Speaker
Masayoshi Nagata
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1043.pdf
Plasmoid reconnection in transient coaxial helicity injection on HIST
M. Nagata, A. Fujita, Y. Ibaraki, Y. Kikuchi and N. Fukumoto
Graduate School of Engineering, University of Hyogo, Himeji, Hyogo 671-2280, Japan
The Spherical Torus (ST) is a promising candidate for an advanced fusion reactor due to the
compactness. Elimination of the central solenoid coil to allow an approach to lower aspect ratio
configurations requires for the non-inductive plasma start-up and the transient coaxial helicity
injection (T-CHI) is a leading candidate for its method. One of the most important issues in
T-CHI is whether it can establish a current sufficient in closed flux surfaces for succeeding
current drive and heating. Understanding the flux closure during the start-up process is the
primary purpose of the T-CHI experiment on the Helicity Injected Spherical Torus (HIST:
R=0.30 m, a=0.24 m, A=1.25) [1]. Also, the CHI provides a good platform for pursuing MHD
relaxation and magnetic reconnection physics. Magnetic reconnection is an essential element in
understanding of self-organization phenomena such as sawtooth oscillations and Taylor
relaxation in fusion plasmas and also eruptive mass ejection of solar flares in astrophysical
plasmas. To prove the flux closure issue in the CHI start-up, we have investigated the fast
magnetic reconnection driven by multiple plasmoids [2].
Here, we report that in the formation process of T-CHI start-up plasma with the plasmoid
reconnection, (i) two or three small-size plasmoids are generated in elongated toroidal current
sheet with the full width δ ~0.05 m, a long length L=0.6-1 m and a high density ne=
0.3-2x1020 m-3, (ii) one of plasmoids becomes a large-scale closed flux surface during the
decay phase, and (iii) in the He discharge, the reconnection rate on the mid-plane is slower
than that in the H2 discharge and the self-generated toroidal magnetic energy (poloidal current)
increases rapidly, leading to the formation of a doublet-type magnetic configuration. The
observation of the regular oscillations of the reconnecting magnetic field Bz, ne and the axial
out flow Vz in the current sheet provides a strong evidence of the plasmoid reconnection.
These findings could verify that the plasmoid reconnection in the elongated current layer in
the presence of the strong toroidal field allows the fast flux closure in the T-CHI.
References
[1] M. Nagata, et al., Phys. Plasmas 10, 2932 (2003).
[2] F. Ebrahimi and R. Raman, Phys. Rev. Lett. 114, 205003 (2015).
