Study of ion sheath expansion and anisotropy of the electron parallel energy distribution in the CASTOR tokamak

Abstract

A novel diagnostic, the tunnel probe, is used to investigate the edge plasma of the CASTOR tokamak. Comparison with conventional small, cylindrical Langmuir probes (typical of those usually employed for turbulence measurements in magnetized plasmas) demonstrates the superiority of the tunnel probe. The collectors of the tunnel probe are concave, eliminating in theory all uncertainty of the effective collecting area, and thereby rendering the measurement of parallel ion current density, electron temperature, and floating potential more reliable. Two tunnel probes, mounted back-to-back in a Mach probe arrangement, are used to investigate directional asymmetries of the plasma parameters. The tunnel probes are used as standard Langmuir probes by applying voltage sweeps simultaneously to all their internal conductors. The measured electron temperature is higher on the electron side than on the ion side, and the floating potential is lower. The observed asymmetries, measured at low density and collisionality in CASTOR, could be consistent with a hot tail of non-thermal electrons flowing in the counter-current direction. The ratio of ion saturation currents to the internal conductors of the tunnel probe provides a second independent measurement of electron temperature whose directional asymmetry is less pronounced, in agreement with recent theoretical predictions that tunnel probes should be less sensitive to non-thermal electrons than Langmuir probes in certain conditions.

Bi-Maxwellian electron energy distribution function in the vicinity of the last closed flux surface in fusion plasma

“…Additional error in estimation of the parallel power flux density by using equation 3can be caused by uncertainties in evaluating the ion saturation current density sat J . As it was shown [12] in the case of small size probe the ion current at large negative probe potential do not saturate due to the sheath expansion, but increases almost linearly. This effect is less pronounced for bigger cylindrical probes with diameter ~3-5 mm.…”

“…In fusion plasmas, the assumption for a Maxwellian EEDF is generally valid. However, experimental evidence does exist suggesting non-Maxwellian distributions in tokamak SOL plasmas [12]. In ASDEX, the electron temperatures measured by a Langmuir probe assuming a Maxwellian EEDF are about twice as high as those determined by Thomson scattering [13,14].…”

Novel Technique for Direct Measurement of the Plasma Diffusion Coefficient in Magnetized Plasma

“…For each magnetic field value, the probe was placed at three different radial positions: two inside the SOL (r = 75 mm and r = 85 mm) and one just beyond the Last Closed Flux Surface, inside the confined plasma region (r = 65 mm). T e ranges from 10 eV at 90 mm to 42 eV at 57 mm [5]. For each radial position of the probe, an h scan was performed on a shot-by-shot basis.…”

Electric Probes in Tokamaks: Experience in Tore Supra

“…This means that the effective collecting area of most convex probes used in tokamaks can be highly uncertain, perhaps depending sensitively on the very quantities one wants to measure (and even ones that are not measured, such as T i ). It was demonstrated in [21] that the collecting area of small probes can be more than twice their geometrical projection along B. Many theories for ion collection by convex probes in plasmas whose degree of magnetization varies from zero to infinite have been elaborated.…”

“…Plasma flows into the open orifice and the ion flux is distributed between the tunnel and the back plate. The favourable properties of the tunnel probe were demonstrated during dedicated experiments in the CASTOR tokamak [21]. The original motivation for developing the tunnel probe was to invent a method to make measurements of electron temperature fluctuations.…”