Current neutralization and focusing of intense ion beams with a plasma-filled solenoidal lens. I

Abstract

The response of the magnetized plasma in an axisymmetric, plasma-filled, solenoidal magnetic lens, to intense light ion beam injection is studied. The lens plasma fill is modeled as an inertialess, resistive, electron magnetohydrodynamic (EMHD) fluid since characteristic beam times τ satisfy 2π/ωpe,2π/Ωe≪τ≤2π/Ωi (ωpe is the electron plasma frequency and Ωe,i are the electron, ion gyrofrequencies). When the electron collisionality satisfies νe≪Ωe, the linear plasma response is determined by whistler wave dynamics. In this case, current neutralization of the beam is reduced on the time scale for whistler wave transit across the beam. The transit time is inversely proportional to the electron density and proportional to the angle of incidence of the beam with respect to the applied solenoidal field. In the collisional regime (νe>Ωe) the plasma return currents decay on the normal diffusive time scale determined by the conductivity. The analysis is supported by two-and-one-half dimensional hybrid particle-in-cell simulations.