Ion and electron heating via magnetic reconnection during merging/compression plasma startup in ST40

Abstract Here, we report the latest results on the high-field application of a merging/compression (M/C) plasma startup in the ST40 spherical tokamak, using 96-channel 2D ion Doppler tomography and 30-channel Thomson scattering diagnostics installed over the past three years. The new findings and achievements are summarised as follows: (i) the new 96-channel 2D ion Doppler tomography clearly revealed ion heating in the downstream of the reconnection JET and its confinement/transport process. (ii) Electron density also increases in the same region, while electrons are heated around the X -point (or magnetic axis) and the electron temperature profile forms a peaked profile. (iii) Quasi-steady sustainment/confinement of M/C startup plasma current and ion temperature has been demonstrated. (iv) Based on a series of M/C experiments, I p ∝ I MC scaling was increased to I p ∼ 600 kA. (v) Full pressure profile measurements lead to the evaluation of the energy conversion ratio of the dissipated magnetic energy to the reconnecting field B rec and it was found that ∼ 30 % of released magnetic energy was converted to ion thermal energy. (vi) The previously demonstrated Δ T i ∝ B rec 2 scaling has been successfully upgraded to Δ U i = 1.5 Δ ( n e κ B T i ) ∝ B rec 2 for the first time and Δ U i ∼ 10 kJ m − 3 which was 10 times higher than the previous record in MAST was explored.