Plasma exhaust and density control in tokamak fusion experiments with neutral beam or ICRF auxiliary heating

Abstract

Particle exhaust studies have been carried out with the pump limiter ALT-II in the TEXTOR tokamak, under ohmic conditions as well as with NBI and with ICRF auxiliary heating, and the pumping effectiveness is shown to meet the requirements for a fusion reactor. Quantitative measurements of Dα emission, made with a CCD camera, have been used to determine the particle efflux from the plasma. Roughly one third of the Dα emission occurs in a diffuse `halo' that surrounds the limiter belt. The particle confinement time is less than the energy confinement time by a factor of typically 4. Modelling in 2-D of plasma and neutral flows in the TEXTOR boundary has been performed. The source of D+ ions can be related to the Dα emission by a factor that is found to depend on the location of the emission and on the discharge density. The predicted total Dα emission agrees with the measurements within a factor of about 2. Pumping of ALT-II allows for density control; with NBI, the density can be increased well beyond the ohmic limit without the discharge ending in disruption. The plasma particle efflux and the pumped flux both increase with density as well as with heating power. The exhaust efficiency is typically ∼2%, with the highest values observed in high density NBI discharges. Higher exhaust rates are observed with NBI than with ICRF. Plasma and neutral flows in the ALT-II scoops have been simulated, making use of a simple plasma model. The scoop may be viewed as a non-linear amplifier of the plasma particle flux; the amplification is found to range from about 2 to 3 for most cases. Flow reversal in the scoop is found in some of the NBI cases and particularly in the highest density case.

Scaling of plasma turbulence suppression with velocity shear

“…The main reciprocating probe is located at the outer midplane of the tokamak and the second probe is at the top. TEXTOR edge plasmas are toroidally symmetric due to the ALT-II belt limiter [21]. The data for the turbulent measurements are digitized at 1 MHz with a 10 bit digitizer and filtered by low pass 500 kHz anti-aliasing filters.…”

Enhanced particle confinement and turbulence reduction due toE Bshear in the TEXTOR tokamak

“…The H α emission in TEXTOR during polarization has been analysed with a calibrated CCD camera [41] observing the ALT-II limiter. The camera signal is integrated over a large area, including an entire limiter blade and parts of the wall, to increase the signal to noise ratio and to account for the contribution of extended poloidal 'halos', which have been shown to contain about 50% of the signal in low plasma density discharges such as these.…”

Particle collection with the ergodic divertor of Tore Supra

“…For this purpose, two systems have been developed to manage the plasma-wall interaction, namely limiters and divertors. Whereas limiters have demonstrated extensively their ability to control the density [1][2][3][4][5] they suffer from the presence of a leading edge, upon which the heat load can reach intolerable values during high power discharges and from an insufficient decoupling between the plasma core and the edge, which can lead to significant impurity penetration. This decoupling is achieved with axisymmetric poloidal divertors and has led to good results in terms of confinement, impurity screening and power deposition [6].…”