Ion Cyclotron Range Of Frequencies Antenna Research Articles

Detection of ion cyclotron emission by using an ion cyclotron range of frequency antennas-based diagnostic system in experimental advanced superconducting tokamak

In the experimental advanced superconducting tokamak (EAST), a novel ion cyclotron range of frequency (ICRF) antenna-based diagnostic system is designed to measure ion cyclotron emission (ICE) driven by high-energy ions. The diagnostic system includes ICRF antenna straps, a three-tune impedance matching system, a coaxial switching system, a direct current block, and a data acquisition and storage system. Using the coaxial switching system, the ICRF antenna can be switched from the heating mode to the coupling mode between two discharges. In the 2023 EAST experiment campaign, core ICE was observed using the ICRF antenna-based diagnostic system during neutron beam injection heating, and the obtained results agreed well with the signal detected by the previous high-frequency B-dot probe-based diagnostic system.

Characterization of SOL profiles and turbulence in ICRF-heated plasmas in EAST

Scrape-off layer (SOL) profiles and turbulence in ion cyclotron range of frequency (ICRF)-heated plasmas are investigated by the reciprocating probe diagnostic system (FRPs) and gas puff imaging (GPI) diagnostic in EAST. A radio-frequency (RF) sheath potential reaching up to 100 V is identified proximate to the ICRF antennas. Notably, the amplitude of this RF sheath potential escalates in response to rising ICRF power and inversely with plasma density. When a RF sheath is present in the far SOL, a pronounced density ‘shoulder’ forms in front of the ICRF antennas, while the ‘shoulder’ fade away as the antenna and associated RF sheath shift outwards. A strong E r shear is revealed by measurements from both FRPs and GPI. Analysis of the poloidal wave number-frequency spectrum reveals suppression of high-frequency turbulence in the far SOL due to the RF sheath. This effect is manifested in the reduced autocorrelation time τ c and reduced average blob size δ blob of the SOL plasma. Intriguingly, the poloidal propagation direction of the low-frequency turbulence reverses from the electron to the ion diamagnetic drift direction at the RF sheath location. A surge of tungsten impurity is potentially attributed to the heightened interaction between the SOL plasmas and the wall material. Shifting the ICRF antennas outward, to alleviate heat spots, results in the relocation of the RF sheath to the shaded region of the main limiter. This shift amplifies the radial velocity of blobs in the far SOL and concurrently diminishes the SOL density when compared to conditions without ICRF injection. The properties of ion saturation current fluctuations are consistent with the stochastic model predictions.

Studies of the material erosion and deposition during the wall conditioning processes using quartz crystal microbalance in EAST

To study material erosion and deposition during different wall conditioning processes in the Experimental Advanced Superconducting Tokamak (EAST), two newly developed quartz crystal microbalances (QMBs) at the mid-plane of port C (C-QMB) and port J (J-QMB) with the same radial radius were used for in-situ and real-time measurements during the 2021 autumn experimental campaign. It was found that the material deposition was mainly influenced by the location of the antenna and the power of the ion cyclotron range of frequency (ICRF), rather than the location of the gas inlet, during the wall cleaning with helium plasma and siliconization assisted with ICRF plasma. The higher energy and density of plasma could result in higher deposition rates near the antenna. While, during lithium (Li) coating, the material deposition was greatly influenced by the location of the Li oven due to the deposition of unionized Li atoms. The ICRF power influenced the material deposition behavior far away from the Li ovens. Besides, a stronger effect on impurity suppression was observed with a more uniform deposition trend of Li coating. This necessitates further study on the influence of factors such as the location of the Li oven and ICRF antenna, the temperature of ovens, and ICRF power on the homogeneity of wall conditioning.

Design of power and phase feedback control system for ion cyclotron resonance heating in the Experimental Advanced Superconducting Tokamak

Ion cyclotron range of frequency (ICRF) heating system is an important auxiliary heating method in the experimental Advanced Superconducting Tokamak (EAST). In EAST, several megawatts of power are transmitted with coaxial transmission lines and coupled to the plasma. For the long pulse and high power operation of the ICRF waves heating system, it is very important to effectively control the power and initial phase of the ICRF signals. In this paper, a power and phase feedback control system is described based on field programmable gate array (FPGA) devices, which can realize complicated algorithms with the advantages of fast running and high reliability. The transmitted power and antenna phase are measured by a power and phase detector and digitized. The power and phase feedback control algorithms is designed to achieve the target power and antenna phase. The power feedback control system was tested on a dummy load and during plasma experiments. Test results confirm that the feedback control system can precisely control ICRF power and antenna phase and is robust during plasma variations.

Observation of ion-cyclotron-range-of-frequency wave emission in electron-cyclotron-resonance-heated tokamak plasma

Emissions of ion-cyclotron-range-of-frequency (ICRF) waves have been observed in a solely electron-cyclotron-resonance-heated plasma, namely no fast-ion plasma, on the JT-60U tokamak for the first time. The mode frequencies are around the ion cyclotron frequency at the low-field-side plasma edge. The waves are considered to be electromagnetic and standing waves from poloidal and toroidal phase differences measured with ICRF antennas. Wave characteristics of the ICRF emissions such as frequency, polarization and propagation are similar to those of edge-ion cyclotron emissions (ICEs) driven by neutral-beam-injected fast ions although driving sources are completely different. From analysis of the resonance conditions, this ICRF wave emission is considered to be driven by the electron-cyclotron-resonance-heated fast electrons via the Cherenkov resonance condition. When ICE is used for fast ion diagnostics, such fast-electron-driven ICRF wave emission may complicate its interpretation during electron cyclotron resonance heating.

Pre-conceptual studies of a travelling wave array antenna for the EAST

One of the primary problems of ion cyclotron range of frequency (ICRF) systems in magnetic confinement experiments is the coupling of a large amount of radio frequency wave power through the plasma cut-off layer within the voltage limits of the antenna system. Travelling wave array (TWA) antennas have higher coupling than conventional ICRF antennas, which is manifested in a sharp and optimized k // spectrum. As a pre-study of TWA applications in tokamaks, a TWA antenna with six consecutive straps and double-fin capacitors was conceptually designed for the experimental advanced superconducting tokamak (EAST). The antenna geometry was optimized to seek a low reflection coefficient for EAST ICRF heating scenarios. The design and simulation results of the TWA antenna are briefly presented. The results of the frequency sweep in vacuum show that a bandwidth of approximately 3 MHz with S 11 < −30 dB can be obtained. The peak of the k // power spectrum is adjusted to ∼3–4 m−1 at the frequency of 34–36 MHz. In addition, the properties of the power flow and the characteristics of the wave field are also discussed by modelling the plasma facing the TWA antenna using a cold plasma medium. The results in this study may provide some reference and guidance for the study of TWA antennas and other ICRF antennas in magnetic-confined fusion devices like EAST or the Chinese Fusion Engineering Test Reactor.

Investigation of ICRF-NBI synergetic heating induced fast ion distribution and transport in EAST tokamak

In magnetic confinement fusion plasmas, radio-frequency wave heating in the ion cyclotron range of frequencies (ICRF) and neutral beam injection (NBI) are two main heating methods. Their synergetic heating has long been a key topic in fusion research. In this work, we clarify the basic principles of ICRF high harmonic heating and the synergetic heating between ICRF and NBI. Then, we perform a series of experiments on EAST tokamak and carry out the corresponding TRANSP simulations. The results indicate that the ICRF-NBI synergetic heating not only significantly increases the plasma parameters (including poloidal beta, plasma stored energy, ion temperature and neutron yield), but also generates a large number of energetic particles and develops an energetic particle tail in its distribution function. For instance, the ICRF third harmonic heating with 1 MW of power can increase the energy of NBI fast ions from 60 to 600 keV. By changing the hydrogen minority concentration, improving the ICRF and NBI heating power, using the on-axis ICRF heating or optimizing the NBI injection angle, the ICRF-NBI synergetic heating effect can be further enhanced, accompanied with an increase of fast ion energy. Moreover, by using the fast ion distribution as input in the orbit tracing code, the transport and loss of energetic particles are calculated. The results show that the initial positions of the lost energetic particles are on the low field side, and their orbits are mainly trapped orbits. The loss of energetic particles is mainly located in the middle and upper plane of the main limiter, ICRF and LH antenna limiters. The lost of these energetic particles are considered as one of the main reasons why hot spots occur on the limiters.

First experimental results with new ICRF antenna in EAST

A new ion cyclotron range of frequency (ICRF) antenna, instead of a B-port antenna, is installed at the N-port to improve the power coupling in the 2021 experimental advanced superconducting tokamak (EAST) experimental campaign. In all the experiments, a frequency of 37 MHz is used, corresponding to hydrogen minority heating of deuterium plasma at B 0 = 2.5 T. The maximum coupling resistance is up to 10 Ω, which is close to two times the resistance of the original two antennas in EAST. The plasma stored energy is increased by 30 kJ MW−1, and significant core ion and electron heating are observed. For long pulse operation, we have achieved 1.5 MW/41 s and 1.1 MW/61 s with the new antenna in the high poloidal beta plasma discharge. ICRF power up to 1.8 MW is routinely coupled to the plasma for pulse lengths up to 21 s.

Preliminary evaluation of inverse scattering-based plasma-profilometry application to fusion plasmas

A new diagnostic method able to perform plasma profilometry is investigated in this work to retrieve the plasma electron density profile in front of the Ion Cyclotron Range of Frequencies (ICRF) antennas. As a reference scenario for our numerical study, the Divertor Tokamak Test (DTT [1]) ICRF antennas and plasma will be considered. Specifically, the profilometry needs to solve an inverse scattering problem, which is non-linear and ill-posed. In some recent papers [2, 3], plasma imaging profilometry in compact plasma reactors, such as the electron cyclotron ion sources (ECRIS), has been proposed and tested numerically by means of proof-of-concept examples. In particular, proper formulation of electromagnetic inverse scattering techniques have been proposed, requiring measurements of the reflection coefficient in presence of a metallic reflecting surface or, alternatively, both the reflection and transmission coefficients through two antennas facing one each other. In this paper, we would like to investigate this method also for large-size (scale-length) fusion reactors by addressing the profilometry of DTT-like plasma, assuming a very high-frequency probing regime (∼0.5 THz) for the accessibility of both O and X-modes in the DTT plasma (electron density up to 1020 m−3 and magnetic field up to 9 T). To this aim, we adopt COMSOL Wave Optics® module based on beam propagation method (or slowly varying envelope approximation) for simulating the wave propagation and to determine the probing frequencies. A proper formulation allowed to reconstruct a tiny scrape-off plasma layer, approximated as an isotropic medium, thanks to the high probing frequency.

Conceptual design and optimization of an ITER-type ICRF antenna on CFETR

The Chinese Fusion Engineering Testing Reactor (CFETR) plans to use an ITER-type antenna and couple ∼30 MW ion cyclotron range of frequencies (ICRF) power to the plasma. In this paper, the physical design of a CFETR antenna in the midplane port is carried out. Parameter scans were performed to study the optimized toroidal and poloidal numbers of straps as well as the optimized geometric sizes of the straps. The coupling resistance, power spectrum, maximum voltage in the resonant transmission line of the strap and parallel electric field in the antenna vicinity are used to determine the performance of the studied antennas. It is shown that four poloidal substraps (i.e. quadruplets) and six toroidal strap columns arranged in half of the antenna allows the antenna to have the best coupling capability. To improve the coupling capability of the proposed antenna model, local gas puffing methods, as well as various antenna phasings, are studied. It is indicated that the coupling resistance can be increased by a factor of three for all studied antenna phasings when applying the midplane gas puffing with a gas puff rate in the order of 4 × 1023 el s-1. The toroidal phasings suitable for heating include (0, pi, 0, pi, 0, pi) and (0, pi, pi, 0, 0, pi).

Observation of direct power deposition of ICRF in EAST plasma boundary

Waves in the ion-cyclotron range of frequencies (ICRF) used for plasma heating are known to interact strongly with the scrape-off layer (SOL) plasma, potentially limiting ICRF performance. We present the evidence of direct power deposition of ICRF in the Experimental Advanced Superconducting Tokamak (EAST) plasma boundary, with two ICRF antennas and 12 MW of ICRF source power in support of high-power and long-pulse operation. By applying Retarding Field Analyzer (RFA) diagnostic, a sharp rise in average collected ion parallel energy was obtained during the period of ICRF heating as the RFA probe stayed still at 2 mm behind the main limiter which indicates that a certain amount of power is deposited near the limiter instead of being absorbed by the main plasma. A significant increase of the target floating potential in electron temperature measured by Divertor Langmuir Probes arrays was observed. The wall temperature in the far SOL was also enhanced. As a consequence, an enhancement of the carbon and tungsten impurity concentration was observed both at the plasma boundary and core. Besides, plasma radiation, the impurity concentration of the boundary and core increased, and the temperature rise of the first wall was also observed, which are intuitive and powerful pieces of evidence of ICRF direct power deposition at the boundary.

Overview of the ICRF antenna coupling experiments on EAST

Experimental verification of ion cyclotron range of frequencies (ICRF) wave coupling is crucial to gain insight into the physics of the improvement of ICRF antenna coupling. The progress of ICRF antenna coupling experiments on EAST has been reported since 2007. The dependence of the loading resistance on the target plasma parameters, operational regimes and synergic heating scenarios on EAST are reviewed. Results show that the antenna coupling resistance R c in L-mode regime meets the relation of R c ∝ exp(−1.5k ∥ x c), which is in good agreement with the simulation results with a perfectly matched layer method, where k ∥ is the dominant launched parallel wave number and x c is the distance of the ICRF antenna and fast wave cut-off layer. Low plasma current, gas puff in the magnetic connections with the ICRF antenna and lower hybrid wave/electron cyclotron heating are beneficial for improving ICRF antenna coupling. The antenna coupling has a strong response during large edge-localized modes (ELMs) but it becomes steady state during ELM free. The study of ICRF antenna coupling under various parameters is helpful to optimize the ICRF wave heating scenario and understand the physical mechanisms which influence the ICRF wave coupling in plasmas.

Validation of high-fidelity ion cyclotron range of frequencies antenna coupling simulations in full 3D geometry against experiments in the ASDEX Upgrade tokamak

We address the validation of finite elements ion cyclotron range of frequencies (ICRF) antenna coupling simulation against experiments performed in the ASDEX Upgrade tokamak. Measurements of the loading resistance in ICRF-heated, magnetically-perturbed 3D plasma discharges are compared against numerical predictions of the RAPLICASOL code. To this end, the 3D induction field and the 3D density profile are modeled by concatenating the PARVMEC, BMW and EMC3-EIRENE codes. The 3D density is input to RAPLICASOL, where full-wave simulations are performed on a finite element mesh retaining full 3D geometry in the ICRF antenna model and the plasma description. The results are further compared with RAPLICASOL simulations using a 1D density profile as measured at the outboard midplane in the same experiments. We find that simulations using a 1D density profile overestimate the change in loading resistance by a factor of ∼197 − 248%, while simulations using the full 3D density profile are in agreement with experiments within a factor ∼16%.

Metallic impurity content behavior during ICRH-heated L-mode discharges in EAST

This study uses the diversity of materials at known locations in the experimental advanced superconducting tokamak to extract, local information on which plasma-surface interaction processes are dominant during ion cyclotron resonance heating (ICRH) (near vs far field effects). Metallic impurity content is indicated by the intensity of impurity line emissions observed with a fast-time-response extreme ultraviolet spectrometer, normalized to line-averaged plasma density, i.e. Iimp/ne. Parametric dependencies are explored over scans of ICRH and LH (lower hybrid) power and for different toroidal phasings between straps, strap power balance, and magnetic configurations. This diversity of behavior is interpreted as the signature of different physical processes. Before 2017, as only the upper divertor region contained tungsten (W), the W content in the core used to increase a lot when moving from lower to upper single null configurations, and was correlated with the total injected power rather than the ICRH power. Molybdenum, covering the part of the inner wall facing one ion cyclotron range of frequencies (ICRF) antenna appear sensitive to this antenna power and phasing benchmarked by modeling suggesting a probable interaction due to residual ICRF waves crossing the plasma. Materials close and magnetically-connected to an active antenna show better correlation with ICRF antenna electrical tuning than those which are far away, or not connected. This is particularly the case with W since 2018, because the limiter tiles of the LH launchers were changed from graphite to tungsten. In these latter conditions, it is shown that W sources at the mid-plane (equatorial plane) contribute to a significant fraction of the core contamination by tungsten (25% in ohmic regime, and more during ICRH).

Development of a compact ICRF antenna for high-power and long-pulse plasma heating in the KSTAR

For the high-power and long-pulse ion cyclotron range of frequencies (ICRF) heating of the KSTAR plasma, we developed the compact ICRF antenna (CIA). The target injection power of CIA is 2 MW for 300 s. In order to continue injecting the power into plasma even if drastic instantaneous changes occur in the plasma condition, such as ELM events, we adopted the internal conjugate-T method for the load resilience. Between antenna heads and the junction point, impedance transformers were inserted to satisfy the condition of conjugate-T in a limited space keeping the electric field on the transformer low enough. To reduce the risk of water leakage into the vacuum chamber, only the backsides of antenna heads are water-cooled in the in-vessel region.

ICRF coupling in ASDEX upgrade magnetically perturbed 3D plasmas

The RF properties of the four ion cyclotron range of frequencies (ICRF) antennas in the ASDEX Upgrade tokamak are characterized in H-mode magnetically perturbed 3D discharges. An n = 2 magnetic perturbation (MP) field is applied and rigidly rotated, which allows diagnosing the separatrix displacement and consequent coupling change. We find the antenna loading resistance to be coherently modified by the resulting non-axisymmetric plasma equilibria, thus becoming a function of the applied MP field poloidal mode spectra. We perform a detailed statistical analysis, which correlates the change in loading resistance to the fast wave R-cutoff layer movements. From it, a 1D scaling is derived that differs from previous studies evaluated in pure axisymmetric plasma conditions. This experimentally derived scaling is used to predict the average loading resistance change of the ITER ICRF antenna under applied MPs. ICRF coupling simulations using measured 1D density profiles are performed with the RAPLICASOL code, in order to investigate the predictive capabilities of numerical state of the art tools. We find that both 1D conventional scaling laws and 1D numerical simulations fail to capture the 3D physics, and can substantially overestimate the measured loading resistance change up to a factor of ∼3.

IShTAR: A test facility to study the interaction between RF wave and edge plasmas.

Existence of high electric fields near an RF antenna launcher causes a number of parasitic phenomena, such as arcing and impurity release, which seriously deteriorate the performance of an Ion Cyclotron Range of Frequencies (ICRF) heating scheme in fusion devices. Limited accessibility of the near antenna region in large-scale fusion experiments significantly complicates the associated experimental studies. The IShTAR test facility has been developed with the requirement to provide a better accessibility and diagnosability of plasmas in the direct vicinity of an ICRF antenna. The purpose of this work is to give a detailed description on the experimental setup and the available diagnostics. Furthermore, the paper will demonstrate the capability of the experiment to study phenomena near an ICRF antenna launcher which are relevant for large-scale fusion ion cyclotron resonance heating systems.

An overview of the in-vessel ICRF-diagnostics in the ASDEX Upgrade tokamak

Radio Frequency (RF) waves in the Ion Cyclotron Range of Frequency (ICRF) are successfully used to heat fusion plasmas. For a better understanding of the ICRF wave propagation, absorption and other processes in the plasma, the ICRF-related in-vessel diagnostics has been extended recently in the ASDEX Upgrade tokamak.A number of high frequency B-field probes have been installed which, arranged as an array, pick up the magnetic field components of the radiated wave, and can thus be used to characterize the radiation spectrum of the ICRF-antennas during plasma discharges. Dual channel logarithmic RF-detectors with phase detection capability have been designed and installed to track the fast variations of the probe signals during events like edge localized modes (ELMs). The same probes are also used to measure the ion-cyclotron emission (ICE) of plasmas. A fast, open source data acquisition system, which digitizes the RF-signal with 125 MSamples/s and 14 bit amplitude resolution, records the ICE signals. To allow ICE measurements during ICRF operation notch filters are used to suppress the ICRF signal by 50 dB. In order to characterize drive mechanisms and consequences of RF sheaths on the ICRF-antenna structures, 12 antenna limiter tiles are equipped with shunts to allow the measurements of RF- and DC-currents during operation, with some of the locations also covered by the spectroscopic lines of sight measuring the emission of neutral tungsten at 400.9 nm (WI). The antenna-embedded X-mode reflectometry with 3 channels operating simultaneously in the extended U-band helps to study ICRF wave coupling, whereas the RF current probes in the 3-strap ICRF antennas provide information on the phasing between central and outer straps.

Progress on an ion cyclotron range of frequency system for DEMO

An Ion Cyclotron Range of Frequency (ICRF) system can provide power for a number of tasks, experimentally verified on present machines: heating and current drive, first wall conditioning, plasma startup, removing central impurities, controlling sawteeth and current ramp down assist. The system has a high plug-to-power efficiency and most of the components external to the machine are sturdy, with industrial steady state capability.Traditional ICRF antenna systems are often characterized by a high operating voltage and high power density. Low power density and low voltage however provides a bonus in terms of reliability. Therefore, travelling wave type antennas have been proposed (Ragona and Messiaen, 2016). They can be integrated in the blanket and use only a limited number of feeders.The effect on the tritium breeding ratio of such an antenna incorporated in the blanket, including the feeders, is small. The k// spectrum is peaked and the dominant k// value can be optimized for coupling and bulk absorption, while avoiding the generation of coaxial modes in the edge. The coupling can be further enhanced with gas puffing near the antenna. Assuming the ITER-2010-low density profile, 50 MW can be coupled with a voltage on the antenna components of about 15 kV.

Scrape-off layer density tailoring with local gas puffing to maximize ICRF power coupling in ITER

The coupling of ion cyclotron range of frequencies (ICRF) power to the plasma depends critically on the scrape-off layer (SOL) density since the fast wave is evanescent below the cut-off density. The ICRF power coupling can be improved by increasing the SOL density locally in front of the antenna by means of local gas puffing/fueling. To understand the influence of local gas puffing on the SOL and ICRF coupling and to find the optimized gas valve positions to maximize ICRF coupling in ITER, the 3D SOL code EMC3-EIRENE is used to calculate the SOL density, and the ICRF antenna codes ANTITER and FELICE are then used to calculate the coupling resistances. Purely deuterium plasma is simulated and the total gas puff rate for all studied cases is 4.5e22 el/s. The divertor gas puffing case is considered as the reference case. The density and temperature profiles in the reference case are well fitted to the standard ITER profiles (both for the low and medium density) with proper transport parameter profiles. The gas source is then switched to other local positions of the main chamber while all other simulation parameters are kept the same. The simulation results indicate that midplane gas puffing increases the antenna coupling resistance most significantly (by 150%-200%) for both ICRF antennas. This increase is at the same level as long as the gas valve is located toroidally close to the antenna, no matter if the gas valve is right to or left to the antenna. Outer top gas puffing increases the coupling resistance less significantly (by 100%–150%) for the antenna with good magnetic field line connections to the gas valve, but the increase is at a much smaller level (by ∼30%–60%) for the other antenna with only partial field line connections to the valve. The simulations thus confirm for ITER a behavior similar as seen experimentally in current devices and strongly suggest that ITER should modify the existing main chamber injection configuration to bring one of the four planned injection points closer to the antennas.