M. Bacal

Photodetachment diagnostic techniques for measuring negative ion densities and temperatures in plasmas

Photodetachment diagnostic techniques can help determine densities and temperatures of negative ions in a variety of scientific devices in which these ions are one of the major charged particle species. This method has been extensively used in the development of hydrogen negative ion sources as well as other devices. In order to obtain spatial resolution, a photodetachment diagnostic technique is used with an electrostatic probe that detects the currents of photodetached electrons.

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.

Method for extracting volume produced negative ions

The extraction of H− ions from a volume source is greatly improved by the presence of a weak magnetic field parallel to the plasma electrode that magnetically insulates this electrode from the bulk plasma. When biased positive, the plasma electrode depletes the electron population in the neighboring region. To maintain plasma neutrality, negative ions from the main plasma replace the electrons in this region. Thus a high fraction of negative ions builds up in front of the plasma electrode. The implications of these phenomena for negative ion beam formation are discussed.

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.

Plasma driven superpermeation of hydrogen through group Va metals

A barrier for the absorption of hydrogen molecules generated by a nonmetallic film on a metal surface may result in superpermeation of suprathermal hydrogen particles. Virtually all the sticking or implanted particles may permeate through the metallic membrane, with a permeation flux depending neither on the metal temperature nor on its thickness. Niobium and vanadium membranes with oxicarbide monolayer films were immersed in a hydrogen plasma in order to study their interaction with hydrogen ions in the energy range from a few eV to 250 eV, controlled by changing the membrane bias. A stable superpermeation flux was observed up to ion energies of tens of eV without special efforts to maintain the surface film. We found that the nonmetal monolayer film remains absolutely stable under the sputtering, including chemical sputtering, by hydrogen particles of energy less than ≈50 eV. Thus superpermeability appears to be an intrinsic feature of the investigated systems of “nonmetallic film–metal–hydrogen particl...

Photodetachment technique for measuring H− velocities in a hydrogen plasma

This article reports work in progress on laser diagnostics of negative‐ion transport velocity in H−‐ion volume sources. The plasma dynamics after the laser shot is discussed in detail, and the effect of the potential perturbation on the H− velocities is evaluated. A method of evaluation of the H− transport velocity from single‐laser‐beam photodetachment experiments is proposed. To substantiate this method, two‐laser‐beam photodetachment experiments have been effected. The velocities thus determined are pressure dependent; they correspond to H− energies in the range 0.23–0.08 eV.