Acceleration of ion rotation during internal reconnection events in the versatile experiment spherical torus (VEST)

Abstract

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-I p 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 S NTV and ion temperature T i are related via .

Line integrated density measurements on the Versatile Experiment Spherical Torus (VEST) using frequency sweep interferometer

“…IRE is a relaxation phenomenon similar to minor disruption in large aspect ratio tokamak. It involves thermal quenching, current spiking and subsequent current quenching, which also occur frequently in the VEST spherical tokamak [34,36]. The temporal evolution during 316-320 ms of (a) plasma current, (b) line emissions of hydrogen, carbon, and oxygen, (c) poloidal B dot probe at the outboard, and (d) LID measured by 94 GHz interferometer, is shown in figure 14.…”

Electron Temperature Measurements Using a Two-Filter Soft X-ray Array in VEST

“…Figure 7 plots the Fourier spectra of both SXR and Mirnov coil signals. The Mirnov coils in VEST [20] are located at four different toroidal positions on the outboard midplane. The Mirnov signal in Figure 7b was obtained from the toroidal position closest to where the SXR array was installed.…”

Observation of MHD-correlated blobs during internal reconnection events in VEST

“…The edge safety factor of VEST is 7-10, and a typical internal inductance is 0.3 to 0.7, which places VEST plasma in a kink and tearing mode unstable operation region [18]. The most common MHD instability that precedes IREs in VEST is tearing modes (likely n = 1, m = 3-4, where n refers to a toroidal mode number, and m is a poloidal mode number [19]). A typical electron density and temperature in VEST plasmas are 10 19 m 3 and 100 eV [20].…”

“…Magnetic probes and a visible fast camera are used as main diagnostics in the study. In magnetics, 69 magnetic probes in VEST (poloidal array 65, toroidal array 4) are located on the chamber wall to measure poloidal magnetic perturbations with a sampling rate of 250 kSamples/s [19]. The poloidal magnetic probe on the low field side (LFS) chamber wall at R = 0.8 m and Z = 0 m is mainly used for the analyses and denoted as Ḃθ,LFS .…”