Generation of Energetic Electrons during Spherical Tokamak Merging in UTST

Ushiki, Tomohiko; Inomoto, Michiaki; Yamasaki, K.; Xiaohong, Guo; Sugawara, T.; Matsuyama, Kiminori; Koguchi, Haruhisa; Yamada, Teppei

doi:10.1585/pfr.11.2402100

Cited by 5 publications

(6 citation statements)

References 16 publications

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“…Figure 4 shows 2D electron temperature and density profiles during discharges 21374-21380 (P6 coils are used to shift the vertical position of Thomson scattering measurement 50 ) after the fast reconnection event (1:2 m < r sep < 1:3 m for the vertical scan shots at t ¼ 8; 9; 10; 11; 12 ms). The electron temperature profile forms a characteristic peaked structure around the X-point where localized electron acceleration and heating by the parallel electric field are reported in the high guide field laboratory merging (driven reconnection) experiment in UTST (University of Tokyo Spherical Tokamak), [59][60][61] while the electron density increases downstream. In contrast to the no guide field experiment in MRX 27,28 where electron energy gain is quickly transported downstream, the higher toroidal field in MAST strongly…”

Section: -2mentioning

confidence: 91%

Recent progress of magnetic reconnection research in the MAST spherical tokamak

Tanabe

Yamada²,

Watanabe

et al. 2017

Physics of Plasmas

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Section: -2mentioning

confidence: 91%

Recent progress of magnetic reconnection research in the MAST spherical tokamak

Tanabe

Yamada²,

Watanabe

et al. 2017

Physics of Plasmas

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“…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.…”

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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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Section: Mhd Simulation Of Merging Fueling Methods Used For St Plasmamentioning

confidence: 99%

“…Additionally, Tri-Alpha Energy Inc. has succeeded in generating field-reversed configuration (FRC), with good confinement performance, by merging two FRC plasmas [5,6]. In recent years, merging experiments of spherical torus (ST) plasmas have also been carried out at UTST, and reports have been submitted on the generation of high energy electrons during the merging process [7].The merging fueling method [1] aims to supply particles to large plasma via the merging of two ST plasmas with different sizes. The purpose of this preliminary research was to analyze the process of axially translating and colliding the small secondary plasma pushed by the magnetic pressure difference to the large plasma by using the three-dimensional (3-D) MHD simulation, and to determine research tasks in the merging fueling method.…”

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MHD Simulation of Merging Fueling Method Used for ST Plasma

Koike

Takahashi

Mizuguchi

et al. 2018

Plasma and Fusion Research

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Three-dimensional resistive MHD simulation was carried out in order to develop a method to fuel the plasma core by merging two Spherical Torus (ST) plasmas. A relatively small ST plasma was translated into the axial direction by external magnetic field control and made to collide with the larger ST. In this calculation, ballooning instability occurred in both plasmas during translation, while the magnetic surface was observed to collapse from the Poincare plot of the magnetic lines. It was observed that the magnetic lines in the core regions of the two STs were connected to each other and that the merging was completed. Keywords: advanced fusion, spherical torus plasma, fueling, MHD simulation, merging, magnetic reconnection DOI: 10.1585/pfr.13.1203008As ITER construction progresses, the establishment of fueling technology to fusion burning plasmas is still a key unresolved issue. Neutral particles injected as fuel are ionized in the periphery and have difficulty reaching the core under the high temperature conditions of 10 keV or more, such as those of burning plasma. For this reason, it is urgent to develop a method of supplying particles to the burning plasma core. Under these circumstances, the concept of the merging fueling method is proposed [1].Progress in plasma merging experiments via the magnetic reconnection process was taken as the background to this proposal [1]. Historically, merging experiments of spheromak have been carried out at TS-3/4 [2, 3], SSX [4], and others. Additionally, Tri-Alpha Energy Inc. has succeeded in generating field-reversed configuration (FRC), with good confinement performance, by merging two FRC plasmas [5,6]. In recent years, merging experiments of spherical torus (ST) plasmas have also been carried out at UTST, and reports have been submitted on the generation of high energy electrons during the merging process [7].The merging fueling method [1] aims to supply particles to large plasma via the merging of two ST plasmas with different sizes. The purpose of this preliminary research was to analyze the process of axially translating and colliding the small secondary plasma pushed by the magnetic pressure difference to the large plasma by using the three-dimensional (3-D) MHD simulation, and to determine research tasks in the merging fueling method. There are several reports on the reproduction of the plasma merging phenomenon by MHD simulation [8][9][10]. However, the 3-D simulation of two large and small ST collision phenomena has not been carried out so far.In this paper, we clarify the change of the macroscopic author's e-mail: t09306025@gunma-u.ac.jp plasma structure, especially in the translation/collision process in the category of the ordinary MHD model. Additionally, by considering the development of the fueling method, it was necessary to simulate and predict whether the particles in the smaller secondary plasma could reach the larger main plasma core. For this reason, a particle trajectory calculation was performed based on the MHD simulation. Thus, it was clarified...

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Generation of Energetic Electrons during Spherical Tokamak Merging in UTST

Cited by 5 publications

References 16 publications

Recent progress of magnetic reconnection research in the MAST spherical tokamak

Recent progress of magnetic reconnection research in the MAST spherical tokamak

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

MHD Simulation of Merging Fueling Method Used for ST Plasma

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