Effects of finite pulse length, magnetic field, and gas ionization on ion beam pulse neutralization by background plasma

Abstract

This paper presents a survey of the present theoretical understanding of plasma neutralization of intense heavy ion beams. Particular emphasis is placed on determining the degree of charge and current neutralization. We previously developed a reduced analytical model of beam charge and current neutralization for an ion beam pulse propagating in a cold background plasma. The model made use of the conservation of generalized fluid vorticity. The predictions of the analytical model agree very well with numerical simulation results. The model predicts very good charge neutralization during quasi-steady-state propagation, provided the beam pulse duration is much longer than the electron plasma period. In the opposite limit, the beam pulse excites large-amplitude plasma waves. If the beam density is larger than the background plasma density, the plasma waves break, which leads to electron heating. The reduced-fluid description provides an important benchmark for numerical codes and yields useful scaling relations for different beam and plasma parameters. This model has been extended to include the additional effects of a solenoidal magnetic field, gas ionization and the transition regions during beam pulse entry and exit from the plasma. Analytical studies show that a sufficiently large solenoidal magnetic field can increase the degree of current neutralization of the ion beam pulse. However, simulations also show that the self-magnetic field structure of the ion beam pulse propagating through background plasma can be complex and non-stationary. Plasma waves generated by the beam head are greatly modified, and whistler waves propagating ahead of the beam pulse are excited during beam entry into the plasma. Accounting for plasma production by gas ionization yields a larger self-magnetic field of the ion beam compared to the case without ionization, and a wake of the current density and self-magnetic field are generated behind the beam pulse. Beam propagation in a dipole magnetic field configuration and background plasma has also been studied. r

Physics of Neutralization of Intense High-Energy Ion Beam Pulses by Electrons

“…In previous studies 28 , we focused on the nonlinear case, where the plasma density, p n , is comparable with or smaller than the beam density, b n , and the degree of current neutralization is arbitrary. The results of the theory agree well with particle-in-cell simulations and thus confirm the analytical formulas for the general nonlinear case, p b n n 48 . This section briefly reviews the major conclusions of that study and serves as basis for discussions of the additional effects of gas ionization, and solenoidal and dipole magnetic fields in subsequent sections.…”

Controlling charge and current neutralization of an ion beam pulse in a background plasma by application of a solenoidal magnetic field: Weak magnetic field limit

“…Particle-in-cell simulations show that structure of the self-electric and self-magnetic fields excited by the beam in the presence of whistler and lower-hybrid waves becomes rather complex, 27,29 and will be discussed in future publications. Coupling of helicon waves to the beam ion oscillations can lead to the development of the modified two-stream instability.…”

“…29 Lower-hybrid waves were observed in PIC simulations. 27,29 Note that for relativistic beams there is an extra factor 1 / ␥ b in Eq. ͑61͒ compared with the derivation based on the lower hybrid frequency, Eq.…”

“…With a further increase in the magnetic field value, the beam pulse can excite strong electromagnetic perturbations, including whistler waves, corresponding to longer wavelengths, 18,27 and lower-hybrid-like or helicon waves, 28,29 corresponding to shorter wavelengths. Both wave perturbations propagate nearly perpendicular to the beam propagation direction.…”

Stability properties of the electron return current for intense ion beam propagation through background plasma

“…The background plasma is needed to neutralize the ion beam space charge and beam current so that it can be transported and efficiently focused either ballistically or by the remnant unneutralized self-magnetic field or applied magnetic field (Roy et al, 2005;Logan et al, 2007;Sefkow et al, 2007;Welch et al, 2007). The ion beam current is neutralized by the opposing background plasma electron return current (Kaganovich et al, 2007(Kaganovich et al, , 2005(Kaganovich et al, , 2001, which implies that the plasma electrons inside the beam flow with average velocity v e0 = Z b (n b /n 0 )v b relative to the electrons outside of the beam. Here, v b is the beam velocity, Z b is the charge state of the beam ions, and n b and n 0 are the beam density and background electron densities, respectively.…”