VUV absorption spectroscopy of a Penning surface-plasma negative hydrogen ion source.

Abstract

The demand for energetic, high-current H$\sp-$ beams is ever-growing. Because H$\sp-$ is efficiently neutralized at high energies, these beams are ideally suited to applications where energetic neutral beams of particles are required to propagate across magnetic fields. Prime examples are neutral-beam heating of magnetic fusion plasmas and directed-energy weapons for ballistic missile defense. Such applications place demanding requirements on sources of H$\sp-$ ions, particularly with respect to the parameters of beam current, brightness, quiescence, reliability, and duty-factor. A class of sources that holds great promise for meeting these stringent requirements is the surface-plasma source (SPS), and in particular, the Penning type of SPS. It has long been conjectured that atomic hydrogen plays an important role in both H$\sp-$ formation and transport in these sources. Understanding the interdependence of atomic hydrogen properties and those of H$\sp-,$ and how this relationship might be exploited to improve source performance is the motivation for this research. An overview of SPS's is presented. Previous measurements on the discharge are reviewed. Absorption spectroscopy, the diagnostic technique used to gather all of the data presented here, is discussed. Techniques that may potentially be used to measure the properties of H$\sp-$ in the discharge are discussed. The two absorption spectrometers used in this experiment are described. Measurements of ground-state atomic hydrogen density and temperature in a Penning SPS are presented. These measurements are the first of this kind for this type of discharge. An upper limit on the H$\sp-$ density in the extraction region of the source is measured by the application of a novel diagnostic technique: the hydrogen atom density following H$\sp-$ photodetachment by a Nd:YAG beam is measured and compared to the equilibrium atomic density. A simple model is derived that describes the dependence of the atomic temperature on the externally-controlled parameters of discharge current and H$\sb2$ gas flow. The measured atomic density is considered in light of the widely-accepted hypothesis of the mechanism for H$\sp-$ formation. The measured upper limit of the H$\sp-$ density is used to infer the potential of the discharge plasma relative to the source anode.