Spontaneous Formation of Plasmoid during Early Magnetic Reconnection Phase of Two Merging Tokamaks

Abstract

A new type of spontaneous formation of plasmoid is observed in our toroidal plasma merging experiment on TS‐6 using a newly developed high‐resolution magnetic probe array, which has axially gyro‐scale spatial resolution (ΔZ ∼ 1.1 cm). In the axially elongated new machine, the plasmoid is formed near the mid‐plane with a configuration of two X‐points in the early phase of magnetic reconnection when the initial two flux tubes (merging tokamaks) are still in their respective formation region. The plasmoid is then merged with the major upstream plasmas at the two X‐points, which finally triggers the increment of reconnection speed. © 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Global ion heating/transport during merging spherical tokamak formation

“…As reported in [26], the new experiment TS-6 enables flexible diagnostics access for full-2D imaging measurement both for in situ probes [41] and optical diagnostics [40]. Figure 3 shows the highlight of experimentally measured full-2D (r − z) ion temperature profile during plasma merging (B rec ∼ 0.02 T and B t ∼ 0.1 T at r ∼ 0.2 m): reconnection heating/transport model (left), measured 2D ion temperature profile T i in three major time frames at t = 65 μs, 75 μs and 85 μs (before, during and after merging) with poloidal flux contour lines (middle) and 1D vertical profile of radially averaged T i at r ∼ 0.12 ± 0.02 m at each z position (right).…”

“…Figure 1 illustrates typical plasma scenario in the TS-6 ST [26,40,41]. TS-6 has a cylindrical vacuum vessel (φ 0.75 m × 1.44 m), center stuck with φ0.11 m (slim-CS (30 turns × 2) and TF coil (12 rods)), two pairs of poloidal field (PF1 (four turns) and PF2 (three turns)) coils inside the chamber and a couple of equilibrium field coils (EF: 234 turns) at the top and bottom of the device as shown in figure 1(a).…”

“…Although MAST experiments provided the finest heating profile as a Frontier of reconnection heating physics using high resolution Thomson scattering and ion Doppler tomography [39], further detailed investigation with the information of magnetics was not available in MAST due to the absence of proper magnetic diagnostics to visualizes the double-axis configuration of merging flux tubes during reconnection. To explore the new Frontier in reconnection heating, we made further diagnostics upgrade in TS-6: 96CH/320CH ion Doppler tomography [26,40] and ∼200CH magnetic probes [41] with gyro-scale spatial resolution (Δr ∼ 10 mm and Δz ∼ 20 mm, and Δr = 20 ∼ 60 mm and Δz = 10 ∼ 25 mm, respectively) for clearer 2D imaging measurement. Here we report our new finding from the full-2D ultra high-resolution imaging measurement of ion heating/transport process during merging/reconnection startup.…”

MHD-FiT: MHD-based dynamic reconstruction of tokamak plasma configuration

“…These pick-up coils have a cylindrical hollow structure with 300 turns and dimensions of 5 mm in length, 1.9 mm outer diameter, and 1 mm inner diameter. Additionally, there are pickup coils for measuring the toroidal component of the magnetic field (B t ), which have an elliptical structure with 200 turns, 5 mm length, 2 mm outer diameter, and 1 mm inner diameter [15,29].…”