Investigation of merging/reconnection heating during solenoid-free startup of plasmas in the MAST Spherical Tokamak

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

doi:10.1088/1741-4326/aa6608

Cited by 25 publications

(26 citation statements)

References 44 publications

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“…However, the size of the centersolenoid coil must be reduced owing to the small space near the central axis. One center-solenoid-free start-up method is the merging formation that has been developed in the TS-3/4 [2], UTST [3], and START/MAST [4] devices. In the merging formation, two STs are inductively formed using poloidal-field coils and are then merged into one ST via magnetic reconnection [5] of the poloidal fields of the two initial STs.…”

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confidence: 99%

Separated Double-Current Layers in a High-Guide-Field Reconnection Experiment

Kondo

Inomoto

Xiaohong

et al. 2017

Plasma and Fusion Research

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Magnetic reconnection in the presence of a high-guide-field is utilized to heat the plasma in the merging startup of a spherical tokamak configuration. The reconnection current layer between two spherical tokamaks was gradually compressed, and a quasi-steady current-sheet thickness was obtained. During the compression phase, the current layer split transiently into two separated layers, which provided a flattened magnetic-field-region near the X-point. This modification may affect the electron-energization mechanism in the merging start-up of a spherical tokamak in a high-guide-field. The spherical tokamak (ST) concept [1] provides attractive features such as high toroidal beta and a high bootstrap-current fraction. However, the size of the centersolenoid coil must be reduced owing to the small space near the central axis. One center-solenoid-free start-up method is the merging formation that has been developed in the TS-3/4 [2], UTST [3], and START/MAST [4] devices. In the merging formation, two STs are inductively formed using poloidal-field coils and are then merged into one ST via magnetic reconnection [5] of the poloidal fields of the two initial STs. This rapidly converts magnetic energy to plasma kinetic/thermal energy.In the merging start-up of an ST, a strong toroidal magnetic field, which is perpendicular to the reconnecting magnetic field, acts as a "guide field," magnetizing the plasma particles even at the reconnection X-point and changing the global/microscopic behavior of reconnection [6]. A unique feature of guide-field-assisted reconnection is that electrons are effectively accelerated along the guide field via the reconnection electric field. The maximum energy attained by the electrons is affected both by the reconnection electric field and the ratio of the toroidal (guide) magnetic field to the poloidal (reconnection) magnetic field [7,8]. In this study, we investigate the detailed structure of the reconnection magnetic field around the Xpoint of a high-guide-field reconnection event in the UTST merging experiment. Figure 1 shows the magnetic flux surfaces observed at an early phase of an ST merging experiment via a 2D pickup-coil array whose locations are indicated by the small "x"s in the figure. The time evolutions of the reauthor's e-mail: kondo@ts.k.u-tokyo.ac.jp connected magnetic flux and of the reconnection electric field are shown in Figs. 2 (a) and (b). The plasma merging is completed within ∼60 µs because the poloidal magnetic flux contained in each of the initial STs is limited. The reconnection electric field reached ∼100 V/m in the middle of the merging period. This high electric field accelerates the electrons in the region wherein the poloidal magnetic field is much weaker than that of the toroidal magnetic field. The detailed structure of the reconnection magnetic field B r was measured via a 1D pickup-coil array at intervals of 1 cm, as shown by the small red circles in Fig. 1. The time evolution of the current-layer thickness obtained using the fitting of the Harris-type fu...

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confidence: 99%

Separated Double-Current Layers in a High-Guide-Field Reconnection Experiment

Kondo

Inomoto

Xiaohong

et al. 2017

Plasma and Fusion Research

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“…During merging/reconnection, it typically has double magnetic axes at the top and bottom of the device and a X-point around the midplane. The radial component of poloidal magnetic field B r is one of the most important components of the reconnecting magnetic field B rec which determines the amplitude of reconnection heating and final temperature obtained by merging plasma startup [22,27]. Toroidal current density j t typically has opposite polarity around Xpoint, forming a current sheet [22,25], and the structure disappears after merging.…”

Section: Merging Plasma Startup Of Spheri-mentioning

confidence: 99%

“…electron heating is small [25,26] (ions are heated globally but electron heating is localized around X-point); and (iv) the maximum heating rate depends on the amplitude of the reconnecting component of magnetic field: B rec (B p for tokamaks) [27]. Significant plasma heating to temperature exceeding 100 eV was demonstrated in many merging experiments such as TS-3 [3], START [28], C-2U [16] and MAST [29].…”

Section: Introductionmentioning

confidence: 99%

“…The high field merging/reconnection experiment in MAST resulted in ion temperatures of ∼1 keV and bulk electron heating to temperatures upto hundreds of eV through ion-electron energy relaxation [29][30][31], successfully exceeding radiation losses due to low-Z impurities to achieve plasma duration times of over 100 ms in the central solenoid (CS)-free startup [27,31]. As a promising startup scenario for spherical tokamak, the high field merging experiment in MAST was also successfully combined with additional heating by NBI and a solenoid (hybrid startup scenario) to establish H-mode and higher/longer flat-top plasma current (typically hundreds of milliseconds) [27,31,32]. In the MAST merging experiments, which typically operated in high guide field conditions B t /B rec > 3 with B t ∼ 0.6 T and B rec ∼ 0.1 T [33], better ion energy confinement after merging helps to combine high temperature merging plasma startup with long pulse scenarios.…”

Section: Introductionmentioning

confidence: 99%

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Recent Progress in High Resolution 2D Imaging Measurements of Reconnection Heating during Merging Plasma Startup in TS-3

Tanabe

Cao

Tanaka

et al. 2019

Plasma and Fusion Research

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We present recent results from high resolution 2D imaging measurement of reconnection heating during central solenoid (CS)-free merging startup of spherical tokamak plasmas in TS-3 using an ultra-high resolution 2D ion Doppler tomography and in-situ 2D magnetic probe arrays. The new high-resolution 2D ion Doppler tomography diagnostic has successfully resolved the formation of fine structure during magnetic reconnection and it has been found that magnetic reconnection increases the ion temperature inside a current sheet as well as in the downstream region of an outflow jet. The maximum ion temperature is obtained during a current sheet ejection event and the double peak structure of ion heating becomes clearer after the end of reconnection. The maximum ion heating increases with the reconnecting magnetic field. The high temperature region in the downstream typically propagates vertically along a closed flux surface and MAST-like fine structure formation has successfully been reproduced in this laboratory experiment for the first time.

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“…21(b)); (iii) the larger toroidal magnetic field produces the higher electron temperature around the X−point. While, magnetic filed has negligible effect on the ion heating; (iv) the electron and ion temperatures form triple peaks during the relaxation phase 33,34 . F. Guo reported the results on the non-thermal particle acceleration in the magnetic reconnection, using the particle-in-cell (PIC) simulation technique.…”

Section: Reconnection and Particle Accelerationmentioning

confidence: 99%

Summary of the fundamental plasma physics session in the first AAPPS-DPP conference

Hao

Diamond

2019

Rev. Mod. Plasma Phys.

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Understanding of fundamental plasma physics contributes to progress in plasma science in many fields, such as magnetic confinement fusion, solar/space physics and application of plasma techniques. Intersecting common fundamental problems stimulate cross-fertilization of different topical areas. This paper summarizes highlights of progress reported in the Fundamental Plasma Physics Session of the 1st conference of Association of Asia Pacific Physical Societies-Division of plasma physics (AAPPS-DPP). Progress in the following fields is reviewed: (i) basic plasma theory; (ii) self-organization; (iii) reconnection and particle acceleration; (iv) magneto-hydrodynamics (MHD) instabilities and energetic particle physics; (v) magnetic confinement physics fundamentals; and (vi) exploiting fundamentals for performance.

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Investigation of merging/reconnection heating during solenoid-free startup of plasmas in the MAST Spherical Tokamak

Cited by 25 publications

References 44 publications

Separated Double-Current Layers in a High-Guide-Field Reconnection Experiment

Separated Double-Current Layers in a High-Guide-Field Reconnection Experiment

Recent Progress in High Resolution 2D Imaging Measurements of Reconnection Heating during Merging Plasma Startup in TS-3

Summary of the fundamental plasma physics session in the first AAPPS-DPP conference

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