B.C. Gregory

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.

Improvements in recycling and impurity control obtained by divertor biasing

Experiments in which the divertor plates are biased with respect to the grounded vacuum chamber were conducted in the Tokamak de Varennes, with ohmic discharges and boronized walls. A comprehensive set of diagnostics was used and those effects relating to plasma-surface interactions and their consequences, namely recycling and impurity production, are reported here. The main conclusion is that the bias voltage actively controls the balance between the particle fluxes to the walls and the diverters, with a weak influence on the confinement. The quantities associated with plasma surface interactions, i.e. the impurity influx, the plasma contamination and the global recycling coefficient, increase as a function of the voltage applied between the plates and the wall. Typically, the quantities of interest vary by a factor of two or more in the range from -240 to 290 V. The poloidal fluxes to the upper or lower divertor are enhanced by negative or positive biasing, respectively, in agreement with the sign of the E*B drift. For negative biasing the reduction in the wall flux and increase in the poloidal flux combine to improve greatly the divertor efficiency: the divertor pressure and the impurity retention time are increased by factors of up to 5 and 8, respectively. The above effects, together with the SOL density profiles and the biasing current-voltage characteristic, are consistent with a model in which the radial transport in the SOL is limited by the ion mobility; a mobility of 0.04 m2/V.s is found. Besides allowing active density and impurity control, it is suggested that biasing could facilitate the attainment of a radiative divertor by injection of well retained impurities in the divertor

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.

Chemical Impurity Production in the Tokamak de Varennes

Plasma contamination due to the generation of impurity molecules has been studied by mass spectrometry and by visible emission spectroscopy in the Tokamak de Varennes. The dominant effects are carbon monoxide formation, which is correlated with the residual water vapor pressure in the vacuum chamber, and the formation of C{sub 1}, C{sub 2}, and C{sub 3} hydrocarbons. The measured molecular fluxes are sufficient to account for a large part of the plasma impurity content. Visible spectroscopy indicates that the plasma is significantly affected by these chemical impurity sources. The molecules appear to originate mainly from the stainless steel walls rather than from the graphite limiters.

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

Impurities in TdeV with and without conditioning by trimethylboron/helium glow discharge

Abstract Boronisation by glow discharge with 30% B(CH3)3 in He was applied in the TdeV tokamak. Plasma current and density scans were performed before and after the process; the impurity influxes (visible spectroscopy and mass spectrometry), the plasma contamination (VUV spectroscopy and effective ion charge Z eff ) and the radiated power P rad were measured. The density limit was investigated. The lifetime of the conditioning effect was correlated with surface analysis of wall samples. Without boronisation, Z eff , P rad and a large fraction of the particle recycling were determined by oxygen from the residual gas. The density limit was (3.5−4.0)×10 19 m -3 and a shrinkage of the plasma radius occured at low current and high density. With boronisation, oxygen is reduced several fold, and ( Z eff −1 ) and P rad are reduced by ≈60%. Also, plasma shrinkage is eliminated and the density limit is increased to ≈5×10 19 m -3 . The lifetime of the effect seems to be mostly determined by saturation of the boron layer by oxygen.

Retention of intrinsic gases in TdeV's biasable divertor

The screening effect of the divertor plasma against the backflow of gas into the plasma core is studied on TdeV, a double-null tokamak with biasable neutralizing plates. Accumulation of intrinsic impurities in the divertor chamber is observed, and negative biasing of the plates causes a strong enhancement of the effect. The divertor gas composition is correlated with the line radiation emitted in the vicinity of the plates and with the plasma impurity content. Using a simple particle balance model, it is concluded that H 2 flows unimpeded through, the divertor slit, but that the backflow of other gases is significantly weaker than predicted by the vacuum conductance, e.g. by a factor of ≥4 in the case of CO.

Fueling and gas evolution studies on the Tokamak de Varennes

Abstract Fueling and gas evolution have been investigated in the Tokamak de Varennes using time-resolved mass spectrometric gas analysis during and after the shot, gas flow measurements, and standard plasma diagnostics. The evolution of the H and impurity content, both in the plasma and the neutral gas, is used to compare the recycling behaviour of H2 and CO. An accounting of the exhaust of H2 and other gases after the shot is given, together with a discussion of the outgassing mechanisms. The scaling of the fueling efficiency with density and current has been studied and shows a remarkable increase at high current due to improved particle confinement and possibly an increased recycling coefficient.

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.

Magnetohydrodynamic equilibrium of tokamak plasmas with reversed current layers

An MHD code is used to find a sequence of equilibria where negative skin current penetrates progressively toward the magnetic axis, on which the toroidal current density (positive) and kinetic pressure are maintained constant. The initial equilibrium with no skin current has a central safety factor close to unity and an edge safety factor about three. Equilibria have been found for currents down to 5 per cent of the initial value (total negative current about half total positive current). For total currents less than a third of the initial current, the plasma becomes smaller because of the creation of a poloidal separatrix on the small major radius side. The vertical field necessary for equilibrium decreases more slowly than the total plasma current. This is explained by a large increase in the poloidal beta accompanied by a smaller increase in the internal inductance. The use of such a fast rampdown sequence to provide a controlled termination for an energetic tokamak plasma is discussed.