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Ion Cyclotron Radio Frequency (ICRF) antennas operating under high voltage to couple high power to fusion plasmas are at risk of electrical arcing. The standard Voltage Standing Wave Ratio (VSWR) system does not protect low impedance areas, which are used in antennas to achieve load tolerance to variations in plasma loading during Edge Localised Modes (ELMs). The Scattering Matrix Arc Detection System (SMAD) was designed to create additional protection for these areas, and 4 complete systems were implemented and tested on the ITER-like antenna (ILA) on JET. This paper describes the performance under relevant experimental conditions of load tolerance and/or high voltage and electrical arcing causing generator trip events.

Operating ion cyclotron resonance heating (ICRH) antennas at high power density puts them at risk of arcing which reduces the coupled power to the plasma because the perturbed impedance match triggers the voltage standing wave ratio (VSWR) based generator trip system but even worse might damage the antenna beyond repair because of the arc's localised energy deposition. New antennas are designed to operate in a load tolerant way which creates low impedance zones that are especially at risk since the existing VSWR protection systems are less sensitive to arcs in these areas. To protect these low impedance areas, a new arc protection system referred as scattering matrix arc detection (SMAD) has been proposed. This paper describes the basic operating principle and implementation in hard- and software for the JET ITER-like antenna (ILA), with testbed and preliminary JET commissioning results.

The ion cyclotron resonance frequency (ICRF) heating system on JET is currently being upgraded to validate new matching concepts in view of coupling ICRF power to ITER-like plasmas and to further increase the total additional heating power on JET. The present paper reports on first testbed results from the new JET ITER-like antenna as well as on the first use of the newly installed hybrid couplers between two of the existing A2 antennas. Several other on-going improvements, such as improved trip management system, external conjugate-T matching circuit and arc detection systems are also discussed.

ITER's Ion Cyclotron Range of Frequencies (ICRF) system [1] comprises two antenna launchers designed by CYCLE (a consortium of European associations listed in the author affiliations above) on behalf F4E for the 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.

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.

RF arc detection is a key operational and safety issue for the ICRF system on ITER. Indeed the high voltages inside the antenna put it at risk of arcing, which could cause substantial damage. This paper describes the various possibilities explored by circuit simulation and the strategy now considered to protect the ITER ICRF antenna from RF arcs.

In this paper, significant results in the heating and current drive domains obtained at JET in the past few years following systems upgrade and dedicated experimental time, will be reviewed. Firstly, an overview of the new Ion Cyclotron Resonance Frequency (ICRF) heating capabilities will be presented i.e. results from the ITER‐Like ICRF antenna (ILA), the use of External Conjugate‐T and 3dB hybrid couplers to increase the ICRF power during ELMy H‐mode. Furthermore, experiments to study the influence of the phasing of the ICRF antenna on power absorption and coupling will be described. Looking at Low Hybrid (LH) issues for ITER, the effect of the location of gas injection on the LH coupling improvement at large launcher‐separatrix distances will be discussed as the possibility to operate at ITER‐relevant power densities. Experiments to characterise the LH power losses in the Scrape‐Off‐Layer (SOL) and to determine the LH wave absorption and current drive using power modulation will be shown. Finally, plas...

The commissioning of the ITER‐Like ICRF Antenna (ILA) [1] on JET plasmas from May 2008 to April 2009 in various conditions (33, 42 and 47 MHz, L‐ and H‐mode, antenna strap‐plasma separatrix distances of ∼9 to 17 cm) has provided relevant information for future antenna design and operation. The maximum power density achieved was 6.2 MW/m2 in L‐mode with strap to plasma separatrix distance of ∼9–10 cm at 42 MHz on the lower half of the ILA extrapolating to 8 MW/m2 if the full generator power had been available. Efficient (trip‐free operation) ELM tolerance was obtained both at 33 and 42 MHz on a large range of ELMs with strap voltages up to 42 kV and a maximum power density of 4.1 MW/m2. The paper reviews these achievements as well as remaining issues.

Following an overview of the ITER Ion Cyclotron Resonance Frequency (ICRF) system, the JET ITER‐like antenna (ILA) will be described. The ILA was designed to test the following ITER issues: (a) reliable operation at power densities of order 8 MW/m2 at voltages up to 45 kV using a close‐packed array of straps; (b) powering through ELMs using an internal (in‐vacuum) conjugate‐T junction; (c) protection from arcing in a conjugate‐T configuration, using both existing and novel systems; and (d) resilience to disruption forces. ITER‐relevant results have been achieved: operation at high coupled power density; control of the antenna matching elements in the presence of high inter‐strap coupling, use of four conjugate‐T systems (as would be used in ITER, should a conjugate‐T approach be used); operation with RF voltages on the antenna structures up to 42 kV; achievement of ELM tolerance with a conjugate‐T configuration by operating at 3Ω real impedance at the conjugate‐T point; and validation of arc detection sys...

The Scattering Matrix Arc Detection System (SMAD) has been fully deployed on all 4 sets of Resonant Double Loop (RDL), Vacuum Transmission Line (VTL) and Antenna Pressurised Transmission Lines (APTL) of the JET ICRF ITER‐Like Antenna (ILA) and this has been indispensable for operating at low (real) T‐point impedance values to investigate ELM tolerance. This paper describes the necessity of the SMAD vs VSWR (Voltage Standing Wave Ratio) protection system, SMAD commissioning, problems and a number of typical events detected by the SMAD system during operation on plasma.

Ion Cyclotron Resonance Frequency (ICRF) antennas achieve maximum power transfer from RF generator to plasma load by establishing an impedance match. The JET ITER‐Like Antenna (ILA) consists of 4 mutually coupled resonators that need to be matched simultaneously and whose resonant states need to be accurately controlled by several available actuators to couple maximum power and to achieve optimal ELM resilience. The operation of the matching system is described. Experimental operation revealed some phenomena and sensitivities that did not surface from simulation alone and that should be taken into account for the design of future ICRF antennae systems.

A set of simulation/interpretation tools based on transmission line theory and on the RF model developed by M. Vrancken [2] has been developed to study the ITER‐like Antenna (ILA) at JET. For given tuning element settings, the unique solution of the equations governing the ILA circuit requires solving a system of coupled linear equations relating the voltages and currents at the antenna straps and other key locations. This computation allows cross‐checking predicted values against measured experimental ones. Further more, a minimization procedure allows improving the correspondence with the quantities measured in the circuit during shots, thus coping with unavoidable errors arising from uncertainties in the measurements or from inaccuracies in the adopted RF model. Typical applications are e.g. fine‐tuning of the second‐stage of the ILA circuit for increased ELM‐resilience, cross‐checking the calibration of the measurements throughout the circuit and predicting the antenna performance and matching conditions in new plasma scenarios.

The RF characteristics of the JET‐ITER Like (JET‐IL) antenna relevant for operation on plasma have been assessed using full wave three Dimensional (3D) electromagnetic CST® Microwave Studio (MWS) simulations, measurements of the full 8‐port antenna strap array S/Z‐matrix, and RF circuit modeling. These efforts are made in parallel with the high voltage testing of the antenna inside a vacuum tank and the hardware implementation of a RF (Radio Frequency) matching feedback control system prior to installation of the antenna on the JET tokamak.

The paper summarizes the activities carried out and difficulties encountered assembling and testing the JET‐EP ITER‐like ICRH antenna. First results from low power as well as high power testing and commissioning activities are presented.

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.

A load-tolerant External Conjugate-T (ECT) impedance matching system for two A2 Ion Cyclotron Resonance Heating (ICRH) antennas has been successfully put into operation at JET. The system allows continuous injection of the RF power into plasma in the presence of strong antenna loading perturbations caused by Edge Localized Modes (ELMs). Reliable ECT performance has been demonstrated under a variety of antenna loading conditions including H-mode plasmas with Radial Outer Gaps (ROG) in the range of 4-14 cm. The high resilience to ELMs predicted during the circuit simulations has been fully confirmed experimentally. Dedicated arc detection techniques and real-time matching algorithms have been developed as a part of the ECT project. The new Advanced Wave Amplitude Comparison System (AWACS) has proven highly efficient in detection of arcs both between and during ELMs. The ECT system has allowed the delivery of up to 4 MW of RF power without trips into plasmas with Type-I ELMs. Together with the 3dB system and the ITER-Like Antenna (ILA), the ECT has brought the total RF power coupled to ELMy plasma to over 8 MW, considerably enhancing JET research capabilities. This paper provides an overview of the key design features of the ECT system and summarizes the main experimental results achieved so far., 30 pages, 23 figures

This paper summarizes the operational experience of the ion cyclotron resonant frequency (ICRF) ITER-like antenna on JET aiming at substantially increasing the power density in the range of the requirements for ITER combined with load resiliency. An in-depth description of its commissioning, operational aspects and achieved performances is presented.

The paper summarizes the continuous developments made to the ion cyclotron resonant frequency (ICRF) system at JET in order to improve the reliability of the power coupled to plasma. It details the changes and improvements made to the system so that more power is coupled during ELMy plasmas as well as increasing the power density to demonstrate reliable operation in the range of the requirements for ITER. Results obtained using the conventional matching (stubs and trombones) system, 3 dB couplers and the conjugate-T scheme with variable matching elements outside the wave launching structure (external conjugate-T) and inside the wave launching structure (ITER-like antenna) are described. The presence of the three different approaches to load resilient ICRF systems at JET creates a unique opportunity to compare these methods under very similar plasma conditions and to assess the results of ICRF power delivery to ELMy plasmas, an important issue for ITER. The impact of the availability of increased levels of reliable ICRF power on plasma physics studies in JET is illustrated.

Recent progress on heating and current drive on JET is reported. Topics discussed are: high power coupling of ICRF/LH at ITER relevant antenna/launcher‐separatrix distances, succesfull demonstration of 3 dB couplers for ELM tolerance of the ICRF system, influence of ICRF on LH operation, rotation studies in plasma without external momentum with standard and enhanced JET toriodal field ripple, studies of different ICRF heating schemes and of NTM avoidance schemes using Ion Cyclotron Current Drive. A brief outlook on future plans for experiments at JET is given.