Research Topics of the NBI group
1. Plasma production and heating by use of particles
1-a. Study of fueling, heating and potential formation by neutral beam injection
Fueling, heating and potential formation by using neutral beam injection (NBI) have been investigated. The neutral beam is produced by accelerating hydrogen atom with high energy (~ 25 keV) and injected into the plasma. Figure 1 shows the schematic view of the NBI system. In GAMMA 10, seven injectors and three power supply systems are equipped and the injectors are installed at the central-cell, anchor cell and plug/barrier cell.
Fig. 1 Schematic view of the NBI system
1-b. Study of fueling and transport by hydrogen ice-pellet injection and super-sonic molecular beam injection
Particle fueling and its transport have been investigated by using hydrogen ice-pellet injection and super-sonic molecular beam injection. We aim the improvement of plasma performance such as plasma density using these techniques. In the GAMMA 10 central-cell, a pellet injector and molecular beam injector are installed near the mid-plane. Figure 2 shows the time evolution of plasma light emission cloud produce by the pellet injection captured with high-speed camera.
Fig. 2 Photograph of hydrogen ice-pellet obtained by high-speed camera.
2. Plasma diagnostics and transport analysis based on the particle and visible emission from the plasma
2-a. Hot ion measurement using neutral particle energy analyzer
High-energy charge-exchange neutral particles emitted from hot ion plasmas have been measured with a neutral particle energy analyzer installed at the central-cell of GAMMA 10. Figure 3 shows photograph of the neutral particle energy analyzer.
Fig. 3 Picture of neutral particle energy analyzer
2-b. Neutral particle transport analysis based or Balmer-alpha emission measurements and Monte-Carlo simulation
Light emission from the neutral hydrogen atoms (Balmer-aa) has useful information of neutrals in the plasma. There is a number of a line-emission detectors installed in GAMMA 10. We have been studying neutral particle transport in the plasma, based on the absolutely calibrated Ha measurement and neutral transport simulation using the Monte-Carlo code “DEGAS”. Figure 4 shows the results of the simulation. ((a) a mesh model used for the simulation code; (b) cross-section view of Ha emission obtain from the simulation, (c) simulation result of the camera view).
Fig. 4 Example of neutral particle transport simulation analysis (a) mesh model used for the simulation code; (b) cross-section view of Ha emission obtain from the simulation, (c) simulation result of the camera view.
2-c. Two-dimensional image analysis of light emission from the plasmas using high-speed camera
Plasma behavior, such as fluctuations in the boundary plasmas and effect of the plasma heating and fueling systems has been investigated by observing the two dimensional image of light emission from the plasma. A high-speed camera with the frame rate of 100,000 fps (100 kHz) and two medium-speed cameras with 1,000 fps have been installed and the visible emission from the central-cell and end-cell plasmas are observed. Figure 5 shows the still image of the light emission due to the plasma- material interaction on the target plates install at the end cell.
Fig. 5 Example of visible image of plasma material interaction on the target plates captured at the GAMMA 10end-cell.
3. Plasma-wall interactions, development of the future technology
3-a. Divertor simulation studies using end-mirror plasmas
In order to utilize a merit of open magnetic field configuration of tandem mirror device, divertor simulation studies have been started using end-mirror plasmas of GAMMA 10. Figure 6 shows the schematic view of experimental arrangement for divertor simulation experiments and the photograph of the interior of the end vacuum chamber.
Fig. 6 Schematic view of divertor simulation experiment and photograph of the interior of the end-cell vacuum chamber.
Figure 7 shows the results of heat-flux measured near the end-mirror exit of GAMMA 10. In a standard hot-ion-mode plasmas with only ICRF heating, a heat-flux density of 0.6 MW/m2 has been achieved and superimposing ECH pulse (300 kW, 25 ms) onto the RF plasma attained the maximum heat-flux density of 9MW/m2 This value almost equal to the heat-flux level expected in the ITER divertor, which gives a clear prospect of generating the required heat-flux density for divertor studies by building up heating systems to the end-mirror cell.
3-b. Fundamental studies for direct energy conversion
High-energy charged particles are produced by nuclear fusion reaction. In GAMMA 10, fundamental research of direct energy conversion of plasma flow from the end-cell has been performed based on the collaboration program between PRC and University of Kobe. Figure 8 show the principle of direct energy conversion from the open-ended plasma devices.
Fig. 8 Principle of direct energy conversion from the fusion plasmas.