Low-impedance, 2.8-MHz, pulsed bunching system for the Los Alamos Proton Storage Ring

The Proton Storage Ring (PSR) now under construction at Los Alamos National Laboratory is a device designed to accept a proton beam from the 800-MeV linear accelerator (LAMPF) and accumulate the protons to provide short, extremely high intensity bursts that drive a high-flux pulsed-neutron source. These bunches, which circulate at a 2.8-MHz frequency, are longitudinally confined by an rf electric field produced at that frequency by a ferrite-loaded coaxial quarter-wave resonator driven by a unique low-impedance rf amplifier. The buncher develops a 14-kV peak amplitude across the resonator gap and is energized in 3 ms-long pulses at rates up to 24 Hz. A principal design challenge exists because of high circulating beam currents (up to 45 A) that would, in a system driven by a conventional high-impedance amplifier, produce an induced voltage phased to interefere with the bunching action and much higher than that of the rf drive. The chosen solution is to feed the cavity from a low-output impedance amplifier whose final stages operate in Class A with a push-pull common-anode configuration. With this approach, similar to the rf system proposed for the colliding-beam accelerator (now cancelled) at BNL, we can achieve an effective system impedance of about 25 Ω and can limit beam-induced effects to an acceptable level. This paper discusses the design of the cavity and amplifier and reports results from preliminary tests