Ion Cyclotron Resonance Heating System Research Articles

ICRH operations during the JET tritium and DTE2 campaigns

The JET-ILW pure tritium and deuterium–tritium (DTE2) experimental campaigns took place in 2021–2022. Tritium (T) and deuterium–tritium (D–T) operations present challenges not encountered in present day tokamaks (Horton et al 2016 Fusion Eng. Des. 109–111 925–36). This contribution focuses on ion cyclotron resonance heating (ICRH) operations in tritium and deuterium–tritium plasmas, starting with a summary of the program of improvements to the ICRH system which spanned a few years prior to these experiments. Procedures were implemented to address specific constraints from tritium and deuterium–tritium operations (tritium safety and reduced access to the RF generator area) and increase the system reliability and power availability during plasma pulses. Operation of the upgraded real time RF power control system that maximises the launched power while taking into account limitations from the system or antenna coupling is described. We also report on the result from dedicated pulses performed to assess the potential harmful impact of the 2nd harmonic tritium resonance in the plasma, close to the inner wall, when using the standard central hydrogen minority ICRH scheme. During DTE2, the ITER-like antenna was not used because water leaked from an in-vessel capacitor into the vessel on day-2 of the experimental campaign. The lessons learnt from this incident are highlighted. Finally, the ICRH plant adjustments required to safely perform ion cyclotron wall cleaning discharges are described.

Design and operation of a load-tolerant ICRH system in Experimental Advanced Superconducting Tokamak

Ion Cyclotron Resonance Heating (ICRH) has been a dependable tool for sturdy plasma heating with high RF power of several megawatts. However, a sudden increase in the reflected power during ICRH heating experiments is a problem that should be solved for future fusion experimental devices. To solve this issue, a load tolerant matching network has been designed for the ICRH system in EAST. The matching network includes a 3-stub tuner impedance matching system with conjugate-T structure, 30 Ω to 50 Ω transmission line and center grounded antenna strap. By maintaining a low reflection ratio in the network for a wide range of resistance, this matching network can allow sturdy high-power operations without fast impedance matching in EAST. In our matching network, the two arms of a conjugate-T are designed to have a λ/2 length difference which could mitigate current imbalance and antenna poloidal phasing outside of the control problem. The T-point corresponds to the maximum point of the standing wave voltage, which could greatly improve the input impedance of the antenna.

Development of a real-time impedance matching system for ion cyclotron resonance heating in experimental advanced superconducting tokamak

To achieve stable operation of an ion cyclotron resonance heating (ICRH) system in the Experimental Advanced Superconducting Tokamak (EAST), a real-time impedance matching system needs to be established to respond to antenna load variation during long pulse discharges. A new impedance matching method based on capacitors was proposed in this study. By considering the reflected voltage of the transmission line as the feedback parameter, the real-time impedance-matching system can quickly control the motors based on a programmable logic controller to determine the minimum reflection voltage. A real-time impedance matching system was successfully used on the test platform in the laboratory and on the ICRH system in EAST. A significant result is that we can match the variable impedance within 1 s by suitably adjusting the motor controller to ensure high-power and long-pulse operation of the ICRH system to satisfy the requirements of the EAST experiment.

Multipactor predictions for ion cyclotron resonance heating antennas of the tokamak WEST *

During the plasma experimental campaigns performed on the tokamak WEST, a performance-limiting observation has been made on the Ion Cyclotron Resonance Heating (ICRH) system composed of three identical antennas. When only one antenna is powered while the remaining two are off, an increase in the internal pressure is measured within the non-powered antennas. The internal pressure of the non-powered antennas exceeds a safety pressure interlock whose threshold is Pa, and consequently, the application of the RF power is prohibited to avoid plasma creation within these antennas. In this paper, the multipactor phenomenon caused by the inter-antennas’ coupling is shown to be responsible for the pressure increase observed in the non-powered antennas. Additionally, an analytic estimation of the pressure rise caused by the multipactor is compared to that observed experimentally.

Design of an optimized load-resilient conjugate T for the ICRH system in the LHD using a novel hybrid circuit/3DLHDAP code and experimental results

It is crucial to correctly predict the S-matrix with plasma and set the optimal impedance matching device in the ion cyclotron resonance heating (ICRH) antenna system design. In this paper, a hybrid circuit/3DLHDAP code to verify the S-matrix measurements in the presence of plasma and optimize the load-resilient conjugate-T circuit for Large Helical Device (LHD) ICRH antennas has been developed and benchmarked. The variation of S-matrices for handshake form (HAS) and field-aligned-impedance-transforming (FAIT) antenna systems with density, magnetic field and coupling distance during heating obtained by the code’s simulations agrees with that of with the LHD ICRH experiments. The mutual coupling of toroidally aligned HAS antennas is larger than that of poloidally aligned FAIT antennas over a wide range of densities. When the density and coupling distance increase, under a magnetic field on the magnetic axis of 2.75 T and 1.0 T, within a certain density change range, at the minimum voltage position with vacuum injection, the change rule of the antennas’ Sa_minV_ 11 and Sa_minV_ 22 with density is opposite to that with coupling distance, which means that under certain conditions, adjusting the coupling distance may make up for the S-parameters changes caused by plasma density variation, keeping the minimum voltage position fixed, and may make impedance matching easier to achieve during long-pulse operation. Based on obtaining the S parameters, conjugate-T circuits for the HAS and FAIT antennas are designed with the hybrid circuit/3DLHDAP code, which can keep the reflection coefficients low without controlling impedance matching device over a wide range of plasma parameters region. The related results in this paper may provide some guidance for the high-power long-pulse operation of the ICRH antenna system on the fusion device.

Optimization of the operational domain for ICRH scenarios in WEST from statistical analysis

In the WEST tokamak, the Ion Cyclotron Resonance Heating (ICRH) system plays a substantial role in increasing the plasma temperature. However, its efficiency can be lowered by two main phenomena: ripple-induced fast ion losses and technical limits on the antenna current. In this work, a new technique using IR thermography was employed to measure the flux of particles coming from the trapping of fast ions in the toroidal magnetic field ripple. These measurements provided the opportunity to fit parametric scaling laws in order to predict the ion flux intensity and the total power loss for experiments with a plasma current of kA and a major radius of the cyclotron resonance layer of m depending on the heating power and the line-averaged electron density. Furthermore, another semi-empirical parametric scaling was developed to evaluate the coupling resistance depending on controllable parameters such as the line averaged electron density and the radial outer gap between the separatrix and the ICRH antenna. These laws were used to define an operational domain from the database of previous experiments made during campaign C4. The settled operational domain suggests that high power ( MW) and high electron density ( m−3) discharges are suitable for optimized steady-state high confinement scenarios in WEST using ICRH.

Multipactor-Triggering Powers’ Modelling of a WEST ICRH Antenna During RF Conditioning

During radio frequency (RF) conditioning of the ion cyclotron resonance heating (ICRH) system on WEST, under vacuum, pressure rises are measured inside the antennas. A proposed hypothesis for the cause of this phenomenon is multipactor, an electron multiplication process taking place in RF devices under vacuum. Modelling multipactor conditions in a resonant antenna, i.e., under high standing wave (SW) ratio, requires particular precautions. This article proposes a methodology to determine the multipactor conditions in such a context. Using the proposed approach, the operational conditions, expressed as the generator’s RF powers, of the appearance of multipactor inside a WEST ICRH antenna during its RF conditioning under vacuum, are deduced. Calculations are performed for two different surface states: when the surface is baked but nonconditioned, and when it is baked and fully conditioned. For the nonconditioned surface state, it is shown that when only one side of the under-conditioning ICRH antenna is powered, the off-mode side is prone to multipactor when its capacitors are tuned. Detuning the capacitors on the off-mode side suppresses almost the multipactor on the nonpowered side for the frequency range of interest. Moreover, it is shown that the fully conditioned antenna surface state can suppress the appearance of multipactor in some regions of the antenna.

High-order coupling of shear and sonic continua in JET plasmas

A recent model coupling the shear-Alfvén and acoustic continua, which depends strongly on the equilibrium shaping and on elongation in particular, is employed to explain the properties of Alfvénic activity observed on JET plasmas below but close to the typical frequency of toroidicity-induced Alfvén eigenmodes (TAEs). The frequency gaps predicted by the model result from high-order harmonics of the geodesic field-line curvature caused by plasma shaping (as opposed to lower-order toroidicity) and give rise to high-order geodesic acoustic eigenmodes (HOGAEs), their frequency value being close to one-half of the TAEs one. The theoretical predictions of HOGAE frequency and radial location are found to be in fair agreement with measurements in JET experiments, including magnetic, reflectometry and soft x-ray data. The stability of the observed HOGAEs is evaluated with the linear hybrid magnetohydrodynamic/drift-kinetic code CASTOR-K, taking into account the energetic-ion populations produced by neutral beam injection and ion cyclotron resonance heating systems. Wave-particle resonances, along with drive/damping mechanisms, are also discussed in order to understand the conditions leading to HOGAEs destabilisation in JET plasmas.

Three-dimensional modeling and simulation of RF cavity for a transmitter with bandwidth of 40–100 MHz at 100-kilowatt level

A 1.5 MW final power amplifier is to be developed for ion cyclotron resonance heating (ICRH) system for the China fusion engineering testing reactor (CFETR) project. First, the feasibility of driver power amplifier with input matching circuit and output resonator cavity and a newly developed tetrode was cross-verified through three-dimensional (3D) simulations and experiments. A single coaxial cavity in parallel with the tetrode covered the frequency range of 40 to 100 MHz at a 100-kilowatt level. In contrast to the previous method of deducing matching parameters using RF circuit and transmission line theory, the use of 3D transmitter simulations provided more accurate matching parameters of tuning components at different operating frequencies. A new phenomenon of high frequency oscillation between the short strip inductor and the shield cavity was found in the input simulation results, which was subsequently verified by experiments. Specific reasons for dividing the output into two parts of high and low frequency were found through output circuit simulations. One is that the length of short stub decreases with the increase of operating frequency, thus decreasing the frequency resolution if all frequencies were using low frequency port. The other is that the coupling capacitor increases sharply with the increase of frequency due to parasitic effect in the low output. Further, the simulation facilitated the study of parasitic effects on matching parameters caused by strip-inductance, inter electrode capacitances of tetrode, and RF cavity. Thus, facilitating the design of final power amplifier in the future. Finally, the amplifier installed with the new tetrode DB968 achieved the design objectives in the test experiment with 150 and 110 kW power outputs at 48 and 100 MHz, respectively.

Impedance matching system using triple liquid stub tuners for high-power ion cyclotron resonance heating in EAST tokamak.

Ion cyclotron resonance heating (ICRH), one of the main auxiliary methods, for high-power and long-pulse plasma heating had been developed in Experimental Advanced Superconducting Tokamak (EAST). An impedance matching system, one important part of ICRH, had been developed for high-power injection and transmitter protection by reducing the reflected power from the antenna. The input impedance in the outlet of the stub tuner can be measured by voltage-current probes installed on the coaxial transmission line between the antenna and triple liquid stub tuners, and the optimum liquid levels in the stub tuners can be calculated based on the input impedance. The calculation and adjustment process of the optimum liquid levels are described comprehensively in this article. Finally, impedance matching had been achieved between two shots during EAST experiments. In the near future, a real-time impedance matching system will be developed to prevent large variations of the ICRH antenna impedance and achieve steady-state and long-pulse operation with the ICRH system.

Numerical Design of RF Antennas for Ion Cyclotron Resonance Heating in ECRIS Environment

In this paper we present the numerical design and simulation of RF antennas to be employed in Ion Cyclotron Resonance Heating (ICRH) systems working in ECRIS environment. A 3D full-wave numerical model, based on the coupling between COMSOL FEM solution of Maxwell equations and the MATLAB-computed non-homogeneous plasma dielectric tensor, has been employed in order to study the performances of several ICRH antennas. Results in terms of S-parameters, on-axis electric field and RF absorbed power inside the plasma chamber have been obtained and compared between the chosen antenna geometries. The presented study will permit to better understand the fundamental aspects of ion dynamics in ECRISs as well as allowing the design of a proper matching network between the RF amplifier and the antenna, necessary to cope with the plasma properties’ fast variations. Further ion kinetic simulations are ongoing.

Analytical edge power loss at the lower hybrid resonance: ANTITER IV validation and application to ion cyclotron resonance heating systems

In the ion cyclotron range of frequency (ICRF), the presence of a lower hybrid (LH) resonance can appear in the edge of a tokamak plasma and lead to deleterious edge power depositions. An analytic formula for these losses is derived in the cold plasma approximation and for a slab geometry using an asymptotic approach and an analytical continuation near the LH resonance. The way to minimize these losses in a large machine like ITER is discussed. An internal verification between the power loss computed with the semi-analytical code ANTITER IV for ion cyclotron resonance heating (ICRH) and the analytic result is performed. This allows us to check the precision of the numerical integration of the singular set of cold plasma wave differential equations. The set of cold plasma equations used is general and can be applied in other parameters domain.

Thermal Analysis of Main Components in The Design of High-power Tetrode

High-power tetrode is a key component in ion cyclotron resonance heating system, as the final amplifier component to increase the RF power to the megawatt level. Thermal analysis is of important significance to the development and use of high-power tetrodes. In this paper, the trajectory of particles during the operation of the tetrode is obtained by CST particles studio, and its energy conversion is analyzed through the simulation results. Then the grid, anode, and support structure of the tetrode are analyzed thermally in Ansys Workbench, and the results are helpful to the design of the heat dissipation structure of the tetrode.

Development of mock-up Ion cyclotron resonance heating system with a new algorithmic approach for automatic impedance matching

This paper presents the development of a mock-up Ion Cyclotron Resonance Heating (ICRH) system along with a new algorithmic approach for fast automatic impedance matching. The mock-up is developed for low power (few watts) and frequency (182.5 MHz) i.e. five times the ICRH frequency of SST-1. The size of the mock-up is considerably reduced to be fit on the test bench using it. The main objective of the work is to develop different matching approaches in order to get the optimum speed of matching. Reducing the size of the mock-up allows frequent implementation and testing of new algorithms which is generally difficult to implement in the real system of tokamak because of its size and complexity. The system consists of indigenously developed components such as 3 dB hybrid coupler, 20 dB directional coupler, tapped transmission line, rigid coaxial stub tuners, line stretcher, service stub, RF antenna, movable water tank based load. The developed system is capable of automatically match any arbitrary load in the range, (35–260)Ω, resistive + (-j25 to +j220)Ω, reactive or VSWR in the range 1.5–4.0 within 200 ms. The matching system also consists of power detectors which automatically sense the forward and reflected power using coupler and the feedback controlled moving coaxial stubs, line stretchers respond as per developed algorithms. The matching algorithm incorporates the various matching steps based on VSWR which have been interpreted for the controller program to control the devices used in the matching network. The developed system is tested using an RF antenna facing the movable saltwater tank that emulates the plasma-like loading behavior. The developed setup is also useful for various high power RF applications like RF heating, sputtering, plasma generation, RADAR, etc. in the RF frequency range.

WEST actively cooled load resilient ion cyclotron resonance heating system results

Three identical new WEST ion cyclotron resonance heating (ICRH) antennas have been designed, assembled then commissioned on plasma from 2013 to 2019. The WEST ICRH system is both load-resilient and compatible with long-pulse operations. The three antennas have been successfully operated together on plasma in 2019 and 2020, with up to 5.8 MW of coupled power. The load resilience capability has been demonstrated and the antenna feedback controls for phase and matching have been developed. The breakdown detection systems have been validated and successfully protected the antennas. The use of ICRH in combination with lower hybrid has triggered the first high confinement mode transitions identified on WEST.

Impact of suprathermal ions on neutron yield in the pre-DT phase of ITER operation

An assessment of neutron production during the pre-fusion power-operation (PFPO) phase has been carried out for a representative set of plasma scenarios predicted by the ITER Research Plan. A range of heating systems, namely neutral beam injection (NBI) (hydrogen), electron cyclotron resonance heating (ECRH), and ion cyclotron resonance heating (ICRH) are planned to be used for PFPO studies in helium, hydrogen, and mixed hydrogen–helium plasmas. Fast ions (protons and 3He) originating from NBI and ICRH systems can increase neutron production in PFPO plasmas by directly interacting with intrinsic Be impurities or through secondary processes, as also evidenced at JET. The generation of fast ions in ITER PFPO scenarios has been modelled using the ASTRA-NBI and TORIC-SSFPQL codes. A significant impact of the synergy between hydrogen NBI and hydrogen-minority ICRH on neutron production in helium plasmas is reported. In addition, the stability of the toroidicity-induced Alfvén eigenmodes (TAE) is analyzed for PFPO plasmas with a high pressure of suprathermal ions and a weak reversed shear. The possible impact of sawtooth oscillations and TAEs on neutron production is discussed, based on a linear stability analysis.

Energy-angle distribution of the ions in the RF sheath of ICRH antennas

Radio-frequency sheaths forming at the surface of ion cyclotron resonance heating (ICRH) actuators in fusion experiments are associated with enhanced impurity sputtering from ICRH plasma-facing components (PFCs). The minimization of impurity fluxes from the ICRH PFC is a vital task for the usage of ICRH systems. Capturing the ion kinetics is important for an accurate description of plasma–material interactions, because the ion dynamics plays a crucial role in RF sheaths. Here, we present a hybrid particle-in-cell (hPIC) model able to capture the kinetic behavior of the ions. We analyze the kinetic ion energy-angle distributions (IEADs) impacting the RF antenna and its dependence on different plasma and RF sheath parameters. In particular, the IEAD dependence on RF frequency and magnetic field alignment is investigated. Using hPIC, we simulated a case emulating the latest experimental campaign from JET. The simulation showed that under specific plasma and RF parameters, the kinetic motion of ions results in a cusp formation in the IEAD.

Preliminary RF characterization of DEMO in-port ion cyclotron heating system for toroidal and poloidal geometry variations

Ion cyclotron Resonance Heating (ICRH) is one of the auxiliary heating schemes considered for the future DEMOnstration Power Plant (DEMO). The ICRH system in DEMO should couple 50 MW using three equatorial ports. Compared to the ITER one, the current DEMO port design has a smaller toroidal and a larger poloidal extent.One of the ICRH pre-conceptual design options for DEMO, an in-port antenna based on the current ITER design, is investigated using the ANTITER II code. To make optimal use of the port larger poloidal extent, the possibility to use quadruplets instead of the ITER triplets is explored. The impact on the power coupling is evaluated for two different antenna geometry considering a possible toroidal extension of the antenna in the blanket. The ITER matching-decoupling system is adapted to the new antenna front geometries considered.

Ion cyclotron resonance heating fast and slow wave excitation and power deposition in edge plasmas with application to ITER

The antenna power coupling to the plasma centre and its possible deleterious coupling to the plasma edge are key parameters in an ion cyclotron resonance heating system. The influence on these parameters by the confluence between the slow and the fast magnetosonic waves is studied for the case of large machines. Until now, the modelling of the scrape off layer region has been calculated by ANTITER II, which contains only the fast wave description and where the confluence with the slow wave (S wave) is approximated by the Alfvén resonance. In the present study, a complete modelling of the slow and fast waves is made by ANTITER IV. The modelling by the two codes is compared and shows the important role of the Alfvén and the lower hybrid resonances for the excitation of large fields and associated power deposition at the edge of the plasma even far from the antenna location. The ANTITER IV modelling is thereafter applied to the case of the ITER antenna with a reference density profile and heating parameters. A comparative study is made for the edge power deposition and the excitation of large fields for different toroidal phasing cases of the antenna. This study also takes into account the tilting of the antenna array with respect to the total magnetic field in front of the antenna. If the Faraday screen is field-aligned, the excitation of the S wave occurs at the wave confluence; however, in the case of non-alignment the antenna directly excites the S wave. This effect is studied and quantified. All edge effects, even the direct excitation of S waves, can be strongly reduced by tailoring the current distribution in the straps of the antenna array. Resulting cases for the minimisation of edge power deposition in ITER and the reactor are studied.

Robust control of q-profile and βp using data-driven models on EAST

• A novel H ∞ robust control scheme is developed for the current profile control using data-driven models. • A decoupling technique enables the control scheme to exhibit multi-functional characteristics. • An anti-windup technique can compensate the effects from moderate time-delays and actuator dynamics. • An efficient procedure is provided to support the H-mode steady-state operation on EAST. • Three general performance indexes are proposed to characterize the current profile control performance. A new robust feedback controller for the safety factor profile and poloidal plasma pressure parameter has been developed using a two-time-scale data-driven model. The model describes the linear plasma responses of the ι profile and β p with respect to auxiliary heating & current drive (H&CD) powers, around a typical plasma equilibrium in an H-mode steady-state plasma discharge on EAST. The feedback controller comprises a carefully designed low-pass filter for the timescale separation, a decoupling module and three local controllers synthesized from the H ∞ norm optimization and the singular value decomposition. The actuators are the lower hybrid current drive (LHCD) system at 4.6 GHz and the ion cyclotron resonance heating (ICRH) system at 33 MHz. Taking into account the actuation dynamics, an anti-windup technique is employed to condition the controller online aiming to attenuate the negative effects from moderate time delays and power saturations. Extensive nonlinear closed-loop simulations with the METIS code suggest that high β p and negative central magnetic shear that characterize advanced tokamak plasma scenarios can be achieved and sustained on EAST with good tracking performance and reasonable robustness via the proposed control scheme. The feedback control of the core ι profile and β p with a range of time delays, power saturations and varying weighting functions are evaluated numerically, compared and discussed. The control robustness to plasma parameter uncertainties including the line-averaged plasma density 〈 n ¯ e 〉 , the H-mode enhancement factor H factor and the effective charge number Z eff are assessed and analyzed.