C. Boucher

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.

Modelling of radial electric fields and currents during divertor plate biasing on TdeV

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

Effects of divertor plate biasing on radial and poloidal edge fluxes in the TdeV

The divertor plates of TdeV, a tokamak with a double-null divertor and closed divertor chambers, have been electrically biased with respect to the walls. The authors discuss the resulting effects on the edge electron density profile, on the neutral pressures and impurity fluxes in the main vacuum chamber and the divertor chambers, and on the plasma flow to the divertors. As a function of the bias voltage, which was varied between-180 V and +160 V, the electron density scrape-off width and the wall impurity influxes increase monotonically; the flows to the top and bottom divertors vary strongly, in qualitative agreement with an E*B/B2 rotation, but not symmetrically. With negative biasing, the electrostatic barrier and the rotation combine to give a strong improvement of the divertor efficiency

Edge turbulence during ergodic divertor operation in Tore Supra

We report new measurements of turbulence during ergodic divertor (ED) operation. At low density, some de-correlation of the turbulence is observed with a decrease of the long timescale structures. It is shown that the typical time involved is compatible with a de-correlation mechanism through radial separation of the B field lines by the ED, with an associated parallel length of the order of the distance between two modules of the ED. This observation reinforces the conclusion drawn in [1] and based on computer simulations. The situation changes when the density is increased: the turbulence level is found to increase. At the highest density, the structure of the turbulent signal is modified and the bursty behaviour suppressed by the ED at low density reappears. These observations lead to the conclusion that the turbulence measured at high density is not sensitive to the ED stabilization effect. This indicates that it could be carried by the ions.

Measurement of flow in the scrape-off layer of TdeV

Two techniques are used to monitor the flow in the scrape-off layer of Tokamak de Varennes (TdeV); one is based on a new multipin Langmuir/Mach probe called Gundestrup while the other depends on the measurement of the upstream/downstream asymmetry of the power absorbed by a test limiter inserted into the plasma edge. Gundestrup has been used to measure the components of velocity parallel and perpendicular to the magnetic field as a function of the radial electric field. Both components vary linearly with the radial electric field and inversely as the magnetic field ( U ‖ ∝ E r / B θ and U ⊥ ∝ E r /B ). The pattern of power deposition on the test limiter implies that the flow is in the same direction as that measured by Gundestrup and the e-folding length for the power deposition is in agreement with Gundestrup measurements of temperature and density. The test limiter observations indicate that the flow reverses just inside the separatrix.

Measurement of pre-sheath flow velocities by laser-induced fluorescence

Abstract For the first time, the pre-sheath ion flow velocity has been measured using the Doppler shift of laser-induced fluorescence in singly-ionized argon ions. The velocity shows a monotonic increase, from a value of about 0.15 of the sound speed VS far from the target to 0.5 of Vs at a distance of 5 mm from the surface. The temperature, the floating potential and the density are calculated from cylindrical probe measurements taken in the same region under identical conditions. These experimental results are compared with those from a 1D isothermal single-ion fluid model of the pre-sheath and a kinetic electron/fluid ion model. Both models agree well with the density profile, but underestimate the potential change and overestimate the velocity. In addition, the bulk flow velocity has been independently determined from “Mach probe” measurements, using various candidate theories to relate the Mach number to the ratio of the upstream to downstream saturation currents. Comparison with the optical measurements indicate that the probe models which include viscosity provide reasonable agreement with our Mach probe data.

Measured plasma parameters in TdeV's closed poloidal divertors

Abstract Measurements in TdeV in the double null configuration show strong asymmetries between the upper and lower divertors. The divertor to which the ion ∇ B drift is directed has a higher density, which varies little with the central line-average density. The electron temperature also exhibits a poloidal variation which depends on the direction of the toroidal magnetic field. The divertor plasma shows a double peak structure which evolves as a function of the central density, with the outer peak varying more rapidly. We have measured flow reversal near the separatrix in the bottom divertor when the ion ∇ B drift is directed away from the X -point. Divertor plate biasing allows us to control the flux of plasma in the SOL; under negative biasing there is a strong increase of the electron density in the active divertor, the one favoured by the E × B flow. Calculations of the energy deposition derived by flush-mounted probes agree well with measurements of the heat load derived from the temperature increase of the divertor tiles.

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.

Modification of the plasma edge properties during divertor plate biasing experiments on TdeV

A laser-ablated fast particle diagnostic has been used for the measurement of n e (r) from the wall to 50 mm inside the TdeV separatrix. Divertor plate biasing has been found to modify substantially the n e ( r ) profile. The e-folding length at the separatrix, ν n , has been evaluated as a function of the biasing potential, and the results suggest a modification of the radial transport as a function of the radial electric field. The “LaBombard model” using different diffusivities ( D ÷ ) and mobilities (μ ÷ ) for ions and electrons cannot reproduce the dependence of ν n as a function of biasing. A simple model using the same D ÷ and μ ÷ for electrons and ions has been developed which gives a reasonable fit to the experimental measurements for D ÷ =3.2 m 2 /s and μ ÷ =0.039 m 2 /s V. Also, the particle confinement time τ p has been estimated with the help of the model, suggesting that negative biasing of the divertor plates increases τ p .