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# 27th IAEA Fusion Energy Conference - IAEA CN-258

22-27 October 2018
Mahatma Mandir Conference Centre
Asia/Kolkata timezone
CONFERENCE MATERIAL NOW AVAILABLE!

## Investigation of fine structure formation of guide field reconnection during merging plasma startup of spherical tokamak in TS-3U

24 Oct 2018, 08:30
4h
Mahatma Mandir Conference Centre

Poster

### Speaker

Dr Hiroshi Tanabe (University of Tokyo)

### Description

We present the latest results of high-resolution 2D imaging measurement of merging/reconnection heating during the central solenoid (CS)-free plasma startup of spherical tokamak using a new 96CH 2D ion Doppler tomography diagnostics. In the last decade, magnetic reconnection research made a major progress such as (a) achievement of $\sim$1keV plasma heating in MAST both for ions and electrons, (b) demonstration of $B_{rec}^2$ scaling of ion heating ranging 0.01keV < $T_i$< 1.2keV with 0.01T< $B_{rec}$ < 0.15T in many plasma merging experiments based on outflow heating mechanism and (c) elucidation of fundamental heating characteristics: localized electron heating around $X$-point mostly by current sheet dissipation and global ion heating downstream where kinetic energy of outflow jet dissipates. Namely in the last three years, it was found that reconnection heating forms fine structure under high guide field condition of $B_t$ > $3B_{rec}$. From 2017, the formation process of the fine structure has been investigated in TS-3U ($B_t\sim5B_{rec}$) with direct measurement of magnetic field profile and high-resolution 2D imaging measurement of ion temperature profile using a new 96CH ion Doppler Tomography. As a new finding, it was found that ion temperature increases inside the current sheet as well as downstream. The high temperature region around the $X$-point is affected by Hall current $j_{Hall}$ from the decoupling of ions and electrons, the characteristic heating profile rotates poloidally toward $j_{Hall}\times B_t$ direction. This characteristic is clearer in high field side ($B_t$ depends on major radius in tokamak configuration) and with higher mass ratio (enhancement of $j_{Hall}\times B_t$ due to the larger scale length than current sheet width). While at the end of merging, ion heating downstream is surrounded by closed flux surface formed by reconnected field lines and forms another fine structure. The high temperature profile downstream propagates vertically and finally forms poloidally double-ring-like structure under the influence of better toroidal confinement with higher guide field which strongly suppresses perpendicular heat transport ($\chi^\parallel/\chi^\perp\sim$ 2($\omega_{ci}\tau_{ii})^2$>>10). This work was supported by JSPS KAKENHI Grant Numbers 15H05750 and 17H04863, and NIFS Collaboration Research Program NIFS16KLER048.
Country or International Organization Japan EX/P3-22

### Primary author

Dr Hiroshi Tanabe (University of Tokyo)

### Co-authors

Ms Asuka Sawada (University of Tokyo) Mr Hironori Hatano (University of Tokyo) Prof. Michiaki Inomoto (The University of Tokyo) Ms Moe Akimitsu (University of Tokyo) Mrs Qinghong Cao (University of Tokyo) Mr Takumi Hayashi (University of Tokyo) Prof. Yasushi Ono (University of Tokyo)