Physics of gas breakdown for ion beam transport in gas

Summary

Detailed analysis, experiments, and computer simulations are producing a new understanding of gas breakdown during intense ion beam transport in neutral gas.Charge neutralization of beam micro clumps is shown to limit the net clump potentals to a non-zero valueφ _min, which can lead to divergence growth and axial energy spreading. At pressures ≳1 Torr, plasma shielding should substantially reduce this effect.Current neutralization has been studied in experiments on the GAMBLE II accelerator. The importance of fast electrons (knock-ons and runaways) has been established in IPROP simulations, which are in agreement with the experiments. For light-ion fusion parameters with pressures ≳1 Torr, very small net current fractions (≪1%) appear feasible, permitting ballistic transport in gas.Self-pinched transport requires higher net current fractions (≥2%) and preliminary IPROP code results indicate that this appears achievable for small-radius intense beams in lower-pressure gases (≲1 Torr). Several self-pinched transport concepts look promising. The importance of these results for both light-ion fusion and heavy-ion fusion is discussed.