Development of Negative Ion Based NBI System for JT-60U
N. Umeda, N. Akino, N. Ebisawa, L. Grisham1), S. Hikita, A. Honda, T. Itoh, M. Kawai, M. Kazawa, M. Kusaka, N. Kusanagi, M. Kuriyama, P. Lee2), K. Mogaki, T. Ohgo, H. Oohara, F. Satoh, H. Seki, N. Seki, Y. Tanai, R. Toyokawa, K. Usui and H. Yamazaki
1) Japan Atomic Energy Research Institute
2) Naka Fusion Research Establishment, Princeton Plasma Physics Laboratory Sauthwestern Institute of Physics, CNNC
Abstract.
The negative ion based neutral beam injection (N-NBI) system for JT-60U has been developed in JAERI in order to demonstrate core plasma heating and non-inductive current drive with a higher plasma density, and has operated since 1996. The aim of N-NBI is to inject the neutral beam power of 10MW at 500keV for 10sec and the injection power of 5.2MW at 350keV has already achieved with deuterium beam. High energy deuterium beam injections at around 350keV into JT-60U plasmas have confirmed the current drive efficiency of 1.3E19 A/Wm2 and electron heating of core plasma, which are consistent with the theoretical prediction.[1]
However some issues to be solved have remained for increasing beam power moreover. The first is spatially source plasma non-uniformity which brings about a higher heat loading onto the accelerator grid, caused by a perveance mismatching of beam acceleration. Ion saturation currents around the plasma grid with ten Langmure probes and a spatial distribution of arc currents were measured and large spatially non-uniformity of the source plasma has been found. As a countermeasure against the plasma non-uniformity, arc current resistors, which are connected in series to each of eight filament groups, have been changed independently to improve the non-uniformity. The change of the arc resistors has shown a sign of improvement. Another countermeasure is to change an arc discharge mode between space charge limited mode and emission limited one through changing the filament cathode temperature. The effect of the arc discharge mode affected the plasma uniformity is being investigated. In order to estimate easily the source plasma uniformity, two dimensional beam profile measurement equipment, which is made of a copper plate of 110cm in length and 50cm in width, was newly installed at just downstream of the ion source. The beam deposition profile is measured by both an lnfra-red camera and thermo-couples. The equipment is retractable in the time for injecting the beam into JT-60U.
Another issue for increasing negative ion current is to datermine a relation between the amount of cesium vapor injected into the arc chamber and the temperature of plasma grid facing to source plasma. Negative ion production is affected greatly by the temperature of the plasma grid, and the optimum grid temperature depends on the amount of cesium. As a preliminary result, an optimum temperature of the grid is about 200 degree for a few grams and about 250 degree for 4-5grams.
[1] T. Oikawa, et, al, "Reactor Relevant Current Drive and Heating by N-NBI on JT-60U", to be presented in 8th IAEA conference in Italy