Theoretical analysis of wire-guided ion beam transport

Ion beam transport using a current carrying wire to provide an azimuthal confining magnetic field has been proposed for inertial confinement fusion applications. The authors discuss several aspects of this transport scheme investigated theoretically to determine whether or not the scheme is viable. An important issue is how much of the injected ion beam current can be successfully transported. Propagating beam ions execute betatron-like oscillations around the wire, and must have sufficient angular momentum to avoid collision with the wire. The fraction of the injected ion beam with sufficient angular momentum and with small enough radial excursion to match pellet dimensions is determined by the extraction ion diode focusing geometry, the wire size, and the current carried by the wire. A second issue is the length of the standoff, or ballistic drift region, between the end of the guide wire and the target. The 1/r dependence of the guide magnetic field causes ion trajectories to bend inward sharply close to the wire, indicating that the standoff could be severely limited. Because of packing constraints, the standoff determines the maximum number of beams that can be delivered to the target, which in turn determines the amount of current each beam must carry. Packing constraints also determine the positioning of the return current wires, which produce magnetic fields that can strongly influence the trajectories of beam ions. The advantages and disadvantages of the wire guided scheme are discussed and compared to the standard z-discharge transport scheme