M. Vervier

Overview of radiative improved mode results on TEXTOR-94

The radiative improved (RI) mode is a tokamak regime offering many attractive reactor features. In the article, the RI mode of TEXTOR-94 is shown to follow the same scaling as the linear ohmic confinement regime and is thus identified as one of the most fundamental tokamak operational regimes. The current understanding derived from experiments and modelling of the conditions necessary for sustaining the mode is reviewed, as are the mechanisms leading to L-RI mode transition. The article discusses the compatibility of high impurity seeding with the low central power density of a burning reactor, as well as RI mode properties at and beyond the Greenwald density.

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

High confinement and high density with stationary plasma energy and strong edge radiation cooling in the upgraded Torus experiment for technology oriented research (TEXTOR-94)

An overview of the results obtained so far for the radiative I-mode regime on the upgraded Torus Experiment for Technology Oriented Research (TEXTOR-94) [Proceedings of the 16th IEEE Symposium on Fusion Engineering (Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1995), Vol. 1, p. 470] is given. This regime is obtained under quasistationary conditions with edge neon seeding in a pumped limiter tokamak with circular cross section. It combines high confinement and high β (up to a normalized beta, βn=2) with low edge q values (down to qa=2.8) and high density even above the Greenwald limit together with dominant edge radiative heat exhaust, and therefore shows promise for the future of fusion research. Bulk and edge properties of these discharges are described, and a detailed account is given of the energy and particle confinement and their scaling. Energy confinement scales linearly with density as for the nonsaturated Ohmic Neo-Alcator scaling, but the usual degradation with total power remains. No deleterious effects of the neon seeding on fusion reactivity and plasma stability have been observed.An overview of the results obtained so far for the radiative I-mode regime on the upgraded Torus Experiment for Technology Oriented Research (TEXTOR-94) [Proceedings of the 16th IEEE Symposium on Fusion Engineering (Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1995), Vol. 1, p. 470] is given. This regime is obtained under quasistationary conditions with edge neon seeding in a pumped limiter tokamak with circular cross section. It combines high confinement and high β (up to a normalized beta, βn=2) with low edge q values (down to qa=2.8) and high density even above the Greenwald limit together with dominant edge radiative heat exhaust, and therefore shows promise for the future of fusion research. Bulk and edge properties of these discharges are described, and a detailed account is given of the energy and particle confinement and their scaling. Energy confinement scales linearly with density as for the nonsaturated Ohmic Neo-Alcator scaling, but the usual degradation with total power ...

Overview of experiments with radiation cooling at high confinement and high density in limited and diverted discharges

An overview is presented of recent experiments with radiating mantles on limiter and divertor machines, realizing simultaneously high confinement and high density at high-radiation levels. A variety of operational regimes has been observed and the characteristics of each are documented. High-performance plasmas (i.e. edge localized mode (ELM)-free H-mode confinement quality and normalized beta values simultaneously) with radiating mantles have been demonstrated under quasistationary conditions during the maximum flattop time of the machine (equal to tens of confinement times) on DIII-D and TEXTOR-94. Maximum values for up to 4 and for the advanced tokamak confinement-stability product up to 13, have been obtained in very high confinement mode (VH-mode) like discharges with radiating mantles in DIII-D. There is a striking similarity between improved ohmic confinement discharges (with or without Ne seeding) and radiating mantle discharges, indicating a possible common origin for the confinement improvement observed. Possible scenarios for the application of radiating mantles on larger machines such as JET and JT-60U are indicated.

Electron cyclotron resonance heating on TEXTOR

The 110 GHz and the new 140 GHz gyrotron systems for electron cyclotron resonance heating (ECRH) and ECCD on TEXTOR are described and results of ECRH experiments with the 110 GHz system are reported. Central ECRH on Ohmic plasmas shows the presence of an internal electron transport barrier near q = 1. This is confirmed by modulated ECRH experiments. A central barrier is also indicated by ECRH in radiatively improved (RI) mode discharges and up to two barriers are seen with ECRH during the current ramp phase. ECRH control of sawteeth is reported for both Ohmic and RI mode target plasmas.

Comparison of the performance of ICRF antennas with and without Faraday shield on TEXTOR

After the first demonstration of successful operation with ion cyclotron resonance heating (ICRH) on TEXTOR using an antenna without Faraday shield (FS), a further comparison of an antenna with FS and an antenna without FS has been made over a wide range of plasma conditions (including the improved confinement regime on TEXTOR), in zero phase and pi phase operation, using various heating scenarios. No restriction in the operational conditions was found to exist for the screenless antenna. A theoretical analysis shows that antenna side limiters are sufficient to protect the antenna conductor from the plasma and that the plasma in front of the antenna can take over the role of the FS without introducing additional losses. The impact of the operation with both antennas on plasma temperature, density, energy, impurities, etc., is discussed in detail

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

Study of the ITER ICRH system with external matching by means of a mock-up loaded by a variable water load

A mock-up of the complete antenna array (24 straps grouped in 8 triplets) of the ICRH system with external matching for ITER has been constructed with a length reduction factor of 5. At a frequency increased by the same factor the electrical properties of the full-scale system can be measured in the presence of non-dispersive medium. A movable water tank in front of the array simulates variable plasma loading. Measurements of the matching performances of various external circuit configurations and of the scattering matrix of the system show (i) the non-negligible effect of mutual coupling on load resilient matching by Conjugate T (CT) or hybrid leading to coupling between the matching actuators and the generators and asymmetry in power distribution, (ii) good load resilience of a single CT for the right choice of configuration and number of matching parameters, (iii) the large number of matching solutions for coupled CTs and (iv) the benefit of passive power distribution to the straps. This has been successfully tested in the case of the complete array. The power is passively distributed among the upper half and the bottom half of the 24 radiating straps of the antenna plug. The 4 top and 4 bottom triplets are, respectively, set in parallel outside the antenna plug near a voltage anti-node by means of T junctions. The load resilient matching (VSWR <1.3 for an antenna loading variation of about 1–8 Ω m−1) is then obtained by a 4-parameters single CT configuration or a hybrid. The maximum voltage along the line remains equal to the one in the antenna plug and there is a fair power share between the straps. A straightforward robust matching procedure of the complete array is described.The effective radiation resistance of different toroidal and poloidal phasing conditions is measured and compared. The paper also underlines the significant influence of the presence of the electrostatic screen and the resulting increase in the recess of the straps on the reduction of the coupling to the load and of the mutual coupling between the straps.

Status of the ITER IC H&CD System

The ITER Ion Cyclotron Heating and Current Drive system will deliver 20 MW of radio frequency power to the plasma in quasi continuous operation during the different phases of the experimental programme. The system also has to perform conditioning of the tokamak first wall at low power between main plasma discharges. This broad range of requirements imposes a high flexibility and a high availability. The paper highlights the physics and design requirements on the IC system, the main features of its subsystems, the predicted performance, and the current procurement and installation schedule.

Confinement transitions with radiation cooling in TEXTOR-94

New experiments performed at high power with radiation cooling on TEXTOR-94, upgraded for long pulses, have shown the following. (i) Improved confinement conditions at high density (for which the plasma is mainly thermal) lasting for several seconds up to the present maximum flat top time of the upgraded machine with a low and stationary central impurity content. Note that the radiation occurs near the plasma edge, mainly inside the last closed magnetic flux surface. (ii) The existence of confinement transitions during the radiatively cooled phase of the discharge: the already observed transitions from higher to lower confinement regimes have been confirmed but, remarkably, transitions from lower to higher confinement are now also present, leading to an improvement of the confinement time of up to a factor of three.