Numerical analysis of the spatial nonuniformity in a Cs-seeded H− ion source

Abstract

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.

Particle in Cell Simulation of Low Temperature Laboratory Plasmas

“…This rules out that the first possibility can explain the H beam non-uniformity observed in the experiments. To examine the second possibility, a systematic numerical study on the neutral transport has been done [103][104][105]. Production and transport processes of H ¼ atoms are numerically simulated using a 3D Monte Carlo transport code in the realistic 3D geometry of the JAEA 10A source shown in fig.20.…”

Performance of multi-aperture grid extraction systems for an ITER-relevant RF-driven negative hydrogen ion source

“…It consists of the area of the chamfer surrounding the apertures, as well as the flat areas between the apertures. Although the mean free path of the negative ions in the boundary layer near the PG is of the order of some tens of centimetres [17], the projected range is much shorter due to the movement within the magnetic filter field and the charge exchange collisions. Hence, only negative hydrogen ions created in a certain distance-much smaller than the mean free path-from the aperture can be extracted.…”

“…Caesium forms a layer with a low work function at the source surfaces on which impinging hydrogen neutrals and ions are converted to negative ions; these are accelerated back into the plasma by the sheath potential. As the binding energy of the electron is rather low (0.75 eV), the mean free path of a negative hydrogen ion in the main source plasma with an electron temperature of 5-10 eV [6] is only in the range of a few centimetres [17]. Hence, only negative ions created near the PG apertures can be extracted, but they have to be bent back to the apertures by magnetic fields and charge exchange collisions with hydrogen neutrals [18].…”

“…The meniscus forms a convex lens and so determines the initial beamlet quality. For positive ion extraction, where the space charge of the positive ions can be easily compensated by the fast electrons, the dependence of the meniscus shape on the plasma parameters and the extraction potential is well understood; for negative ion extraction, however, the situation is more complex as the space charge of the negative ions must be compensated by slow positive ions (H + , H + , H + , but also Cs + in the case the last few years by modelling [17,26] and by dedicated experiments at IPP [8]. A part of this effort is reported in this paper.…”

Magnetic filter field dependence of the performance of the RF driven IPP prototype source for negative hydrogen ions

“…This low electron temperature minimizes the collissional losses of negative hydrogen ions. As the binding energy of the electron is rather low (0.75 eV), the mean free path of a negative hydrogen ion in the main source plasma is only in the range of a few cm [23]. Hence, only negative ions created near the plasma grid apertures can be extracted, here the mean free path is considerably larger (several tens of cm) due to the lower electron temperature and density.…”