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内容記述 |
The heliotron/torsatron devices are regarded as an attractive candidate for a fusion reactor because of their steady state operation without inductive plasma currents. In these devices major disruptions induced by current driven instabilities can be avoided. However, they are susceptible to pressure driven instabilities such as interchange modes and ballooning modes, which limit high-β operation. Recent theoretical study suggests that such instabilities also contribute to enhance anomalous transport and deteriorate the confinement. In the Compact Helical System (CHS), magnetic fluctuations have been studied by the use of poloidal and toroidal arrays of magnetic probes. It has been found that the fluctuation modes depend on magnetic configuration, beta value, direction of beam induced current during NBI heating (which can change the magnetic shear), and so on. Among these fluctuations, periodic, burst type m/n=2/1 (m: poloidal mode number, n: toroidal mode number) modes observed in a low-β , NBI (co-injected) plasma have shown the strongest activity.<br /> The oscillations appear periodically, typically every 4 milliseconds, and their frequency generally decreases from 40kHz to 15kHz during a growing phase. The mode is considered to be an interchange instability. However, the mode, propagating initially in the ton diamagnetic drift direction, reverses the direction (to the electron diamagnetic drift direction) in the decaying phase. In addition, the role of the magnetic fluctuations on confinement has not been clarified yet. Local and direct measurements of the internal structure is necessary for further investigation.<br /> We have applied the heavy Ton beam probe (HIBP) for the first time to measure MHD instabilities in helical system. The HIBP is a unique and powerful technique which can directly measure the electric potential and its fluctuations in high temperature plasmas. It has been successfully applied to tokamaks, mirrors and bumpy tori in which toroidal field component is dominant on the beam path and the beam trajectories are basically two dimensional in the same poloidal plane. First application of HIBP to a helical device was done in the ATF torsatron, preliminary data has been reported also for MHD studies. HIBP measurements in helical devices are not so simple as in tokamak because of its three dimensional beam trajectories.<br /> The CHS HIBP has two sets of beam deflectors to control the primary and secondary Ton beams independently so that the injection angle of the secondary beam into the energy analyzer is kept constant during a radial scan. This method makes the observation area wider and improve accuracy in determining a plasma potential. Because an observation point is determined by the combination of four deflector voltages, it is inevitably sensitive to the accuracy of beam line alignment and fringing field of the deflectors. Experimental calibration is necessary to verify the accuracy of the total system. A movable detector and a gas ionization method have been applied to calibrate the beam trajectory and observation points. Sets of deflector voltages to observe locations along a radial scan line were experimentally obtained. The results agree well with the calculation. The movable detector is also used to optimize the focusing condition of the primary beam in the plasma region. Total calibration procedures with these methods have been successfully carried out.<br /> In applying HIBP to measure the local space potential and its fluctuations during MHD activities, various non local effects (path integral effects along the beam trajectories) have to be examined. The effects of beam deflection and acceleration (or deceleration), which are caused by the fluctuating vector potential, on local potential fluctuation measurements are evaluated using the experimental data from HIBP and magnetic probes. By taldng those effects into account, the radial structures of the m/n = 2/1 burst type MHD oscillation have been derived. The potential fluctuation has a strong peak around q = 2 surface in the growing phase and its amplitude is about 40 volts at maximum. The oscillation frequency decreases from 20 kHz to 10 kHz and the phase difference between the potential fluctuation and a Mirnov coil signal varies about 90 degree during the growing phase. The mode is propagating in the Ton diamagnetic direction. At the end of the growing phase, the mode structure abruptly changes and the potential fluctuation is suppressed everywhere. The magnetic perturbation decays slowly at the constant frequency of 5 kHz (decaying phase). The maximum amplitude is larger than that in the growing phase. The propagation is in the electron diamagnetic direction and the mode appears to be fixed to the Er×Bt plasma rotation (Er: radial electric field, Bt: toroidal magnetic field) determined by the electrostatic potential. By considering those characteristics, the mode is considered to be an m/n = 2/1 interchange mode in the growing phase, although the propagation velocity and the growth rate are not fully explained. The mode structure in the decaying phase is completely different and is suggesting m = 2 island formation at the q = 2 rational surface.<br /> In conclusion, the application of HIBP for the study of MHD instabilities in a helical plasma has been successfully carried out. Radial structure of the potential fluctuation associated with the m/n = 2/1 burst type interchange instability is experimentally clarified. The result demonstrates a new diagnostic approach to MHD fluctuation studies in helical plasmas. |
要旨・審査要旨 / Abstract, Screening Result (299.0 kB)
