R Koch

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.

Ray-tracing analysis of ICRH power-deposition profiles in non-circular large Tokamaks

Using self-consistent initial conditions obtained from the full-wave solution of the field radiated by an ion-cyclotron resonance heating (ICRH) antenna, the ray-tracing technique is applied to generate r.f. power-deposition profiles in non-circular large Tokamaks such as JET and NET/INTOR. An analytic expression has been used to simulate the shape of the flux surfaces, which fit reasonably well to the numerical solutions of the Grad-Shafranov equation describing the Tokamak equilibrium, by adjusting three free parameters: the flux-surface shift Delta , the elongation ratio kappa and the triangularity parameter delta . For an elliptic INTOR plasma, it is found that the focussing of rays is much reduced and when the absorption layer is located in the center, the r.f. power density figures are lower approximately by a factor of 1.9 compared to that obtained in an equivalent circular case. This reduction in power density is not so significant when the power is deposited off-center, as demonstrated by an example treated for the JET D-shaped plasma.

Low frequency heating and flow driven by the dynamic ergodic divertor in tokamaks

The profile and dissipation of the field excited by dynamic ergodic divertor (DED) coils in tokamak plasmas are calculated, and an estimate is made of the poloidal/toroidal velocities driven by this field. The coils are idealized as an inboard sheet current composed of a toroidal sequence of helical line current segments expanded in Fourier series with poloidal/toroidal mode numbers M/N, and mode amplitudes depending on feeding. Numerical calculations with cylindrical and toroidal codes show maxima of field dissipation due to Alfv´ en wave mode conversion effect taking place at the rational magnetic surfaces where q = M/N. The effects of toroidicity and ion collisions in the dielectric tensor in the upper DED frequency range described (f = 5–10 kHz) are found to be very important in absorption calculations. At the q = 3 resonant magnetic surface typical for DED coil design, it is estimated that ponderomotive forces produced by 20 kW of dissipation can drive local toroidal and poloidal flows of respective orders 8 km s −1 and 10 km s −1 in the TEXTOR tokamak.

Ray-tracing modelling of ICRH at arbitrary cyclotron harmonics in large hot plasmas

A ray-tracing model for studying ion cyclotron resonance heating at higher cyclotron harmonics in very hot large plasmas is presented. It includes the full hot dielectric tensor for bi-Maxwellian distribution functions with parallel drift retaining temperature effects to all orders and allows for non-Maxwellian distribution functions. The model only retaining first-order temperature corrections underestimates the electron damping on the Bernstein wave. Highly energetic tails efficiently absorb fast wave power at high cyclotron harmonics. Significant alpha -particle absorption is to be expected in D-heating D-T scenarios.

A comparison between ICRF theory and experiment

The status of, successively, RF modeling theory, experimental interpretation methods and comparison between theory and experiment is discussed. As far as the recent developments in theory are concerned, it is found that full wave models appear to be consistent with ray-tracing models and to yield comparable results when applied to selected cases where both are applicable. Methods for including consistent gradient terms in the wave equation have now been found; however, a unified understanding of the impact of such corrections is desirable. From the experimental point of view several procedures have been developed based on sawteeth analysis, modulation or simulation, to evaluate the fraction of the power coupled to the plasma bulk and the power deposition profiles. Comparison between experiments and theory on these subjects as well as for coupling indicates that the ICRF physics is well understood. Tail analysis has been done extensively only for PLT; further work is going on for JET and TEXTOR, which clearly reveals their presence and the importance of their role. chi e analysis in the presence of RF has also been done; however, several of the presently available thermal diffusion models seem to be able to fit the experimental data. It is concluded that, in the ICRF field, theory nowadays provides quite reliable tools and that extensive comparisons with experiments should be pursued in order to provide the basis for the understanding of the interplay between heating, velocity diffusion, confinement and edge effects.