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

Abstract

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.

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

“…It first computes the surface impedance at the antenna box aperture for deuterium-tritium (D-T) plasma with the electron density profile shown in figure 3. This allows us to solve the Maxwell's equations with the appropriate boundary conditions in the antenna box and in the plasma profile [8]. The active radiated power is obtained from (1) and evaluated at the antenna front or equivalently from the EMF method applied on the surface of the straps.…”

“…This code includes a plasma description by the code FELICE [10]. The modeling of the ITER antenna array by TOPICA and ANTITER II is compared in [8]. The results of both codes are qualitatively and even quantitavely very similar.…”

Fast wave current drive in DEMO

“…One can immediately identify 2 regions of dominant electron absorption with potential for efficient CD: f<25MHz, where none of the plasma ions is resonant inside the plasma and f=70-75MHz, where the second harmonic T absorption is shifted to the high-field side and the harmonic resonances of D and -particles have not yet entered the plasma. [5] for an antenna identical to the one being designed for ITER but placed in DEMO (R ant =11.45m). One readily sees that the f=72MHz case gives the highest CD value (0.037A/W) with a maximum driven current around =0.5-0.8m, followed by the f=25MHz case (0.032A/W), which features more central current-drive.…”

Study of a distributed ICRF antenna system in DEMO

“…Here only the mutual coupling with the first two neighbours (on each side) are considered. Exploiting the symmetry of the system means that all values of the self-inductance L i =L and mutual couplings M i+1 =M 1 , M i+2 =M 2 , for i = 1, …, N str , are respectively the same and all other elements of the [5]). The array is configured as combline with a tuning reactance on each strap and can be described by an array impedance matrix Z a such that 1/Z a = 1/Z s +1/Z c where 1/Z c is the admittance matrix of the tuning capacitors.…”