Ion Heating and Rotation Acceleration during Bursting MHD Events on Versatile Experiment Spherical Torus

Abstract

MHD instabilities in magnetically confined plasma significantly degrade the confinement and possibly leads to disruption. Plasma rotation and its shear have stabilization effects on various MHD instabilities such as NTMs (Neoclassical tearing modes) and RWMs (Resistive wall modes). On the other hand, MHD instabilities can have a significant influence on plasma rotation. While it is an important piece toward a thorough understanding of the interaction between MHD instabilities and plasma rotation, its experimental exploration has been limited to a few phenomena and still remains to be incomplete. In this thesis, detail dynamics of plasma rotation and ion temperature are investigated in bursting MHD activity on VEST (Versatile Experiment Spherical Torus). For this purpose, a novel ion Doppler spectroscopy (IDS) is newly developed with a superb temporal resolution ranging from 0.2-1 ms. This IDS capability permits to investigate the bursting MHD activity with fast crash time (~0.3ms). Interestingly, significant toroidal rotation acceleration as well as ion heating is observed in the bursting MHD activity. In order to explain the sudden spin-up phenomenon, several candidate mechanisms are discussed and compared with the experimental result. The fast IDS system is carefully designed and installed to measure the plasma rotation and ion temperature in VEST. A high throughput spectroscopic system is successfully developed by employing the transmission grating and by matching the etendue across the whole of the optical system. Fast temporal resolution can be attained to 1 ms for 10 spatial channels and 0.2 ms for one spatial channel. The diagnostic is based on the line emission of intrinsic carbon impurity and it can cover the R>0.5 m. The measured spectra by the IDS system are averaged spectra over the line-of-sight. Doppler tomographic inversion technique is employed to extract the local plasma rotation and ion temperature from the line integrated spectra. However, the inversion problem is a highly ill-posed problem and the reconstructed results sometimes have an unphysically oscillating feature. Second-order Tikhonov regularization method is utilized to properly estimate the local plasma properties which are believed to be continuous. This method is based on the minimization of the cost function which includes not only the accuracy of measurement but also the smoothness of the solution. Also, appropriate quantification of the uncertainties of the IDS system is analyzed by considering the propagation of errors of fitted parameters of spectra based on the numerical Monte Carlo (MC) method. Bursting MHD activities are usually observed in the low q Ohmic discharge with low prefill gas in VEST. These events are characterized by a spike in the plasma current, loop voltage and a burst of magnetic fluctuations with a high frequency above 10 kHz. Magnetic fluctuation shows a very fast exponential growth with a typical time constant of tens of microsecond (~100τA). Interestingly, fast IDS diagnostic reveals that significant toroidal rotation acceleration in the counter-Ip direction as well as ion heating are observed in the bursting MHD activity. The rotation and ion temperature are changed globally in the same manner in the entire of the plasma volume, which clearly shows that some kind of rotation torque and ion heating are acting on the plasma, instead of locally enhanced plasma transport. The rotation and ion temperature increase very fast with the time scale of < 300 μs and then recover slowly with the confinement time scale. Several physical mechanisms to account for the sudden spin-up are discussed in terms of temporal behavior, the direction of torque and torque amplitude. The considered mechanisms are reconnection outflows, toroidal electric field by reconnection process, enhanced electron loss, and NTV (Neoclassical toroidal viscosity) torque with offset rotation. Among them, NTV torque by non-axisymmetric magnetic fluctuation from the MHD activity seems to a most probable mechanism. Strong plasma rotation accelerations during the MHD activities are experimentally investigated for the first time. This observation will help us understand the effect of MHD event in plasma rotation and temporal dynamics of MHD event. In particular, this result suggests the possibility of the interplay between MHD instability and induced plasma rotation such as the self-organization process.