Ions Beams and Ferroelectric Plasma Sources

Abstract

Near-perfect space-charge neutralization is required for the transverse compression

of high perveance ion beams for ion-beam-driven warm dense matter experiments,

such as the Neutralized Drift Compression eXperiment (NDCX). Neutralization can be accomplished by introducing a plasma in the

beam path, which provides free electrons that compensate the positive space charge of the ion beam. In this thesis, charge neutralization of a 40~keV, perveance-dominated Ar$^+$ beam by a Ferroelectric Plasma Source (FEPS) is investigated.

First, the parameters of the ion beam, such as divergence due to the extraction optics, charge neutralization fraction, and emittance were measured. The ion beam was propagated through the FEPS plasma, and the effects of charge neutralization were inferred from time-resolved measurements of the transverse beam profile. In addition, the dependence of FEPS plasma parameters on the configuration of the driving pulser circuit was studied to optimize pulser design.

An ion accelerator was constructed that produced a 30-50~keV Ar$^+$ beam with pulse duration $<$300~$\mu$s and dimensionless perveance $Q$ up to 8$\times$10$^{-4}$. Transverse profile measurements 33~cm downstream of the ion source showed that the dependence of beam radius on $Q$ was consistent with space charge expansion. It was concluded that the beam was perveance-dominated with a charge neutralization fraction of approximately zero in the absence of neutralizing plasma. Since beam expansion occurred primarily due to space charge, the decrease in effective perveance due to neutralization by FEPS plasma can be inferred from the reduction in beam radius.

Results on propagation of the ion beam through FEPS plasma demonstrate that after the FEPS is triggered, the beam radius decreases to its neutralized value in about 5~$\mu$s.

The duration of neutralization was about 10~$\mu$s at a charging voltage $V_{FEPS}$~=~5.5~kV and 35~$\mu$s at $V_{FEPS}$~=~6.5~kV. With $V_{FEPS}$~=~6.5~kV, the transverse current density profile 33~cm downstream of the source had a Gaussian shape with $x_{rms}$=5~mm, which corresponds to a half-angle divergence of 0.87$^\circ$. The measurements show that near-perfect charge neutralization with FEPS can be attained. No loss of ion beam current was detected, indicating the absence of a neutral cloud in the region of beam propagation, which would cause beam loss to charge exchange collisions. This provides evidence in favor of using FEPS in a future Heavy Ion Fusion accelerator.

The FEPS discharge was investigated based on current-voltage measurements in the pulser circuit. Different values of series resistance and storage capacitance in the pulser circuit were used. The charged particle current emitted by the FEPS into vacuum was measured from the difference in forward and return currents in the driving circuit. It was found that FEPS is an emitter of negative charge, and that electron current emission begins approximately 0.5~$\mu$s after the fast-rising high voltage pulse is applied and lasts for tens of $\mu$s. The value of the series resistance in the pulser circuit was varied to change the rise time of the voltage pulse; plasma density was expected to decrease with increasing values of resistance. However, the data showed that changing the resistance had no significant effect. The average charge emitted per shot depends strongly on the value of the storage capacitance. Lowering the capacitance from 141~nF to 47~nF resulted in a near-complete shut-off of charge emission, although the amplitude of the applied voltage pulse was as high, and rise time as short, as when high-density plasma was produced. Increasing the capacitance from 141~nF to 235~nF increased the average charge emitted per shot by a factor of$~$2.