Final transport of heavy ion beams for inertial confinement fusion

A heavy ion beam propagating through a hard or near vacuum (< 10⁻³ torr) is subject to eraittace growth due to the phase space distortions caused by enharmonic self-forces. If the distortion remains uncorrected, an enlarged focal spot results. Analytic predictions of an increase in rms emittance and focal spot size are given for the hard vacuum case and are shown to agree with computer simulations for initially cold beams. Some effects of beam preconditioning in the region from accelerator output to final focusing magnet are discussed. For the case of near vacuum transport (10⁻⁴–10⁻³ torr), prediction of final focal spot size requires consideration of additional effects such as beam stripping to high charge state and charge neutralization by electrons which are produced internally in the vessel and/or are injected from outside. Despite the deleterious effect of ion stripping, charge neutralization substantially improves the transport of intense beams with respect to hard vacuum ballistic transport. With the use of particle simulation codes we have delineated the acceptable ranges of beam energy, current and initial charge state and reactor size and temperature for efficient transport of heavy on beams to the pellet.