7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Transformation of energy conversion by active control of in-plane electric fieldin high-guide-field magnetic reconnection
Michiaki Inomoto1, Taiju Suzuki2, Hiroshi Tanabe1, Yasushi Ono1
1
Graduate School of Frontier Sciences, the University of Tokyo,
2
Graduate School of Engineering, the University of Tokyoe-mail (speaker): inomoto@k.u-tokyo.ac.jp
Merging formation [1-3] is one of the candidates forcenter-solenoid-free start-up scheme of high-beta
spherical tokamak (ST) plasma. Two initial STs areformed inductively by using outer poloidal field coils
and merge through magnetic reconnection that provides
efficient conversion from magnetic to kinetic/thermalenergy. Dominant heating source during magnetic
reconnection is the ion outflow with ExB drift velocity.
Figure 1 (a) shows the magnetic flux surfaces during
merging of two ST plasmas. The electric and magnetic
field components in the reconnection downstream regionis illustrated in Fig. 1 (b). Here, the electric field consistsof toroidal inductive field and dominant poloidal static
field generated by the charge separation [4]. Thus it isexpected that the motion of charged particles isconverted into parallel acceleration when the static
poloidal electric field is artificially suppressed. In thisstudy, a control system of the static electric field by using
multiple electrodes (see Fig. 1 (a)) and fast switching
devices was installed on the UTST experiment todemonstrate the transformation of energy conversion
process in magnetic reconnection.
Figure 2 shows the evolution of radial profiles of (a)toroidal and (b) axial electric field in open condition withelectrodes disconnected. The inductive toroidal electricfield up to 100 V/m was induced in the wide area of thedownstream region (R<0.35 m), and the static axial
electric field reached 6000 V/m in the downstream
region due to the high toroidal magnetic field. In theclose conditions with electrodes connected, the static
poloidal electric field was significantly suppressed asshown in Fig. 2 (d). The radial profiles of the axial andparallel electric field at t=9.502 ms are shown in Fig. 3(a)(b). The axial static electric field was largely
suppressed about to half in the downstream region. Theparallel electric field shows some increase near theX-point (R>0.25 m). This increase in the parallel electricfield is considered to enhance the electron acceleration asindicated by the SXR emission profile shown in Fig. 3(c). Note that this enhancement was not observed in thefar downstream region (R<0.3 m) because the parallel
electric field in the open condition changed its sign
because of the excessive static electric field. This parallel
electric field reversal is possibly required to form a
toroidal current density profile suitable for the merged
ST equilibrium.
References[1] Y. Ono, et al. 2012 Plasma Phys. Control. Fusion 54,124039[2] M. Inomoto, et al. 2015 Nucl. Fusion 55, 033013[3] M. Gryaznevich, et al. 2022 Nucl. Fusion 62, 042008[4] M. Inomoto, et al., 2019 Nucl. Fusion 59, 086040

Figure 1 (a) Magnetic flux surface during merging
and (b) cartoon of the magnetic and electric fieldcomponents in the reconnection downstream region.

Figure 2 Spatiotemporal plot of (a,c) toroidal(inductive) electric field and (b,d) poloidal (static)
electric field measired at z=0 for open condition(a-b) and for close condition (c-d).

Figure 3 Radial profile of (a) in-plane electric filedm(b) parallel electric field and (c) SXR intensity foropen and close conditions.