A particle fueling into magnetically confined plasma is necessary to increase and sustain the plasma density. Measurements of electron and neutral hydrogen densities are important to study the fueling effect because fueling cause a lot of neutral gas influx to target plasma. In GAMMA 10, neutral beam injector (NBI) is used for fueling to plasma in the central cell. We measured time dependent radial profiles of electron and neutral hydrogen density to research into the change of the plasma and to explain the cause of these changes during neutral beam injection.
In GAMMA 10, as diagnostic systems for measuring electron and neutral hydrogen density profiles, arrays of absolutely calibrated H_α line-emission detectors and microwave interferometers are installed near the midplane of the central cell. Each vertical and horizontal H_α detector array has 12 channel. The spatial profiles of H_α line integrated brightness measured by using these arrays. An Algebraic Reconstruction Technique (ART), which is the technique of reconstructing a local value from a line integral value, is used in order to obtain the radial profiles of the H_α emissivity. Neutral hydrogen density can be obtained from comparing the H_α emissivity obtained by ART and collisional-radiative model (CR-model) calculation results using electron density and the temperature.
The neutral beam is injected into a ICRF-heated hot ion mode plasma at the central cell. In the standard hot ion mode plasma, the electron density, the electron temperature and the ion temperature is 2x10¹² cm^-3, 100 eV and 5 keV, respectively. The increase of peripheral electron density by 30 % and the decrease of peripheral neutral hydrogen density by 8 % are observed just after the beginning of the neutral beam injection, while usual peripheral electron density is about 5x10¹¹ cm^-3 and peripheral neutral hydrogen density is about 5x10⁹ cm^-3. Then the increase of peripheral neutral hydrogen density by 30 % is observed about 10ms after the beginning of neutral beam injection. These observation results could be explained from the effect of the two kinds of gas caused by NBI, that consist of recycling gas from vacuum vessel and cold gas from ion source of NBI. Therefore it is thought that the gas plays important roles in the change of plasma during NBI. In this paper, the detailed behavior of electron and neutral densities during NBI are presented and their analytical result will be discussed.