Electron Cyclotron Resonance Heating Research Articles

ICRF plasma production at hydrogen minority regime in LHD

This study aim is to develop further an ion cyclotron range of frequencies (ICRF) method of plasma production in stellarators based on the minority heating. The previous studies demonstrate production of low density plasma (9.5 × 1017m−3) at low power of up to 0.2 MW. The higher ICRF heating power experiments become possible after introducing a programmable ICRF power ramp up at the front of the ICRF pulse. With this trick, all the shots went with the antenna voltage within the safe range. Increase of the ICRF power predictably results in increase of the density of produced plasma. Without pre-ionization the plasma density achieved was 6 × 1018 m−3 which is 6 times higher than in previous experiments. However, the electron temperature was not high, the light impurities were hot fully stripped, and there were no recombination peaks after termination of the ICRF pulse. Plasma density is too low to provide good conditions for efficient plasma heating. For the reference, the ICRF heating of high density cold plasma prepared by electron cyclotron resonance heating is performed. Both electrons and ions were heated to high temperatures, and this plasma state is sustained. The antenna–plasma coupling was much better which result in larger heating power with the lower antenna voltage.

Thermo-mechanical and quasi-optical analysis of the ECRH equatorial launcher steerable mirror towards the CFETR

This study reports a prototype design of the front steerable mirror (SM) in the Electron Cyclotron Resonance Heating (ECRH) system, which is used to validate the manufacturing technology of the large-scale quasi-optical mirror in the China Fusion Engineering Test Reactor (CFETR). As a plasma-oriented component, the SM will experience more neutron heat and plasma radiation heat than the remote focusing mirrors, and the simulation results indicate that the peak temperature of the SM would then reach 292.5 °C, the maximum deformation and the maximum equivalent stress would be 1.16 mm and 446 MPa, respectively. The beam exhibits little distortion because the thermal deformation gradient around the wave center is small, and the quasi-optical analysis shows that the position of the wave heating plasma should be shifted by -6 mm along the local Z-axis, while the amplitude of the central electrical field would increase by 0.3%, from 294.7 kV/m to 295.5 kV/m.

Distinctive Features of Anomalous Plasma Radiation in Experiments on Electron Cyclotron Heating Using Extraordinary Wave at the Second Resonance Harmonic in a Wide Density Range

A detailed interpretation of the effect of anomalous radiation is given, which was first observed at the TEXTOR tokamak in experiments on electron cyclotron resonance heating by the extraordinary wave at the second resonance harmonic in a wide range of plasma densities.

Absorption of Microwaves During Plasma Heating at the Second Harmonic of Electron Cyclotron Resonance in Tokamaks and Stellarators: Linear Theory and Experiment

We study the microwave absorption during electron cyclotron resonance heating (ECRH) by the extraordinary wave at second harmonic (X2 mode) in the T-10 tokamak and TJ-II stellarator in a wide range of plasma densities, and compare experiments with the classical formulas for the absorption of the injected ECR power. Empirical relations for the absorption efficiency and for the critical plasma density {{n}_{{{text{cr}}}}}, which separates the regions with full and partial absorption of the injected ECR power, are obtained using the numerical simulation of the heat transport with the transport model of canonical profiles. It is shown that in both devices, the range of densities exists, where the absorption predicted by the classical formulas is almost full, while according with the empirical formula, only a small fraction of the power is absorbed. The obtained relations allow ones to optimize the conditions of ECRH in toroidal systems for magnetic plasma confinement.

Broadband Alfvénic excitation correlated to turbulence level in the Wendelstein 7-X stellarator plasmas

During the first operational phase (OP1) of the Wendelstein 7-X (W7-X) stellarator, poloidal magnetic field fluctuations, , were measured in several different plasma scenarios with a system of Mirnov coils. In the spectrograms, multiple frequency bands close together in frequency are observed below f = 600 kHz. Furthermore, a dominant feature is the appearance of a frequency band with the highest spectral amplitude centred between kHz. The fluctuations are observed from the beginning of most W7-X plasmas of OP1, which were often operated solely with electron cyclotron resonance heating. The fluctuations show characteristics known from Alfvén waves and possibly Alfvén eigenmodes (AEs). However, the fast particle drive from heating sources, which is generally a driver necessary for the appearance of AEs in magnetic confinement plasmas, is absent in most of the analysed experiments. A characterization of the Alfvénic fluctuations measured during OP1 plasmas is possible using a newly developed tracking algorithm. In this paper, we extensively survey the different spectral properties of the fluctuations in correlation with plasma parameters and discuss possible driving mechanisms. The correlation studies of the dynamics of the possible ellipticity induced AEs indicate that Alfvén activity in the frequency interval between kHz could be excited due to an interaction with turbulence, or profile effects also affecting the turbulence amplitude.

The Formation and Stationary Maintenance of Peaked Pressure Profiles in Turbulent Tokamak Plasmas

The theoretical model for increasing peaking of radial distribution of plasma thermal energy and plasma energy confinement time improvement due to the formation of current profiles those provide the increased value of the safety factor qb at the outer plasma boundary is considered. The model for the formation and stationary maintenance of the peaked current density profiles those ensure the maintenance of plasma column equilibrium state with increased qb value and conservation of q0=1 at the magnetic axis is developed. The model is based on the significant ramp down of the Iind current maintained by the inductor with simultaneous central ECR heating and electron cyclotron current drive (ECCD) switching on. It is shown by the computer simulations of the self-consistent time evolution of turbulent plasma for the conditions of T-10 and T-15MD tokamaks that in the shot scenarios with three- or four-fold decrease in the Iind current at the ECRH stage of shots it was achieved an approximately two-fold enhancement of the electron and ion temperatures at the axis of the plasma column as compared to the standard shots with the same heating powers.

The Formation and Stationary Maintenance of Peaked Pressure Profiles in Turbulent Tokamak Plasmas

The theoretical model for increasing peaking of radial distribution of plasma thermal energy and plasma energy confinement time improvement due to the formation of current profiles those provide the increased value of the safety factor {{q}_{b}} at the outer plasma boundary is considered. The model for the formation and stationary maintenance of the peaked current density profiles those ensure the maintenance of plasma column equilibrium state with increased {{q}_{b}} value and conservation of {{q}_{0}} approx 1 at the magnetic axis is developed. The model is based on the significant ramp down of the {{I}_{{{text{ind}}}}} current maintained by the inductor with simultaneous central ECR heating and electron cyclotron current drive (ECCD) switching on. It is shown by the computer simulations of the self-consistent time evolution of turbulent plasma for the conditions of T-10 and T-15MD tokamaks that in the shot scenarios with three- or four-fold decrease in the {{I}_{{{text{ind}}}}} current at the ECRH stage of shots it was achieved an approximately two-fold enhancement of the electron and ion temperatures at the axis of the plasma column as compared to the standard shots with the same heating powers.

Combined analysis of laser interferometer and microwave reflectometer for a consistent electron density profile on HL-2A

In order to acquire a consistent and accurate electron density profile for HL-2A tokamak, a combined analysis of the laser interferometer and microwave reflectometer based on the mapping of density profiles to a common equilibrium magnetic coordinate has been proposed. In this approach, the edge density profile measured locally by microwave reflectometer is taken as a supplementary boundary condition in the procedure of inverting line-integrated data from interferometer diagnostic. The density profile inside that boundary is reconstructed by a Gaussian Process Tomography (GPT) method [1] within the Bayesian framework, by which the uncertainty of the result can be quantified via the confidence interval of a posterior probability. In the simulation benchmark, two representative cases of density distribution with different profile shapes, e.g. peaked and flat, are tested and the factors that may affect the accuracy of the results are investigated, providing a useful reference for practical applications. The application of this approach to actual data proves its ability to resolve the dynamic evolution of density profiles. Specifically, the effect of Electron Cyclotron Resonance Heating (ECRH) on electron particle transport observed in HL-2A experiments has been confirmed. The combined analysis approach developed is expected to provide a robust means to study the confinement performance and particle transport on HL-2A, with the focus on the core region.

Suppression of anomalous impurity transport in NBI-heated W7-X plasmas

Radial impurity density profiles in two Wendelstein 7-X (W7-X) experiments heated by neutral beam injection (NBI) are analyzed with respect to their impurity transport properties. Local impurity densities are derived from charge exchange reactions with the W7-X NBI system using simulated neutral densities cross-validated with beam emission spectroscopy. Impurity profiles of argon and carbon are found to show an evolving central accumulation inside half radius. The properties of the underlying impurity transport are assessed using the one-dimensional transport code pySTRAHL. Comparisons between simulation and experiment indicate transport dominated by anomalous diffusion outside half radius. The observed central impurity accumulation is found to match best simulations with purely (neo-)classical transport in the accumulation region. This data implies a suppression of the anomalous impurity transport channel to below 35% of the (neo-)classical one. Experimental data is found to be matched best when invoking a time evolving, inward propagating zone where anomalous impurity transport is suppressed. An additional central power deposition into a plasma with central impurity accumulation via electron cyclotron resonance heating is found to affect the (neo-)classical transport components in case of operation times below 200 ms only. For longer operation times, it is found to re-introduce an altered level of anomalous diffusion. The existence of an inward directed anomalous pinch as an alternative explanation for the central impurity accumulation cannot explain the observed profiles.

Evaluation and validation of radial impurity density profiles from CXRS using neutral beam modelling in W7-X

Absolute radial impurity density profiles in the Wendelstein 7-X stellarator (W7-X) are derived from charge exchange recombination spectroscopy using modelling of the neutral beam. The approach is validated via cross comparisons of the neutral beam attenuation, the radial localization of ion temperature measurements, as well as a comparison of the effective plasma charge determined by other diagnostics. The latter implies the validity of the obtained absolute density levels. The simulation based approach novel to W7-X allows to assess the neutral beam halo population, introducing corrections to the shape and amplitude of determined impurity density profiles. To illustrate the capabilities of the derived impurity density profiles, the particle transport properties in a W7-X discharge heated by electron cyclotron resonance heating were assessed. Density profiles for various impurities are found to be flat, consistent with dominant anomalous diffusion in the range 0.1–5.0 m2 s−1. This range of the anomalous transport in such plasmas is in line with other impurity diagnostics at W7-X.

Steady-State Operation Control of Gyrotrons in Support of Long Pulse Plasma Operation in EAST

In order to realize the 1000 s long pulse operation of the ECRH (Electron Cyclotron Resonance Heating) system on EAST (Experimental Advanced Superconducting Tokamak), we developed an ECRH steady-state operation control system. Four working modes were designed, namely manual restart mode, automatic restart mode, timed alternate operation mode, and protection trigger alternate operation mode. The FPGA-based NI cRIO was used as the control device, and the LabVIEW was used to write the program. The system has a quick response and accurate timing sequence, and can realize the long-pulse steady-state operation of the gyrotrons in EAST ECRH system. The steady-state operation control system has been verified in nearly two rounds of EAST experiments, helping EAST achieve the 1056 s long pulse discharge.

Short-pulse frequency stabilization of a MW-class ECRH gyrotron at W7-X for CTS diagnostic

At the Wendelstein 7-X stellarator, a 174 GHz Collective Thomson Scattering (CTS) diagnostic will be implemented. One of the 140 GHz Electron Cyclotron Resonance Heating (ECRH) gyrotrons will be operated at around 174 GHz in a higher cavity mode, using it as source for the CTS mm-wave probing beam. To prevent any damage to the CTS receiver, a notch filter cuts out the high-power gyrotron signal at the entrance of the receiver. The bandwidth of the gyrotron signal determines the notch filter bandwidth. First proof-of-principle experiments on frequency stabilization were conducted on W7-X ECRH gyrotrons employing Phase-Locked Loop techniques. The gyrotron output frequency was controlled with the accelerating voltage, which is applied between the anode and cathode of the gyrotron diode-type Magnetron Injection Gun. Frequency stabilization experiments with 10 ms pulses were conducted at the gyrotron nominal frequency of 140 GHz as well as at 174 GHz. It is concluded that the gyrotron frequency could be stabilized for at least 3 ms at 140 GHz and 8 ms at 174 GHz. In the frequency spectrum, a clear main peak of the gyrotron frequency at 140 GHz with a full -15 dB linewidth of below 500 Hz was achieved.

Development of the electron cyclotron resonance heating system for Divertor Tokamak Test

The Divertor Tokamak Test (DTT) facility, whose construction has started in Frascati (Italy), will be equipped with an ECRH (electron cyclotron resonance heating) system including 32 gyrotrons as microwave power sources. The procurement of the first batch of sources with 16 MW total power, based on 170 GHz/≥ 1 MW/100 s vacuum tubes, is in progress and will be available for the first DTT plasma. The system is organized into four clusters of 8 gyrotrons each. The power is transmitted from the Gyrotron Hall to the Torus Hall Building (THB) by a quasioptical transmission line (TL), mainly composed of large mirrors shared by eight beams coming from eight different gyrotrons and designed for up to 1.5 MW power per single beam, similar to the TL installed at the stellarator W7-X. One of novelties introduced in the DTT system is that the mirrors of the TLs are embodied in a vacuum enclosure, using large metal seals, mainly to avoid air absorption and risk of arcs. The main reason is to reduce the risk of air breakdown, maintaining a pressure of 10−5 mbar far away from the Paschen minimum. The TL estimated volume is between ∼70 and ∼85 m3. The direct connection of the TL to the tokamak vacuum vessel has been evaluated, and different solutions have been proposed in order to prevent a possible impact on DTT operations. The microwave power is injected into the tokamak using independent single-beam front-steering launchers, real-time controlled and located in the equatorial and upper ports of four DTT sectors. In-vessel piezoelectric walking drives are the most promising candidates for the launcher mirror movement considering their compactness and capability to operate in an environment with strong magnetic field under ultra-high vacuum. The DTT ECRH system design, presented here, is based mainly on existing and assessed solutions, although the challenging adaptations to the DTT case are considered.

Experimental study of core MHD events in thousand-second improved confinement plasma on the EAST tokamak

Recently, stationary plasma with a world-record pulse length of 1056 s was achieved on the Experimental Advanced Superconducting Tokamak (EAST). In this work, the core magnetohydrodynamics (MHD) events as well as mode coupling processes have been investigated in EAST long pulse operation with electron heating dominant and pure radio frequency wave heating low collisionality, by using several diagnostics and the nonlinear numerical code M3D together. A saturated m/n = 1/1 kink mode was observed in the core region, where a stable internal transport barrier was found in the electron temperature channel. The frequencies and 2D structures of these modes were studied by a combination of soft X-ray (SXR) imaging and electron cyclotron emission diagnostics. The frequency of the m/n = 1/1 mode exhibited a feature of chirping down with time, and this chirping rate corresponds to the rate of electron diamagnetic drift frequency change. A twisted pattern (‘Taichi structure in shape’) was reconstructed by SXR tomography of the m/n = 1/1 mode. The perturbations of electron temperature and density caused by m/n = 1/1 are different in size, the latter one being much smaller. The destabilization of m/n = 1/1 was due to the strong central heating combined with electron cyclotron resonance heating and lower hybrid current drive. In the presence of the m/n = 1/1 mode, a negative current was generated on the magnetic axis, which anomalously broadened the core current profile. A m/n = 3/2 tearing mode triggered by m/n = 1/1 mode was also observed. The m/n = 3/2 mode has a smaller frequency than the m/n = 1/1 mode, and carries an m/n = 3/2 island with detectable size. A novel 3D MHD model that separately evolves the plasma density and temperature is applied to the m/n = 3/2 triggered by m/n = 1/1. It is found that a toroidal current density at the q = 1.5 surface, caused by non-axisymmetric density perturbation during the m/n = 1/1 nonlinear growth phase, was generated and hence the destabilization of the m/n = 3/2 tearing mode. Both the modeled electron temperature and density perturbations agree well with experimental observations. In the last section of this paper, the interaction between the m/n = 1/1 mode and fast electrons as well as the active control of this mode are also presented.

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.

Second-harmonic electron-cyclotron resonance heating: A possible impact from mode coupling near the resonance.

The short-wavelength paraxial asymptotic technique, known as Gaussian beam tracing, is extended to the case of two linearly coupled modes in plasmas with resonant dissipation. The system of amplitude evolution equations is obtained. Apart from purely academic interest, this is exactly what happens near the second-harmonic electron-cyclotron resonance if the microwave beam propagates almost perpendicularly to the magnetic field. Because of non-Hermitian mode coupling, the strongly absorbed extraordinary mode may partly transform into the weakly absorbed ordinary mode near the resonant absorption layer. If this effect is significant, it could impair the well-localized power deposition profile. The analysis of parameter dependencies gives insight into what physical factors affect the power exchange between the coupled modes. The calculations show a rather small impact of non-Hermitian mode coupling on the overall heating quality in toroidal magnetic confinement devices at electron temperatures above 200 eV.

Progress on the conceptual design of the antennas for the DTT ECRH system

One of the main goals of the Divertor Tokamak Test (DTT) facility is to reach a ratio of power crossing the separatrix over the major radius of about 15 MW m − 1, as the one expected in DEMO. For this purpose, up to 45 MW of external additional heating power shall be coupled to the plasma, provided by Electron Cyclotron Resonance Heating (ECRH), Ion Cyclotron Resonance Heating and Neutral Beam Injection. The foreseen total ECRH power installed at full power shall be 32 MW, generated using 1 MW/170 GHz gyrotrons, for 100 s. The present ECRH system foresees two launching antennas per DTT sector, based on the front-steering concept. The equatorial antenna is dedicated to plasma heating and current drive and the upper antenna to the stabilization of MHD instabilities. This paper focuses on the latest design concept for these two antennas and on the definition of the ex-vessel matching optics unit of the last section of the evacuated Transmission Line (TL). The design has been updated to be compatible with the insertion of CVD diamond windows, to separate the vacuum environment of DTT from the one of the TL. This choice requires adding corrugated waveguide sections between the last mirror of the TL and the first mirror inside the port, requiring some adaptation of the in-vessel optics and of the supporting structure. The possibility to modify the steering range for the launching mirror has been also investigated to be compatible with the new design of the first wall and for the upper antenna, to reach the q = 2 surface in the new plasma scenario.

Design and experimental results of a 28 GHz, 400 kW gyrotron for electron cyclotron resonance heating

A high-power 28 GHz gyrotron has been successfully developed at the Institute of Applied Electronics, China Academy of Engineering Physics. This gyrotron was designed for electron cyclotron resonance heating (ECRH) in the spherical tokamak XL-50. A diode magnetron injection gun was designed to produce the required gyrating electron beam. The gyrotron operates in the TE8,3 mode in a cylindrical open cavity. An internal quasi-optical mode converter was designed to convert the operating mode into a fundamental Gaussian wave beam and separate the spent electron beam from the outgoing microwave power. A tube has been built and successfully tested. The operational frequency of the tube is 28.1 GHz. For beam parameters at an accelerating voltage of 71 kV and beam current of 16 A, the gyrotron has delivered an output power of 400 kW, with a pulse length of 5 s. The output efficiency is about 50% with a single-stage depressed collector. The gyrotron has been installed on the XL-50 and has played an important role in the ECRH experiments.

Triple Langmuir probe calibration in TOMAS ECRH plasma

In the TOMAS device, a triple Langmuir probe is used to measure the electron temperature and density. The accuracy of this measurement depends on correct determination of the effective collecting area of the probe, which depends on complex plasma transport processes. The probe can be calibrated by electron cyclotron resonance heating experiments using the cut-off density of the ordinary wave (O-wave). This threshold only depends on the frequency of the injected wave, and the occurrence of this phenomenon is clearly visible in the temperature evolution. The value of density is consequently known at this point and can be used to calibrate the density measurements of the triple Langmuir probe.

Preliminary design and thermo-mechanical analysis of the ECRH equatorial launcher focusing mirror (M1) towards the CFETR

The Electron Cyclotron Resonance Heating (ECRH) Equatorial Launcher (EL) is under research towards the China Fusion Experimental Tokamak Reactor (CFETR). In the latest pre-research model, 6MW of electromagnetic waves would be guided to the plasma by two focusing mirrors (M1 & M2) and one front plate steering mirror (SM). Since the M1 will experience intense heat generation from ohmic dissipation and neutronic irradiation, a specific water-cooling structure has been designed for it to reduce the thermal deformation, allowing the M1 to experience less mechanical pressure during long pulses. The simulation demonstrates that water can efficiently dissipate ∼0.3 kW of neutronic heat and ∼30 kW of ohmic loss heat. As a result, the maximum deformation and equivalent stress are less than 0.4 mm and 160 MPa, respectively, which satisfactorily meets the system requirements.