The role of an in-plane electric field during the merging formation of spherical tokamak plasmas

Abstract

Axial merging of two torus plasmas is utilized as a centre-solenoid free start-up scheme of a high-beta spherical tokamak plasma, in which magnetic reconnection under a strong guide field plays dominant roles on energy conversion and equilibrium formation. Ion heating source in magnetic reconnection is the plasma outflow with E×B drift velocity in the downstream region where the reconnected field lines flow out. Since the inductive reconnection electric field is almost parallel to the magnetic field particularly in the inboard-side downstream region of magnetic reconnection under a strong guide field, large electrostatic field in the poloidal plane is spontaneously formed to sustain steady plasma outflow motion in the downstream region. In the spherical tokamak plasma merging experiment, self-generated electrostatic field in the downstream region does not always balance with the inductive electric field to make the total electric field strictly perpendicular to the total magnetic field. Excess of the electrostatic field will provide even faster outflow plasma velocity than the magnetic field line motion and quick reversal of toroidal plasma current to form convex flux surfaces.