated, the upgraded RF buncher has delivered the specified performance at LANSCE