The Naval Research Laboratory Modified Betatron Accelerator and assessment of its results*

Abstract

In this paper the experimental results of the Naval Research Laboratory (NRL) Modified Betatron Accelerator (MBA) [Phys. Rev. Lett. 64, 2374 (1990)] are briefly summarized and assessed. These results show that the addition of the strong focusing (SF) field made a substantial improvement in the confining properties of the device, at least in the intermediate time scale, and that the SF field together with the finite resistivity of the vacuum chamber wall is responsible for the trapping of the beam. There is extensive experimental evidence, suggesting that the slow beam loss during acceleration is caused by the cyclotron resonances. Reduction of several field errors together with operation at higher toroidal and SF field led to beam energies in excess of 20 MeV, while the trapped current was above 1 kA. Preliminary beam extraction experiments with 12 resonant coils have shown that a ∼80 G field disturbance is required to extract the beam from the MBA.