Ion beam pulse neutralization by a background plasma in a solenoidal magnetic field

摘要

Ion beam pulse propagation through a background plasma in a solenoidal magnetic field has been studied analytically. The neutralization of the ion beam current by the plasma has been calculated using a fluid description for the electrons. This study is an extension of our previous studies of beam neutralization without an applied magnetic field. The high solenoidal magnetic field inhibits radial electron transport, and the electrons move primarily along the magnetic field lines. For high-intensity ion beam pulses propagating through a background plasma with pulse duration much longer than the electron plasma period, the quasi-neutrality condition holds, n_e = n_b + n_p, where n_e is the electron density, n_b is the density of the ion beam pulse, and n_p is the density of the background plasma ions (assumed unperturbed by the beam). For one-dimensional electron motion, the charge density continuity equation partial deriv ρ/partial deriv t + ▽ • j = 0 combined with the quasi-neutrality condition [ρ = e(n_b + n_p — n_e) = 0] yields j = 0. Therefore, in the limit of a strong solenoidal magnetic field, the beam current is completely neutralized. Analytical studies show that the solenoidal magnetic field starts to influence the radial electron motion if ω_(ce) ≥ω_(pe)β (where ω_(ce) = eB/mc is the electron gyrofrequency, ω_(pe) is the electron plasma frequency, and β = V_b/c is the ion beam velocity relative to the speed of light). This condition holds for relatively small magnetic fields. For example, for a 100 MeV, 1 kA Ne~+ ion beam (β = 0.1) and a plasma density of 10~(11) cm~(-3), B corresponds to a magnetic field of 100 G.