Abstract
In heavy-ion inertial-confinement fusion systems, intense beams of ions must be transported from the exit of the final-focus magnet system through the fusion chamber to hit spots on the target with radii of about 2 mm. For the heavy-ion-fusion power-plant scenarios presently favored in the U.S., a substantial fraction of the ion-beam space charge must be neutralized during this final transport. The most effective neutralization technique found in numerical simulations is to pass each beam through a low-density plasma after the final focusing. To provide quantitative comparisons of these theoretical predictions with experiment, the Virtual National Laboratory for Heavy Ion Fusion has completed the construction and has begun experimentation with the neutralized-transport experiment. The experiment consists of three main sections, each with its own physics issues. The injector is designed to generate a very high-brightness, space-charge-dominated potassium beam, while still allowing variable perveance by a beam aperturing technique. The magnetic-focusing section, consisting of four pulsed quadrupoles, permits the study of magnet tuning, as well as the effects of phase-space dilution due to higher-order nonlinear fields. In the final section, the converging ion beam exiting the magnetic section is transported through a drift region with plasma sources for beam neutralization, and the final spot size is measured under various conditions of neutralization. In this paper, we discuss the design and characterization of the three sections in detail and present initial results from the experiment.
Highlights
The final transport section in a heavy-ion inertialconfinement fusion system poses major challenges
From the very early days of heavy-ion fusion (HIF), final focusing has been a subject of intense study [2,3,4], with perhaps the most comprehensive study being that of HIBALL-II [5]
The experiment uses a beam that is orders of magnitude lower in current than a driver for heavyion fusion, a careful choice of parameters guarantees that the transport physics closely matches that of a full-scale system
Summary
The final transport section in a heavy-ion inertialconfinement fusion system poses major challenges. From the very early days of heavy-ion fusion (HIF), final focusing has been a subject of intense study [2,3,4], with perhaps the most comprehensive study being that of HIBALL-II [5] These studies assumed that the chamber could have a sufficiently high vacuum that beam ions would experience no forces other than their collective space charge during the final transport to the target. In this final drift section, the beam space charge acts to enlarge the focal spot, so the beam species, current, and energy in early studies were chosen to make the spacecharge blowup manageable. Detailed numerical simulations of driver-scale systems using the Olson parameters show good spot sizes [7]
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