P. Dumortier

Performance of the ITER ICRH system as expected from TOPICA and ANTITER II modelling

The performance on plasma of the antennas of the proposed ITER ICRF system is evaluated by means of the antenna 24 × 24 impedance matrix provided by the TOPICA code and confirmed and interpreted by the semi-analytical code ANTITER II (summarized in an appendix). From this analysis the following system characteristics can be derived: (1) a roughly constant power capability in the entire 40–55 MHz frequency band with the same maximum voltage in the eight feeding lines is obtained for all the considered heating and current drive phasings on account of the broadbanding effect of service stubs. (2) The power capability of the array significantly depends on the distance of the antenna to the separatrix, the density profile in the scrape-off layer (SOL) and on the strap current toroidal and poloidal phasings. The dependence on phasing is stronger for wider SOL. (3) To exceed a radiated power capability of 20 MW per antenna array in the upper part of the frequency band, with a separatrix–wall distance of 17 cm and a conservative short decay plasma edge density profile, the system voltage stand-off must be 45 kV and well chosen combinations of toroidal and poloidal phasing are needed. (4) On account of the plasma gyrotropy and of poloidal magnetic field, special care must be taken in choosing the optimal toroidal current drive and poloidal phasings.The ANTITER II analysis shows furthermore that important coaxial and surface mode excitation can only be expected in the monopole toroidal phasing, that strong wave reflection from a steep density profile significantly reduces the coupling even if the separatrix is closer to the antenna and that the part of the edge density profile having a density lower than the cut-off density pertaining to the considered phasing does not significantly contribute to the coupling.

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.

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.

Overview of transport, fast particle and heating and current drive physics using tritium in JET plasmas

Results are presented from the JET Trace Tritium Experimental (TTE) campaign using minority tritium (T) plasmas (n(T)/n(D) 2 MA) and monotonic q-profiles. In CH discharges the gamma-ray emission decay times are much lower than classical (tau(Ts) + tau(alpha s)), indicating alpha confinement degradation, due to the orbit losses and particle orbit drift predicted by a 3-D Fokker-Planck numerical code and modelled using TRANSP.

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

ELECTRON CYCLOTRON RESONANCE HEATING ON TEXTOR

Abstract TEXTOR is equipped with two gyrotrons at 110 and 140 GHz, respectively. Both share a single power supply and a confocal quasi-optical transmission line. They cannot be operated simultaneously. The 110-GHz gyrotron with limited power and pulse length (300 kW; 200 ms) has been used in a first series of experiments on electron cyclotron resonance heating (ECRH) and electron cyclotron current drive (ECCD) and for collective Thomson scattering (CTS) diagnostics of energetic ions. In the future the 110-GHz gyrotron will be operated exclusively for CTS diagnostics, while for ECRH and ECCD, the newly installed 140-GHz, high-power (800-kW), long-pulse (>3-s) gyrotron is now available. The highlights of first ECRH experiments with the 110-GHz gyrotron are reported. These include observations of internal transport barriers with ECRH on various target plasmas: in the current plateau phase of both ohmic and radiation improved mode (RI-mode) discharges. In addition, sawtooth control by localized ECRH is demonstrated. First results on CTS include the observation of the slowing down of energetic ions and of the redistribution of energetic ions in sawtooth crashes.

The influence of plasma-edge properties on high confinement discharges with a radiating plasma mantle at the tokamak TEXTOR-94

The radiative improved mode obtained on the limiter tokamak TEXTOR-94 combines the possibility of power exhaust by a radiating plasma boundary (with a fraction of the radiated power with respect to the total input power up to 90% with neon or argon cooling) with improved energy confinement (as good as in the ELM-free H-mode in divertor tokamaks) at high plasma densities (line-averaged central-electron density equal to or even above the Greenwald density limit nGW) in quasi-stationary discharges. An overview is given of the substantial changes in plasma-edge properties occurring at high radiated power levels . These changes are characterized by a reduction of the plasma-edge density and temperature, a reduction of particle transport out of the confined plasma volume and an increase of the penetration depth of deuterium and impurity atoms. As a consequence, the particle confinement time increases and the electron-density profiles steepen. The transition to improved confinement takes place as soon as the density peaking reaches a critical threshold. An internal transport barrier is observed in the bulk of RI-mode plasmas (at r=a6 0:6) characterized by an increase of the pressure gradient and of the shear of the toroidal velocity compared to discharges without additional impurity seeding. The dilution at the plasma boundary is strongly increased by the seeded impurities whereas the central dilution is only weakly affected.

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.