Articles you may be interested inModeling of neutrals in the Linac4 H− ion source plasma: Hydrogen atom production density profile and Hα intensity by collisional radiative modela) Rev. Sci. Instrum. 85, 02B118 (2014); 10.1063/1.4833016 Role of positive ions on the surface production of negative ions in a fusion plasma reactor type negative ion source-Insights from a three dimensional particle-in-cell Monte Carlo collisions model Phys. Plasmas 20, 113511 (2013); 10.1063/1.4834475 Effect of non-uniform electron energy distribution function on plasma production in large arc driven negative ion sourcea) Rev. Sci. Instrum. 83, 02A719 (2012); 10.1063/1.3673485 Effect of energy relaxation of H 0 atoms at the wall on the production profile of H − ions in large negative ion sourcesa) Rev. Sci. Instrum. 79, 02A503 (2008); 10.1063/1.2802583 Monte Carlo simulation of negative ion production in the negative hydrogen ion source Rev. Sci. Instrum. 71, 883 (2000)The production and transport processes of 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 the Cs-seeded condition to obtain a spatial distribution of surface-produced H − ions. In this analysis, the amount of H 0 atoms produced through dissociation processes of H 2 molecules is calculated from the electron temperature and density obtained by Langmuir probe measurements. The high-energy tail of electrons, which greatly affects H 0 atom production, is taken into account by fitting a single-probe characteristic as a two-temperature Maxwellian distribution. In the H 0 atom transport process, the energy relaxation of the H 0 atoms, which affects the surface H − ion production rate, is taken into account. The result indicates that the surface H − ion production is enhanced near the high-electron-temperature region where H 0 atom production is localized.

The H− ion production and transport processes are numerically simulated to clarify the origin of H− beam nonuniformity. The three-dimensional transport code using the Monte Carlo method has been applied to H0 atoms and H− ions in the large “JAERI 10A negative ion source” under the Cs-seeded condition, in which negative ions are dominantly produced by the surface production process. The results show that a large fraction of hydrogen atoms is produced in the region with high electron temperature. This leads to a spatial nonuniformity of H0 atom flux to the plasma grid and the resultant H− ion surface production. In addition, most surface-produced H− ions are extracted even through the high Te region without destruction. These results indicate a correlation between the production process of the H− ion and the spatial nonuniformity of the H− ion beam.

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Structure of electric potential near the extraction region in a negative ion source is investigated analytically with the effect of strong surface H(-) production. The potential profile is analyzed one dimensional by solving the plasma-sheath equation, which gives the electric potential in the plasma region and the sheath region near the wall self-consistently. The potential profile depends on the production rate and the temperature of negative ions. As the production rate becomes large and the negative ion energy becomes small, the potential near the extraction region decreases. The negative potential peak is formed near the plasma grid (PG) surface for the case of large amount and low energy surface production. As a result, negative ions are reflected by this negative potential peak near the PG and returned to the PG surface. This reflection mechanism by the negative potential peak possibly affects the negative ion extraction.