Recent Results of the University of Maryland Research Program on Collective Ion Accelerators

Two collective ion acceleration schemes are being studied at the University of Maryland: (a) the electron ring method (ERA) and (b) collective acceleration with linear beams. In the ERA, a hollow, axially moving (??z ?? 0.2) electron beam is formed by the cusp method. This beam is to be stopped for ion loading by resistive wall interaction and a fast magnetic mirror system. The rings are then accelerated by the magnetic expansion method over a distance of 1.4 m. During stopping experiments with resistive walls, a rapid growth of the radial beam width was observed which led to severe particle loss. This effect is attributed to radial off-centering in the cusp, mismatch in the equilibrium radius due to self fields, and the negative mass instability. It can be suppressed by the use of cusp correction coils, proper boundaries and Landau damping with scattering foils. In the linear-beam experiment, proton beams of 16 MeV energy, peak currents of 10 kA, pulse length of 3 ns, focused to 1 cm diameter have been obtained. Slowwave structures to increase the ion energy are studied.