Wire-guided transport of intense ion beams

Light ion inertial confinement fusion requires beam transport over distances of a few meters for isolation of the diode hardware from the target explosion and for power compression by time-of-flight bunching. This paper evaluates a wire-guided transport system that uses the azimuthal magnetic field, produced by a current driven through a thin wire, to radially confine the ion beam. Ion orbits are studied to determine the injection efficiency (i.e., the fraction of the beam which is transported) under various conditions. Some ions hit the wire because of too small angular momentum at injection; others hit the wire or are lost to large radius during transport because of chaotic orbit behavior induced by a small number of return-current wires close to the beam envelope. For a multimodular scheme (10–30 beams), individual transport system are packed around the target at some standoff distance. The fraction of the beam which is lost in this field-free standoff region is also evaluated under various conditions. The standoff efficiency is then combined with the injection efficiency to give the dependence of the total transport efficiency, ηt, on diode, focusing, transport, and standoff parameters. It is found that ηt can be as large as about 60% for parameter values which appear to be achievable.