RF power is widely used as an auxiliary heating method in fusion devices. This paper reviews the relevant theoretical considerations for the ion cyclotron, lower hybrid and electron cyclotron ranges of frequency, and presents the history, the state of the art, and the plans and prospects for antennas and transmission lines for RF heating. Reactor-relevant concerns are discussed, and the information needed to develop realistic antenna designs for a reactor environment is assessed.

Power transmission and coupling for radiofrequency heating of plasmas

The 2D (radial/poloidal) spatial topology of RF-induced convective cells developing radially in front of ion cyclotron range of frequency (ICRF) antennae is investigated, in relation to the spatial distribution of RF currents over the metallic structure of the antenna. This is done via a Green's function, determined from the ICRF wave coupling equations, and well-suited to open field lines extending toroidally far away on both sides of the antenna. Using such formalism, combined with a full-wave calculation using the 3D antenna code ICANT (Pecoul S. et al 2000 Comput. Phys. Commun. 146 166–87), two classes of convective cells are analysed. The first one appears in front of phased arrays of straps, and depending on the strap phasing, its topology is interpreted using the poloidal profiles of either the RF current or the RF voltage of the strip line theory. The other class of convective cells is specific to antenna box corners and is evidenced for the first time. Based on such analysis, general design rules are worked out in order to reduce the RF-sheath potentials, which generalize those proposed in the earlier literature, and concrete antenna design options are tested numerically. The merits of aligning all strap centres on the same (tilted) flux tube, and of reducing the antenna box toroidal conductivity in its lower and upper parts, are discussed.

https://iopscience.iop.org/article/10.1088/0029-5515/45/8/002/pdf

RF current distribution and topology of RF sheath potentials in front of ICRF antennae

Measurements of low power ( 1 mW) antenna loading are used to study the coupling of a compact loop antenna structure to plasmas in the divertor configuration in DIII-D heated by neutral beam injection (NBI) or electron cyclotron heating (ECH). When a transition to the H-mode regime occurs during NBI, the antenna loading resistance drops by approximately a factor of two. This coupling decrease is due to a steepening of the edge density profile near the separatrix, accompanied by a reduction in edge density in the scrape-off layer. During edge localized modes, the opposite effects occur, and the antenna coupling increases transiently. The loading measurements are compared with theoretical calculations which take into account the measured density profiles as well as the conducting side-walls of the recessed antenna housing. Absolute agreement between the theoretical and the experimental results is obtained, including the correct dependence on the density, antenna position, RF frequency and antenna geometry. The theoretical interpretation of the results is discussed, together with the technological implications for future high power experiments.

https://iopscience.iop.org/article/10.1088/0029-5515/30/4/001/pdf

Coupling of fast waves in the ion cyclotron range of frequencies to H-mode plasmas in DIII-D

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.

https://iopscience.iop.org/article/10.1088/0741-3335/54/7/074012/pdf

Physics and engineering results obtained with the ion cyclotron range of frequencies ITER-like antenna on JET

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.

https://iopscience.iop.org/article/10.1088/0029-5515/46/7/S11/pdf

Key results of long pulse ICRH operation in Tore Supra

Antennas, vacuum feedthroughs, Faraday shields, and antenna materials for application in the ICR frequencies have been analysed and tested. The emphasis on the RDL (resonant double loop) antenna.(AIP)

ICRF antenna and feedthrough development at the Oak Ridge National Laboratory

Ion cyclotron resonance heating (ICRH) is expected to become the dominant contributor to the supplemental heating needs of plasmas in future toroidal devices. The severity of the environments that will be imposed on the ICRH antennas by the plasma in such devices requires the investigation of different approaches to providing adequate life and reliability in addition to the necessary coupling. This work, which is part of the National Ion Cyclotron Heating Program, addresses these issues. The rf coupling capabilities and characteristics of various antennas have been measured. The tested antenna configurations include the simple loop antenna operated at resonant lengths as used on Alcator-C, the cavity antenna proposed for Doublet III-D, and the resonant double loop, asymmetric resonant double loop, and U-slot antennas. Models of the voltage, magnetic fields outside the structure, and current have been correlated with the measurements made on these antennas. From these measurements and from typical observations of ICRH coupling in tokamaks, we are studying power and frequency limitations on each antenna and the causes of the limitations. A comparison of the technology, performance, and power limitations of each type of antenna is presented.

/pdf/experimental-measurements-of-the-ion-cyclotron-antennas-5g52md5bot.pdf

Experimental measurements of the ion cyclotron antennas' coupling and rf characteristics

Frequently, high-power pulsed ion cyclotron range of frequency experiments are limited by breakdown at the vacuum feedthrough This paper describes the development and testing of vacuum feedthroughs to increase both reliability and capability The ultimate goal of the program is to develop a continuous-wave feedthrough for the next generation of fusion experiments A feedthrough concept currently under investigation consists of a simple, cylindrical alumina ceramic brazed between tapered coaxial conductors A prototype has been tested to voltage levels in excess of 100 kV for 100-ms pulses and 70 kV for 5-s pulses at 28 MHz Insertion-voltage-standing-wave ratios are <115:1 for frequencies below 450 MHz An upgraded water-cooled version being fabricated for use on TEXTOR will be described

/pdf/radio-frequency-vacuum-feedthroughs-for-high-power-icrf-1n1j91vryl.pdf

Radio frequency vacuum feedthroughs for high-power ICRF heating applications

The Oak Ridge National Laboratory is designing, fabricating, and testing antennas for TFTR and Tore Supra. The antennas will deliver 4 MW per port to the hot plasma. The antennas’ designs are nearly complete; they are backed by the results of extensive modeling, analyses, and tests of the electrical, thermal, and structural characteristics of the antennas that were carried out during the design process.

/pdf/the-design-of-high-power-icrf-antennas-for-tftr-and-tore-7i0ctkczr1.pdf

The design of high‐power ICRF antennas for TFTR and Tore Supra

The rf technology program at the Oak Ridge National Laboratory (ORNL) is developing the components needed for heating in the ion cyclotron range of frequencies (ICRF) on future fusion machines, such as the Compact Ignition Tokamak. Components produced in this program are already in use on experiments throughout the world, including Doublet III-D (DIII-D), TMX-U, Alcator-C, and TEXTOR. Designs have been developed for implementation on Tore Supra and ASDEX. Recent developments in vacuum feedthroughs, compact loop antennas, high-current capacitors, and folded waveguides are discussed in this paper.

T.L. Owens

Papers

ICRF antenna and feedthrough development at the Oak Ridge National Laboratory

Experimental measurements of the ion cyclotron antennas' coupling and rf characteristics

Radio frequency vacuum feedthroughs for high-power ICRF heating applications

The design of high‐power ICRF antennas for TFTR and Tore Supra

ICRF heating technology development activities at Oak Ridge National Laboratory