No abstract

High Mach flows and their formation mechanism in tokamak divertor regions have been studied. Numerical calculations using a two dimensional edge plasma simulation code (the B2-EIRENE code) have been made for the JT-60U open divertor geometry with discharge parameter ranges of relatively low input power and L mode experiments. In the high density case, typical formation of an X point MARFE, which is accompanied by plasma detachment, has been observed. At the same time, high Mach flows appear near the ionization front, where the plasma static pressure rapidly drops and is far away from the target plates. Redistribution from static pressure to dynamic pressure without a large momentum loss is shown to be a possible cause of the high Mach flows observed in the simulation. In addition, a simple 1-D analytic model along the field lines has been applied and the results are compared with the B2-EIRENE results in order to identify the formation mechanism of high Mach flows.

No abstract

The production mechanism of negative ions has been investigated experimentally in a cesium seeded volume type negative ion source. The cesium light emission intensity, Cs ion volume density, plasma parameters, H- current, and work function of the plasma grid were measured. The change in plasma parameters was small even in a high Cs ion density region(up to 1012 cm-3) which was also confirmed by zero-dimensional simulation. Upon heating the plasma grid, the work function decreased and the negative ion current increased to become three times larger than that of the low plasma grid temperature condition. This increase was in good agreement with the decrease of the work function of the plasma grid surface. Therefore, it is considered that the Cs contribution for the negative ion production is the surface effect in the negative ion source.

Two multidimensional Monte Carlo simulation codes—(a) neutral (H2,H) transport code and (b) negative ion (H−) transport code—have been developed. This article focuses on the recent simulation results by the neutral transport code for the H− production in a large, hybrid negative ion source, “Camembert III.” Two-dimensional spatial profiles of vibrationally excited molecules H2(v) and H− production are obtained for a given background plasma profile. Both H2(v) and H− ions are mainly produced near the filaments in the driver region. However, the H− source density has double peak in its spatial structure, while the density profile of H2(v) is characterized by the “mushroom” structure with a single peak. These results indicate a large potential of the neutral transport code, not only for the understanding of underlying physics, but also for designing ion sources, including complicating effects of geometry, spatial and velocity distribution of particles, and atomic and wall processes.