Formation and Extraction of H– Ions from Penning Discharge with Metal Hydride Cathodes in LMF and HMF Modes
Abstract
The use of metal hydride cathodes in Penning discharges offers a promising approach to the efficient production of negative hydrogen ions under low- and ultralow-pressure conditions. In this work, we summarize and extend our experimental studies on the influence of metal hydride elements on the characteristics of Penning discharge with axial extraction of negative ions. At low residual pressure, the introduction of metal hydride cathodes enables plasma generation exclusively from hydrogen released by the cathode material, without any external gas injection. This feature opens the way to the development of compact, gas-feed-free ion sources with excellent gas utilization. The negative ions extraction mechanism is shown to depend strongly on the operating mode of the Penning discharge, determined by the magnetic field strength. In the low magnetic field mode, the expansion of the anode layer toward the discharge axis and the associated negative space charge suppresses efficient extraction, limiting it mainly to paraxial ions and those formed near the extraction aperture. In contrast, in the high magnetic field mode, the anode layer becomes thin, and the central plasma region is essentially field-free, enabling the extraction of negative ions from the entire discharge cross-section. Furthermore, the effect of increasing the anode diameter is investigated. Enlarging the anode diameter increases the plasma volume surrounding the metal hydride cathodes, leading to improved plasma uniformity, more homogeneous cathode heating, and a more uniform hydrogen release. These effects enhance the conditions for negative ions formation and result in a higher extracted current. The results demonstrate the feasibility and advantages of metal-hydride-based Penning ion sources for efficient production of negative ions under low-pressure operation.
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