Ion Cyclotron Resonance Frequency Antenna Research Articles

Theoretical Study of the Impedance Matching of the Ferrite Tuner System for ICRF Heating

A theoretical study on a ferrite stub tuner antenna system for ion cyclotron resonance frequency (ICRF) heating is discussed in this paper. High amounts of radio frequency power can be reflected at the antenna as a result of impedance mismatches arising from large changes in the plasma resistance during L- to H-mode transitions or edge localized modes. A fast-response ferrite stub tuner network has been proposed to mitigate these reflections by rapidly varying the impedance to match the rapid load changes on the ICRF antenna. This study numerically shows the influence relationship of the normalized mechanical length and the ratio of the ferrite part of two ferrite stubs on the regulating range of a biasing magnetic field of two stubs. A prematching stub can be used to reduce the standing-wave voltage on the ferrite tuners. The analysis of the ideal position and length of the prestub as well as the distance between the ferrite network and prestub are presented. Numerical simulations demonstrate that selecting ideal values for mechanical length and the ratio of the ferrite part of two ferrite stubs plays an important role in the impedance matching performance of the triple ferrite system during a large variation in plasma resistance.

Design of a multi-voltage probe system for ICRF antenna coupling resistance measurement on EAST

A multi-voltage probe array system is designed to measure the coupling resistance of an ion cyclotron resonance frequency antenna. In the process of the antenna coupling resistance data extraction, the minimization algorithm, the original Levenberg–Marquardt algorithm, is replaced by the Broyden–Fletcher–Goldfarb–Shanno algorithm to achieve more stable and accurate results. Moreover, a simple model of the multi-voltage probe array was applied to simulate the performance of the Kalman filter, and to optimize the distance and position of the probes and probe number to mitigate the influence of the system noise on the rebuilt results. During the EAST experiment in 2019, a four-voltage probe array was applied to measure the coupling resistance of line 6 during high confined mode discharge. The measurement results by the multi-voltage probe array system and the voltage/current probe pair show a good agreement.

Design and development of resonant loop antenna for mock‐up ion cyclotron resonance frequency system of tokamak

This paper presents the design and fabrication of a prototype antenna used in ion cyclotron resonance frequency (ICRF) heating system of the tokamak. The prototype incorporates a similar design as of the ICRF antenna in a tokamak. It has been fabricated for 2 kW power handling capability at 182.5 MHz. The study incorporates the theory and design calculation of ICRF antenna using equivalent circuit analysis. The presented work formulates design calculation for optimum performance, which has been applied in fabrication. The fabricated antenna has been tested, where results are found as, reflection coefficient of − 24.65 dB, directivity of 2.95 dBi, and gain of 2.5 dB. The obtained results after fabrication are found as per the desired and in agreement with the simulation results. The work presented here is useful for the design and fabrication of high-power antenna in HF and VHF range.

Wave coupling simulation of ICRF antenna in EAST relying on the perfectly matched layer technique

A coupling model for the waves in ion cyclotron resonance frequency (ICRF) in tokamak plasmas is described. The simulation is performed based on the finite element method. A cold plasma dielectric tensor is used. A perfectly matched layer (PML) is applied to describe the ICRF wave single-pass absorption in the core plasma. The selection of PML characteristic parameters is optimized in a wide wave frequency range and validated for plane waves and ICRF waves. The dependencies of the coupling resistance on the plasma-antenna gap, average plasma density, and plasma profiles are given. The simulation results are in good agreement with the experimental results.

Impact of localized gas injection on ICRF coupling and SOL parameters in JET-ILW H-mode plasmas

Recent JET-ILW [1,2] experiments reiterated the importance of tuning the plasma fuelling in order to optimize ion cyclotron resonance frequency (ICRF) heating in high power H-mode discharges. By fuelling the plasma from gas injection modules (GIMs) located in the mid-plane and on the top of the machine instead of adopting the more standardly used divertor GIMs, a considerable increase of the ICRF antenna coupling resistances was achieved with moderate gas injection rates (<1.5×1022e/s). This effect is explained by an increase of the scrape-off-layer density in front of the antennas when mid-plane and top fuelling is used. By distributing the gas injection to optimize the coupling of all ICRF antenna arrays simultaneously, a substantial increase in the ICRF power capability and reliability was attained. Although similar core/pedestal plasma properties were observed for the different injection cases, the experiments indicate that the RF-induced impurity sources are reduced when switching from divertor to main chamber gas injection.

ICRF physics aspects of wall conditioning plasma characterization in TEXTOR

This paper focuses on encouraging results obtained on the characterization of RF produced plasmas during pulsed-mode wall conditioning discharges in ion cyclotron resonance frequency (ICRF) regime in the limiter tokamak TEXTOR. Recent Ion Cyclotron Wall Conditioning (ICWC) experiment carried out in TEXTOR tokamak, lead to the identification of various dependences of the antenna-plasma coupling efficiency on the plasma parameters for possible ICWC-discharge cleaning in ITER at half field. Our ICWC experiments emphasize on (i) study of antenna coupling during the mode conversion scenario, (ii) reproducible generation of ICRF plasmas for wall conditioning, by coupling RF power from one or two ICRF antennas and (iii) effect of application of an additional (along with toroidal magnetic field) stationary vertical (BV≪BT) or oscillating poloidal magnetic field (Bp≪BT) on antenna coupling and relevant plasma parameters.

Plasma and antenna coupling characterization in ICRF-wall conditioning experiments

Ion Cyclotron Wall Conditioning (ICWC) discharges, in pulsed-mode operation, were carried out in the divertor tokamaks ASDEX Upgrade (AUG) and JET to simulate the scenario of ITER wall conditioning at half-field (AUG) and full-field (JET). ICWC-plasma and antenna coupling characterization results obtained during the Ion Cyclotron Resonance Frequency (ICRF)-Wall Conditioning experiments performed in helium-hydrogen mixture in AUG and helium-deuterium mixtures in JET are presented here. Safe operational regimes for optimum ICWC in ITER could be explored for different magnetic fields. Satisfactory antenna coupling in the Mode Conversion scenario along with reproducible generation of ICRF plasmas and reliable wall conditioning were achieved by coupling RF power from one or two ICRF antennas at two (AUG, JET) different resonant frequencies. These results are in qualitative agreement with the predictions of 1-D TOMCAT code. Present study of ICWC indicates towards the beneficial effect of application of an additional (along with toroidal magnetic field) stationary vertical (BV≪BT) magnetic field on antenna coupling and plasma parameters. The results obtained from JET and AUG tokamaks, presented in this paper, emphasizes the proposed phenomenological schemes for further development of ICWC in superconducting tokamaks.

Heat loads on JET plasma facing components from ICRF and LH wave absorption in the SOL

In JET, lower hybrid (LH) and ion cyclotron resonance frequency (ICRF) wave absorption in the scrape-off layer can lead to enhanced heat fluxes on some plasma facing components (PFCs). Experiments have been carried out to characterize these heat loads in order to: (i) prepare JET operation with the Be wall which has a reduced power handling capability as compared with the carbon wall and (ii) better understand the physics driving these wave absorption phenomena and propose solutions for next generation systems to reduce them. When using ICRF, hot spots are observed on the antenna structures and on limiters close to the powered antennas and are explained by acceleration of ions in RF-rectified sheath potentials. High temperatures up to 800 °C can be reached on locations where a deposit has built up on tile surfaces. Modelling which takes into account the fast thermal response of surface layers can reproduce well the surface temperature measurements via infrared (IR) imaging, and allow evaluation of the heat fluxes local to active ICRF antennas. The flux scales linearly with the density at the antenna radius and with the antenna voltage. Strap phasing corresponding to wave spectra with lower k∥ values can lead to a significant increase in hot spot intensity in agreement with antenna modelling that predicts, in that case, an increase in RF sheath rectification. LH absorption in front of the antenna through electron Landau damping of the wave with high N∥ components generates hot spots precisely located on PFCs magnetically connected to the launcher. Analysis of the LH hot spot surface temperature from IR measurements allows a quantification of the power flux along the field lines: in the worst case scenario it is in the range 15–30 MW m−2. The main driving parameter is the LH power density along the horizontal rows of the launcher, the heat fluxes scaling roughly with the square of the LH power density. The local electron density in front of the grill increases with the LH launched power; this also enhances the intensity of the LH hot spots.

Mechanical Design and First Engineering Commissioning of ICRF Antenna for EAST

A new loop and fold wave-guide ion cyclotron resonance frequency (ICRF) antenna with a power output of 3 MW, which can operate at a frequency in the range of 30 MHz to 110 MHz, was designed. The design of key components of the new ICRF antenna and the characters of the new prototype ICRF antenna are presented. The thermo-mechanical analysis of both the Faraday shield and the current straps was conducted, and the stresses due to heat loads are studied in detail with different cooling-water velocities considered. In addition, the movability of prototype ICRF antenna under vacuum condition by the driving system was tested. An engineering commissioning was successfully performed on the prototype ICRF antenna using the original transmitter. The results are close to the expected.

Commissioning of the ITER-like ICRF antenna for JET

The new JET ion cyclotron resonance frequency (ICRF) ITER-like antenna (ILA), which was assembled during 2006, was commissioned on the JET RF testbed prior to installation on the JET torus. The 4 resonant double loops (RDL) of the ILA were tested at high power at 42 MHz up to 42 kV for 5 s in 10 min intervals. Low power matching studies using a saltwater load placed in front of the ILA have allowed testing and optimizing proposed matching algorithms on single RDLs, paired RDLs and finally on the full array. The upper limit of the frequency range of the ILA appears to be limited to 47–49 MHz due to the effect on the electrical lengths of the connection between the capacitors and the conjugate T point. Capacitor position scans have allowed obtaining the necessary data to confirm the RF model of the RDL which is necessary for the scattering matrix arc detection. The latter is deemed necessary in order to detect arcs at the low impedance conjugate T of the circuit. The antenna was installed onto JET during August 2007 and commissioning on plasma started May 2008. At present the commissioning of the ILA on JET is ongoing in a series of dedicated experimental campaigns.

ICRF wall conditioning and plasma performance on EAST

ICRF (ion cyclotron resonance frequency) wall conditioning was performed on EAST superconducting tokamak. It included ICRF cleaning in-between shots, boronization associated with ICRF plasmas, and ICRF oxidation with an ITER-relevant full metallic wall material. A dedicated ICRF antenna was developed for wall conditioning on EAST. The RF frequency is 30 MHz, and maximum output power of RF system is 300 kW. The duty time of pulsed ICRF wave was set at 0.3 s on and 1.2 s off in presence of the toroidal magnetic field of 1–2 T. Optimization of start-up, ramp-up and plasma shaping for single and double-null configurations were obtained following the ICRF wall conditioning. High density, high plasma current and long pulse discharges were achieved on EAST.

Recent developments in ICRF antenna modelling**This paper is an expanded version of material originally presented at the 20th IAEA Fusion Energy Conference, Vilamoura, Portugal, 1–6 November 2004 (Vienna: IAEA), CD-ROM file FT/PT-18.

The antennas presently developed for ion cyclotron resonance frequency (ICRF) heating of the ITER plasma consist of a tightly packed array of a large number of radiating straps, in order to deliver a high power density without exceeding radio-frequency voltage standoffs. A recently developed three-dimensional electromagnetic commercial software has enabled important progress in the coupling analysis and optimization of such demanding systems. Approximations allowing these codes to convincingly model the effect of antenna loading by a magnetized plasma are discussed. It is shown that, for the purpose of antenna design calculations, a slab of ordinary dielectric with a high relative permittivity, KD, of the order of (c/VA)2, where VA is the plasma Alfvén velocity, can reproduce the main features of wave reflection and refraction by the plasma edge. Multi-layered dielectrics permit more refined approximations. The numerical application of the approximation is illustrated by a simulation of the JET ‘A2’ ICRF arrays, which fairly qualitatively reproduces the experimental frequency dependence of the coupling resistance. The same loading approximation is applied to the design of realistic experimental test-bed conditions for antenna prototypes or mock-ups, using water as a means of creating plasma-relevant antenna loading. Application to a scaled mockup of the ITER antenna is briefly presented.

Capacitance of an array of thick rods parallel to the ground - application to the ion cyclotron resonance frequency antenna of the Joint European Torus

A variational evaluation of the electrostatic capacitance of an array of thick metal rods parallel to the ground is presented. A straightforward procedure allowing unlimited numerical accuracy is given, as well as a simple analytical approximation sufficient for all practical purposes. The method is applied to the Faraday shield of the JET ICRF antenna, where it yields a correction of order 40% to the classical thin wire formula.