Negative Hydrogen Ion Production Research Articles

Resonant charge transfer in the scattering of hydrogen atoms from a metal surface

Electron transfer and the production of negative hydrogen ions in the reflection of hydrogen atoms from a metal surface is studied theoretically. Starting from the total Hamiltonian of the metal–atom system, the time development of the one‐electron density matrix is determined. This is shown to be related to the time‐dependent coupling of the affinity level of the moving atom with the metal states. In the limit of weak coupling, the occupation probability of the affinity level can be obtained in a closed form. This theory is applied to the problem of negative hydrogen ion production at a W(110) surface, both with and without Cs coverage, and the results are compared with available experimental data and predictions of existing theories.

Intense negative hydrogen ion production in high-power diodes

Some peculiarities of using negative ions for accelerators and CTR are examined. Highpower beams of fast atoms obtained by charge exchange of negative ions can have advantages over positive ion beams for inertial confinement fusion (ICF) since they do not experience any deviations, instabilities, and scattering. H− current up to 7 kA in a 100–200 ns pulse with densities up to 200 Å/cm2 and a voltage of 0.8 MV were obtained in experiments on magnetically insulated diodes at the Lebedev Physical Institute. Diagnostics of high-power H− beams are described. Experimental results on cathode plasma formation by surface discharge on a dielectric initiated by a bipolar prepulse or by laser illumination are presented. The dependence of H− yield on cathode current was determined and experiments on plasma stabilization are described. It is concluded that 1 kÅ/cm2 of H− can be obtained.

Negative ion formation at a barium surface in contact with a hydrogen plasma

Measurements on the production of negative hydrogen ions at a barium surface, in contact with a hydrogen plasma, are presented and discussed. In spite of the high work function of barium compared to more conventional cesiated converter surfaces, considerable yields of negative ions were produced. Conversion efficiencies up to 4% were achieved. No negative barium ions were observed.

Negative Ions and Energy Distribution in a Hydrogen Plasma Device

The production of negative hydrogen ions, H-, has been experimentally studied using a multipole-type hydrogen-plasma device. The dependence of the density of H- on the heater current, the discharge current and the hydrogen gas pressure was investigated by measuring the energy distribution. The density of H- was determined from the height of a sharp peak of the second derivative of the langmuir probe characteristic, which corresponded to the H- current. The optimum value of these parameters could be found for the production of H-. Primary electrons contributed largely to the production of H-, whereas thermal electrons had little effect.

Sputtering of negative hydrogen ions by cesium bombardment

The production of negative hydrogen ions sputtered from a low work function converter surface has been investigated. Hydrogen and cesium admitted into the vacuum chamber are chemisorbed on a polycrystalline molybdenum target. H−, Mo−, and e− are sputtered from this cathode by Cs+ ions in the energy range 150–1000 eV. Angular and parallel energy distributions of H−, Mo−, and e− are measured as a function of hydrogen gas pressure, cesium coverage, and incident ion energy. For optimum coverage, the H− ion temperature varies from 0.65% and 0.35% of the incident Cs+ bombarding energy for Cs+ ion energies of 250 and 1000 eV, respectively. The secondary electrons have a temperature of 0.04% of the bombarding energy almost independent of Cs+ energy. The spreads increase with decreasing target coverage and are independent of surface roughness. The optimum H−, Mo−, and e− yields are also measured as a function of hydrogen pressure and incident Cs+ bombarding energy. The optimum H− ion yield is 0.41 at a Cs+ ion energy of 750 eV. By extrapolating the H− ion yield at low Cs+ bombarding energy, a Cs+ ion threshold energy of 120 eV may be estimated. This indicates a binding energy of hydrogen smaller than 3.6 eV.

A model for the dependence of H - ion production on the converter potential in a surface plasma source

An elementary model for the production of negative hydrogen ions on a W(110) converter surface in a hydrogen plasma is presented. The calculated equilibrium cesium coverage is far from optimum in a high density plasma. The optimum converter potential for H - ion production is ∼ -200 V with respect to the plasma.

Vacuum ultraviolet emission and H- production in a low pressure hydrogen plasma

The vacuum ultraviolet emission from a low pressure hydrogen plasma has been used to study the production mechanism for vibrationally excited ground state molecules thought to be important in negative hydrogen ion production. The present results are in agreement with theoretical calculations by Hiskes (1980). The rate of production of molecules with v>or=5 has been estimated and is sufficient to allow H- production in such plasmas to be via these molecules.

Application of anomalous diffusion in production of negative ions

The production of negative hydrogen ions is investigated in the reflex-type negative ion sources. When anomalous diffusion in the positive column was found by Hoh and Lehnert [Phys. Fluids 3, 600 (1960)], it was pointed out that the large particle loss produced by anomalous diffusion is compensated for by the larger particle production inside the plasma. In the present experiments anomalous diffusion was artificially encouraged by changing the radial electric field inside the reflex discharge. Apparent encouragement of negative ion current by the increase of the density fluctuation amplitude is observed. Twice as much negative ion current was obtained with the artificial encouragement as without. On the other hand, the larger extracted negative ion current was observed with a lower electron temperature, which is calculated from the anomalous diffusion coefficient derived from a simple nonlinear theory. This result is consistent with Wadehra’s calculated results [Appl. Phys. Lett. 35, 917 (1979)].

Photoelectric work function measurement of a cesiated metal surface and its correlation with the surface-produced H− ion flux

For application in plasma heating, fueling, and current drive of magnetic fusion devices, high current negative deuterium ion sources for intense neutral beam injectors are being developed using efficient production of negative hydrogen isotope ions on low work function metal substrates imbedded in hydrogen plasmas. In order to investigate the correlation between work function and negative hydrogen ion production, photoelectron emission from a cesiated metal surface, which is immersed in a hydrogen plasma with an electron density less than 5×1010/cm3, was measured in the photon energy range of 1.3 to 4.1 eV. The work function determination was based on Fowler’s analysis, and at the optimum coverage a work function of less than 1.5 eV was observed for a Cs–Cu surface. Measured values of work functions for different Cs coverages were compared to the negative hydrogen currents produced at the metal surface in the discharge; the surface production of negative hydrogen ion current is monotonically increasing with decreasing work function.

Production of negative hydrogen ions by sputtering adsorbed hydrogen from a cesiated molybdenum surface

We have investigated the production of negative hydrogen ions by bombarding hydrogen adsorbed onto a cesiated molybdenum surface with positive cesium ions. The negative hydrogen yields are measured as a function of hydrogen gas pressure, cesium coverage, and energy of the incident cesium ions. The optimum yield obtained was 0.4 for 750-eV cesium ions bombarding hydrogen adsorbed onto the molybdenum surface covered with 0.65 monolayers of cesium. It is observed that the sticking coefficient of hydrogen decreases as the cesium coverage increases. We measure the energy distribution of the negative hydrogen ions as they leave the surface. The energy spread is about 0.5% of the incident cesium-ion energy. We compare our results for the production probability of negative hydrogen ions with the results obtained by Hiskes and Schneider.

Negative hydrogen ion production by low energy hydrogen atom bombardment of surfaces

A hydrogen furnace was used to irradiate a molybdenum surface with a flux of low energy hydrogen atoms. A small fraction of the hydrogen atoms were observed to be backscattered as negative ions when Cs was evaporated onto the surface.

Use of a Hall accelerator in the production of negative hydrogen ions in cesium vapor

Atomic hydrogen particles from a Hall accelerator were utilized in low-energy (<2 keV) negative-hydrogen-ion-production experiments in cesium vapor. Cell thicknesses in the range 8×1011 to 6×1015 Cs atoms/cm2 were studied for H− current densities on the order of 1 mA/cm2. Anomalously, large H− fractions of up to 30–60% were observed at intermediate cell thicknesses when protons were not removed from the dominantly H0 incident beam. These observations indicate either the presence of excitation of the H0 beam or the existence of two space-charge phenomena: a focusing effect inside of the cesium cell, due to a buildup of positive space charge in the beam line, and a divergence effect as the H− beam emerges from the cesium cell due to a lack of space-charge neutralization.

Mechanisms of formation of diatomic and triatomic hydrogen negative ions

The rates of production of diatomic and triatomic negative hydrogen ions were studied in a mass spectrometer as a function of pressure and of isotopic composition. A hollow cathode duoplasmatron was used as the ion source. At 1 torr D2 the relative ion currents are D−:D−2:D−3=1:10−6:10−7. H/D isotope effects in the formation of partially or totally deuterated analogs of H−2 and H−3 were observed and provided additional insight on the mode of formation of these molecular ions.

A Pulsed Negative Hydrogen Source for Currents up to One Ampere

During the 2nd Symposium on Ion Sources and Formation of Ion Beams, we reported on the development of our Mk II pulsed double slit magnetron source for the production of negative hydrogen ions.1 The source was capable of yielding beam currents up to 125 milliamperes, corresponding to current densities of 1.25 A/cm2. In order to increase negative hydrogen beam intensities by an order of magnitude (this would be quite useful for initial high energy neutral injector systems on Tokamaks), a larger, Mk III magnetron has been constructed, with the number of slits increased up to six. The idea was to utilize in a more efficient way the plasma width. In addition, such a source geometry will be more adaptable for beam formation and acceleration than single slit structures. With three extraction slits, we obtained a negative hydrogen yield of 300 mA with current densities of 1.2 A/cm2; preliminary results with six extraction slits showed beam currents in excess of half an ampere with averaged current densities in excess of 0.75 A/cm2.

Transformation of the proton source of an U-120 cyclotron for negative hydrogen ion production

The proton source of an U-120 cyclotron was transformed to generate a H − ion current. By modifying the discharge chamber and using different ways of gas feeding, internal beams of H − were obtained of the order of one mA pulsed peak value. The modified source can deliver efficiently also positive ions, which are accelerated as usually in the cyclotron.

Design Considerations for High-Intensity Negative Ion Sources

The production of negative hydrogen ions (H-) by an ion source similar to that employed in many cyclotrons has been studied. The negative ions are extracted directly from the arc plasma in a direction normal to the magnetic field without use of a charge-exchange medium. In an attempt to improve the negative-ion yield, the arc-discharge chamber has been modified to conform to assumptions made as to the probable formation mechanism of the observed negative ions. Outputs of negative hydrogen ions in ex cess of 5 mA have been obtained. Ion-source operating parameters are discussed, as well as considerations for the utilization of this discharge geometry by negative-ion cyclotrons.

Negative Helium Ions

Negative helium ions have been produced with an ion source previously used for negative hydrogen ion production. Positive helium ions pass through a charge-exchange tube filled with hydrogen gas and the resulting negative ions are magnetically analyzed. The momentum spectrum indicates peak yields of negative helium ions in excess of 3.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ ampere under the conditions of this experiment. The yield of negative helium ions as a function of converter gas density suggests a possible mechanism for the creation of the negative helium ion.