Multi-point measurement using two-channel reflectometer with antenna switching for study of high-frequency fluctuations in GAMMA 10.
A two-channel microwave reflectometer system with fast microwave antenna switching capability was developed and applied to the GAMMA 10 tandem mirror device to study high-frequency small-amplitude fluctuations in a hot mirror plasma. The fast switching of the antennas is controlled using PIN diode switches, which offers the significant advantage of reducing the number of high-cost microwave components and digitizers with high bandwidths and large memory that are required to measure the spatiotemporal behavior of the high-frequency fluctuations. The use of two channels rather than one adds the important function of a simultaneous two-point measurement in either the radial direction or the direction of the antenna array to measure the phase profile of the fluctuations along with the normal amplitude profile. The density fluctuations measured using this system clearly showed the high-frequency coherent fluctuations that are associated with Alfvén-ion-cyclotron (AIC) waves in GAMMA 10. A correlation analysis applied to simultaneously measured density fluctuations showed that the phase component that was included in a reflected microwave provided both high coherence and a clear phase difference for the AIC waves, while the amplitude component showed neither significant coherence nor clear phase difference. The axial phase differences of the AIC waves measured inside the hot plasma confirmed the formation of a standing wave structure. The axial variation of the radial profiles was evaluated and a clear difference was found among the AIC waves for the first time, which would be a key to clarify the unknown boundary conditions of the AIC waves.
Internal Electric Field Structure of Launched Fast Magnetosonic Waves in the DIII-D Tokamak
Results are obtained, for the first time in a major tokamak plasma, of the spatial electric field structure of a launched fast magnetosonic wave (FW). The FW launch directionality necessary for current drive has been confirmed at a position close to the FW antenna. The FW electric field profile is determined using X-mode reflectometry. Electric field profiles, calculated using a full-wave numerical code, exhibit many features common to the experimental results. However, differences remain regarding density dependence that suggest alternative damping mechanisms may exist. {copyright} {ital 1998} {ital The American Physical Society}
Stability of ICRF-heated plasmas in the GAMMA 10 tandem mirror
Abstract Recent experiments related to macro- and micro-instabilities in the GAMMA 10 tandem mirror are described. A stability boundary of a flute interchange mode was revealed experimentally. The pressure anisotropy becomes strong with an increase in the β value in the central cell. The critical β of the central cell for the flute interchange mode depends on the pressure anisotropy. The Alfven ion-cyclotron (AIC) mode which is driven by the anisotropy in the velocity space is spontaneously generated well below the theoretically predicted absolute boundary. The performance of the macro- and micro-stabilities in ICRF-heated plasma of the central cell is related to the pressure anisotropy.