Spontaneous Formation of Closed Toroidal Flux Surfaces
by Electron Cyclotron Heating

T. Maekawa, H. Tanaka, T. Yoshinaga and M. Uchida 

Recently, the spherical tokamak (ST) concept attracts considerable interest
in the magnetic fusion experiments since it can realize high beta plasmas in
a compact shape. For future ST fusion plants, however, removing the central
Ohmic solenoid is considered to be a crucial step, since STs have only a
limited space at the center column of the devices to ensure the virtue of
low aspect ratio. Usually, toroidal currents for the Tokamak equilibrium are
generated and maintained by inductive voltage from flux swing of the central
solenoid. We need a non-inductive method. Electron cyclotron heating and
current drive (ECH/ECCD) is potentially an attractive candidate for this
purpose since plasma initiation, and current start-up and ramp-up might be
realized simultaneously. We have attempted ECH experiments in the Low Aspect
ratio Torus Experiment device at Kyoto University and found that toroidal
currents (6 kA) are generated by ECH alone under steady vertical fields (80
Gauss) and toroidal fields (450 Gauss), where plasma current increases
rapidly in the time scale of a few milliseconds. Thus, a closed field
configuration of toroidal flux surfaces is spontaneously generated from an
open field configuration before ECH.