Nonlinear charge and current neutralization of an ion beam pulse in a pre-formed plasma

Abstract

The propagation of a high-current finite-length ion beam in a cold pre-formed plasma is investigated. The outcome of the calculation is the quantitative prediction of the degree of charge and current neutralization of the ion beam pulse by the background plasma. The electric and magnetic fields generated by the ion beam are studied analytically for the nonlinear case where the plasma density is comparable in size with the beam density. Particle-in-cell simulations and fluid calculations of current and charge neutralization have been performed for parameters relevant to heavy ion fusion assuming long, dense beams with length lb≫Vb/ωb, where Vb is the beam velocity, and ωb is the electron plasma frequency evaluated with the ion beam density. An important conclusion is that for long, nonrelativistic ion beams, charge neutralization is, for all practical purposes, complete even for very tenuous background plasmas. As a result, the self-magnetic force dominates the electric force and the beam ions are always pinched during beam propagation in a background plasma.

Overview of theory and modeling in the heavy ion fusion virtual national laboratory

“…The analytical theory has been derived using the assumption of long charge bunches and conservation of generalized vorticity. The analytical results agree well with the results of numerical simulations for ion beam charge and current neutralization [12]. The numerical simulations also show complex phenomena, including collective wave excitations, during beam entry into the plasma.…”

“…We also recently adapted a model which includes emittance growth in the chamber [58] and self-pinching in the chamber [12,59,60], and which captures some of the understanding gleaned from particle simulations and analysis of chamber propagation [40,55]. These models have been incorporated into the systems code IBEAM, thereby improving the agreement between systems models and particle simulations.…”

“…Models for charge and current neutralization have been studied both analytically and numerically during ion beam entry, propagation, and exit from the plasma. A suite of codes has been developed for calculating the degree of charge and current neutralization of the ion beam pulse by the background plasma [12]. The code suite consists of two different codes: a fully electromagnetic, relativistic particle-in-cell (PIC) code, and a relativistic Darwin model for long charge bunches.…”

“…Examples include: detailed analytical and nonlinear perturbative simulation studies of collective processes, including the electron-ion two-stream instability [2][3][4][5][6][7], and the Harrislike temperature-anisotropy instability driven by T ⊥b T b [8][9][10][11]; development of a selfconsistent theoretical model of charge and current neutralization for intense beam propagation through background plasma in the target chamber [12][13][14][15]; development of a robust theoretical model of beam compression dynamics and nonlinear beam dynamics in the final focus system using a warm-fluid description [16]; development of an improved kinetic description of nonlinear beam dynamics using the Vlasov-Maxwell equations [2,[17][18][19][20], including identification of the class of (stable) beam distributions, and the development of…”

“…Plasma: The propagation of a high-current finite-length ion charge bunch through a background plasma is of interest for many applications [12][13][14][15], including heavy ion fusion. Models for charge and current neutralization have been studied both analytically and numerically during ion beam entry, propagation, and exit from the plasma.…”