MAGNETIC-CONFIGURATION SCANS IN H-1 HELIAC

The results of the experimental study of the magnetic configurations i n the H-1 heliac are presented. The shape of the flux surfaces and the rotational transform in H-1 can be controlled by varying external coi l currents. Electron beam magnetic mapping has been performed to show the existence of closed nested flux surfaces and to observe the effect of small errors in coil alignment on the vacuum magnetic structure in H-1. Langmuir probes have been used to study the electron density pro files in a current-free collisional RE-sustained plasma (n(e) less tha n or equal to 4x10(12)cm(-3), T-e less than or equal to 15 eV). In sta ndard magnetic configuration and for the present moderate RF power lev els, the highest central density is achieved at rather low magnetic fi eld (0.07 T). This regime is characterised by peaked density profiles that appear to have a maximum coincident with the position of the vacu um magnetic axis. When a lowest-order m = 1, n = 1 resonance is introd uced inside the outermost magnetic surface a strong asymmetry in both the vacuum magnetic structure and the plasma density profiles is obser ved. We observed low frequency (2-3 kHz) density fluctuations having l ow radial mode numbers and internal parallel plasma current localised in the regions of highest density gradient. These fluctuations are eff ectively suppressed by an increase of the magnetic field.