Design, performance, and grounding aspects of the International Thermonuclear Experimental Reactor ion cyclotron range of frequencies antenna

Abstract

ITER's Ion Cyclotron Range of Frequencies (ICRF) system [Lamalle et al., Fusion Eng. Des. 88, 517–520 (2013)] comprises two antenna launchers designed by CYCLE (a consortium of European associations listed in the author affiliations above) on behalf of ITER Organisation (IO), each inserted as a Port Plug (PP) into one of ITER's Vacuum Vessel (VV) ports. Each launcher is an array of 4 toroidal by 6 poloidal RF current straps specified to couple up to 20 MW in total to the plasma in the frequency range of 40 to 55 MHz but limited to a maximum system voltage of 45 kV and limits on RF electric fields depending on their location and direction with respect to, respectively, the torus vacuum and the toroidal magnetic field. A crucial aspect of coupling ICRF power to plasmas is the knowledge of the plasma density profiles in the Scrape-Off Layer (SOL) and the location of the RF current straps with respect to the SOL. The launcher layout and details were optimized and its performance estimated for a worst case SOL provided by the IO. The paper summarizes the estimated performance obtained within the operational parameter space specified by IO. Aspects of the RF grounding of the whole antenna PP to the VV port and the effect of the voids between the PP and the Blanket Shielding Modules (BSM) surrounding the antenna front are discussed. These blanket modules, whose dimensions are of the order of the ICRF wavelengths, together with the clearance gaps between them will constitute a corrugated structure which will interact with the electromagnetic waves launched by ICRF antennas. The conditions in which the grooves constituted by the clearance gaps between the blanket modules can become resonant are studied. Simple analytical models and numerical simulations show that mushroom type structures (with larger gaps at the back than at the front) can bring down the resonance frequencies, which could lead to large voltages in the gaps between the blanket modules and perturb the RF properties of the antenna if they are in the ICRF operating range. The effect on the wave propagation along the wall structure, which is acting as a spatially periodic (toroidally and poloidally) corrugated structure, and hence constitutes a slow wave structure modifying the wall boundary condition, is examined.

Conceptual study of an ICRH traveling-wave antenna system for low-coupling conditions as expected in DEMO

“…This is shown in figure 11(c). To benefit from the substantial r A increase with a longer strap without too large strap voltage increase, one can either (i) split the strap in sections with parallel feeding (as the strap triplets fed by a four-port junction in the ITER antenna design [11,12]) or (ii) use central grounding with push-pull feeding. These solutions also have the advantage of maintaining nearly constant the current amplitude all along the strap.…”

Maximization of ICRF power by SOL density tailoring with local gas injection

“…In ITER there are significant uncertainties in the density profiles predicted in the SOL, which lead to large uncertainties in the predicted coupled power (between 10 MW and 20 MW per antenna limited by the installed generator power) using antenna simulation codes [5] and taking into account the various operating limits in the antenna system (e.g. maximum voltage at the straps and in the transmission lines, radio-frequency (RF) field at the 3-port junction, etc) [6]. Some years ago, it was proposed to use local gas injection [7] as a tool to tailor the electron density in front of the ICRF antennas during the pulses and hence maximize or control the antenna coupling resistance.…”

Radiofrequency and mechanical tests of silver coated CuCrZr contacts for the ITER ion cyclotron antenna

“…The ITER Ion Cyclotron Resonance Heating and Current Drive (ICRH&CD) system is designed to couple to the plasma 20 MW (10 MW each from two antennas) of RF power in the 40-55 MHz frequency range in continuous wave operation (3600 s) [5][6][7]. The RF power, generated by nine 3 MW sources (four per antenna and one spare) [8], is carried though pressurized coaxial lines to 8 feeds per antenna.…”