Electron Cyclotron Resonance Heating Plasma Research Articles

Characterization of ECRH plasmas in TOMAS

To improve the plasma performance and control the density and plasma quality during the flat top phase, wall conditioning techniques are used in large fusion devices like W7-X and in JT60-SA. To study the performance of electron cyclotron wall conditioning, numerous experiments were performed on the TOroidally MAgnetized System, which is operated by LPP-ERM/KMS at the FZ-Jülich. It is a facility designed to study plasma production, wall conditioning, and plasma–surface interactions. The produced electron cyclotron resonance heating plasmas are characterized in various conditions by density and temperature measurements using a movable triple Langmuir probe in the horizontal and the vertical direction, complemented by video and spectroscopic data, to obtain a 2D extrapolation of the plasma parameters in the machine. A way to calibrate the triple Langmuir probe measurements is also investigated. These data can be used to determine the direction of the plasma drift in the vessel and identify the power absorption mechanisms. This will give more insight in the plasma behavior and improve the efficiency of wall conditioning and sample exposure experiments.

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.

Recent progress of the ECRH system and initial experimental results on the J-TEXT tokamak

In order to broaden the range of the plasma parameters and provide experimental conditions for physical research into high-performance plasma, the development of the electron cyclotron resonance heating (ECRH) system for the J-TEXT tokamak was initiated in 2017. For the first stage, the ECRH system operated successfully with one 105 GHz/500 kW/1 s gyrotron in 2019. More than 400 kW electron cyclotron (EC) wave power has been injected into the plasma successfully, raising the core electron temperature to 1.5 keV. In 2022, another 105 GHz/500 kW/1 s gyrotron completed commissioning tests which signifies that the ECRH system could generate an EC wave power of 1 MW in total. Under the support of the ECRH system, various physical experiments have been carried out on J-TEXT. The electron thermal transport in ECRH plasmas has been investigated. When ECRH is turned on, the electron thermal diffusivity significantly increases. The runaway current is elevated when a disruption occurs during ECRH heating. When the injected EC wave power is 400 kW, the conversion efficiency of runaway current increases from 35% to 75%. Fast electron behavior is observed in electron cyclotron current drive (ECCD) plasma by the fast electron bremsstrahlung diagnostic (FEB). The increase in the FEB intensity implies that ECCD could generate fast electrons. A successful startup with a 200 kW ECW is achieved. With the upgrade of the ECRH system, the J-TEXT operational range could be expanded and further relevant research could be conducted.

Measurements of magnetic field fluctuations in TJ-II plasmas with new in-vessel helical arrays of magnetic coils.

This paper describes two new helical arrays of magnetic coils recently installed inside the TJ-II vacuum vessel. Their main objective is the precise measurement of the spatial periodicity of the magnetohydrodynamic perturbations usually found in the TJ-II plasmas. Given the high probability of coil failures due to the harsh plasma environment and in view of the extremely difficult access to the TJ-II vessel interior for maintenance, the coil system has been divided in two quasi-identical helical arrays. Both arrays consist of 32 triaxial sensors measuring orthogonal components of the local magnetic field along an ideal helical path whose trajectory runs close to the plasma edge. A description of the main characteristics of coils and arrays as well as their nominal positioning along an ideal helical path, inside the vessel, is given. A precise experimental determination of the real spatial orientation of the coils is performed by comparing the signals measured in current ramp-up and ramp-down experiments with calculations based on a filamentary model for the TJ-II magnetic coils. After this fine calibration procedure, it is possible to analyze the dependence of the amplitude of the measured magnetic field and its fluctuations as a function of the coil distance to the last closed flux surface. The study of the phase evolution of the parallel and perpendicular oscillatory components is also enabled. Finally, two examples of mode number determination are shown. One corresponds to a low frequency mode appearing in pure electron cyclotron resonance heating plasma, and the other one shows several modes observed during combined injection of both co and counter neutral beams and identified as shear Alfvén waves.

2D distributions of potential and density mean-values and oscillations in the ECRH and NBI plasmas at the TJ-II stellarator

2D plasma potential ϕ distribution was measured in the electron cyclotron resonance heating (ECRH) and neutral beam injection (NBI) plasmas of the TJ-II stellarator with the heavy ion beam probe for the whole radial range and wide area of the poloidal angle, and supported by Langmuir probe data at the edge. The whole operation domain for the on-axis ECRH was explored ( = 0.45–0.8 ×1019 m−3, P EC = 220–470 kW), in addition, NBI plasmas with = 0.9–1.3 × 1019 m−3 and P NBI = 510 kW were studied. In ECRH plasmas the density ramp-up is accompanied by the evolution of the potential from the bell-like to the Mexican hat profile, while the density profiles were flat or slightly hollow. The potential has the positive peak at the centre, and LFS-HFS (low field—high field sides) and up-down symmetry. Equipotential lines are consistent with vacuum magnetic flux surfaces. In the high-density NBI scenario, the ϕ profile was fully negative with a minimum up to −300 V at the centre, while at low-density ECRH plasma, ϕ has a maximum up to +0.9 kV at the centre. Fluctuations of potential and density are stronger in low-density scenarios and not poloidally symmetric. At the mid-radius (area of the maximum density), root mean square (RMS) of fluctuations were up to ϕ ∼ 15 V at LFS vs ∼20 V at HFS; RMS n e ∼ 2% at LFS vs ∼3% at HFS. In the NBI plasmas with the density rise, the asymmetry decreases and finally vanishing at = 1.2 × 1019 m−3. 2D distribution of the NBI-induced Alfvén eigenmodes (AEs) shows asymmetric ballooning structure: contrary to broadband turbulence, AE-associated potential perturbation dominates in the LFS with a factor up to 1.7 respect to the HFS. The electrostatic mode, excited in ECRH plasmas by suprathermal electrons also shows asymmetric structures: density perturbation dominates in the top-bottom direction compared to LFS-HFS direction.

Progress on the development of key technologies for the fourth generation ECR ion source FECR

A 4th generation ECR (Electron Cyclotron Resonance) ion source FECR (First 4th generation ECR ion source) is under development at IMP. Aiming to be operated with the microwave power of 20 kW at 45 GHz, FECR is equipped with a fully superconducting Nb3Sn magnet and conventional parts durable for high power ECR plasma heating and optimum for intense beam production and extraction. Breakthroughs on the Nb3Sn superconducting magnet, high power density plasma chamber, high temperature oven, and so on have been successfully demonstrated recently. The prototype Nb3Sn cold mass has been successfully tested. The plasma chamber with an innovative structure of micro-channel cooling structure has been tested which enables SECRAL-II ion source operated continuously for more than 1,100 hours with ∼300 eμA Kr26+ for routine operation at the microwave power level of ∼5 kW. A high temperature inductive heating oven has also been developed and used for intense uranium beam production. This talk will present the recent progress on the development of key technologies for the 4th generation ECR ion source.

On whistler-wave instability driven by butterfly-like electron distribution in a mirror magnetic trap

When electron cyclotron resonance (ECR) plasma heating is operated at a frequency exceeding the minimum electron cyclotron frequency in an open trap, the power deposition zone is shifted from the trap center towards higher magnetic fields. In this case, ECR heating may result in the formation of ‘butterfly-like’ distribution functions of fast electrons characterized by holes in a velocity space formed by particles that can not reach the heating zone while moving adiabatically from the trap center. In this paper, we develop a basic three-dimensional (1d-real + 2d-velocity space) kinetic model that allows us to describe the effects of the butterfly-like distributions on the whistler wave instability. We calculate a linear amplification gain for a whistler wave propagating along the magnetic field in an essentially inhomogeneous plasma, and compare the results with the well-studied case of anisotropy-driven whistler instability. The proposed theory is primarily aimed at the interpretation of recent experimental data accumulated in laboratory studies of kinetic instabilities and stimulated emission of ECR plasma discharges in mirror traps.

Observation of ICRH effect on toroidal rotation for Ohmic and ECH plasmas in KSTAR

Toroidal rotation behaviors are investigated in KSTAR when ion cyclotron resonance heating (ICRH) is applied in Ohmic and electron cyclotron resonance heating (ECH) plasmas. The ICRH induces the core toroidal rotation to the co-current direction, and H-mode transition is achieved by ICRH in ECH plasmas. Distinctive behaviors of the toroidal rotation during L- to H-mode transition triggered by ICRH and influences of edge localized modes in toroidal rotation are discussed.

Soft X-Ray Spectra of Plasma in High-Power ECR Heating Regime

The effect of the induced longitudinal (along the toroidal magnetic field) electric field on the shape of soft X-ray spectra in the regime of electron cyclotron resonance (ECR) heating of plasma in the L-2M stellarator is considered. Causes of the appearance of non-Maxwellian two-slope spectra of soft X-rays measured in the axial ECR plasma heating regimes at the introduced microwave powers of 200–400 kW are analyzed. In the authors' opinion, the two-temperature spectral shape arises due to the fact that radiation coming from two plasma regions falls in the field of view of the spectrometer when measuring the spectra along the central chord. A radiation fraction comes from the axial plasma region; another fraction comes from the heating region which in the high-power ECRH regime is approximately at half the plasma radius, rather than at the plasma center.

Improved performance of electron cyclotron resonance heating by perpendicular injection in the Large Helical Device

A real-time interlock system for power injection in electron cyclotron resonance heating (ECRH) was developed to be applied to Large Helical Device (LHD) plasma. This system enabled perpendicular injection, thus improving the performance of ECRH more than has ever been achieved before in LHD. Perpendicular propagation of the electron cyclotron wave at 77 GHz became more insensitive to the effect of refraction in comparison to the conventional oblique propagation. The achieved central electron temperature in the case of perpendicular injection was approximately 2 keV higher than that in the case of standard oblique injection for a central electron density of 1 × 1019 m−3 by 1 MW injection. With such improved performance of ECRH, high-density ECRH plasma of 8 × 1019 m−3 was successfully sustained after the injection of multiple hydrogen ice pellets for the first time in LHD.

Longitudinal Electric Field in the Ohmic and Electron Cyclotron Resonance Regimes of Plasma Heating at the L-2M Stellarator

A method for determining the longitudinal electric field from the spectrum of soft X-ray radiation is used to measure the radial distribution of the longitudinal electric field. In the ohmic heating regime, statistically reliable spectra of soft X-ray radiation were measured at the L-2M stellarator. It was obtained that the longitudinal field is distributed approximately uniformly over the radius. Comparison of the measured soft X‑ray radiation (SXR) spectra with the model bremsstrahlung spectrum of magnetized plasma in the longitudinal electric field has shown that at energies W > 3 keV, the measured spectrum deviates markedly not only from the Maxwellian one but also from the model spectrum of magnetized plasma in an electric field. Using SXR spectra that were measured in the regime of electron cyclotron resonance (ECR) plasma heating (without the ohmic-heating current), estimates are made of the longitudinal electric field that may occur in plasma due to drag currents, which are created during ECR heating. The dependence of the electric field on the plasma density was measured and it was shown that it agrees with the dependence of the difference current flowing through the plasma on the density.

RF plasma simulations using the TOMATOR 1D code: a case study for TCV helium ECRH plasmas

The 1-dimensional reaction-diffusion-convection code TOMATOR-1D describes plasma production by RF waves inside a tokamak using the Braginskii continuity and heat balance equations. The model simulates self-consistent radial density and temperature profiles for magnetised plasma mixtures of hydrogen and helium. The model reproduces the density profiles of X2 electron cyclotron resonance heating (ECRH) plasmas on TCV and proposes a Bohm-like poloidal magnetic field dependent scaling for anomalous diffusion and a convection scaling that results from drifts in the toroidal magnetic field configuration. A relation is proposed between the anomalous diffusion and the outward convection in toroidal plasmas. It is found that the EC absorption efficiency decreases at higher power, which is understood from the acceleration of electrons beyond the optimal energy for the electron impact ionisation of helium. A dramatic increase of the absorption efficiency is seen at intermediate vertical magnetic field values of which results in the highest density plasmas. Losses along the field lines in the vertical direction become dominant at higher fields which effectively reduces the plasma density in these discharges. To arrive at predictive capabilities towards ECRH plasmas on JT-60SA and ITER, the proposed scalings, subsuming dependencies on the torus major radius and the toroidal field strength, need to be validated in a multi-machine study.

Measurements of Microwave Power Absorbed during ECR Plasma Heating at the L-2M Stellarator

The results are presented from measurements of the power absorbed in plasma of the L-2M stellarator in experiments on the electron cyclotron resonance (ECR) plasma heating at the second harmonic of the gyrofrequency. Hydrogen plasma was created and heated in the stellarator vacuum chamber as a result of the resonance absorption of microwave power supplied by the gyrotrons in the pulse-periodic operation regime. The total energy of the toroidal plasma column and the absorbed power were measured using the diamagnetic diagnostics. The shielding effect of the metal vacuum chamber on the diamagnetic signal measurements was taken into account. It was ascertained that, during the axial ECR heating, up to 90% of the inputted gyrotron power is absorbed in the plasma that is consistent with the theoretical estimates.

2D full-wave simulations of different scenarios of ECR plasma heating at the L-2M stellarator

Different scenarios of electron cyclotron resonance (ECR) plasma heating under the conditions typical of L-2M experiments (ηe(0) = 1.75 × 1013 cm−3, Te(0) = 1 keV, λ0 = 0.4 cm) are simulated using a 2D full-wave model with allowance for the nonlocal (differential) thermal correction to the plasma dielectric tensor. It is shown that, under central ECR heating, due to the specific shape of the resonance surface ω0 = 2ωce, a significant fraction of the input microwave power is deflected downward and escapes onto the chamber wall. During off-axis heating at the midradius of the plasma column, about one-half of the microwave power is reflected upward from the resonance surface. Optimal conditions for the deposition of the microwave power in plasma are achieved under ECR heating at the vacuum magnetic axis, when the microwave beam is incident normally onto the resonance surface. In this case, the microwave power is almost completely (≈99.5%) absorbed by the plasma, while the coefficient of microwave reflection into the aperture of the incident microwave beam amounts to ∼0.1%, which agrees with results of 1D full-wave simulations.

A SCENARIO OF PULSED ECRH WALL CONDITIONING IN HYDROGEN FOR THE WENDELSTEIN 7-X HELIAS

A new wall conditioning scenario in hydrogen atmosphere for the Wendelstein 7-X stellarator-helias (W7-X) has been proposed and tested in part. Feasibility of the proposed scenario has been demonstrated on W7-X at OP1.2b experimental campaign during investigation of a chain of ultra-short pulsed electron cyclotron resonance heating plasma discharges.

A Scanning SXR Spectrometer Used in Experiments on ECR Plasma Heating at the L-2M Stellarator

Abstract—A diagnostic complex that is based on a scanning soft X-ray spectrometer has been developed at the Prokhorov General Physics Institute. The diagnostic complex is intended to measure the radial profiles of the plasma electron temperature in the L-2M stellarator. The maximum counting rate of the spectrometer is 2.0 × 105 pulses/s. It allows measuring of SXR spectra in the energy range of γ-photons of 1–80 keV. The spectral resolution of the spectrometer measured using a source of radioactive radiation with energy of E = 5.89 keV (55Fe) was ΔE = 320 eV (FWHM). The scanning system makes it possible to measure SXR spectra along the chords lying in the angular range from –13.5° to +26.5°. Negative and positive angles correspond to chords below and above the equatorial plane of the facility, respectively. The radial resolution of the spectrometer at the plasma axis is ~1.7 cm. The first results of spectral measurements in experiments at electron cyclotron resonance plasma heating at the L-2M stellarator are presented.

ECRH effect on the electric potential and turbulence in the TJ-II stellarator and T-10 tokamak plasmas

Electric field Ε or electric potential φ plays a key role in the transport and turbulence of toroidal plasmas. It is believed that mean radial Er suppresses the turbulence eddies via E × B shear, while oscillatory Er (zonal flows and geodesic acoustic modes, GAM) presents the mechanism of the turbulence self-regulation. Various aspects of the electron cyclotron resonance heating (ECRH), e.g. variation of power PECRH value and deposition effect on the static and oscillatory components of potential were studied in two machines of similar size by heavy ion beam probe (HIBP), operating now on the T-10 tokamak and TJ-II stellarator. HIBP measures in a wide density range = (0.3–5) × 1019 m−3 and in various magnetic configurations in Ohmic and ECRH plasmas on T-10, and in ECRH and NBI-heated plasmas on TJ-II. With ECRH, the potential evolves towards the positive direction. This extra potential Δφ increases with PECRH increase, while Δφ decreases with plasma density raise. ECRH excites the broadband electrostatic oscillations in low-density TJ-II plasma, while in high-density T-10 plasma, this effect is opposite. In T-10 GAM frequency fGAM increases with PECRH in accordance with theoretical dependence on electron temperature (fGAM ∼ ), and GAM amplitude increases with PECRH. ECRH affects to NBI-excited Alfvén eigenmodes (AEs): the steady frequency AEs transform to the chirping modes. In the low-density TJ-II plasmas, strong ECRH produces suprathermal (ST) electrons, exciting the electrostatic ST-modes. Dual HIBP measures the stable long-range potential correlations in TJ-II, resembling spatially localized low-frequency zonal flows in the core of ECRH plasmas. Finally, various aspects of the ECRH effects on the mean potential, broadband electrostatic turbulence, and on quasicoherent modes, including GAMs, AEs and ST-modes, are summarized.

Wall conditioning by ECRH discharges and He-GDC in the limiter phase of Wendelstein 7-X

Wendelstein 7-X (W7-X) relies on wall conditioning to control the density and the impurity content of the plasma. Wall conditioning in the first operation campaign of W7-X consisted of baking at 150 °C during 1 week prior to operation, glow discharge conditioning (GDC) in helium (He) and electron cyclotron resonance heating (ECRH) discharges. Additionally, the usage of He-GDC was limited to avoid sputtering and migration of metallic plasma facing components. This presented a unique opportunity for studying the applicability of ECRH discharges for initial wall conditioning on a stellarator, albeit in the carbon limiter configuration. A single envelope curve is observed in the normalised outgassing data that takes into account all ECRH discharges. This illustrates that the majority of discharges operates at the limits of a radiative collapse. Hydrogen recycling dominated the fuelling of ECRH discharges throughout while CO outgassing was found strongest at the start of the campaign. A reduction of recycling was observed throughout the campaign. Temporarily depleting the walls from H and impurities was possible by He-GDC. It was shown that the recycling coefficient in -ECRH plasmas could be reduced and the pulse duration significantly extended by He-’recovery’ ECRH plasmas. Good wall conditions were defined by normalised outgassing values below mbar kJ−1. In absence of -GDC, more than 311 cumulated discharge seconds of ECRH discharges are needed for obtaining lasting low outgassing levels. A release model with two trapping reservoirs could reproduce the normalised outgassing trend, including ECRH and GDC plasma wall interactions.

A Full Electromagnetic Particle-in-Cell Code To Model Collision-less Plasmas in Magnetic Traps

A lot of plasma physics problems are not amenable to exact solutions due to many reasons. It is worth mentioning among them, for example, nonlinearity of the motion equations, variable coefficients or non lineal conditions on known or unknown borders. To solve these problems, different types of approximations which are combinations of analytical and numerical simulation methods are put into practice. The problem of plasma behavior in numerous varieties of a minimum-B magnetic trap where the plasma is heated under electron cyclotron resonance (ECR) conditions is the subject of numerical simulation studies. At present, the ECR minimum-B trap forms the principal part of the multi-charge ion sources. There are different numerical methods to model plasmas. Depending of both temperature and concentration, these can be classified in three main groups: fluid models, kinetic models and hybrid models. The fluid models are the most simple way to describe the plasma from macroscopic quantities, which are used for the study of highly collisional plasmas where the mean free path is much smaller than size of plasma (l_mfp > L) from the solution of the Boltzmann or Vlasov equation, respectively [2]. For kinetic simulations there are different method to solve the Boltzmann or Vlasov equation, being the Particle-In-Cell (PIC) codes one the most popular. The hybrid model combine both the fluid and kinetic models, treating some components of the system as a fluid, and others kinetically; which are used for the study of plasmas, may use the PIC method for the kinetic treatment of some species, while other species (that are Maxwellian) are simulated with a fluid model. In this work, a scheme of the relativistic Particle-in-Cell (PIC) code elaborated for an ECR plasma heating study in minimum-B traps is presented. For a PIC numerical simulation, the code is applied to an ECR plasma confined in a minimum-B trap formed by two current coils generating a mirror magnetic configuration and a hexapole permanent magnetic bars to suppress the MHD instabilities. The plasma is maintained in a cylindrical chamber excited at TE_111 mode by 2.45 GHz microwave power. In the obtained magnetostatic field, the ECR conditions are fulfilled on a closed surface of ellipsoidal type. Initially, a Maxwellian homogeneous plasma from ionic temperature of 2 eV being during 81.62 ns, that correspond to 200 cycles of microwaves with an amplitude in the electric field of 1 kV/cm is heated. The electron population can be divided conditionally into a cold group of energies smaller than 0.2 keV, a warm group whose energies are in a range of 3 − 10 keV and hot electrons whose energies are found higher than 10 keV.

Two-slope soft X-ray spectra observed in experiments on electron cyclotron resonance plasma heating in the L-2M stellarator

In experiments on the generation and electron cyclotron resonance heating (ECRH) of plasma in the L-2M stellarator, non-Maxwellian two-slope soft X-ray (SXR) spectra were observed. The temperatures of the thermal and epithermal components of the spectra were measured as functions of the heating power and plasma density. A hypothesis based on the experimental results is suggested to explain the formation mechanism of two-slope SXR spectra in the ECRH experiments at the L-2M stellarator. The measured SXR spectra are compared with the results of numerical simulations.