Enhancement of plasma confinement and observation of channels of negative toroidal current in a small tokamak

Abstract

The SINP tokamak is a small, ohmically heated tokamak (R0=30 cm, a=7.5 cm) with discharge durations of ≈1-2 ms under normal operating conditions. The role of the vertical magnetic field (B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</sub> ) on the total discharge duration was investigated at low filling pressures and at low to moderate toroidal magnetic fields. It was observed that for certain values of B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</sub> the discharge duration extended to ≈3-5 ms and that this increase in duration occurred mainly after the onset of decay of the plasma current (after it crosses its peak value) [1,2]. During this extended phase, the plasma current remains approximately constant for ≈1 ms. Estimates also indicate an improvement in the energy confinement times by nearly 2-5 times (with respect to the standard SINP discharges). Further observations in the extended current phase show the presence of negative toroidal currents (i.e., toroidal current channels flowing opposite the main current) at localized radial locations accompanied by enhanced loss of high energy electrons and qualitative changes in the edge fluctuations. It is also found that the negative current channels and the extended phase occur simultaneously and that the termination of the negative currents leads to termination of the extended phase as well. The enhanced high energy electron losses measured using a NaI(Tl) detector, indicates the role of beam plasma instabilities, playing a pivotal role in the improvement of plasma confinement. Estimates indicate favorable conditions for triggering the generation of return currents causing the onset of the filamentation instability [2], which is particularly interesting since such instabilities have normally not been reported in tokamak plasmas, to the best of our knowledge. The return currents can also explain the observation of negative currents at localized radial locations. This paper presents experimental results supporting the above. In addition, characterization of the edge fluctuations using spectral and nonlinear analysis tools will also be presented.