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Four loop antennas are installed in the Compact Helical System (CHS) heliotron/torsatron in order to externally excite Alfven eigenmodes (AEs). These antennas are used in two different operation methods: one is as a set of four external loop antennas, and the other two pairs of electrodes inserted near the plasma boundary. For these two operations, magnetic perturbations in the frequency range of 10–250 kHz are generated in the plasma. The plasma response for the applied magnetic perturbations is measured by magnetic probe arrays. Information of AEs can be derived from the analysis of the response. In the excitation experiment of AEs using the “electrode method,” magnetic fluctuations are resonantly enhanced in a certain electron density during the afterglow of a neutral beam heated plasma, where no energetic ions exist. The fluctuations have the toroidal mode number n=1 and the frequency lies just above the lower bound of the toroidicity-induced Alfven eigenmode (TAE) gap in the plasma core region. The excited fluctuations are thought to be related to TAEs.

In tokamak plasmas, the confinement of energetic ions depends on the magnetic field structure. If the plasma pressure is finite, the equilibrium current (i.e., the Pfirsch-Schluter current and diamagnetic current) flows in the plasma to maintain the magnetohydrodynamic (MHD) equilibrium. These plasma currents generate poloidal and toroidal magnetic field and alter the field structure. Moreover, if we consider the non-axisymmetry of magnetic field structures such as toroidal field (TF) ripples, the non-axisymmetric component of the equilibrium current can alter TF ripples themselves. When the plasma beta becomes high, the changes in the field structure due to the equilibrium current might affect the confinement of energetic ions significantly. We intend to clarify how these currents alter the field structure and affect the confinement of alpha particles in high-beta plasma. The MHD equilibrium is calculated using VMEC and the orbits of fusion alpha particles are followed by using the fully three-dimensiona...

The kinetic effect of trapped energetic particles (EPs), arising from perpendicular neutral beam injection, on the stable low-n peeling modes in tokamak plasmas is investigated, through numerical solution of the mode's dispersion relation derived from an energy principle. A resistive-wall peeling mode with m/n=6/1, with m and n being the poloidal and toroidal mode numbers, respectively, is destabilized by trapped EPs as the EPs' pressure exceeds a critical value βc*, which is sensitive to the pitch angle of trapped EPs. The dependence of βc* on the particle pitch angle is eventually determined by the bounce average of the mode eigenfunction. Peeling modes with higher m and n numbers can also be destabilized by trapped EPs. Depending on the wall distance, either a resistive-wall peeling mode or an ideal-kink peeling mode can be destabilized by EPs.

A new technique to measure the first and second derivatives of the ion temperature profile has been developed by using a charge exchange spectroscopy system with space modulation optics. The space observed is scanned up to ±3 cm with a cosine wave modulation frequency up to 30 Hz by shifting the object lens in front of the optical fiber bundle by 0.5 mm with a piezoelement. The first and second derivatives of ion temperature are derived from the modulation component of the ion temperature measured by using Fourier series expansion.

The multichannel motional Stark effect (MSE) diagnostic system in JT-60U has been upgraded to measure density fluctuation profile. A 16-channel fast-sampling digitizer has been added in order to measure photomultiplier-tube signals at measurement frequency of 0.5–1MHz. The new system works as a MSE and beam emission spectroscopy diagnostic. Spatially resolved electron density fluctuation profile measurement in various operation regimes is presented. In the core plasma, density fluctuation induced by rotation of tearing mode islands was observed. Temporal evolution of the fluctuation frequency agrees with that measured by Mirnov coils (poloidal and toroidal mode numbers: 2 and 1, respectively). The phases of the fluctuations on either side of the q=2 surface are inverted, which is consistent with electron cyclotron emission. These measurements show that the density fluctuation is caused by a rotating magnetic island structure induced by the tearing mode. In the scrape-off layer of a H-mode plasma with edge...