Dynamics of Ion Beam Charge Neutralization by Ferroelectric Plasma Sources.

Ferroelectric Plasma Sources (FEPSs) can generate plasma that provides effective space-charge neutralization of intense high-perveance ion beams. Here we present experimental results on charge neutralization of a high-perveance 38 keV Ar$^{\mathrm{+}}$ beam by a FEPS plasma. By comparing the measured beam radius with the envelope model for space-charge expansion, it is shown that a charge neutralization fraction of 98{\%} is attainable. The transverse electrostatic potential of the ion beam is reduced from 15 V before neutralization to 0.3 V, implying that the energy of the neutralizing electrons is below 0.3 eV. Near-complete charge neutralization is established $\sim $5$\mu $s after the driving pulse is applied to the FEPS, and can last for 35 $\mu $s. It is argued that the duration of neutralization is much longer than a reasonable lifetime of the plasma produced in the sub-$\mu $s surface discharge. Measurements of current flow in the driving circuit of the FEPS suggest that plasma can be generated for tens of $\mu $s after the high voltage pulse is applied. This is confirmed by fast photography of the plasma in the 1-meter long FEPS on NDCX-II, where effective charge neutralization of the beam was achieved with the optimized FEPS timing.