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Alfven eignemode experiments have been carried out by using the negative-ion-based neutral beam in JT-60U. For bursting instabilities in the frequency range of Alfvn Eigenmodes, two types of mode activity have been observed. In one type called Abrupt Large-amplitude Event (ALE), enhanced transport of the energetic ions was detected. The enhanced transport showed the clear energy dependence based on measurements performed using a natural diamond detector. In another type of bursting event called the fast Frequency Sweeping (fast FS) mode, frequency up-down chirping is observed on the time-scale of a few ms, much faster than equilibrium changes in the background plasma. This frequency chirping was well reproduced by the non-linear simulation code, MEGA, for magnetohydrodynamics (MHD) with energetic particles. The slow up-frequency sweeping of the Reversed-Shear-induced Alfven Eigenmode (RSAEs), due to the slow evolution of qmin was also observed. These experiments were analyzed by using the NOVA-K code. It is shown that the most unstable modes are localized around the minimum in the magnetic safety factor (qmin) in the analysis.

A new simulation method that improves the conservation properties of δf particle simulation for the collisionless Boltzmann equation is presented. When the distribution function is divided into a reference distribution specified in advance and a variation distribution, the time evolution of the variation distribution is described by an advection term in the phase space and a source term associated with the reference distribution. The time evolution of the Klimontovich distribution function of the δf method was investigated. It is shown that the errors in the Monte Carlo estimate of the source term in the δf method deteriorate the conservation properties. In the new simulation method, errors in the Monte Carlo estimate of the source term are corrected with a complementary fluid model. An example of the complementary fluid model is presented for bump-on-tail instability. The simulation results are compared with those of the conventional δf method. It is demonstrated that particle, momentum, and energy are well conserved with the new simulation method.

This article describes recent progress in nonlinear simulation studies of Alfven eigenmodes, especially of the toroidicity-induced Alfven eigenmode (TAE). Early simulations showed clearly that wave-particle trapping saturates the growth of TAE, but experimental results such as recurrent bursts and steady saturation remain unexplained. Recent simulations, which take account of particle source and collisions, can solve the mystery of these qualitatively different behaviors.

Fast frequency sweeping (fast FS) mode in a JT-60U plasma is investigated with particle-magnetohydrodynamic hybrid simulation. A new kind of energetic particle mode (EPM) is found near the plasma center at a frequency close to the central frequency of the fast FS mode. Frequency sweeping close to that of the fast FS mode takes place. The ratio of the mode damping rate to the linear growth rate is consistent with spontaneous hole-clump pair creation.