Autoneutralization of space charge dominated beams for heavy ion fusion

Several heavy ion beams, each with current in the kiloampere range and particle energy in the gigaelectronvolt range, must be focused onto a millimetre size spot to provide the power required for ignition of high gain, radiation driven targets for inertial confinement fusion. It is difficult to achieve this condition for three reasons: (a) The repulsive space charge forces within individual beams and the mutual space charge repulsion among the beams, which are strongest near the focal spot, prevent multiple beams from focusing onto a small spot. (b) Beam current increases from beam head to tail, so the beam-beam repulsion is time dependent. This prevents multiple beams from being focused onto a small spot throughout the entire pulse. (c) Radiation from the beam heated target photoionizes some beam ions into higher charge states, and these ions are expelled from the beam by the space charge force. To minimize these three effects, it is proposed to place a metallic cylinder in front of the focal spot and to cover the cylinder, at the beam entrance end, with a plastic film of submicron thickness. As the beams pass through the film, the beam space charge force draws electrons from the film to co-move with the beam, `autoneutralizing' the ion beams inside the cylinder. Although the film strips beam ions into higher charge states, particle-in-cell simulations show that autoneutralization is so effective that the residual radial electric field in the neutralized beam is considerably less than that in the unneutralized beam. The application of this scheme thereby results in a great improvement in focusing quality. Beams with higher charge state or lower mass ions, which have the advantages of reducing both the length of the high energy section of induction linacs and the strengths of the final-focusing magnets, can therefore be used