G Van Oost

Confinement and profile changes induced by the presence of positive or negative radial electric fields in the edge of the TEXTOR tokamak

Edge radial electric fields were induced in the edge of the TEXTOR tokamak by means of a polarization electrode in order to study their influence on the plasma edge profiles and its confinement. The studies include the generation of H-mode behaviour with either positive or negative polarity. Particle confinement (τp) of deuterium and of impurity ions as well as energy confinement (τE) are investigated. For positive fields which remain below the threshold for the L-H transition, an interesting regime of reduced particle confinement without noticeable energy confinement loss is found. A strong asymmetry in the edge density profiles with respect to the electric field sign is observed at these low polarization voltages. Above the threshold, H-mode behaviour with increased energy confinement and especially particle confinement can be produced with either polarity of the applied electric field. It is, however, found that, whereas the energy confinement in positive H-modes is at least as good as that in negative ones, the ratio τp/τE is about three times lower in the former case

Edge biasing in tokamaks

An overview is presented of recent results on the creation of electric fields in the edge of limiter or divertor tokamaks. The practical implementation and theoretical basis of several schemes, generating radial and/or poloidal fields, are outlined. The manipulation of edge and scrape-off-layer profiles and the control of particle exhaust is discussed. Contributions of biasing experiments to H-mode physics are highlighted. Some prospects for biasing in next generation tokamaks are finally given.

PREDICTING THE RADIAL ELECTRIC FIELD IMPOSED BY EXTERNALLY DRIVEN RADIAL CURRENTS IN TOKAMAKS

H mode behaviour is usually linked to the existence of radial electric fields or to their shear at the edge of tokamaks. The mechanisms are investigated by which such fields are induced in the plasma edge and by which the profile shaping is obtained when radial currents are imposed by electrode polarization. Earlier detailed experimental field measurements are successfully compared with a theoretical conductivity model in which neoclassical non-ambipolar transport and mobility through ion-neutral collisions are predominant. Strong neoclassical viscosity in the bulk plasma allows significant fields to develop only at the very edge of the plasma. There, a delicate balance between viscosity and ion-neutral friction takes place, which strongly affects the magnitude of the fields and the spatial location and the threshold condition for L-H field bifurcation. It is also shown how to verify experimentally the neoclassical diffusion coefficients in the plateau regime

Improved confinement with edge radiative cooling at high densities and high heating power in TEXTOR

Improved confinement is achieved on TEXTOR under high power conditions (up to 4 MW of additional heating with NBI-co+ICRH, NBI-co+counter or NBI-co+counter+ICRH) with edge radiative cooling employing silicon or neon as the radiating impurities. It is shown that in quasi-stationary conditions up to 85% of the input power can be radiated. Such high power fractions offer the possibility of utilizing these techniques to facilitate the power exhaust problem for a Tokamak reactor. Discharges with edge radiative cooling exhibit enhanced confinement properties at high densities, e.g. at a central line averaged electron density of 7.5*1013 cm-3, an enhancement factor of 1.7 over ITER L89-P confinement scaling is obtained with an edge q value as low as 2.7. Stable discharges have been obtained even with the q=2 surface located inside the radiating zone. Furthermore, for radiatively cooled discharges heated with balanced NBI-co+counter with or without ICRH, supershot-like peaked electron density profiles, with central density values above 1.0*1014 cm-3 are observed. The present results show that there is no impurity accumulation in the centre and the Ne and/or Si concentration is so low that the reactivity of the plasma remains unaffected

EFFECT OF ANTENNA PHASING AND WALL CONDITIONING ON ICRH IN TEXTOR

Four new low field side antennae grouped in pairs have been installed on TEXTOR. It is found that the interaction with the wall (density rise, impurity generation) is significantly reduced when operating each pair out of phase ( pi ) as opposed to in phase (0). The beneficial effect in the pi configuration is obtained without drop in plasma loading. This experimental property is shown, from theory, to be explained by the judicious choice of the geometrical configuration. A further improvement in the wall interaction is made possible by an appropriate choice of wall conditioning (wall carbonization with liner at 400 degrees C or, above all, boronization). As a result record low values of Prad/Ptotal were achieved during ICRH. The large reduction in wall interaction during ICRH allows routine long pulse (>1 s) ICRH operation at the maximum power level available ( equivalent to 2.5 MW).

Ion cyclotron resonance heating on TEXTOR

Ion cyclotron heating on TEXTOR has now reached the Megajoule level. The heating scenario is normally mode conversion but occasionally minority heating in a D-(H) plasma. With appropriate wall conditioning by carbonization more than 1 MW of RF power has been injected for long pulse durations ( approximately 1 s). The ICRF heated plasma is characterized by a quasi-stationarity of all plasma parameters, little if no impurity increase and a loop voltage reduction resulting in the total power coupled to the plasma reaching six times the remaining ohmic power input. Evidence of the coupling of the RF power to the plasma is obtained from the increase of the thermal load on the limiters and central energy deposition is supported from analysis of the sawtooth heating rate.

TRANSPORT AND IMPROVED CONFINEMENT IN HIGH POWER EDGE RADIATION COOLING EXPERIMENTS ON TEXTOR

A stationary high level of edge radiation ( gamma =Prad/Ptot up to ~90% with peak radiation up to ~1 MW/m3) has been obtained in TEXTOR by using silicon and/or neon as radiating impurities. The confinement and neutron reactivity are not degraded but can even be improved at high plasma densities. Stationary reactor relevant heating and radiated power flows with a figure of merit fH/qa=0.6 have been achieved. The interpretation of these results shows a reduction of the bulk transport in the presence of edge radiation cooling. The properties of the radiatively cooled discharges are interpreted or modelled mainly by the self-consistent radiative transport code RITM, and also by the codes TRANS and PRETOR. From these modelling studies an enhancement of the bulk confinement is found in terms of the reduction of the convective losses and the decrease of the edge electron temperature, which results in a peaking of the current profile. The code RITM also predicts self-consistently the detailed properties of the radiating layer for different injected impurities as a function of their incoming flux, and shows that the optimal conditions to obtain confinement improvement as well as minimum fuel dilution by the radiating impurity are obtained at high density

High power ICRH and NB heating results in TEXTOR

Neutral beam injection (NBI-co and NBI-counter), ICRH and their combinations have been compared on TEXTOR for what concerns the heating and their effect on the energy anisotropy and the confinement. In most of the cases, stationary heated plasma conditions have been obtained with boronized wall. The main results are: (i) Production of a hot ion mode with NBI-co. It is characterized by a large energy anisotropy, by a large noninductively driven toroidal current and by the stabilization of the sawteeth (of the monster type) due to the hot ion tail. (ii) Significant enhancement of these effects when ICRH is added to NBI-co. The neutron yield is also increased by the addition of ICRH. Peak electron and ion temperature around 3 keV have been obtained and 70% of the total current has been non inductively driven. (iii) Large excess of the total energy content (up to 2.3) with respect to the L-mode scaling predictions has been obtained with the combinations NBI-co+ICRH or NBI-co+NBI-counter. (iv) Up to 6 MW of additional power has been coupled to the plasma leading to beta p=1.5 and beta t equal to 70% of the Troyon limit.

Experimental and theoretical investigation of waves and resonances of a bounded hot electron, cold ion magnetized plasma

A plasma column is produced by diffusion along a steady magnetic field by a helical microwave discharge source located at one end of a linear machine. Waves in the plasma are electrostatically excited by a split-cylindrical, slow-wave structure. Movable well-balanced RF double probes are used to measure RF electric field amplitude as a function of frequency. This frequency domain includes the Trivelpiece-Gould (T-G) mode (fLH<f<fpe), surface wave mode, electrostatic ion cyclotron mode (fci<f<fLH) and ion-acoustic mode (f<or approximately=fci). Experimentally observed resonances and dispersion curves of the bounded plasma system are in good agreement with theory for ion-acoustic waves, surface waves and T-G mode.

Excitation of global modes in TEXTOR and comparison with theory

The excitation of fast-wave global eigenmodes is investigated both experimentally for TEXTOR and theoretically using a slab coupling mode. A brief comparison with a cylindrical model shows that in the TEXTOR situation no essential geometrical effect is left out by the slab code. The resonant behaviour associated with eigenmodes is studied for several signals: the resistance and inductance of the antennae the r.f. magnetic and electric field components picked up by probes at the plasma edge, and the power coupled to a fast-wave antenna used as a receiver. It is shown that all the major features of these signals linked to the existence of eigenmodes can be explained using the theoretical models.