An experimental investigation of the plasma properties of the edge region of the ISX-B tokamak using multiple langmuir probes

The method of decomposing the Fourier components of the double-probe characteristic has been extended to include higher-order terms in the expansion parameter V_applied/T_e and finite circuit resistance. The system uses a 10-kHz driving voltage, which, together with analog filtering of the current, allows measure-ments of electron temperature (2 eV < T_e < 40eV) and electron density (3 x 10⁹ cm^-3 n_e < 2 x 10 ¹³ cm^-3). Electron density calculations include corrections for magnetic field effects, probe potential, and finite sheath thickness. Fluctuation levels can be deduced from the frequency structure between the harmonics of the driving frequency. It has proven necessary to correct for these fluctuation levels in the calculation of both T_e and n_e. Diffusion coefficients are estimated from the decay lengths measured behind the limiter face, taking into account fluxes perpendicular to the wall and fluxes parallel to the limiter.

Experimental results obtained on ISX-B show that there are no significant temperature gradients along the field lines. However, variations of up to 50% are seen in the electron density, which may be due to the local recycling effects. Changes in local edge parameters when neutral beam heating is initiated are evident. In particular, the results show that the density decay lengths increase dramatically during the beam-heated phase of the discharge, implying an increase in particle diffusion. The direction of beam injection plays a minor role in determining particle confinement, especially if the recycling at the limiter surface changes with beam direction. Parameters in the scrape-off vary linearly with the central line-averaged electron density showing some evidence of saturation in the parallel particle flux at high densities. Parameters measured at the limiter demonstrate the V effects of plasma plugging. Differences between high and low energy confinement regimes are also evident at the plasma edge. Measurements made during fueling studies with the pumped limiter are used to determine qualitatively the exhaust efficiency of the limiter. Finally, data taken during pellet injection experiments help to determine the role of fast ion ablation in the scrape-off layer in beam-heated discharges.