Bifurcated merging operations of two spherical tokamak plasmas for reconnection heating and magnetic helicity injection

Abstract In tokamak plasma merging experiments, high-power ion heating is achieved only when the current sheet thickness is compressed to ion gyroradius ρ i . The magnetic reconnection converts 40%–50% of the reconnecting magnetic field energy ( B rec 2 /2 μ 0 ) into the ion heating energy ( W i ) with the scaling law of W i proportional to B rec 2 . If the compressed current sheet thickness is larger than ρ i the magnetic reconnection converts only 5%–10% of B rec 2 /2 μ 0 into W i . The high-power ion heating can be understood from the experimental finding that the poloidal electric field due to electrostatic quadrupolar potential in the downstream region increases linearly with both B rec and the guide field B t .