Plasma studies on a hollow cathode, magnetic multipole ion source for neutral beam injection

Abstract

A series of Langmuir probe studies has been undertaken on the hydrogen plasma of an 8×8‐cm extraction area hollow cathode, magnetic multipole ion source intended for use in neutral beam injection systems. Five movable probes were utilized to measureplasma density, electron temperature,plasma potential, and primary electron density as a function of position in the plasmagenerator and performance of the ion source. The measurements indicate that at discharge current below 150 A, a fairly uniform plasma over the source volume, with a potential positive with respect to the anode, is generated. The performance of the plasmagenerator at low currents resembles the filament discharge, magnetic multipole confinement ion source. The ion source in this case has good efficiency and plasma uniformity, but atomic species fractions of less than 70%. As the discharge current is increased, the plasma separates into a high‐density, high‐ionization, positive potential region near the cathode, and a lower‐density, negative potential plasma near the extraction grids. This plasma near the acceleration electrodes is characterized by a relatively low primary electron to plasma electron density ratio of about 2%. The performance of the source significantly increases over the uniform volume plasma case with atomic species output of typically 85% and gas utilization efficiencies of 40%–65%. The high‐current, hollow cathode discharge produces a plasma configuration and output performance similar to that of the duoPIGatron ion source, but with lower noise levels.