Laboratory study of diffusion region with electron energization during high guide field reconnection

Abstract

Floating potential profile was measured around the X-point during high guide field reconnection in UTST merging experiment where the ratio of guide field (Bg) to reconnecting magnetic field (Brec) is Bg/Brec>10. Floating potential measurement revealed that a quadrupole structure of electric potential is formed around the X-point during the fast reconnection phase due to the polarization by inductive electric field. Also, our floating potential measurement revealed the existence of parallel electric field in the vicinity of the X-point. While field-aligned components of inductive electric field (E∥ind) and electrostatic electric field (E∥es) cancel out with each other away from the X-point, E∥ind exceeds E∥es around the X-point, indicating the deviation from ideal MHD criterion within the region. The diffusion region extends in the outflow region and the scale length of region is an order of ion skin depth, which is quite different from the VTF experiment result. Based on the measured magnetic field and electric field profile, our particle trajectory analysis indicates that fast electrons with energies over 300 eV are produced within 1 μs around the X-point in the non-ideal MHD region. These results indicate that production of fast electrons or electron heating are expected to be observed in the vicinity of the X-point.

Recent progress of magnetic reconnection research in the MAST spherical tokamak

“…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…”

Generation of Energetic Electrons during Spherical Tokamak Merging in UTST

“…(1), (2), and (3), respectively, as follows: Because the poloidal magnetic field reconnects during plasma merging in the UTST experiment, the induced reconnection electric field is in the toroidal direction. Therefore, in the ST merging experiment, in which a strong toroidal field is applied by an external coil, the reconnection electric field becomes field aligned near the X-point where the poloidal field is negligible [14]. The typical ratio of the toroidal (guide) field to poloidal (reconnection) field is 10 -20 in the UTST experiment, which is much higher than those in previous TS-3/4 experiments or numerical studies.…”

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

“…When the polarity of I PF is reversed, the negative current contributes to detachment of the plasma rings from PF coils and pushes the two plasma tori vertically toward midplane. In addition to the rapid increment of plasma current, magnetic reconnection occurs with characteristic bright structure [43][44][45] around the X-point at t ∼ 75 µs. Merging finishes roughly around t ∼ 80 µs and finally a spherical tokamak configuration is formed with additional shaping by separation coil current I sep .…”