Characterization of a compact electron cyclotron resonance plasma source under negative DC biasing and microwave plasma cathode operation

Abstract To realize future long-lifetime negative hydrogen ion sources, two approaches were investigated using a compact coaxial ECR plasma source: direct negative DC biasing and a microwave plasma cathode (MPC) configuration. In noble gases such as argon, negative biasing extracted fast electrons from the ECR plasma, enhancing ionization and increasing density. In contrast, molecular gases like hydrogen and nitrogen showed density reduction, and optical emission spectroscopy indicated that extracted electrons were energetic enough for excitation but not for ionization. To overcome this limitation, the ECR plasma was utilized as an MPC to selectively supply fast electrons downstream. This configuration achieved overdense hydrogen plasma with spatial uniformity, unlike the diffusion-dominated case of direct biasing. Because the discharge was sustained by fast electrons from the ECR region, its behavior resembled filament-driven arcs. Replacing filaments with MPCs may thus enable long-lifetime, uniform, and controllable microwave-driven negative hydrogen ion sources.