A. M. Bruneteau

Measurement of H- density in plasma by photodetachment.

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.

Negative ion production in hydrogen plasmas confined by a multicusp magnetic field

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 multipole, and in the absence of magnetic containment fields. It is shown that the hybrid magnetic multipole configurations, characterized by a relatively low lifetime for wall losses of primary 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.

Hydrogen vibrational population distributions and negative ion concentrations in a medium density hydrogen discharge

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 ...

PRESSURE AND ELECTRON-TEMPERATURE DEPENDENCE OF H- DENSITY IN A HYDROGEN PLASMA

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.

Investigation of a large volume negative hydrogen ion source

The electron and negative ion densities and temperatures are reported for a large volume hybrid multicusp negative ion source. Based 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 n+∝I0.57d. 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−/ne versus the electron temperature is reported. When the negative ion wall loss is negligible, all the data on n−/ne versus the electron temperatures fit a single curve.

Measurement of vibrational populations in low‐pressure hydrogen plasma by coherent anti‐Stokes Raman scattering

Coherent anti‐Stokes Raman scattering (CARS) has been applied to the measurement of vibrational populations in a low‐pressure H2 plasma. We describe the plasma generator and give some particulars of the optical arrangement. For an electron density of 2×1011 cm−3 and a total pressure of 0.13 mbar, the rotational temperature is found to be 475 K. The populations of vibrational states v = 0, 1, and 2 have also been measured. Their distribution is non‐Boltzmann. The influence of pressure and discharge parameters is briefly discussed. The instrument detection sensitivity for a given rovibrational state is about 1012 cm−3.

Measurement of the H− thermal energy in a volume ion source plasma

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.

Effect of positive and negative ion energies on H− destruction by mutual neutralization in low‐pressure plasmas

The effect of plasma potential profile and collisions with neutrals and charged particles upon positive and negative ion energies has been studied in a low‐pressure (2–13.6 mTorr) hybrid magnetic multicusp hydrogen discharge. When the plasma density is lower than 1011 cm−3, the negative ion temperature is always close to the gas temperature, while the positive ion temperature goes down from 1 to 0.1 eV when the pressure goes up. The observed reduction of the relative negative ion density when the pressure is increased is shown to be related to the reduction of positive ion velocities and to the associated increase in the mutual neutralization rate coefficient.

Effect of cesium and xenon seeding in negative hydrogen ion sources

It is well known that cesium seeding in volume hydrogen negative ion sources leads to a large reduction of the extracted electron current and in some cases to the enhancement of the negative ion current. The cooling of the electrons due to the addition of this heavy impurity was proposed as a possible cause of the mentioned observations. In order to verify this assumption, the authors seeded the hydrogen plasma with xenon, which has an atomic weight almost equal to that of cesium. The plasma properties were studied in the extraction region of the negative ion source Camembert III using a cylindrical electrostatic probe while the negative ion relative density was studied using laser photodetachment. It is shown that the xenon mixing does not enhance the negative ion density and leads to the increase of the electron density, while the cesium seeding reduces the electron density.

Investigation of two negative hydrogen and deuterium ion sources: Effect of the volume

We have investigated by probes and by photodetachment the plasma properties of two negative ion sources of different volume. We compared the data relevant to these two sources and identify the effects induced by the change in volume and the isotope effects. The observed scaling law for the electron temperature is in each case Te∼I0.27d. Positive ions are predominantly lost to the walls, but due to the increasing influence of their loss by dissociative recombination when the source volume is increased, the scaling laws are ne∼I0.7d, for the small source, and ne∼I0.59d, for the large source. Atoms limit the production of negative ions and destroy them. Thus we see an isotope effect, with higher negative ion density in hydrogen than in deuterium, and the effect of the source volume, with the negative ion density larger in the small source than in the large one.