Positive potential-side global ion heating during high guide field reconnection in the TS-6 merging spherical tokamak formation experiment

Here we report our new finding of positive potential-side global ion heating during high guide field reconnection in the TS-6 merging spherical tokamak formation experiment. Particle accleration and heating during guide field reconnection is one of the major topic of reconnection studies and the contribution of the quadrapole potential structure and inplane electric field are investigated in many laboratory experiments and numerical simulations. Previously most of studies reported that ions are heated at the lower potential-side of the quadrapole potential region by in-plane electric field E_p and it was proposed that the increment of ion temperature T_i is proportional to the potential drop: △T_i ∝ △Φ. However, our full-2D global ion Doppler tomography imaging of T_i profile revealed that T_i is actually higher in the positive potential side and the structure gets flipped when toroidal field (B_t) polarity is changed. In the sufficiently high guide field condition (B_t ~ 5B_p), inductive/reconnection electric field E_t made a major contribution to ion acceleration as well as electron ones. Around the quadrapole potential region, ions just pass through by E×B drift and △Φ ~ 0 on the orbit. Then, the major contribution comes from parallel electric field E_// = E·B/|B|. The combination of E_t and B_t forms large negative parallel electric field and ions are accelerated to the positive potential-side. After the end of merging, in-plane heat transport process equilibrated the characteristic structure and finally globally hollow ion temperature profile is formed. It is sustained by better plasma confinement by guide field (κⁱ_///κⁱ_⊥ ~ 2(ω_ciτ_ii)² >> 1) in the quasi-steady phase after merging and successfully bridging the double-peak heating characteristics reported in many merging experiments such as MAST and TS-3.