Confinement studies of ECRH plasmas in a toroidal heliac

The SHEILA heliac is a toroidal device of major radius Ro=18.75 cm and mean minor radius a (last closed flux surface) of about 3.1 cm. 24 toroidal field coils displaced 2.5 cm about a poloidal ring coil (radius Ro) form a N=3 period helical axis stellarator. SHEILA has been converted to a flexible heliac by an additional l=1 helical winding about the ring coil. Considerable variation in the magnetic geometry can be obtained by adjusting the current ratio C=Ih/Ir between -0.16 to +0.25, equivalent to the range 0.55≥ι(0)≥1.86, where Ih and Ir are the currents in the helical winding and poloidal ring. The vacuum flux surfaces are generally bean-shaped in cross section, but change considerably with variations in the helical current. The |B| surfaces, however, remain roughly circular, concentric with the poloidal ring coil. Plasma formation by electron cyclotron resonance (ECR) depends critically on the position of the fundamental resonance surface, and also the second harmonic surface at low field strengths, relative to the plasma column and the launching antenna. In this paper we investigate the particle confinement properties of the heliac geometry as the rotational transform is varied using up to 2.5 kW peak 10 ms wide pulses of microwave ECR power at 2.45 GHz. Cool (Te=6-10eV) but highly ionized argon plasmas with very reproducible characteristics are generated by discrete dipole/monopole or helical antennae positioned close to the plasma surface. Depending on the magnetic geometry and field strength the antennae can launch either from the high field side (HFS) or the low field side (LFS) of the fundamental resonance surface. (author) 3 refs., 5 figs