Origin and evolution of spontaneous rotation in plasma under different magnetic field geometry in Tokamak quest

Abstract

In ITER and future fusion reactors, intrinsic rotation will play a key role in stabilizing large scale instabilities, formation of transport barriers and reduction of divertor heat flux in the absence of significant NBI induced rotation at high injection energy. In order to utilize such intrinsic rotation property of plasma for the beneficial of a fusion grade reactors, one must understand the origin and external controlling parameters of this rotation dynamics. ECRH being one of the most indispensible and flexible heating device in tokamak, remains an attractive choice to initiate such rotations. In this paper we describe the spontaneous toroidal rotation of plasma in spherical tokamak QUEST with the help of ECRH. Several vertical magnetic field (B z ) configurations with varying mirror ratio (M) [1] (a measure of field curvature) is applied and evolution of rotation is studied with the help of Doppler spectroscopy of bulk and impurity ions. Significant toroidal rotation (V φ ∼ 20 km/s) is initiated in the open magnetic field configuration even in the initial plasma breakdown phase, which is later sustained in closed magnetic field configuration in the steady state. Rotation velocity is primarily along co-current direction and is proportional to the B z strength and resulting plasma current. High M and B z are demonstrated to be two specific external controls by which, rotation can be initiated in plasma. Response to external gas puff perturbation is investigated as a function of density and transient rotation reversal from co to counter current direction is observed. Rotation dynamics is studied in several different equilibrium configurations like limiter, single null and natural divertor IPN equilibrium [2] in QUEST and the details will be presented in this paper