M. Sasao

Effect of argon additive in negative hydrogen ion sources

Using the photodetachment technique we investigated the hydrogen negative ion density in the extraction region of a magnetically filtered multicusp ion source when argon is added to hydrogen. We found that the negative-ion density goes up at most by a factor of 1.5 when argon is added to low base hydrogen pressure (0.05 or 0.1 Pa), but goes down when argon is added to higher initial hydrogen pressure (0.5 or 1 Pa). Adding argon did not enhance the Werner and Lyman bands in the vacuum ultraviolet, which indicates that argon addition does not increase the production rate of vibrationally excited H2 molecules. The increase of the negative-ion density by adding argon into a low-pressure hydrogen discharge is probably due to the increase of the low-energy electron density.

Development of negative heavy ion sources for plasma potential measurement

A plasma sputter negative ion source was evaluated for its applicability for measuring the plasma potentials in fusion devices. Both the beam current density and the beam energy spread are key issues for this application. The energy spectra of self‐extracted Au− beams from the source were measured under the condition of a constant work function of the production surface. The FWHM increases from 3 to 9 eV monotonically as the target voltage increases from 50 to 300 V, independently from the target surface work function of 2.2–3 eV.

Angle-resolved energy distributions of He ions produced by backscattering of metastable He at low work function metal surface

An apparatus capable of measuring the angle-resolved energy distribution function of ions produced by backscattering at metal surfaces is designed and tested. To exclude noise due to other impurity ions and electrons from the signal of target ions, a water-cooled magnetic deflection-type momentum analyzer is installed on a rotating table in a vacuum chamber. In addition to the beam forming and beam diagnostics components, work function monitoring equipment and an ion source for surface cleaning are installed. Preliminary tests of the apparatus was conducted to measure He+ emission from neutral He beam injection onto a Mo surface. With an improvement of the signal-to-noise ratio, the apparatus can be used to measure the angle-resolved energy distribution function of He− produced at a metal surface by He beam injection.

Neutron diagnostics in compact helical system

An efficient neutron diagnostic system has been established and applied to deuterium experiments on compact helical system. A neutron counter was installed at the center of the torus, and the detection efficiency was about two orders of magnitude higher than the values of most toroidal fusion devices. The effect of the stray magnetic field on the operation of a proportional counter was experimentally confirmed to be negligibly small. The ion temperature, as low as 400 eV, was obtained from neutron measurement with a time resolution of 1 ms. The decay time of neutron emission rate after the termination of a 1% deuterium-doped beam was measured and the confinement time of neutral beam-injected fast ions was estimated. The fast ion confinement was clearly improved by strengthening the toroidal magnetic field.An efficient neutron diagnostic system has been established and applied to deuterium experiments on compact helical system. A neutron counter was installed at the center of the torus, and the detection efficiency was about two orders of magnitude higher than the values of most toroidal fusion devices. The effect of the stray magnetic field on the operation of a proportional counter was experimentally confirmed to be negligibly small. The ion temperature, as low as 400 eV, was obtained from neutron measurement with a time resolution of 1 ms. The decay time of neutron emission rate after the termination of a 1% deuterium-doped beam was measured and the confinement time of neutral beam-injected fast ions was estimated. The fast ion confinement was clearly improved by strengthening the toroidal magnetic field.

Effects of roughness and temperature on low-energy hydrogen positive and negative ion reflection from silicon and carbon surfaces

Angle-resolved energy distribution functions of positive and negative hydrogen ions produced from a rough-finished Si surface under 1 keV proton irradiation have been measured. The corresponding distribution from a crystalline surface and a carbon surface are also measured for comparison. Intensities of positive and negative ions from the rough-finished Si are substantially smaller than those from crystalline Si. The angular distributions of these species are broader for rough surface than the crystalline surface. No significant temperature dependence for positive and negative ion intensities is observed for all samples in the temperature range from 300 to 400 K.

Absolute calibration of neutron counters on the Compact Helical System

A calibration experiment of the neutron counters of the Compact Helical System (CHS) has been carried out in order to determine the calibration factor which relates the neutron‐counter output to the extended neutron source of a torus plasma. The instruments employed in the present experiment include a BF3 proportional counter, a 3He proportional counter, an NE213 organic liquid scintillator. The BF3 counter was installed at the center of the CHS torus and the other two detectors were installed outside the torus. A total of 64 toroidal angles along the magnetic axis of plasma were surveyed by the movement of a 252Cf neutron source. The poloidal cross section of the CHS plasma is elliptical in shape and rotates toroidally with the same period as that of the helical coil. The spatial distribution of 25 local point efficiencies in three poloidal cross sections with elongated orientations of 0°, 45°, and 90° were investigated in order to determine the influence of the plasma profile on the calibration factor. ...

Influence of filament materials on vacuum ultraviolet emission in H− ion source

The increase of H− current is observed, when the tantalum filaments are used instead of the tungsten filaments. In terms of the production rate of vibrational populations of hydrogen molecules with tungsten and tantalum filaments, we measured the Werner and Lyman emission bands (90–170 nm) the vacuum ultraviolet region to obtain the spectra of molecular hydrogen in an ion source, and observed a significant difference of spectrum between the two cases. From the pressure dependence, it is found that the production rate of vibrationally excited hydrogen molecules is well correlated with the H− density measured by photodetachment in hydrogen plasmas.

Reflection properties of hydrogen ions at helium irradiated tungsten surfaces

Nanostructured W surfaces prepared by He bombardment exhibit characteristic angular distributions of hydrogen ion reflection upon injection of 1 keV H+ beam. A magnetic momentum analyzer that can move in the vacuum chamber has measured the angular dependence of the intensity and the energy of reflected ions. Broader angular distributions were observed for He-irradiated tungsten samples compared with that of the intrinsic polycrystalline W. Both intensity and energy of reflected ions decreased in the following order: the polycrystalline W, the He-bubble containing W, and the fuzz W. Classical trajectory Monte Carlo simulations based on Atomic Collision in Amorphous Target code suggests that lower atom density near the surface can make the reflection coefficients lower due to increasing number of collisions.

Angular distributions of surface produced H− ions for reflection and desorption processesa)

A numerical simulation code, Atomic Collision in Amorphous Target, has been run to clarify the effects due to the incident angle of hydrogen flux onto surface collision cascade in the subsurface region of a Cs covered Mo plasma grid. The code has taken into account the threshold energy for negative hydrogen (H(-)) ions to leave the surface. This modification has caused the shift of energy distribution functions of H(-) from that of hydrogen atoms leaving the surface. The results have shown that large incident angle of hydrogen particle tilt the angular distribution of reflection component, while it caused a small effect onto the angular distribution of desorption component. The reflection coefficient has increased, while the desorption yield has decreased for increased angle of incidence measured from the surface normal.

Extraction characteristics of Na and Na2 negative ions from a small multicusp ion source

Negative sodium ions and negative dimer ions of sodium are extracted from a sodium plasma. The extraction characteristics of Na− and Na2− from a low power discharge (about 1 W) through a single aperture (φ1.5 mm) are described. Both extracted currents of Na− and Na2− increase with the temperature of the heat shield inside the discharge chamber. The current ratio of Na− to Na2− is almost constant independent of discharge voltage. By increasing the discharge power, the value of the bias voltage of the plasma electrode that gives the optimum extraction currents of Na− and Na2− increases. The extracted current of Na2− is lower than that of Na− by two orders of magnitude.