High power heating of magnetic reconnection in merging tokamak experimentsa)

Abstract

Significant ion/electron heating of magnetic reconnection up to 1.2 keV was documented in two spherical tokamakplasma merging experiment on MAST with the significantly large Reynolds number R∼10⁵. Measured 1D/2D contours of ion and electron temperatures reveal clearly energy-conversion mechanisms of magnetic reconnection: huge outflow heating of ions in the downstream and localized heating of electrons at the X-point. Ions are accelerated up to the order of poloidal Alfven speed in the reconnection outflow region and are thermalized by fast shock-like density pileups formed in the downstreams, in agreement with recent solar satellite observations and PIC simulation results. The magnetic reconnection efficiently converts the reconnecting (poloidal) magnetic energy mostly into ion thermal energy through the outflow, causing the reconnectionheating energy proportional to square of the reconnecting (poloidal) magnetic field Brec² ∼ Bp². The guide toroidal field Bt does not affect the bulk heating of ions and electrons, probably because the reconnection/outflow speeds are determined mostly by the external driven inflow by the help of another fast reconnection mechanism: intermittent sheet ejection. The localized electron heating at the X-point increases sharply with the guide toroidal field Bt, probably because the toroidal field increases electron confinement and acceleration length along the X-line. 2D measurements of magnetic field and temperatures in the TS-3 tokamak merging experiment also reveal the detailed reconnectionheating mechanisms mentioned above. The high-power heating of tokamak merging is useful not only for laboratory study of reconnection but also for economical startup and heating of tokamakplasmas. The MAST/TS-3 tokamak merging with Bp > 0.4 T will enables us to heat the plasma to the alpha heating regime: Ti > 5 keV without using any additional heating facility.