J. R. Hiskes

Generation of negative ions in tandem high-density hydrogen discharges

An optimized tandem two‐chamber negative‐ion source system is discussed. In the first chamber high‐energy (E>20 eV) electron collisions provide for H2 vibrational excitation, while in the second chamber negative ions are formed by dissociative attachment. The gas density, electron density, and system scale length are varied as independent parameters. The extracted negative ion current density passes through a maximum as electron and gas densities are varied. This maximum scales inversely with system scale length R. The optimum extracted current densities occur for electron densities nR=1013 electrons cm−2 and gas densities N2R in the range 1014–1015 molecules cm−2. The extracted current densities are sensitive to the atomic concentration in the discharge. The atomic concentration is parametrized by the wall recombination coefficient γ and scale length R. As γ ranges from 0.1 to 1.0 and for system scale lengths of 1 cm, extracted current densities range from 8.0 to 80 mA cm−2. The relative negative‐ion yie...

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 ...

Mechanism for negative‐ion production in the surface‐plasma negative‐hydrogen‐ion source

The system parameters and surface adsorption conditions of the surface‐plasma negative‐hydrogen‐ion source are reviewed. A mechanism is developed for the production of negative ions by incident energetic hydrogen atoms backscattering from a cesiated tungsten surface. The active electronic level during the course of the collision is approximated as the sum of the negative electron affinity of the cesium hydride negative molecular ion and the image potential. The model predicts negative‐ion formation for atomic collisions with all alkali‐coated surfaces for alkalis from sodium through cesium. In the case of lithium, the model does not apply due to the breakdown of the image potential approximation close to the surface. Negative‐ion formation is also expected to occur for incident positive ions which backscatter from the substrate tungsten as neutrals. The energy thresholds for incident atoms are established.

MCSCF pseudopotential calculations for the alkali hydrides and their anions

Multiconfiguration self‐consistent‐field calculations have been carried out on the X 1S+ and a 3S+ states of LiH, NaH, KH, RbH, and CsH, and on the X 2S+ states of their respective anions. Pseudopotentials, including core polarization terms, have been used to replace the core electrons, resulting in simple two‐ and three‐electron calculations. Comparisons of the neutral potential curves with experiment and other ab initio calculations (where available) show very good agreement. The agreement with ab initio calculations on LiH− and NaH− is also very good. Adiabatic electron affinities have been calculated for LiH (0.293 ev), NaH (0.316 eV), KH (0.437 eV), RbH (0.422 eV), and CsH (0.438 eV).

Abinitio MC–SCF ground‐state potential energy curves for LiH−, NaH−, and CsH−

We have evaluated the 1Σ+ ground states of LiH, NaH, and CsH and the 2Σ+ ground states of LiH−, NaH−, and CsH−, as well as the 2Σ+ ground state of CsH+, over a wide range of internuclear distances. Multiconfiguration‐self‐consistent‐field wavefunctions were obtained with the optimized valence configuration approach to the description of chemical bonding. Four configurations for the neutral molecules and seven for the negative ions provided satisfactory descriptions. All of the negative‐molecular‐ion ground states are attractive and have lower potential energies than the neutral‐parent‐hydride molecules over the internuclear distances studied. Molecular electron affinities at the equilibrium internuclear separations were found to be 0.283 eV (LiH), 0.278 eV (NaH), and 0.357 eV (CsH).

Recombination and dissociation of H2+ and H3+ ions on surfaces to form H2(v‘): Negative‐ion formation on low‐work‐function surfaces

The recombination and dissociation of H+2 and H+3 ions incident upon metal surfaces leads to H, H2(v‘), and H− products rebounding from the surface. A four‐step model for H+2 ‐ion recombination generates H2(v‘) via resonant electron capture through the b 3Σ+u and X 1Σ+g states. A molecular trajectory analysis provides final‐state H2(v‘) distributions for incident energies of 1, 4, 10, and 20 eV. The calculated H2: H+2 yields compare favorably with the observed yields. A similar four‐step model for incident H+3 proceeds via resonant capture to form the H3(2p 2E’→2p 2A1) ground state, in turn dissociating into H+H2(v_‘), with the fragment molecule rebounding to give the final H2(v‘) distribution. Comparing the final populations v‘≥5 for incident H+2 or H+3 shows that the H+3 ion will be more useful than H+2 for H− generation via dissociative attachment. Molecular ions incident upon low‐work‐function surfaces generate additional H2(v‘) via resonant electron capture through excited electronic states and provi...

Cross sections for the vibrational excitation of the H2 X 1Σ+g(v) levels generated by electron collisional excitation of the higher singlet states

The excitation cross sections, σ(v,v‘), for an H2 molecule initially in any one of the 15 vibrational levels, v belonging to the ground electronic state and excited to a final vibrational level, v‘ are evaluated for direct excitations via all members of the excited electronic singlet spectrum. Account is taken of predissociation, autoionization, and radiative decay of the excited electronic spectrum that leads to a final population distribution for the ground electronic state, X 1Σ+g(v‘). For v=0, account is taken explicitly of transitions via the B, C, B’, and D electronic states in evaluating the cross sections. The additional contribution of excitations via all Rydberg states lying above the D state enhances these cross sections by approximately 10%. For v≳0, cross sections are evaluated taking explicit account of transitions through the B and C states; higher singlet excitations enhance these values by 25%. The choice of the reference total cross sections remains a subjective one, causing the values c...

Optimum extracted negative‐ion current densities from tandem high‐density systems

A tandem high‐density hydrogen negative‐ion‐source system is optimized for the purpose of identifying the maximum possible extracted ion current densities. In the first chamber, vibrational excitation occurs by high‐energy electron excitation (E‐V process); in the second chamber negative ions are formed by dissociative attachment. The electron, atom, and molecular densities are varied together with the length of the second chamber. Electron excitation cross sections to the B1∑u and C1∏u states are calculated as a function of vibrational excitation. The vibrational excitation approaches an asymptotic value as the first‐chamber electron density increases. For a system with scale length R=10 cm the optimum extracted current densities occur for gas densities near 1015 mol cm−3 for first‐chamber electron densities equal to 1013 cm−3, and for second‐chamber electron densities in the 4–6×1012 cm−3 range. For atomic densities equal to one‐tenth the molecular density and for second‐chamber scale lengths as short a...

ELECTRIC AND MAGNETIC DISSOCIATION AND IONIZATION OF MOLECULAR IONS AND NEUTRAL ATOMS

The dissociation and ionization of molecular ions and neutral atoms by the Lorentz force provides a mechanism for trapping energetic particles inside fusion machines without need for ionizing collisions on background gas molecules or arc discharges. At ion energies of interest in fusion work, magnetic dissociation is equivalent to electrostatic dissociation where the equivalent field is E = vB. The range of parameters of interest in thermonuclear research corresponds to effective electric fields up to the order 106 V/cm. The threshold fields for dissociating the successive vibrational levels of all isotopic mixtures of the hydrogen molecular ion, H2+, HD+, HT+, D2+, DT+, T2+, are reported. For the uppermost levels of these (nonrotating) ions the threshold fields range from 0.6 × 105 to 0.9 × 105 V/cm. Rotational effects are discussed in detail. The presence of rotation J introduces J + 1 new thresholds for a particular vibrational level; these thresholds are distributed about the single threshold for the nonrotating ion. The lack of a stepwise dissociation observed in the Riviere-Sweetman experiment can be interpreted as due to a superposition of contributions from rotational states up to J = 4, 5. Methods are discussed for enhancing the populations of the uppermost vibrational levels of the hydrogenic ions. The bound vibrational state belonging to the antibonding electronic state of each of the H2+, D2+, and T2+ ions can be dissociated with fields of the order 104 V/cm; reactions populating these levels are discussed. The formation of neutral hydrogen or deuterium atom beams by the charge exchange reaction provides a population distribution covering all excited states. The equilibrium populations of the excited levels are sufficiently large that the ionization by the Lorentz force competes favorably with the other ionization processes. A 30-keV H atom moving in a 60 kG field is susceptible to ionization down to the n = 8, 9 levels; a 100-keV atom in the n = 6 level is ionized by a 130-kG field. Containment experiments designed on the basis of these data are discussed.

Atomic and cluster injection into D-T mirror fusion power systems

The problem of the penetration of a neutral atomic beam and a cluster ion beam into a fusion plasma is reviewed. The short penetration length of an atomic beam, coupled with certain geometric dependences of the magnet coil costs, implies an optimum injection energy for a D-T mirror fusion power system in the range 200 to 300 keV. The penetration length for a cluster beam is inferred from available data. For certain charge and mass distributions of the cluster fragments, a substantial improvement of beam penetration is possible, allowing for injection as low as 100 keV per deuteron. Additional fragment charge and mass distribution data will be necessary, however, to clarify the role of cluster injection.