Abstract
The neutralized transport experiment (NTX) at the Heavy Ion Fusion Virtual National Laboratory is exploring the performance of neutralized final-focus systems for high perveance heavy ion beams. The final-focus scenario in a heavy ion fusion driver consists of several large aperture quadrupole magnets followed by a drift section in which the beam space charge is neutralized by a plasma. This beam is required to hit a millimeter-sized target spot at the end of the drift section. The objective of the NTX experiments and associated theory and simulations is to study the various physical mechanisms that determine the final spot size (radius ${r}_{s}$) at a given distance ($f$) from the end of the last quadrupole. In a fusion driver, $f$ is the standoff distance required to keep the chamber wall and superconducting magnets properly protected. The NTX final quadrupole focusing system produces a converging beam at the entrance to the neutralized drift section where it focuses to a small spot. The final spot is determined by the conditions of the beam entering the quadrupole section, the beam dynamics in the magnetic lattice, and the plasma neutralization dynamics in the drift section. The main issues are the control of emittance growth due to high order fields from magnetic multipoles and image fields. In this paper, we will describe the theoretical and experimental aspects of the beam dynamics in the quadrupole lattice, and how these physical effects influence the final beam size. In particular, we present theoretical and experimental results on the dependence of final spot size on geometric aberrations and perveance.
Highlights
The topic of final-focus systems for high intensity beams has been an important subject of analytical [1,2,3,4], and experimental [5] efforts since the beginning of the Heavy Ion Fusion project in 1976
In a possible scenario of a final-focus system for a heavy ion fusion (HIF) driver, the beam is transported in the finalfocus section through several strong large aperture magnetic quadrupoles, and is allowed to drift ballistically through neutralizing plasma in a low-density gas onto the target
This paper describes the experiments and associated theory and simulations to study the various physical mechanisms in the magnetic lattice that affect the final spot size at a given distance (f) from the end of the last quadrupole of the neutralized transport experiment
Summary
The topic of final-focus systems for high intensity beams has been an important subject of analytical [1,2,3,4], and experimental [5] efforts since the beginning of the Heavy Ion Fusion project in 1976. The first question, addressed with a combination of simulations and experiment, is whether we can place the focal spot at target, whether we can control the beam in the quadrupole lattice to produce any desired beam size r0 and convergence angle at the entrance to the drift section. This paper describes the experiments and associated theory and simulations to study the various physical mechanisms in the magnetic lattice that affect the final spot size (radius rs) at a given distance (f) from the end of the last quadrupole of the neutralized transport experiment.
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