Speaker
Description
Axial merging of two torus plasmas is utilized to form a high ion temperature plasma in fusion devices through energy conversion by magnetic reconnection. In a spherical tokamak merging experiment [1] in which the guide magnetic field is more than ten times larger than the reconnecting magnetic field, the inductive reconnection electric field is almost parallel to the magnetic field particularly in the inboard-side downstream region where larger guide field is applied. Thus large in-plane electrostatic field is required to sustain the steady plasma outflow motion, but the self-generated electrostatic field does not always balance with the inductive electric field to make the total electric field strictly perpendicular to the total magnetic field, resulting in a residual parallel electric field that drastically change the plasma dynamics.
Experimental results from the soft X-ray (SXR) imaging diagnostic in the UTST device [2] revealed that an intense SXR emission was observed in the inboard-side downstream region in the early phase of the plasma merging, suggesting that the electrons are accelerated by the transient parallel electric field [3]. Simultaneous measurement of inductive and electrostatic fields revealed that the self-generated electrostatic field in the latter merging period is even larger than that just cancels out the parallel component of the inductive electric field [4]. The excess of the electrostatic field contributes to shape change of the in-plane magnetic field because the reconnection current in the toroidal direction needs to be reversed in order to form a final closed magnetic surface.
The in-plane electrostatic field is moderately suppressed by using small electrode pairs inserted in the downstream region, resulting in a late current reversal as well as an enhancement of the parallel electric field. These experimental results suggest that the electric field condition in the guide-field reconnection in the laboratory experiment could be easily controlled to modify the reconnection dynamics of energy conversion and macroscopic change in magnetic structure.
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. Inomoto, et al. 2019 Nucl. Fusion 59, 086040
[4] M. Inomoto, et al. 2021 Nucl. Fusion 61, 116069
