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A simple analytic fluid model based on particle and momentum conservation describes the response of the scrape-off layer to an applied radial electric field. The model explains why the variation of plenum pressure during limiter biasing in TEXTOR shows the opposite behaviour as divertor biasing in Tokamak de Varennes: the mechanical baffles that define the throats of the pump limiter are aligned along the magnetic field, thus preventing collection of the induced perpendicular flux when the drift is directed towards the scoop. Rather, the optimal pumping is expected when the drift is directed away from the scoop due to the increase of the parallel flux needed to satisfy the Bohm-Chodura criterion. Qualitative agreement is obtained with measurements of the parallel flow near the limiter throat, and with measurements of the plenum pressure during negative biasing for both directions of toroidal magnetic field. The results, extrapolated to Tore Supra, predict that the actual design with throats should provide close to optimal pumping with minor modification of the natural radial electric field, but greater performance could be obtained with negative biasing if a toroidally symmetric neutralizer geometry were adopted.

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%

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

A simple model based on non-ambipolar radial transport and planar sheath physics is used to describe the generation of radial electric fields and currents in the scape-off layer of the Tokamak de Varennes (TdeV) during divertor plate biasing. In general, the calculated predictions compare favourably with TdeV results over a variety of plasma conditions and divertor magnetic configurations. Validated by the experiment, the model is used to study the scaling laws of perpendicular ion mobility and to test existing related theories. Finally, the model is proposed as a useful tool for the design and upgrade of biased divertors through optimization of the plate and throat geometry

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

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

The application of a bias voltage to a neutralization plate of the upper divertor with respect to the vacuum chamber in the Tokamak de Varennes (TdeV) influences the plasma well inside the separatrix. In particular, the unbiased Ohmic poloidal rotation edge velocity measured by visible spectroscopy is found to be in the electron diamagnetic drift direction (2-3 km/s) and increases by a factor of two for Vbias = 100 V. This coincides with a major reduction of the microturbulence signal at low frequencies (50 kHz < f < 1 MHz) and for all wave numbers probed (20 cm-1 < k < 60 cm-1), as determined from coherent laser scattering measurements. One possible explanation is that the turbulence signal is simply Doppler shifted to frequencies outside the accessible range. This scenario is, however, difficult to reconcile with some observations. Another explanation invokes a reduction of the turbulence level. The variation of the turbulence signal as a function of the applied bias voltage can indeed be reproduced with a theoretical model based on radial and poloidal decorrelation mechanisms, the latter corresponding to poloidal velocity shear stabilization. This model also explains the steepening of the k-spectrum decay during biasing. Biasing also modifies the electron density profile inside the separatrix. These changes of ∇ne cannot explain the behaviour of microturbulence in terms of their effect on the free energy term driving the instability. On the contrary, the observed turbulence behavior, when explained in terms of stabilization, would agree with the plasma maintaining a steeper electron density gradient

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.

Simulation results obtained with the axisymmetric Princeton Tokamak Simulation Code TSC are compared with experimental results for the time evolution of the Tokamak de Varennes plasma. The code correctly predicts experimental conditions; of special interest are the favourable comparisons of the simulation with experiments during fast current ramp-down (from 250 kA in 40 ms, three times faster than the L/R time).