Recent progress of magnetic reconnection research in the MAST spherical tokamak

Hiroshi Tanabe ,A Meakins,K G Mcclements,B Crowley,N Hawkes,C Z Cheng,R Scannell,T Watanabe,N J Conway,I Fitzgerald,M Gryaznevich,Clive Michael ,Teppei Yamada ,James R Harrison ,Keii Gi ,Michiaki Inomoto ,T O’gorman ,Ryota Imazawa ,Yasushi Ono

doi:10.1063/1.4977922

Abstract

In the last three years, magnetic reconnection research in the MAST spherical tokamak achieved major progress by the use of new 32 chord ion Doppler tomography and 130 channel YAG and 300 channel Ruby Thomson scattering diagnostics. In addition to the previously achieved high power plasma heating during merging, detailed full temperature profile measurements including the diffusion region have been achieved for the first time. 2D imaging measurements of ion and electron temperature profiles have revealed that magnetic reconnection mostly heats ions globally in the downstream region of outflow jet and electrons locally around the X-point. The toroidal field in MAST “over 0.3T” strongly inhibits cross-field thermal transport, and the characteristic peaked electron temperature profile around the X-point is sustained on a millisecond time scale. In contrast, ions are mostly heated in the downstream region of outflow acceleration and around the stagnation point formed by reconnected flux mostly by viscosity dissipation and shock-like compressional damping of the outflow jet. Toroidal confinement also contributes to the characteristic ion temperature profile, forming a ring structure aligned with the closed flux surface. There is an effective confinement of the downstream thermal energy due to a thick layer of reconnected flux. The characteristic structure is sustained for longer than an ion-electron energy relaxation time (∼4 ms), and the energy exchange between ions and electrons contributes to the bulk electron heating in the downstream region. The toroidal guide field mostly contributes to the formation of a localized electron heating structure around the X-point but not to bulk ion heating downstream.

Highlights

Magnetic reconnection is a fundamental process, which converts the magnetic energy of anti-parallel reconnecting fields to kinetic and thermal energies of plasmas through the breaking and topological rearrangement of magnetic field lines.1,2 This process is known as an effective way of converting magnetic energy into plasma energy in proportion to the square of the reconnecting field / B2rec.3 Recent satellite observations of solar flares revealed several important signatures of reconnection heating
In the last three years, magnetic reconnection research in the MAST spherical tokamak achieved major progress through the use of new 32 chord ion Doppler tomography and 130 channel YAG and 300 channel Ruby Thomson scattering diagnostics. 2D detailed imaging measurement of both electron and ion temperature profiles around the diffusion region has been achieved for the first time
With the better confinement of reconnection heating under high guide field conditions (Bt > 0:3 T) and high temperature experimental conditions, which reduces the severe loss by collisional/radiation cooling, the results from MAST clearly revealed the characteristics of reconnection heating, namely, for electron heating as the highly localized peaked structure around the X-point

Summary

INTRODUCTION

Magnetic reconnection is a fundamental process, which converts the magnetic energy of anti-parallel reconnecting fields to kinetic and thermal energies of plasmas through the breaking and topological rearrangement of magnetic field lines. This process is known as an effective way of converting magnetic energy into plasma energy in proportion to the square of the reconnecting field / B2rec. Recent satellite observations of solar flares revealed several important signatures of reconnection heating. V-shaped high electron temperature region was found around the X-line of reconnection as possible evidence of the slow shock structure.6 Those heating characteristics of reconnection are still under serious discussion because of the absence of (or limited) in-situ diagnostics for astrophysical reconnection events. Since 1986, the merging of two toroidal plasmas (flux tubes) has been studied in a number of experiments: TS-3,7,8 START, MRX, SSX, VTF, TS-4,13 UTST, and MAST.16 For those laboratory experiments, evidence of plasma acceleration toward the outflow direction was observed as a split line-integrated distribution function in 0D,17 1D, and 2D bidirectional toroidal acceleration during counter helicity spheromak merging and in-plane Mach probe measurement around the X-point with and without the guide field.. Tokyo collaboration addressed this issue by the temporary repurposing of an existing collecting lens to provide a 32 chord tomographic ion Doppler spectroscopy capability on the midplane with a radial range spanning the diffusion region. Here, this paper addresses the recent major progress of detailed profile measurement of both electron and heating using those fine diagnostics during a high field reconnection experiment in MAST

EXPERIMENTAL SETUP

RECONNECTION HEATING DURING THE MERGING PLASMA STARTUP EXPERIMENT

COMPARISON OF ELECTRON AND ION HEATING

CONCLUSION