Electron Cyclotron Resonance Heating Research Articles

Broadening of electron cyclotron power deposition and driven current profiles caused by dissipative diffractive propagation

Improvements in electron cyclotron resonance heating (ECRH) and current drive (ECCD) predictions are important issues for the design and control of high-performance fusion plasmas in future devices, where these should play a more important role as actuators than in devices to date. A newly developed EC-prediction package based on the quasioptical ray tracing code PARADE revealed in JT-60SA that (i) the radial profiles of both EC power deposition and driven current are broadened and (ii) the net driven current is increased by a few kA/MW, in comparison with conventional predictions due to dissipative diffractive propagation (DDP). The mechanism of DDP is as follows: EC wave beam obliquely passing through the resonant surface is dissipated non-uniformly on its beam cross section, so that the beam trajectory shifts gradually and thus the resonant position also shifts, resulting in the broadened power deposition profile. This novel ECCD and ECRH prediction package based on PARADE is applicable not only to JT-60SA but other existing devices and even, future devices.

Study of Electron Energy Distribution Function, Transport Parameters and Instability Analysis of Argon Plasma confined by a Dipole Magnet

This theoretical study is to investigate production and properties of plasma confined by a dipole magnet. A cylindrical permanent magnet (N40 grade) having a surface magnetic field of nearly 1Tesla is simulated to create a dipole magnetic field. Plasma is created by electron cyclotron resonance heating. Heating is accomplished using microwaves operating in continuous mode at 2.45 GHz and electron cyclotron resonance. The investigation is performed in systems of cylindrical geometries at varying pressures (0.5 mTorr – 1.5 mTorr). The electron temperature lies in the range of 1 - 5 eV, is hotter near the magnets and follows a downstream fashion throughout the region.Electron Energy Distribution Function (EEDF) growth investigation throughout the region was done to understand the role of magnetic field confinement. The EEDF was calculated numerically from steady state Boltzmann equation, considering plasma follows druyvesteyn distribution. Followed by analysing transport parameters such as diffusion coefficient, drift velocity, mobility, and recombination rates in plasma. A comparative study for all plots gave been done with respect to pressure. The instability analysis of elastic collisional frequency with inelastic collisions, real frequency and growth factor with wave vector has been also listed in the study.

Ion heat transport in electron cyclotron resonance heated L-mode plasma on the T-10 tokamak

Anomalous ion heat transport is analyzed in the T-10 tokamak plasma heated with electron cyclotron resonance heating (ECRH) in second-harmonic extra-ordinary mode. Predictive modeling with empirical scaling for Ohmical heat conductivity shows that in ECRH plasmas the calculated ion temperature could be overestimated, so an increase of anomalous ion heat transport is required. To study this effect two scans are presented: over the EC resonance position and over the ECRH power. The EC resonance position varies from the high-field side to the low-field side by variation of the toroidal magnetic field. The scan over the heating power is presented with on-axis and mixed ECRH regimes. Discharges with high anomalous ion heat transport are obtained in all considered regimes. In these discharges the power balance ion heat conductivity exceeds the neoclassical level by up to 10 times. The high ion heat transport regimes are distinguished by three parameters: the ratio, the normalized electron density gradient , and the ion–ion collisionality . The combination of high , high , and −10 results in values of normalized anomalous ion heat fluxes up to 10 times higher than in the low transport scenario.

Impact of T i/T e ratio on ion transport based on EAST H-mode plasmas

At the EAST tokamak, the ion temperature (T i) is observed to be clamped around 1.25 keV in electron cyclotron resonance (ECR)-heated plasmas, even at core electron temperatures up to 10 keV (depending on the ECR heating power and the plasma density). This clamping results from the lack of direct ion heating and high levels of turbulence-driven transport. Turbulent transport analysis shows that trapped electron mode and electron temperature gradient-driven modes are the most unstable modes in the core of ECR-heated H-mode plasmas. Nevertheless, recently it was found that the T i/T e ratio can increase further with the fraction of the neutral beam injection (NBI) power, which leads to a higher core ion temperature (T i0). In NBI heating-dominant H-mode plasmas, the ion temperature gradient-driven modes become the most unstable modes. Furthermore, a strong and broad internal transport barrier (ITB) can form at the plasma core in high-power NBI-heated H-mode plasmas when the T i/T e ratio approaches ~1, which results in steep core T e and T i profiles, as well as a peaked n e profile. Power balance analysis shows a weaker T e profile stiffness after the formation of ITBs in the core plasma region, where T i clamping is broken, and the core T i can increase further above 2 keV, which is 80% higher than the value of T i clamping in ECR-heated plasmas. This finding proposes a possible solution to the problem of T i clamping on EAST and demonstrates an advanced operational regime with the formation of a strong and broad ITB for future fusion plasmas dominated by electron heating.

On the Saturation of the Instability of Induced Ordinary Microwave Scattering in the Edge Transport Barrier of a Tokamak during Electron Cyclotron Resonance Heating of a Plasma

The saturation of the low-threshold parametric decay instability of an ordinary wave during electron cyclotron resonance heating in the edge transport barrier of a tokamak due to the stochastic damping of a daughter two-dimensionally localized oblique Langmuir wave has been considered. It has been found that the saturation of instability in modern devices occurs at a relatively low level and does not affect the energy balance during plasma heating. It has been shown that the efficiency of nonlinear pump under expected conditions of microwave power injection in the International Thermonuclear Experimental Reactor will exceed the maximum efficiency of stochastic damping, which will result in the breaking of amplitude-dependent saturation and can significantly modify the power deposition profile.

Experiment study of m/n = 1/1 mode saturation level in high-βp discharge on the EAST tokamak

The saturated internal kink mode with m/n = 1/1 has been observed in the H-mode discharge of the experimental advanced superconducting tokamak, characterized by high βP and weak magnetic shear in the core. This observation was made using a combination of soft x-ray imaging and electron cyclotron emission diagnostics. It was noted that the repetitive bursting of m/n = 1/1 internal mode (large amplitude, short bursting duration) transitions into a long-lasting continuous one (small amplitude, constant frequency), when βp> 2.2 in H-mode, a much shorter bursting duration, transitioning into a long-lasting continuous one, with smaller amplitude and a constant frequency. Conversely, for typical βp< 1.5 in L/I discharge, the frequency of m/n = 1/1 mode decreases with time, associated with local poloidal electron diamagnetism drift velocity. The benign m/n = 1/1 mode in high βp has an amplitude much smaller than one in L-mode for high βP stabilization effect. By adjusting the power deposition of electron cyclotron resonance heating, it is found that the saturation level of m/n = 1/1 mode is independent of the local electron temperature gradient in H-mode discharge. It underscores that the flat current and, hence, weak magnetic shear are crucial factors influencing the saturation level of the m/n = 1/1 mode for both the off-axis electron cyclotron current driving current-dominant and off-axis bootstrap current-dominant cases. Furthermore, the outward movement with growth of the m/n = 1/1 mode in the process of L-H mode transition and the characteristic time of this process are close to the current diffusion time.

Cascades of Parametric Instabilities in the Tokamak Plasma Edge during Electron Cyclotron Resonance Heating.

We report observations of nonlinear two-plasmon decay instabilities (TPDIs) of a high-power microwave beam, a process similar to half-harmonic generation in optics, during electron cyclotron resonance heating in a tokamak. TPDIs are found to occur regularly in the plasma edge due to wave trapping in density fluctuations for various confinement modes, and the frequencies of both observed daughter waves agree with modeling. Emissions from a cascade of subsequent decays, which indicate a generation of ion Bernstein waves, are correlated with fast-ion generation. This emphasizes the limitations of standard linear microwave propagation models and possibly paves the way for novel microwave applications in plasmas.

Suppression of the m/n = 2/1 tearing mode by electron cyclotron resonance heating on J-TEXT

Abstract Stabilization of tearing modes and neoclassical tearing modes is of great importance for the tokamak operation. Electron cyclotron wave (ECW) has been extensively used to stabilize the tearing modes with the virtue of the highly localized power deposition. Complete suppression of the m/n = 2/1 tearing mode (TM) by electron cyclotron resonance heating (ECRH) has been achieved successfully on the J-TEXT tokamak. The effect of ECW deposition location and power amplitude on the 2/1 TM suppression have been investigated. It is found that the suppression is more effective when the ECW power is deposited closer to the rational surface. Partial suppression of the 2/1 TM by ECRH has been observed and the island width is reduced by 65% of the initial island width. The island rotation frequency is increased when the island width is reduced. The experimental results show that the temperature perturbations increase inside the magnetic island produced by the local heating plays a dominant role on tearing modes suppression. As the ECW power increased, the 2/1 island is suppressed to smaller islands width and the flow shear also plays a stabilizing effect on small magnetic islands. With the stabilizing contribution of heating and the flow shear, the 2/1 TM can be completely suppressed.

Overview of the Recent Experimental Research on the J-TEXT Tokamak

Abstract The J-TEXT capability is enhanced compared to two years ago with several upgrades of its diagnostics and the increase of electron cyclotron resonance heating (ECRH) power to 1 MW. With the application of electron cyclotron wave (ECW), the ECW assisted plasma startup is achieved; the tearing mode is suppressed; the toroidal injection of 300 kW ECW drives around 24 kA current; fast electrons are generated with toroidal injected ECW and the runaway current conversion efficiency increases with ECRH power. The mode coupling between 2/1 and 3/1 modes are extensively studied. The coupled 2/1 and 3/1 modes usually lead to major disruption. Their coupling can be either suppressed or avoided by external resonant magnetic perturbation (RMP) fields and hence avoids the major disruption. It is also found that the 2/1 threshold of external field is significantly reduced by a pre-excited 3/1 mode, which can be either a locked island or an external kink mode. The disruption control is studied by developing prediction methods capable of cross tokamak application and by new mitigation methods, such as the biased electrode or electromagnetic pellet injector. The high-density operation and related disruptions are studied from various aspects. Approaching the density limit, the collapse of the edge shear layer is observed and such collapse can be prevented by applying edge biasing, leading to an increased density limit. The density limit is also observed to increase, if the plasma is operated in the poloidal divertor configuration or the plasma purity is increased by increasing the pre-filled gas pressure or ECRH power during the start-up phase.

Low-frequency whistler waves driven by energetic electrons in plasmas of solely electron cyclotron wave heating

Whistler waves are a type of low-frequency electromagnetic wave common in nature, which is usually associated with energetic electron phenomena. This study presents experimental observations of low-frequency whistler wave instabilities driven by energetic electrons through wave–particle interactions on EXL-50. The energetic electrons are generated by electron cyclotron waves (ECWs) through stochastic heating [Wang et al., J. Plasma Phys. 89, 905890603 (2023)] and do not match the characteristics of the runaway electrons [Shi et al., Nucl. Fusion 62, 086047 (2022)]. In the steady-state plasma of the Energy iNNovation XuanLong-50 (EXL-50), whistler waves within the 30–120 MHz frequency range were observed during electron cyclotron resonance heating. These waves displayed multiple frequency bands, and the frequencies of waves were directly proportional to the Alfvén velocity. Furthermore, it was interesting to find that superposition of lower hybrid wave into ECW resulted in the suppression of these whistler waves. The experimental results may indicate that the whistler waves are driven by energetic electrons (excluding runaway electrons). These discoveries carry significant implications for several areas of research, including the investigation of wave–particle interactions, the development of radio frequency wave current drivers, their potential impact on the electron dynamics in future fusion devices, and even the presence of unusually low-frequency whistler waves in Earth's radiation belts.

Formation of small-scale modes via ECCD injection into KSTAR plasma core

In KSTAR experiments exhibiting sawtooth instability, the formation of multiple flux tubes (MFTs) has been frequently observed when electron cyclotron resonance heating or a current drive is applied near the inversion radius of the sawtooth. On the global scale, these MFTs evolve into a single flux tube mode or dual modes. The modes are observed as multiple Fourier harmonics in the spectrogram. A comprehensive correlation analysis of 2D imaging diagnostic data reveals a notable energy transfer within structures of varying sizes during the global mode transition. Broadband fluctuations are enhanced, and energy transfer between Fourier harmonics occurs in the presence of MFTs. Cross-power spectrum in the presence of multiple Fourier harmonics aligns with power law of inverse cascade. This suggests that energy inverse cascade process can contribute to formation of MFTs.

Feasibility of a collective Thomson scattering diagnostic for burning plasma control on DEMO

Diagnostic systems are essential for burning plasma control and operation of DEMO, but are at the same time challenging in terms of design and integration. The harsh environment around the DEMO plasma, and the space restrictions and need to maximize the first-wall area used for tritium breeding, set limitations on the number and type of diagnostics to be installed. This will focus the efforts on diagnostics needed for control of the DEMO plasma. The robustness and versatility of a microwave-based Collective Thomson Scattering (CTS) diagnostic make it worthwhile to investigate the potential of a DEMO CTS diagnostic. We present the initial exploratory effort on this.The study builds on the experience of existing CTS experiments on e.g. ASDEX Upgrade, and on the recent development of the ITER CTS system, which focuses on measurements of fast ion dynamics in the burning ITER plasma. The initial target of the DEMO CTS diagnostic was to use an Electron Cyclotron Resonance Heating (ECRH) gyrotron beam as the probing source, with the receiving quasi-optical system being a dedicated CTS setup. Based on raytracing calculations including signal-to-noise estimates, it is found that such a setup is not viable. Here, we present studies of alternative solutions, including assessments of which DEMO plasma parameters the CTS diagnostic may contribute to determine.

β-induced Alfvén eigenmodes and fishbone driven by energetic electrons in EAST electron heating dominant plasmas

β-induced Alfvén eigenmodes (e-BAE) and fishbone (e-fishbone) driven by the energetic electrons have been identified on the experimental advanced superconducting tokamak in electron cyclotron resonance heating (ECRH), low hybrid wave (LHW) together with neutral beam injection plasma and in pure radio frequency heating plasma, respectively. The e-BAE is predominantly located near the pedestal region, and its frequency typically displays a chirping characteristic. The frequency is notably associated with the count of energetic electrons with energy between 30 keV and 80 keV driven by LHW and undergoes three distinct phases. A direct correlation is established between the frequency of e-BAE and the global parameters, including electron temperature, Alfvén velocity and βp . The e-fishbone emerges in the core and becomes noticeable shortly after ECRH initiation, in an electron heated dominant and core confinement enhanced discharge. Experimental analyses suggest that the e-fishbone ties closely with both the population and energy distribution of energetic electrons. These observations can provide valuable insights into the behavior of energetic electrons, which play a crucial role in the formation and sustainability of transport barriers within hybrid plasma, both in the internal and pedestal regions.

Characterization of the TJ-II stellarator plasma by means of reciprocating Langmuir probes

Experiments were conducted in deuterium plasma in the TJ-II stellarator by means of swept Langmuir probes mounted on reciprocating probes manipulators. The results were processed using the four-parameter fit, as well as the triple-probe and the first-derivative probe techniques. The parameters determined were the floating potential, the ion saturation current density, the electron temperature and density, and the plasma potential. The results were obtained for two plasma heating techniques – electron cyclotron resonance heating (ECRH) and neutral beam injection (NBI) heating. In the case of ECRH, employing the first-derivative probe technique resulted in finding that the electron-energy distribution function (EEDF) was not Maxwellian, but rather a bi-Maxwellian one with thermal (14-25 eV) and cold (4-5 eV) electrons. In comparison, during NBI heating we found a Maxwellian EEDF with the electron temperature being around 5 eV and slightly increasing in the confined plasma, but always remaining below 15 eV. We present a detailed analysis and discussion of the data for the plasma parameters as acquired by different techniques of using the reciprocating probe manipulator.

Detection of Internal Transport Barrier in the T-10 Tokamak Using Thomson Scattering Diagnostics

At the T-10 tokamak, the Thomson scattering diagnostics was upgraded in 2016. The new system is based on the Nd:YAG laser with the pulse repetition frequency of 100 Hz; the measurements can be performed every 10 ms during the entire plasma discharge at the spatial resolution of up to 5 mm. Using the upgraded diagnostics, the electron temperature measurements were performed during experimental campaigns of the T-10 tokamak in 2016−2018. In the regimes with the electron cyclotron resonance heating, it was demonstrated that the regions with increased temperature gradients form in the plasma, which was interpreted as the formation of internal transport barrier. The ASTRA-code-based simulations of the current profile time evolution made it possible to correlate the positions of the barrier and the rational surface q = 1.

Fault prediction of gyrotron system on test bench using a deep learning algorithm

The gyrotron system is an essential component of the Electron Cyclotron Resonance Heating (ECRH) system, generating high-power millimeter waves that can be utilized for plasma heating and current drive in magnetically confined nuclear fusion. During high-power long-pulse operation, the gyrotron is prone to faults such as arcing, mode jump, and overcurrent, leading to interruptions in continuous system output. These faults not only endanger the safety of the gyrotron but also reduce the overall efficiency and performance of the system. Addressing this issue, we have employed deep learning methods to predict faults that may occur during the gyrotron’s long pulse operation. Using experimental data collected from the megawatt-level continuous wave (CW) gyrotron test bench since 2019, we constructed a dataset comprising 52,772 data points, including 23,241 instances of faulty data. We trained a model based on the Transformer encoder architecture and compared its performance against classical CNN and LSTM models. The results demonstrated that the Transformer model exhibited superior performance in this context, achieving an AUC value of 0.869. The True Positive Rate (TPR) and False Positive Rate (FPR) reached 85.2% and 18.2%, respectively. Subsequent offline testing verified that the trained model could successfully predict faults in advance with second-level accuracy, laying the groundwork for future gyrotron operation management. In conclusion, deep learning has demonstrated its potential application in the prediction of operational faults in the gyrotron system.

Electron cyclotron resonance during plasma initiation

Electron-cyclotron resonance heating (ECRH) is the main heating mechanism in the Wendelstein 7-X (W7-X) stellarator. Although second-harmonic ECRH (X2) has been used routinely for plasma startup, startup at third harmonic (X3) is known to be much more difficult. In this work, we investigate the energy gain of particles during nonlinear wave–particle interaction for conditions relevant to second- and third-harmonic startups in W7-X. We take into account both the beam and the ambient magnetic field inhomogeneities. The latter is shown to significantly increase the mean energy gain resulting from a single wave–particle resonant interaction. In W7-X-like conditions, the improvement in maximum gained energy is up to 4 times the analogous uniform magnetic field case. However, this improvement is not enough to ensure X3 startup. The optimal magnetic field inhomogeneity length scale for average energy gain and start up in W7-X-like conditions is found to be in the range of $1$ to $3\ {\rm km}^{-1}$ . A possibility of using multiple beams with neighbouring resonances is also considered. A considerable enhancement of the energy gain is demonstrated.

Modeling of heating scheme impacts on neon seeded plasmas with the resonant magnetic perturbation application in the large helical device: ECR and NBI heating

Plasma parameter profiles along the effective minor radius of flux surfaces are modeled for the conditions with neon impurity puffing and external resonant magnetic perturbations at the plasma edge in the large helical device. For the electron cyclotron heating scenario, calculations qualitatively reproduce stable steady states with plasma detached from divertor target plates and radiation losses exceeding 90% of the heating power. It is unlikely with neutral beam injection heating that the plasma collapses if the radiation approaches a critical level noticeably below 100%. The latter is explained by the dependence of neutral beam attenuation on the plasma ion effective charge.

Electron plasma diagnostics in ELTRAP by electron cyclotron resonance heating method.

Electron cyclotron resonance heating method of Particle-in-Cell code was used to analyze heating phenomena, axial kinetic energy, and self-consistent electric field of confined electron plasma in ELTRAP device by hydrogen and helium background gases. The electromagnetic simulations were performed at a constant power of 3.8 V for different RF drives (0.5 GHz- 8 GHz), as well as for 1 GHz constant frequency at these varying amplitudes (1 V-3.8 V). The impacts of axial and radial temperatures were found maximum at 1.8 V and 5 GHz as compared to other amplitudes and frequencies for both background gases. These effects are higher at varying radio frequencies due to more ionization and secondary electrons production and maximum recorded radial temperature for hydrogen background gas was 170.41 eV. The axial kinetic energy impacts were found more effective in the outer radial part (between 0.03 and 0.04 meters) of the ELTRAP device due to applied VRF through C8 electrode. The self-consistent electric field was found higher for helium background gas at 5 GHz RF than other amplitudes and radio frequencies. The excitation and ionization rates were found to be higher along the radial direction (r-axis) than the axial direction (z-axis) in helium background gas as compared to hydrogen background gas. The current studies are advantageous for nuclear physics applications, beam physics, microelectronics, coherent radiation devices and also in magnetrons.

Broadening of the power deposition profile in on-axis X2 ECRH experiments in the L-2M stellarator and the low-threshold parametric decay instability of the pump wave

The effect of considerable broadening of the power deposition profile in on-axis X2 electron cyclotron resonance heating experiments observed in the L-2M stellarator is interpreted in the paper as one of the consequences of the low-threshold parametric decay instability of the pump wave, resulting in excitation of ion and electron Bernstein daughter waves. The instability is considered under the specific L-2M experimental conditions and shown to be absolute. The threshold of this instability is exceeded in the experiment. This finally allows explaining, at least qualitatively, the effect of nonlocal microwave power deposition detected in L-2M.