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

Abstract

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...

Measurement of the Local Current Density Inside a Current Sheet Using a Rogowski Coil Array on UTST Merging Plasmas

“…Thus, the axial width is estimated to be around 20 mm. This estimate agrees well with the current sheet thickness obtained from a onedimensional pickup coil array with a spacing of 10 mm [9].…”