Simulations of intense heavy ion beams propagating through a gaseous fusion target chamber

Abstract

In heavy-ion inertial confinement fusion (HIF), an ion beam is transported several meters through the reactor chamber to the target. This standoff distance mitigates damage to the accelerator from the target explosion. For the high perveance beams and millimeter-scale targets under consideration, the transport method is largely determined by the degree of ion charge and current neutralization in the chamber. This neutralization becomes increasingly difficult as the beam interacts with the ambient chamber environment and strips to higher charge states. Nearly complete neutralization permits neutralized-ballistic transport (main-line HIF transport method), where the ion beam enters the chamber at roughly 3-cm radius and focuses onto the target. In the backup pinched-transport schemes, the beam is first focused outside the chamber before propagating at small radius to the target. With nearly complete charge neutralization, the large beam divergence is contained by a strong magnetic field resulting from roughly 50-kA net current. In assisted-pinched transport, a preformed discharge channel provides the net current and the discharge plasma provides nearly complete charge and current neutralization of the beam. In self-pinched transport, the residual net current results solely from the beam-driven breakdown of the ambient gas. Using hybrid particle-in-cell simulation codes, the behavior of HIF driver-scale beams in these three transport modes is examined. Simulations of neutralized ballistic transport, at a few-mTorr flibe pressure, show excellent neutralization given a preformed or photoionized (from the heated target) plasma. Two- and three-dimensional simulations of assisted-pinch transport in roughly 1-Torr Xe show the importance of attaining >1-μs magnetic diffusion time to limit self-field effects and achieve good transport efficiency. For Xe gas pressures ranging from 10–150 mTorr, calculations predict a robust self-magnetic force sufficient for self-pinched transport. The latest simulation results are presented and the important remaining issues for each transport scheme are discussed.

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

“…2. The electron density perturbation due to the ion beam pulse propagation through a background plasma is calculated in two-dimensional slab geometry using the LSP code [22]. The background plasma density is n p ¼ 2. consist of an electron-emitting electrode, a biased foil, or a short plasma plug without any background plasma during further propagation of the ion beam.…”

Survey of collective instabilities and beam–plasma interactions in intense heavy ion beams

“…As noted earlier, to achieve maximum compression, the space charge of the ion beam is neutralized by the propagation of the ion beam pulse through a dense background plasma [1][2][3][4][5][6][7][72][73][74][75][76][77]. In one approach, transverse compression is facilitated by using solenoidal focusing magnets.…”

Results on intense beam focusing and neutralization from the neutralized beam experiment

“…Therefore, additional plasma, the "plasma plug," is externally injected near the chamber entry port, through which the beam passes. Chamber transport using annular and solid plasma regions in the transport chamber has been examined numerically by several investigators [17][18]25]. The general concept studied in this paper consists of an initially-non neutralized beam passing through a finite thickness of plasma and dragging along plasma electrons for partial charge and current neutralization.…”