Spontaneous toroidal rotation of the plasma is observed in the spherical tokamak QUEST with the help of electron cyclotron resonance (ECR) heating and without the use of any externally injected momentum. Several vertical magnetic field ( Bz) configurations with varying mirror ratio (M) (a measure of field curvature) are applied and evolution of rotation is studied with the help of Doppler spectroscopy of bulk and impurity ions. Significant toroidal rotation (Vφ-6km/s) is initiated in the open magnetic field configuration during the initial plasma breakdown phase, which is later sustained ( Vφ- 20km/s) in a closed magnetic field configuration in steady state. Rotational velocity is primarily along the cocurrent direction and is found to be proportional to the Bz strength and the resulting plasma current. High M and Bz are demonstrated to be the two specific external controls by which rotation can be initiated in the plasma. The rotation in open field lines is found to be initiated at the ECR layer in the slablike plasma, which is evolved to produce a sustained rotation in the natural divertor inboard poloidal field null equilibrium in QUEST.
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