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To avoid the excitation of modes in the 20 mm clearance gap between the ITER ICRH antenna plug and the vacuum vessel in the ITER frequency band it was proposed to perform galvanic grounding at 1 m from the antenna plug front. Experimental S-matrix and gap voltage measurements on a reduced-scale mock-up of the ITER ICRH antenna as well as numerical simulations show indeed that the frequency response of the antenna array is perturbed by a TE 0,1 mode resonance in the ITER frequency band (around 45 MHz) in absence of any additional grounding and may lead to large voltages in the clearance gap. They also confirm the efficiency of the proposed grounding solution. Grounding all around the antenna plug periphery (“full” grounding) is compared to grounding with discrete contacts (“partial” grounding). The influence of the position of the grounding is also studied. Eigen modes analysis allows identifying the modes experimentally observed. Finally a practical solution considered for implementation on ITER is validated both by numerical simulations and experimental measurements.

The dielectric load can fairly well simulate the inhomogeneous plasma loading even in absence of description of the anisotropy effects. There is the same determining dependence on the following plasma density and dielectric constant KD profile characteristics of the coupling performances: (i) distance antenna-cutoff layer and position of an optimum density or dielectric constant value layer with respect to the cutoff one (ii) their gradient between this optimum layer and the bulk regions. The best simulation of the considered ITER profiles (for frequency response and quantitative coupling measurements for the different toroidal phasing cases) is obtained by the relation K D ( x ) = 1 + ( ω pi ( x ) ) 2 / ( ω ω ci ( x ) + ω ci 2 ( x ) ) (KD(x) = ɛ1 − ɛ2 for multispecies plasma) which puts the plasma and dielectric cutoff for all toroidal wavenumbers at the same position. Effects of dielectric losses are discussed. Good simulation of the frequency response is also obtained for most of the phasing cases with a homogeneous dielectric as water but a larger value of KD ≅ 200 would cover all cases. Varying the distance antenna-homogeneous dielectric load can scan a large loading domain covering the one expected for ITER. Illustrations from measurements with the ITER mock-up are given.

Each of the two ITER ICRF antennas consists of a close-packed array of 24 straps arranged in a 6 poloidal by 4 toroidal array. Three poloidally adjacent straps (a “triplet” of straps) are fed together through a 4-port junction from one 20 Ohm feeding line. The complete array has to radiate 20 MW of RF power over a frequency range of 40 MHz to 55 MHz and for different toroidal phasings. The RF optimization of the antenna has been performed numerically on one triplet of straps (1/8th of the antenna) [1] , [2] . In parallel a number of reduced-scale mock-ups of one triplet of the ITER ICRH antenna were constructed in order to validate the results of the numerical optimization [1] , [3] . The aim of this work is primarily to benchmark the CST MWS® [4] numerical modeling against numerous measurements done on the mock-up of the 2007 design. Moreover MWS calculates the 3D distribution of the currents and of the fields of the triplet. Hence it gives the possibility to check the fields and current distributions resulting from the optimisation study of the ITER ICRH antenna triplet done by changing geometrical parameters of the straps and antenna box of the mock-up of 2007 design [1] , [2] , [3] . The considered parameters are: strap width, antenna box depth and vertical septum recess with respect to the front of the current strap. The impact of the presence of the Faraday screen is also evaluated. Excellent agreement between modeled and measured S parameters is obtained. Analysis of the fields and currents distributions on the straps is reported. Excellent current balance is confirmed.

TOPICA is an innovative tool for the simulation of the Ion Cyclotron Radio Frequency (ICRF) antenna systems that incorporates commercial-grade graphic interfaces into a fully 3D self-consistent description of the antenna geometry and an accurate description of the plasma; it can be considered as a "Virtual Prototyping Laboratory" to assist the detailed design phase of the antenna system. Recent theoretical and computational advances of the TOPICA code has allowed to incorporate a CAD drawing capability of the antenna geometry, with fully 3D geometrical modeling, and to combine it with a 1D accurate plasma description that takes into account density and temperature profiles, and FLR effects; the profiles are inserted directly from measured data (when available), or specified analytically by the user. The coaxial feeding line is modeled as such; computation and visualization of relevant parameters (input scattering parameters, current and field distributions, etc.) complete the suite. The approach to the problem is based on an integral-equation formulation for the self-consistent evaluation of the current distribution on the conductors. The environment has been subdivided in two coupled region: the plasma region and the vacuum region. The two problems are linked self-consistently by representing the field continuity in terms of equivalent (unknown) sources. In the vacuum region all the calculations are executed in the spatial (configuration) domain, and this allows triangular-facet description of the arbitrarily shaped conductors and associated currents; in the plasma region a spectral representation of the fields is used, which allows to enter the plasma effect via a surface impedance matrix; for this reason any plasma model can be used, and at present the FELICE code has been adopted; special techniques have been adopted to increase the numerical efficiency. The TOPICA suite has been previously tested against assessed codes and against measurements of mock-ups and existing antennas. This work is devoted to an extensive set of comparisons between measured and simulated reflection coefficients (magnitude and phase), both in vacuum and with plasma during ALCATOR C-MOD operation. The comparison demonstrates a very good agreement, leading to a validation of TOPICA as a predictive tool.

This work presents a comprehensive account of the theory and implementation of a method for the self-consistent numerical analysis of plasma-facing ion-cyclotron resonance heating (ICRH) antenna arrays. The method is based on the integral-equation formulation of the boundary-value problem, solved via a weighted-residual scheme. The antenna geometry (including Faraday shield bars and a recess box) is fairly general and three-dimensional (3D), and the plasma is in the one-dimensional (1D) 'slab' approximation; finite-Larmor radius effects, as well as plasma density and temperature gradients, are considered. Feeding via the voltages in the access coaxial lines is self-consistently accounted throughout and the impedance or scattering matrix of the antenna array obtained therefrom. The problem is formulated in both the dual space (physical) and spectral (wavenumber) domains, which allows the extraction and simple handling of the terms that slow the convergence in the spectral domain usually employed. This paper includes validation tests of the developed code against measured data, both in vacuo and in the presence of plasma. An example of application to a complex geometry is also given.

The consequences of sheath (rectified) electric fields, resulting from the different mobility of electrons and ions as a response to radio frequency (RF) fields, are a concern for RF antenna design as it can cause damage to antenna parts, limiters and other in-vessel components. As a first step to a more complete description, the usual cold plasma dielectric description has been adopted, and the density profile was assumed to be known as input. Ultimately, the relevant equations describing the wave-particle interaction both on the fast and slow timescale will need to be tackled but prior to doing so was felt as a necessity to get a feeling of the wave dynamics involved. Maxwell's equations are solved for a cold plasma in a 2D antenna box with strongly varying density profiles crossing also lower hybrid and ion-ion hybrid resonance layers. Numerical modelling quickly becomes demanding on computer power, since a fine grid spacing is required to capture the small wavelengths effects of strongly evanescent modes.

The ongoing design of the ITER Ion Cyclotron Heating and Current Drive system (20 MW, 40-55 MHz) is rendered challenging by the wide spectrum of requirements and interface constraints to which it is subject, several of which are conflicting and/or still in a high state of flux. These requirements include operation over a broad range of plasma scenarios and magnetic fields (which prompts usage of wide-band phased antenna arrays), high radio-frequency (RF) power density at the first wall (and associated operation close to voltage and current limits), resilience to ELM-induced load variations, intense thermal and mechanical loads, long pulse operation, high system availability, efficient nuclear shielding, high density of antenna services, remote-handling ability, tight installation tolerances, and nuclear safety function as tritium confinement barrier. R&D activities are ongoing or in preparation to validate critical antenna components (plasma-facing Faraday screen, RF sliding contacts, RF vacuum windows), as well as to qualify the RF power sources and the transmission and matching components. Intensive numerical modeling and experimental studies on antenna mock-ups have been conducted to validate and optimize the RF design. The paper highlights progress and outstanding issues for the various system components. (C) 2013 ITER Organization. Published by Elsevier B.V. All rights reserved.

The excitation of non‐TEM modes around the ITER ICRF antenna plug can considerably increase the level of RF voltages and currents on the ITER plug. First study of these modes and a solution to avoid them in the ITER ion cyclotron range of frequencies were reported in [1]. In this work a detailed analysis of electrical properties of the ITER ICRF antenna with the plug was studied for different grounding solutions with CST Microwave Studio® [2]. Conclusions of an earlier work [ 1 ] were confirmed on the detailed model of the antenna with the plug. Different grounding contacts (capacitive, galvanic and mixed capacitive‐galvanic) as well as their distribution inside the plug gap were analyzed. It was shown that capacitive and mixed capacitive‐galvanic grounding are less effective because they demand high values of the capacitance and are more sensitive to the frequency and antenna spectrum. In particular a galvanic grounding realized by the contacts put around the perimeter of the plug gap at lm behind the fr...

Simulations of realistic RF antennas have been performed with the finite element analysis software package COMSOL both with dielectric media and cold plasma loading. Results are presented from simulations with a finite (low) plasma density in the antenna box and ITER like density profile in front of the antenna. First steps were made to introduce the presence of sheaths by means of a non‐linear boundary condition. Output characteristics such as scattering parameters and fields have been compared with results from CST Microwave Studio® and the TOPICA code. Good agreement has been found, which illustrates the potential of the present COMSOL‐based modeling to benchmark antenna codes. Furthermore non‐linear conditions can be imposed on boundaries, which is important for sheath effect modeling.

Up to now, classical “water” loads have been used for low power testing of ITER ICRH prototype or mock‐up antennas . A fair description of the antenna frequency response is obtained excepted for the phasing (0 π 0 π). High dielectric loads are requested to improve the antenna response in the low frequency band [1]. In view of laboratory testing, dummy loads are also required to have efficient wave spatial attenuation to avoid standing waves and to minimize load volume. In this paper, barium titanate ceramic powders mixed with water are shown to exhibit very attractive electromagnetic properties. Coupling performance of one triplet of the ITER ICRH antenna to such kind of loads is numerically investigated. The radiated wave attenuation into the load is also characterized. In spite of its frequency dispersion, “barium titanate” loads are shown to allow the characterization of the full scale triplet frequency response on a scaled‐down mock‐up.

Calculations aimed at representing the plasma‐vacuum boundary condition in real space are presented. Some ideas about how this representation can be used for power coupling analysis and antenna modeling are put forward. The relation between tangential electric and magnetic fields on the plasma surface is linear and customary expressed in the spectral domain for every partial wave via 2×2 plasma impedance matrix—Zp(ky,Kz). In this work, the tangential fields Et and Bt on the plasma surface are projected on a suitably chosen set of basis functions. Then the linear operator relating the components Et and Bt expressed in such a way is derived from the known matrix Zp(ky,Kz). It is shown that in this representation the plasma surface has properties similar to the array of ports of a microwave system and can be formally described by a corresponding impedance matrix in the way it is done in microwave analysis. The obtained representation can be exploited for deeper understanding of the RF power coupling to t...

The Tore Supra (CEA) antenna for the ion cyclotron range of frequencies (ICRF) is a so-called resonant double loop antenna with internal matching capacitors. The required capacitance range is crucial for antenna and the tuning-and-matching network design. The capacitance value for a given discharge is also important for antenna operation. We discuss a method to accurately predict the tuning capacitance values in the presence of plasma operation and for efficiently accounting for the capacitors in the performance analysis. The presented method relies on the use of the TOPICA code, a numerical suite developed for prediction and analysis of modern ICRF antenna systems; it is able to handle antennas with a realistic 3D geometry and an accurate 1D plasma model. Very good agreement with the experimental capacitance values is found using TOPICA, providing further experimental validation of the computational algorithm and physics model.

The demand for a predictive tool to help designing ICRH antennas for fusion experiments has driven the development of codes like ICANT, RANT3D, and the early developments and further upgrades of TOPICA code. Currently, TOPICA handles the actual geometry of ICRH antennas (with their housing, etc.) as well as a realistic plasma model, including density and temperature profiles and FLR effects. Both goals have been attained by formally splitting the problem into two parts: the vacuum region around the antenna, and the plasma region inside the toroidal chamber. Field continuity and boundary conditions allow writing a set of coupled integral equations for the unknown equivalent (current) sources; finite elements are used on a triangular-cell mesh and a linear system is obtained on application of the weighted-residual solution scheme. In the vacuum region calculations are done in the spatial domain, whereas in the plasma region a spectral (wavenumber) representation of fields and currents is adopted, thus allowing a description of the plasma by a surface impedance matrix. Thanks to this approach, any plasma model can be used in principle, and at present Brambilla's FELICE code has been employed. The natural outputs of TOPICA are the induced currents on the conductors and the electric field in front of the plasma, whence the antenna circuit parameters (impedance/scattering matrices), the radiated power and the fields (at locations other than the chamber aperture) are then obtained. An accurate model of the feeding coaxial lines is also included. This paper is precisely devoted to the description of TOPICA, whereas examples of results for real-life antennas are reported in a companion paper [1] in this proceedings. ©2005 American Institute of Physics

An innovative tool has been realized for the 3D/1D simulation of Ion Cyclotron Radio Frequency (ICRF), i.e. accounting for antennas in a realistic 3D geometry and with an accurate 1D plasma model. The approach to the problem is based on an integral‐equation formulation for the self‐consistent evaluation of the current distribution on the conductors. The environment has been subdivided in two coupled region: the plasma region and the vacuum region. The two problems are linked by means of a magnetic current (electric field) distribution on the aperture between the two regions. In the vacuum region all the calculations are executed in the spatial domain while in the plasma region an extraction in the spectral domain and an analytical evaluation of some integrals are employed that permit to significantly reduce the integration support and to obtain a high numerical efficiency leading to the practical possibility of using a large number of sub‐domain basis functions on each solid conductor of the system. The p...

An innovative tool has been realized for the simulation of 3-dimensional Ion Ciclotron Radio Frequency (ICRF) antennas in a realistic geometry. The approach to the problem is based on an integral-equation formulation for the self-consistent evaluation of the current distribution on antennas facing a plasma in a slab geometry. The plasma enters the formalism via a surface impedance matrix; for this reason any plasma model can be used (presently the FELICE code has been adopted). A vacuum-term extraction and an analytical evaluation of some integrals are employed that permit to significantly reduce the integration support and to obtain a high numerical efficiency leading to the practical possibility of using subdomain basis functions on each conductor of the antenna system. Calculation of field distributions (both magnetic and electric), useful for sheat considerations, is included. This tool has been implemented in a suite that is modular and applicable to ICRF antenna structures of arbitrary shape. This new simulation tool can assist during the detailed design phase and for this reason can be referred to as a “Virtual Prototyping Laboratory” (VPL). The VPL has been tested against assessed codes and against measurements of mock-up and prototype antennas. The VPL is being used for the ICRF antenna analysis and design of various machine (like IGNITOR and ITER).

Sheaths tend to form when immersing metallic objects in plasmas. As it avoids the need to capture the sheath details, which occur on the Debye length scale while antennas are typically various orders of magnitude larger, the sheath boundary condition due to D'Ippolito and Myra (2006 Phys. Plasmas 13 102508, 2008 Phys. Plasmas 15 102501) offers antenna designers a major reduction in the numerical problem size they face. The sheath boundary condition was derived by making a number of simplifying assumptions to enable finding an analytical approximation of the conditions rapidly oscillating waves have to satisfy beyond the sheath that forms close to such objects. This paper discusses the solution of the cold plasma wave equation for sheath relevant density profiles, e.g. highlighting the role of the orientation of the static magnetic field and of oblique incidence, and underlining the impact the density profile has on the wave physics. It illustrates that the cross-talk between the waves impinging on and those excited at the wall and in the sheath sensitively depends on a number of parameters. The 2 × 2 connection coefficient matrix that is numerically obtained captures the sheath region fast time scale wave physics for a given density profile. When supplemented with a satisfactory model for the slow time scale variation it is a numerical tool that permits upgrading the realism of the fast time scale wave physics contained in the sheath boundary condition and that can help delimiting the range of applicability of simplified models, and assessing if a sufficiently general set of boundary conditions to describe the effect of the sheath can at all be constructed.

By adopting the usual cold plasma dielectric tensor, it is demonstrated that a rapidly oscillating electric field gives rise to slow time scale drifts, which cause density modifications near antennas. In the presence of a strong magnetic field, the poloidal gradients of the field are at the origin of radial displacements of the plasma while radial field gradients have the potential to trigger density inhomogeneity along the antenna. The radio frequency-induced plasma drifts are more prominent at higher power and for more evanescent modes. It is discussed that the usual cold plasma dielectric tensor is derived neglecting nonlinear effects and zero-order drifts, and therefore does not uniformly allow the capture of the wave–particle interaction near the antenna self-consistently, necessitating a more detailed description to capture both wave and particle effects on the one hand, and global wave propagation and local sheath effects, on the other. A strategy is proposed to complement the model with other needed ingredients enabling one to capture the dynamics on the fast and slow time scales.

A minimal two-field fluid approach is followed to describe the radio-frequency (RF) wave propagation in the bounded scrape-off layer plasma of magnetic fusion devices self-consistently with direct current (DC) biasing of this plasma. The RF and DC parts are coupled by non-linear RF and DC sheath boundary conditions at both ends of open magnetic field lines. The physical model is studied within a simplified framework featuring slow wave (SW) only and lateral walls normal to the straight confinement magnetic field. The possibility is however kept to excite the system by any realistic 2D RF field map imposed at the outer boundary of the simulation domain. The self-consistent RF + DC system is solved explicitly in the asymptotic limit when the width of the sheaths gets very large, for several configurations of the RF excitation and of the target plasma. In the case of 3D parallelepipedic geometry, semi-analytical results are proposed in terms of asymptotic waveguide eigenmodes that can easily be implemented n...

ITER as a superconducting fusion machine needs efficient wall conditioning techniques for application in the presence of the permanent high toroidal magnetic field for (i) reducing the in-vessel impurity content, (ii) controlling the surface hydrogen isotopic ratio and (iii) mitigating the in-vessel long-term tritium inventory build-up. Encouraging results recently obtained with ion-cyclotron wall conditioning (ICWC) in the present-day tokamaks and stellarators have raised ICWC to the status of one of the most promising techniques available to ITER for routine inter-pulse and overnight conditioning with the ITER main ICRF heating system in the presence of the permanent high toroidal magnetic field. This paper is dedicated to a milestone experiment in ICWC research: the first simulation of ICWC operation in an equivalent ITER full-field scenario and the assessment of the wall conditioning effect on the carbon wall in the largest present-day tokamak JET. In addition, we address in this paper the following topics: (i) an analysis of the radio frequency (RF) physics of ICWC discharges, (ii) the optimization of the operation of ICRF antennas for plasma startup and (iii) an outlook for the performance of ICWC in ITER using the ICRF heating system. Important operational aspects of the conventional ICRF heating system in JET (the so-called A2 antenna system) for use in the ICWC mode are highlighted: (i) the ability of the antenna to ignite the cleaning discharge safely and reliably in different gases, (ii) the capacity of the antennas to couple a large fraction of the RF generator power (>50%) to low-density (≈1016–1018 m−3) plasmas and (iii) the ICRF absorption schemes aimed at improved RF plasma homogeneity and enhanced conditioning effect. Successful optimization of the JET-ICWC discharge parameters (BT = 3.3 T, f = 25 MHz) resulted in a reliable operation of the JET A2 antennas and a high conditioning efficiency in a scenario imitating closely ITER full-field operation (BT = 5.3 T, f = 40 MHz) with the fundamental ion-cyclotron resonance for deuterium located on-axis. Numerical modelling with the 3D electromagnetic code Micro Wave Studio, a 1D RF full wave code and a 0D plasma code allows extrapolating the results obtained on JET and other present-day tokamaks to ITER and provides good prospects for the use of the ITER ICRF antennas for ICWC purposes.