Theory of H− formation in a weakly collisional plasma

Abstract

A kinetic plasma model for the formation of negative hydrogen ions is developed, with particular emphasis on conditions prevailing in the NIBLI experiment [Nucl. Instrum. Methods A 243, 255 (1986)]. The overall picture of the plasma in the discharge and values of various parameters have been taken from experimental data, notably a series of electrostatic probe measurements. Physically reasonable electron and ion distributions have been assumed. The magnetic field has a mirror inhomogeneity, and analysis of the quasineutral steady states of isotropic ions with electrons having high perpendicular energy, due to gyroresonance, shows that the plasma is divided into two compartments, one with energetic electrons where vibrationally excited hydrogen molecules can be produced, the other with low-energy electrons where negative ions can be produced and maintained. The mutual interaction between the electric field necessary for quasineutrality and the H− formation from electrons and vibrationally excited hydrogen molecules has been studied. The position is determined as a function of the potential by solving an integrodifferential equation. Quasineutral solutions exist only for a limited range of position and potential, which may be related to transitions between subsonic and supersonic motion. Substantial produced fluxes of 10–100 mA/cm2 of H− are found. The influence of various factors on the flux, the magnitude of the potential, and the size of the structure is discussed, as well as a possible autocatalytic effect from the reaction-induced electric field, and the extraction of the negative ions without simultaneous depletion of the plasma in which they are formed.