In a toroidal system, circulating fast ions generated by neutral beam injection affect the beam stopping cross-section of the neutral beam itself. This effect is called “beam particle self-interaction (BPSI)”. In a recent experiment in JT-60U with a 350 keV H⁰ beam, an indication of this BPSI effect was seen for the first time. In a low density discharge at about 1 × 10¹⁹ m^-3, the beam shine-through decreased by about 35% within several hundred msec after beam injection. This result is consistent with a prediction by the BPSI theory.

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A gyrokinetic particle simulation is a powerful tool in studying tokamak microturbulence. A δf method, which is a standard method in recent gyrokinetic particle simulations, dramatically improves an efficiency of a particle simulation by reducing a particle noise, and full torus turbulence simulations are enabled. In this paper, the δf method is reviewed, and issues in full torus gyrokinetic particle simulations are discussed.

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Characteristics of post-disruption runaway electrons with impurity pellet injection were investigated for the first time using the JT-60U tokamak device. A clear deposition of impurity neon ice pellets was observed in a post-disruption runaway plasma. The pellet ablation was attributed to the energy deposition of relativistic runaway electrons in the pellet. A high normalized electron density was stably obtained with n_e^bar/n^GW ˜ 2.2. Effects of prompt exhaust of runaway electrons and reduction of runaway plasma current without large amplitude MHD activities were found. One possible explanation for the basic behavior of runaway plasma current is that it follows the balance of avalanche generation of runaway electrons and slowing down predicted by the Andersson-Helander model, including the combined effect of collisional pitch angle scattering and synchrotron radiation. Our results suggested that the impurity pellet injection reduced the energy of runaway electrons in a stepwise manner.

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