N. Richard

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 detachment and exhaust on the TdeV tokamak

Experimental data, analysis and simulations are used to describe the physics of divertor detachment and He exhaust under detached conditions in TdeV. With increasing density, the plasma is found to detach progressively from the outboard divertor plates with a marked reduction of the ion flux to the plates, the generation of a pressure gradient between an ionization front and the target plate, and strong cross-field transport in the divertor. Local interactions between the divertor plasma and the plates are described, with evidence for carbon sputtering and molecular processes near the divertor plates. Divertor exhaust and retention continue to increase through detachment and He exhaust is not affected although the divertor He enrichment remains low but constant at about 0.2. A moderate density of seems to be sufficient both for efficient peak power load reduction at the divertor plate and good He exhaust through the divertor. Simulations of the edge and divertor plasmas using the B2/EIRENE and DIVIMP codes give reasonable agreement with the measurements and indicate possible divertor geometry improvements

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.

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.

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.

Characterization and control of the power loss and its distribution in TdeV during divertor plate biasing experiments

Effective control of the particle and power flux distributions between the upper and lower divertors has been demonstrated on TdeV, using biasing of the divertor plates, in ohmic discharges with a double null magnetic configuration. Negative biasing of the divertor plates with respect to the walls significantly increases the pressure and the emitted power from the plasma in the active divertor into which E*B drift is directed, but it has little influence on the opposite divertor. This enhanced power dissipation is the result of improved particle retention in the active divertor. The incident power on the plates in the active divertor also increases with biasing, but at a slower rate. The Franck-Condon and charge exchange neutrals contribute significantly to the power loss in the divertors of TdeV in the present operating mode. Positive biasing of the divertor plates has little influence on the total losses from the divertor regions

Divertor plate biasing effects on particle recycling and power loss distribution in TdeV during lower hybrid current drive and heating experiments

Preliminary results concerning the influence of negative biasing of the divertor plates on particle recycling and on power loss distribution in single null discharges of TdeV during lower hybrid (LH) current drive and heating experiments are presented. The beneficial effects of negative biasing of the divertor plates, such as the ability to control power and particle fluxes in the SOL, remain effective in the presence of auxiliary heating and current drive. Up to 0.7 MW of auxiliary power were injected in these experiments. With a negative biasing of 150 V, and the E ⊗ b flow vector pointing towards the outer divertor chamber, a roughly 2 fold increase in the divertor pressure and the radiation from the divertor region is observed.