Techniques have been developed for measurement of the density of H- in a plasma by photodetachment. Photodetachment is detected by the increase in electron density with no change in positive ion density after a light pulse from a ruby laser. The authenticity of photodetachment signals can be assured by their comparison with known cross sections for photodetachment of H-. Interpretations of photodetachment data are less ambiguous than probe interpretations because photodetachment is not affected by the mass of positive ions and is not limited in usefulness by the Debye distance. Photodetachment measurements with time resolution and spatial resolution are straightforward.

The density of negative ions in a low-pressure hydrogen plasma has been investigated as a function of neutral gas pressure, plasma density, and electron temperature. The comparison of the experimental data, obtained by the photodetachment technique, with theoretical results derived from computed reaction rates, seems to indicate that hydrogen negative-ion production occurs mainly through the process of electron attachment on vibrationally excited hydrogen molecules.

The electron and negative ion densities and temperatures are reported for a large volume hybrid multicusp negative ion source. Eased on the scaling laws an analysis is made of the plasma formation and loss processes. It is shown that the positive ions are predominantly lost to the walls, although the observed scaling law is IZ + ~pd.'~. However, the total plasma loss scales linearly with the discharge current, in agreement with the theoretical model. The negative ion formation and loss is also discussed. It is shown that at low pressure (1 mTorr) the negative ion wall loss becomes a significant part of the total loss. The dependence of n-/n, versus the electron temperature is reported. When the negative ion wall loss is negligible, all the data on n-/n, versus the electron temperatures fit a single curve. 8 299.5 American Institute of Physics.

The H -negative ion thermal energy measured using the two-laser-pulse photodetachment technique is reported to be in the range from 0.1 to 0.7 eV for various conditions of volume ion source operation (pressure-from 2 to 7 mTorr, discharge current-from 1.5 to 20 A). The hydrogen pressure has a significant effect in lowering the negative ion temperature, while the increase of the discharge current leads to a rise in T _ . It is found that T _ is a fraction of the electron temperature, Te This fraction is strongly dependent on the gas pressure. T _ scales linearly with the electron temperature and exceeds the highest values predicted by the theory of dissociative attachment. The possible mechanisms for H -ion heating are discussed.

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The vibrational population distribution for hydrogen molecules in a hydrogen discharge has been calculated taking into account electron collisional excitation, molecule-molecule, and wall collisional de-excitation processes. Electronic excitation processes include vibrational excitation by 1 eV thermal electrons acting through the intermediary of the negative ion resonances, and vibrational excitation caused by the radiative decay of higher singlet electronic states excited by a small population of 60 eV electrons in the discharge. The molecules are de-excited by molecular collisions transferring vibrational energy into translational energy, and by wall collisions. The distributions exhibit a plateau, or hump, in the central portion of the spectrum. The relative concentration of negative ions is calculated assuming dissociative attachment of the low temperature electrons to vibrationally excited, non-rotating molecules. The ratio of negative ions to electrons in the discharge is calculated to be of order 1% if the vibrational excitation survives no more than one wall collision, and of order 10% if the excitation survives ten collisions. The possibility is considered that the higher concentrations can be achieved with few wall collisions provided dissociative attachment occurs to highly rotating molecules.

The Balmer β and γ line shapes have been analyzed to determine the relative density and the temperature of hydrogen atoms in magnetic multicusp plasma generators. Results for a 90-V, 4–40-mTorr, 1–18-A conventional multicusp plasma generator and a 50-V, 4-mTorr, 1–15-A hybrid multicusp plasma generator are presented. The relative number density of hydrogen atoms increased smoothly with pressure and discharge current but never exceeded 10%. The absolute atomic number density in a 90-V 10-A discharge varied in proportion with pressure. The atomic temperature (in the 0.1–0.4-eV range) decreased with pressure and slowly increased with the discharge current. The role of atoms in the processes determining the H− temperature and the H2 vibrational and rotational temperatures is discussed. The results confirm that in multicusp negative-ion sources collisional excitation of ground state atoms and molecules by energetic electrons is the dominant process in Balmer-β and -γ light emission.

The density of negative hydrogen ions and the plasma characteristics are investigated as a function of the discharge current and the neutral gas pressure for several configurations of the magnetic multi pole, and in the absence of magnetic containment fields. It is shown that the hybrid magnetic multi pole configurations, characterized by a relatively low lifetime for wall losses of prim~~y electrons (10-7 s) contain high relative densities (10%-50%) of negative ions. The transition from the low density to the high density regime has been observed.