An Experimental Study of Waveguide Coupled Microwave Heating with Conventional Multicusp Negative Ion Source

TL;DR

This study demonstrates that conventional multicusp plasma chambers can produce negative ion beams using 2.45 GHz microwave heating, achieving significant beam currents without chamber modifications, and explores the plasma energy transfer mechanisms involved.

Contribution

It is the first experimental comparison showing microwave heating can effectively produce negative ions in multicusp chambers similar to traditional methods.

Findings

At least one third of negative ion beam current can be achieved with 1 kW microwave power.

The electron to H^- ratio and pressure range are similar for microwave and arc discharge heating.

Microwave energy transfer to plasma electrons occurs mainly off-resonance.

Abstract

Negative ion production with conventional multicusp plasma chambers utilizing 2.45 GHz microwave heating is demonstrated. The experimental results were obtained with the multicusp plasma chambers and extraction systems of the RFdriven RADIS ion source and the filament driven arc discharge ion source LIISA. A waveguide microwave coupling system, which is almost similar to the one used with the SILHI ion source, was used. The results demonstrate that at least one third of negative ion beam obtained with inductive RF-coupling (RADIS) or arc discharge (LIISA) can be achieved with 1 kW of 2.45 GHz microwave power in CW mode without any modification of the plasma chamber. The co-extracted electron to H^- ratio and the optimum pressure range were observed to be similar for both heating methods. The behaviour of the plasma implies that the energy transfer from the microwaves to the plasma electrons is mainly an off-resonance process.