G. Abel

Experimental demonstration of tokamak fuelling by compact toroid injection

The most promising concept for deep fuelling a reactor is by the injection of compact toroid (CT) plasmoids. The first results showing CT fuelling of a tokamak plasma, without any adverse perturbation to the tokamak discharge, are reported. The Compact Toroid Fueller (CTF) device was used to inject a CT-spheromak plasmoid into the TdeV tokamak. Following the CT penetration, the tokamak particle inventory increased by 16%, the loop voltage and the plasma current did not change, and there was no increase in magnetohydrodynamic (MHD) activity. The number of injected impurities was low and dominated by non-metallic elements. The plasma diamagnetic energy and the energy confinement time increased by more than 35%

Biasing of closed double null poloidal divertor plates on TdeV

Abstract Bias voltages applied to the divertor plates in TdeV (Tokamak de Varennes) permit fine control of several main plasma parameters, including the poloidal rotation velocity near the separatrix, microturbulence, the sawtooth period and the heat pulse propagation speed. Biasing also improves the divertor efficiency in either the top or bottom closed divertor chamber depending on polarity, in agreement with E × B drifts. Negative biasing reduces carbon and CO fluxes from the wall, the loop voltage and the X-ray emissivity, all indicative of decreasing impurity contamination.

Comparison of three boronization techniques in TdeV

Preparation of the internal walls of tokamaks by plasma enhanced chemical vapour deposition (PECVD) of boron containing films has now been implemented on several machines since its development on TEXTOR. More recently, such films were deposited on the internal walls of TdeV using not only this procedure but also two new approaches: solid target boronization (STB) which consisted in inserting a low-density boronized carbon-carbon (C-C) composite into the tokamak plasma and TMB fuelling where trimethylboron was used as fuelling gas during the plasma discharge. These approaches resulted in a rapid shot to shot improvement of important parameters such as the volume averaged resistivity and radiated power over the first dozen shots when the boron source is present. Typically, the resistivity is reduced from ∼4.0×10 −7 to ∼2.5×10 −7 Ωm, comparable to the resistivity obtained with PECVD. The radiated power relative to the ohmic power is reduced by a factor of 2 from 20 to 10%. When the boron source, present during STB or TMB fuelling, is removed however, these plasma parameters start increasing. Within a few tens of shots, they have reverted to their preconditioning values, a situation which requires hundreds of shots after PECVD.

Controlled detachment and particle transport in the divertor plasma in TdeV

Abstract At high densities, the plasma detaches from the outboard divertor plates in TdeV. The signatures are a reduction of the ion flux to the divertor plate, movement of the radiating zone from the plate toward the X-point, a pressure gradient between an ionization front and the target plate, and strong cross-field transport in the divertor. A toroidally-viewing TV imaging system allows us to observe local interactions between the divertor plasma and the different divertor plates. As the plasma detaches, the gas pressure in the divertor continues to rise, and there is evidence for molecular processes in the cold plasma near the divertor plates. Auxiliary heating increases the power and particle flow across the separatrix; our results suggest that detachment depends on the energy transported per particle. Simulations using the B2/EIRENE and DIVIMP codes give reasonable agreement with the measurements for the attached phase.

Divertor retention of metallic impurities during neutralization plate biasing on TdeV

Laser ablation injection of aluminium is used to measure the retention of metallic impurities in the lower poloidal divertor of TdeV. A detailed calibration of the ablation process allows the determination of the quantity and velocity distribution of the injected particles. The experiment measures the flow of the injected particles from the divertor to the main plasma. Negative biasing of the divertor neutralization plates is shown to improve the retention in the active divertor by a factor of at least four at -200 V. A simple model is developed to show that the improved confinement is due to the increased poloidal flux to the divertor during biasing

Retention of Ne and N2 in the closed and pumped TdeV divertor with attached and detached plasmas

Abstract We have studied the retention of a recycling impurity, neon, and a wall-pumped impurity, nitrogen, in the closed and cryopumped divertor of the TdeV tokamak, under a variety of heating, puffing, pumping and biasing conditions. Retention times were deduced from either the decay time or the rise time of the plasma impurity content, measured spectroscopically, following either a short puff or a steady injection. For both neon and nitrogen, the compression ratio increases rapidly with the main plasma density. Retention is not degraded by plasma detachment. In density scans, other conditions being kept constant, the compression ratio for neon is found to grow with the divertor neutral pressure as ∼ P D 1.5 . However, by puffing, pumping or biasing, retention and P D can be varied quite independently of each other.

Characterization of various divertor biasing schemes on TdeV

The fact that the inner and outer divertor plates of the Tokamak de Varennes can be biased independently in a double-null or a single-null configuration makes it possible to characterize various biasing schemes. Current injection, in which the potential difference is applied on the same flux surface, and plasma biasing, in which the potential difference appears between the separatrix and the wall, are generic modes of particular interest. Current-voltage characteristic curves and induced electric fields are discussed for each mode. Finally, this study indicates that biasing only one electrode with respect to both the vessel and the other strike points of the separatrix yields a hybrid behaviour resembling current injection when the electrode is negative and resembling plasma biasing when the electrode is positive

First results on plasma-surface interactions in the tokamak de varennes

Abstract Results of plasma-surface interaction studies made during the early phases of operation of the Tokamak de Varennes are summarized. It was found that the desorption of molecules from the internal walls by UV radiation can be used to reduce the base pressure. Auger depth profiling of stainless steel (SS) samples exposed to hydrogen discharge cleaning has been performed. Glow discharges at about 0.1 mbar etch the surface carbon and oxide at a rate of 0.5 nm/h. RF-glow discharges at 10−3 mbar result in a rapid reduction of the oxide and its replacement by a carbide layer (the graphite limiters being the source of the carbon). Long-term wall samples of SS and Si have been profiled by Auger and nuclear analysis. The SS sample has a similar composition to that exposed to the RF-glow conditioning. The Si sample is covered by a 3 nm deposit of metals, C and O, and contains 1016 H/cm2 within 30 nm of the surface; this dose and width are consistent with the history of the sample.

Particle transport in diverted TdeV

The particle confinement time of ohmic double-null discharges in Tokamak de Varennes (TdeV) is determined by two different techniques, the conventional method and a new technique based on the temporal decay of the total core population following the injection of a gas puff. Both methods show a confinement time increasing with density up to a maximum of 13 ms at and decreasing at larger densities, with very little dependence on plasma current. Particle transport is analysed using fast gas puffing and Abel inversion of the seven-chord submillimetre (SMM) interferometer together with the source profiles determined by measurements. The incremental transport coefficients are obtained by testing the standard form of the particle flux function against the data during the transitory period towards equilibrium. Both perturbed diffusion and convection coefficients are found to vary approximately as the inverse of the density and almost proportionally to the plasma current. The equilibrium transport coefficients are then deduced from the experimental equilibrium density profiles and the measured incremental coefficients using a transport model developed from the data. The model is finally used to predict confinement times to be compared with experiment. The effect of divertor plate biasing on transport is also discussed.

Screening of a neon impurity by the edge plasma in the Tokamak de Varennes

VUV spectroscopy and mass spectrometry measurements imply that the concentration of neon — an artificially introduced impurity in the Tokamak de Varennes — decreases in the central plasma early in the discharge. The missing atoms are found in the volume between the plasma and the vessel walls. The measured transport coefficients of neon are similar to those of oxygen, and reduced penetration of recycled neon into the hot plasma by the screening effect of the scrape-off layer plasma is postulated to explain its depletion.