Analysis of electron energy distribution of an arc-discharge H(-) ion source with Monte Carlo simulation.

For optimization and accurate prediction of the amount of H(-) ion production in negative ion sources, analysis of electron energy distribution function (EEDF) is necessary. We developed a numerical code which analyzes EEDF in the tandem-type arc-discharge source. It is a three-dimensional Monte Carlo simulation code with the effects of cusp, filter, and extraction magnets. Coulomb collision between electrons is treated with Takizuka's model and several inelastic collisions are treated with null-collision method. We applied this code to the JAEA 10 ampere negative ion source. The numerical result shows that the order of electron density is in good agreement with experimental results. In addition, the obtained EEDF is qualitatively in good agreement with experimental results.

Numerical analysis of H(-) ion transport processes in Cs-seeded negative ion sources.

The H(-) ion transport processes are numerically simulated to understand the extraction process of surface-produced H(-) ions. The three-dimensional transport code using Monte Carlo method has been applied to calculate the H(-) ion extraction probabilities in the model geometry of the JAEA 10 ampere negative ion source. The roles of (1) filter magnetic field and (2) collisions with neutrals (H(0) atoms and H(2) molecules) on the H(-) ion extraction are systematically studied. The results show that H(-) ions are extracted mainly by the filter magnetic field under the low gas pressure condition. The simulation results of extracted H(-) ion beam intensity in the JAEA 10 ampere negative ion source without the magnetic filter tend to be smaller than the experimental results, especially under the low pressure condition. Further model improvements, e.g., modeling and implementation of the effects of the electric field near the extraction aperture, will be required to understand the extraction process of the H(-) ions under the low gas pressure condition.

Effect of energy relaxation of H(0) atoms at the wall on the production profile of H(-) ions in large negative ion sources.

Production and transport processes of the H(0) atoms are numerically simulated using a three-dimensional Monte Carlo transport code. The code is applied to the large JAEA 10 ampere negative ion source under a Cs-seeded condition to obtain a spatial distribution of surface-produced H(-) ions. In this analysis, we focus on the effect of the energy relaxation of the H(0) atoms at the wall on the H(-) ion production from the H(0) atoms. The result indicates that, by considering the energy relaxation of the H(0) atoms at the wall, the production profile of the surface-produced H(-) ion is well reflected in the production profile of the H(0) atom production.

Uniform H(-) ion beam extraction in a large negative ion source with a tent-shaped magnetic filter.

Based on previous studies on the spatial uniformity of the negative ion beam, the external magnetic filter was replaced to a novel tent-shaped magnetic filter in the JAEA 10 A negative ion source. The line-cusp field configuration on the source chamber was also changed to form a symmetric magnetic field like many of positive ion sources aiming at high proton yield. This magnetic field configuration allows fast electrons emitted from filament cathodes to rotate azimuthally inside the source chamber. The source configuration thus prevents localization of fast electrons due to their B x grad B drift in the filter field. As a result, the H(-) ion beam profile extracted from a wide region of 340 x 170 mm(2) showed reduction of standard deviation from 16% in the original to 7.9% with the tent filter. The negative ion source with the tent filter satisfied the requirement of the beam uniformity for a large negative ion source in the ITER neutral beam injection.

Negative ion production in cesium seeded high electron temperature plasmas.

In experiments on uniformity improvement in a large negative ion source, steep gradients have been observed in the profiles of electron temperature and H(-) ion beam intensity. It has been observed that the gradient in the H(-) ion beam intensity is altered by seeding cesium, though the electron temperature distribution is not affected by Cs. Thus in the Cs seeded condition, the H(-) ion beam intensity is enhanced in local area illuminated by high electron temperature plasmas. A brief analysis suggests possible advantages of high electron temperature plasmas for the negative ion surface production, by enhancement of dissociation to yield proton or atoms as parent particles of the negative ions.