The ITER Ion Cyclotron Heating and Current Drive system will deliver 20MW 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 reqiurements imposes a high flexibility and a high availabiUty. The paper highlights the physics and design reqiurements on the IC system, the main features of its subsystems, the predicted performance, and the current procurement and installation schedide.

Long pulse operation on the Tore Supra tokamak has entered a new phase, characterized by the use of heating power level in excess of 10 MW, during pulses lasting several tens of resistive times. This has been made possible by the use of ion cyclotron range of frequency (ICRF) heating (9 MW coupled to the plasma at 57 MHz), combined with lower hybrid current drive (LHCD: 3 MW at 3.7 GHz) and efficient fuelling techniques (supersonic gas injection, pellets). This paper addresses key technological, operational and physics issues related to the long pulse operation of the Tore Supra ICRF system and required for a reactor: R&D on the ICRF plant, real-time control and safety procedures, integration with other tokamak subsystems, experimental investigation and theoretical modelling of the edge ICRF physics (wave coupling, heat loads on antenna front faces). As far as possible lessons are drawn from the experience gained on Tore Supra for the design and operation of a next-step device.

No abstract

n the framework of the ion cyclotron resonance frequency (ICRF) heating development at CEA Cadarache, a prototype antenna based on the load-resilient electrical layout foreseen for ITER has been built. This prototype was recently tested in Tore Supra. The ITER-like electrical scheme has been validated during fast perturbations at the edge plasma. Clear load resilience properties are reported. Main conclusions and consequences to be learned for the development of ITER antenna are discussed.

In the framework of the ion cyclotron resonance heating (ICRH) development led at CEA Cadarache, an actively cooled Faraday screen (FS) prototype with cantilevered horizontal bars and a slotted box has been designed to increase the heat exhaust capability (for high-power operation), reduce the parallel RF electric field along long field lines and qualify alternative mechanical solutions for ITER (bars are disconnected from the septum to reduce the stress level). The new FS has been installed on an existing ICRH antenna, and was tested during the 2011 Tore Supra experimental campaign. The antenna hosting the new screen exhibits high sensitivity to the edge plasma condition, some instabilities of electrical matching and improved heat exhaust capabilities in accordance with the thermo-mechanical design. RF-induced heat loads derived from IR thermography have been found to be about five times higher in the equatorial plane with the new design compared with the conventional design. The experimental results show that minimizing the parallel RF electric field along long field lines is not enough to reduce the wave–plasma interaction on the screen. This paper summarizes the experimental RF-induced heat load for several plasma scenarios and edge parameters (plasma current, density and heating power level) with emphasis on RF-sheath rectification and E × B convection generated in front of the antenna through the differential biasing of adjacent field lines.

Radio frequency (RF) sheaths are suspected of limiting the performance of present-day ion cyclotron range of frequencies (ICRFs) antennas over long pulses and should be minimized in future fusion devices. Within the simplest models, RF-sheath effects are quantified by the integral V RF = ∫ E ∥ · dl where the parallel RF field E ∥ is linked with the slow wave. On ‘long open field lines’ with large toroidal extension on both sides of the antenna it was shown that V RF is excited by parallel RF currents j ∥ flowing on the antenna structure. In this paper, the validity of this simple sheath theory is tested experimentally on the Tore Supra (TS) ITER-like antenna prototype (ILP), together with antenna simulation and post-processing codes developed to compute V RF. The predicted poloidal localization of high-|V RF| zones is confronted to that inferred from experimental data analysis. Surface temperature distribution on ILP front face, as well as ILP-induced modifications of RF coupling and hot spots on a magnetically connected lower hybrid current drive antenna, indicates local maxima of dc plasma potential in both the upper and lower parts of the ILP. This result, qualitatively conforming to V RF simulations, is interpreted in terms of j ∥ flowing on ILP frame. Once the validation is done, such reliable theoretical models and numerical codes are then employed to provide predictive results. Indeed, we propose two ways to reduce |V RF| by acting on j ∥ on the antenna front face. The first method, more adapted for protruding antennas, consists of avoiding the j ∥ circulation on the antenna structure, by slotting the antenna frame on its horizontal edges and by partially cutting the Faraday screen rods. The second method, well suited for recessed antennas, consists of compensating j ∥ of opposite signs along long flux tubes, with parallelepiped antennas aligned with (tilted) flux tubes. The different concepts are assessed numerically on a two-strap TS antenna phased [0, π] using near RF fields from the antenna code TOPICA. Simulations stress the need to suppress all current paths for j ∥ to substantially reduce |V RF| over the whole antenna height.

Three identical new WEST Ion Cyclotron Resonance Heating (ICRH) antennas have been designed, assembled then commissioned on plasma from 2013 to 2019. The WEST ICRH system is both load-resilient and compatible with long-pulse operations. The three antennas have been successfully operated together on plasma in 2019 and 2020. The load resilience capability has been demonstrated and the antenna feedback controls for phase and matching have been developed. The breakdown detection systems have been validated and successfully protected the antennas. The use of ICRH in combination with Lower Hybrid has triggered the first high confinement mode transitions identified on WEST.

No abstract