Tanabe DOI:10.1088/1741-4326/ab1cdf EX/P3-22 Investigation of Fine Structure Formation of Guide Field Reconnection During Merging Plasma Startup of Spherical Tokamak in TS-3U H. Tanabe1, H. Hatano1, T. Hayashi1, Q. Cao1, A. Sawada1, M. Akimitsu1, M. Inomoto1, and Y. Ono1 1Graduate School of Frontier Science, University of Tokyo, Tokyo, Japan Corresponding Author: H. Tanabe, [email protected] We present the latest results of high-resolution 2D imaging measurement of merging/recon- nection 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 „ 1 keV plasma heating in MAST 2 both for ions and electrons; b) demonstration of Brec scaling of ion heating ranging 0:01 keVă Ti ă 1:2 keV with 0:01 Tă Brec ă 0:15 T 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 Bt ¡ 3Brec. From 2017, the formation process of the fine structure has been investigated in TS-3U (Bt „ 5Brec) 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 jHall from the decoupling of ions and electrons, the characteristic heating profile rotates poloidally toward jHall ˆ Bt direction. This characteristic is clearer in high field side (Bt depends on major radius in tokamak configuration) and with higher mass ratio (enhancement of jHall ˆBt 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 k K 2 transport (χ {χ „ 2, !ciτiiq " 10). This work was supported by JSPS KAKENHI Grant Numbers 15H05750 and 17H04863, and NIFS Collaboration Research Program NIFS16KLER048. Published as a journal article in Nuclear Fusion http://iopscience.iop.org/article/10.1088/1741-4326/ab1cdf H. Tanabe, H. Hatano et al. INVESTIGATION OF FINE STRUCTURE FORMATION OF GUIDE FIELD RECONNECTION DURING MERGING PLASMA STARTUP OF SPHERICAL TOKAMAK IN TS-3U H. Tanabe, H. Hatano, T. Hayashi, Q. Cao, A. Sawada, M. Akimitsu, M. Inomoto and Y. Ono Graduate School of Frontier Sciences, University of Tokyo, Tokyo 113-0032, Japan Email: [email protected] Abstract We present the latest results of high resolution 2D imaging measurement of ion heating during central solenoid (CS)- free plasma startup of high-beta spherical tokamak. A new ultra-high resolution 96CH/320CH ion Doppler tomography was installed on TS-3U and following two fine structures of high guide field reconnection have successfully been resolved for the first time such as (a) tilted ion heating profile caused by the Hall effect around the X-point during the acceleration phase of magnetic reconnection (b) global ion heating and parallel heat transport process at downstream region where reconnected flux forms thick layer of closed flux surfaces, which confine the high temperature region and forms poloidally double-ring- like structure. Under the condition of high guide field limit (Bt > 3Brec), the contribution of higher guide field to suppress ion heating tends to be saturated and the promising upgrade scenario based on tokamak merging with higher guide field ratio is successfully demonstrated. 1. INTRODUCTION Magnetic reconnection is a fundamental process which accelerates/heats plasmas through the restructuring process 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 magnetic field. Magnetic reconnection is observed in many fusion, laboratory and astrophysical plasmas such as sawtooth crashes in tokamaks [1], geomagnetic substorms in Earth’s magnetosphere and solar flares [2]. In the 1990’s, the application of reconnection heating was pioneered in TS-3 and START, with significant ion heating of up to ~ 200eV and several high beta records for spherical tokamak [3-5]. In the last three decades, the energy conversion mechanism was investigated in a number of experiments: MRX [6], SSX [7], VTF [8], TS-4 [9], UTST [10], C-2U [11] and MAST [12, 13]. For all of the laboratory experiments, following common characteristics have been reported: (i) magnetic reconnection heats ions downstream and electrons around the X-point where magnetic field lines reconnect [13, 14], (ii) ions are heated by the thermalization of flow energy of reconnection outflow jet [15] while electrons gain energy mostly by Ohmic dissipation of current sheet [14], (iii) most of the heating energy goes to ions and electron heating is small [16, 17] (ions are heated globally but electron heating is localized area near X-point) and (iv) achieved maximum reconnection heating rate depends on the amplitude of reconnecting component of magnetic field: Brec (Bp for tokamak) [18]. Based on the characteristics, significant plasma heating over 100eV was demonstrated in TS-3 [3], START [19], C-2U [20] and MAST [21]. The high field merging experiment in MAST documented ~ 1keV of global ion heating and bulk electron heating upto hundreds of eV through ion- electron energy relaxation [21-23], successfully exceeding the radiation barrier of low-Z impurities to achieve the duration time over 100ms in the solenoid free startup [18, 23]. As a promising startup scenario for spherical tokamak, the high field merging experiment in MAST also achieved successful connection with other additional heating by NBI and solenoid (hybrid startup scenario) to establish H-mode and longer flat-top plasma current (typically hundreds of milliseconds) [18, 23, 24]. In the MAST merging experiments, which typically operated in high guide field condition Bt/Brec > 3 with higher toroidal magnetic field of Bt ~ 0.6T and Brec ~ 0.1T [25], better toroidal plasma confinement of ion heating after merging is a key to connect the high temperature merging plasma startup to long pulse scenario [13, 18]. However in MAST, due to the absence of in-plane poloidal field measurements during reconnection, investigation of the detailed heating/transport mechanism were not possible. As a post MAST projects, now further upgrade projects were started in ST40 (Bt ~ 3T and Brec > 0.2T: higher than MAST) by Tokamak Energy Ltd. [26] and TS-U (Brec > 0.1T with MAST-like high resolution diagnostics) by univ. Tokyo [27]. In order to investigate further upgraded scenario of tokamak merging with high guide field (Bt > 3Brec), detailed investigations of ion heating/transport process have been done in TS-3 and TS-3U (TS-6). This paper addresses the highlights of the recent results from our laboratory experiments: the new findings of fine structure formation by reconnection heating and full-2D imaging measurement of in-plane heat transport processes. 1 IAEA-CN-EX/P3-22 2. MERGING PLASMA STARTUP OF SPHERICAL TOKAMAK IN TS-3 Figure 1 shows typical features of merging plasma startup in TS-3 [14, 28]. Magnetic reconnection is driven by PF coil current IPF [kA∙turn] and the two plasma rings at the top and bottom of the device (t = 70s) merge together and forms a spherical tokamak after merging (t = 90s) as shown in the high speed camera images and 2 poloidal flux profile. Toroidal current density jt [MA/m ] has opposite polarity around the X-point (current sheet) during magnetic reconnection (t = 76s) and the fast camera detects toroidally ring-like structure where current sheet exists [29]. Ion temperature starts to increase around those phase and forms double peak structure at r ~ 0.15m and r ~ 0.25m where reconnection outflow jets dissipate [14, 16, 28]. FIG. 1. Typical features of merging plasma startup of spherical tokamak in TS-3. Magnetic reconnection is driven by two merging driving coil current (IPF) and it forms toroidally ring-like bright struture around the midplane where the X-point exists. Ion temperature increases during the merging phase and typically forms double peak structure after merging. 3. FINE STRUCTURE FORMATION OF RECONNECTION HEATING Figure 2 shows 2D ion temperature profile measured by a new 96CH 2D ion Doppler tomography which was upgraded from the previous 35CH system as a TS-U project [30, 31]. As illustrated in Fig.1 (a), it spans radially 16CH and axially 6CH (r-z: 16 × 6) to resolve both the detailed structure around the X-point and global profile downstream. During the characteristic three time frames within 10s at t = 70, 75 and 80s (before, during and after merging), ion temperature starts to increase and forms characteristics heating profile. During reconnection at t = 75s, ion temperature increases around the X-point as well as downstream of outflow jet; while after merging at t = 80s, high Ti region downstream aligned with closed flux surface of tokamak configuration. Figure 3 highlights those two characteristic time frames during reconnection (acceleration phase) and after merging (transport/confinement phase). During merging (phase 1), ion temperature profile is affected by the accelerating effect of guide field reconnection as Ti profile around the X-point changes.