Effect of time-of-flight bunching on efficiency of light-ion-beam inertial-confinement-fusion transport schemes

Abstract

The Laboratory Microfusion Facility (LMF) has been proposed for the study of high-gain, high-yield inertial-confinement-fusion targets. The light-ion LMF approach uses a multimodular system with applied-B extraction diodes as ion sources. A number of ion-beam transport and focusing schemes are being considered to deliver the beams from the diodes to the target. These include ballistic transport with solenoidal lens focusing, z-discharge channel transport, and wire-guided transport. The energy transport efficiency ηt has been defined and calculated as a function of various system parameters so that point designs can be developed for each scheme. The analysis takes into account target requirements and realistic constraints on diode operation, beam transport, and packing. The effect on ηt of voltage ramping for time-of-flight beam bunching during transport is considered here. Although only 5 mrad microdivergence calculations are presented here, results for bunching factors of ≤3 show that transport efficiencies of ≳50% can be obtained for all three systems within a range of system parameters which seem achievable (i.e., for diode microdivergence within 5–10 mrad, for diode radius within 10–15 cm, and for diode-ion-current density within 2–10 kA/cm2). In particular, the point design for the baseline LMF system using ballistic transport with solenoidal lens focusing and a bunching factor of 2 was calculated to have ηt=84%. Other factors affecting the overall system efficiency, but not included in the analysis, are also identified and estimated.