The Plasma Parameters of Penning Discharge with Negatively Biased Metal Hydride Cathode at Longitudinal Emission of H– Ions

Abstract

The enhancement of negative ion production in a volume Penning based source could be performed by the application of metal hydride cathode. Hydrogen isotopes are stored there in a chemically bound atomic state and desorbed from the metal hydride under the discharge current impact. Highly vibrationally / rotationally excited molecules H2* are formed by recombination of H-atoms at the metallic surface, which then can be easily converted to H– by dissociative electron attachment without the pre-excitation of a H2 molecule in plasma. Changing the discharge properties opens the way of source design simplification by negative ions extraction along the external magnetic field in comparison with traditional volume sources, where the extraction is performed perpendicular to magnetic field. The separation of negative ions from the extracted in longitudinal direction flux of charged particles was performed by an electromagnetic filter basing on numerical calculations of particles trajectories. The dependence of electron temperature and plasma density on the bias potential is carried out by Langmuir probe method. The measurement of electron energy was performed by an electrostatic energy analyzer. It was shown that the yield of H– ions depends on the electrical bias on the metal hydride cathode with strong dependences on the plasma electrons temperature. The estimation of the bias potential versus Te was performed under the assumption of electron Boltzmann distribution near the cathode. The presence of additional groups of electrons with higher energies distorts the behavior of H– current, but generally the experimental results are in good agreement with estimation based on the physics behind the Boltzmann distribution. The optimum for the effective extraction of H– ions was revealed, when the metal hydride cathode had been electrically biased at -20V and higher, and plasma density reaches the maximum value to 2×109 cm-3.