Acceleration of ion rotation is observed during internal reconnection events (IREs) in the versatile experiment spherical torus. Two IRE discharges with opposite torques, i.e. acceleration or deceleration of ohmic plasmas with intrinsic, counter-Ip rotation, can be generated using different wall conditionings method. When an IRE occurs, acceleration and deceleration of impurity ion rotation as well as well-known ion heating are observed globally via ion Doppler spectroscopy with multiple channels including a single channel with a high temporal resolution of 0.2 ms. Interestingly, ion heating is observed earlier than the ion rotation acceleration, indicating that this phenomenon has a different mechanism from usual magnetic reconnection. We present several possible mechanisms for the rotation acceleration. For several reasons, the ions are thought to be accelerated by a neoclassical toroidal viscosity (NTV) torque rather than mechanisms such as the reconnection outflow and toroidal electric field induced by the current profile change. A simple 0D momentum balance model with NTV torque in the 1/ν collisionality regime agrees well with the experimental results. Furthermore, a positive correlation between ion temperature and plasma rotation is identified, whereby the NTV torque SNTV and ion temperature Ti are related via .
Density sensitivity of intrinsic rotation profiles in ion cyclotron range of frequency-heated L-mode plasmas