G. Van Wassenhove

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.

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.

Application of PM-355 Solid-State Nulear Track Detectors for ion diagnostics in high-temperature plasma experiments

The paper reports on some applications of Solid-State Nuclear Track Detectors, particularly of the PM-355 type, for diagnostics of fusion reaction protons (3 MeV) and other fast ions emitted from high-temperature plasmas. The measurements with the use of these detectors were performed at the PF-1000 Plasma-Focus facility in Warsaw, TEXTOR tokamak in Juelich, and PALS lasersystem in Pargue.The paper presents histograms of fusion proton track diameters, which were obtained on the basis of track diameter analyses. Some examples of pictures of craters induced in the PM-355 detectors by energetic protons, which were emitted from PF-1000 facility and TEXTOR tokamak, are shown. There are also demonstrated craters created by energetic Ta-ions, which were generated in a solid target by the PALS laser beams.

Attenuation measurement of an atomic hydrogen beam as a diagnostic technique to determine the light impurity content of a plasma

The attenuation of a low intensity atomic hydrogen or deuterium beam, injected into the plasma of the ERASMUS Tokamak, was investigated during the whole duration of the discharge, as a function of the beam energy (3-15 keV) and of the radial position of the injection chord. The comparison with attenuations calculated from impurity models adapted to the ERASMUS plasma allows the evaluation of the content of oxygen assuming that this is the only light impurity. The sensitivity of the diagnostic was tested by injection of test impurities, such as oxygen and helium, and during RF heating in the ion-cyclotron range of frequencies.

Review of combined ICRH‐NBI results in TEXTOR

The synergism observed between NBI and ICRH is theoretically interpreted for the interaction at the second and third ion cyclotron harmonic. It is also shown that the performances of supershot‐like discharges obtained with balanced injection can be substantially improved by beam‐RF interaction both at 2 ωCD and 3 ωCD.

Studies of ICRF Discharge Conditioning (ICRF‐DC) on ASDEX Upgrade, JET and TEXTOR

The present paper reviews the recent results achieved in the ICRF‐DC experiments performed in helium/hydrogen mixtures in the non‐circular tokamaks ASDEX Upgrade and JET and first tests of the ICRF discharges in helium/oxygen mixtures in the circular tokamak TEXTOR. Special emphasis was given to study the physics of ICRF discharges. A new recipe for safe and reliable RF plasma production [〈ne〉∼(3–5)×1017 m−3, Te∼(3–5) eV] with improved antenna coupling efficiency (by 1.5–3 times) and improved radial/poloidal homogeneity was proposed and successfully tested: coupling the RF power in the FW‐IBW mode conversion scenario in plasmas with two ion species. The first results on ICRF wall conditioning in helium/hydrogen and in helium/oxygen mixtures are analyzed.