Cyclotron Emission Research Articles

MHD instability dynamics and turbulence enhancement towards the plasma disruption at the HL-2A tokamak

The evolutions of MHD instability behaviors and enhancement of both electrostatic and electromagnetic turbulence towards the plasma disruption have been clearly observed in the HL-2A plasmas. Two types of plasma disruptive discharges have been investigated for similar equilibrium parameters: one with a distinct stage of a small central temperature collapse (sim 5–10%) around 1 millisecond before the thermal quench (TQ), while the other without. For both types, the TQ phase is preceded by a rotating 2/1 tearing mode, and it is the development of the cold bubble from the inner region of the 2/1 island O-point along with its inward convection that causes the massive energy loss. In addition, the micro-scale turbulence, including magnetic fluctuations and density fluctuations, increases before the small collapse, and more significantly towards the TQ. Also, temperature fluctuations measured by electron cyclotron emission imaging enhances dramatically at the reconnection site and expand into the island when approaching the small collapse and TQ, and the expansion is more significant close to the TQ. The observed turbulence enhancement near the X-point cannot be fully interpreted by the linear stability analysis by GENE. Evidences suggest that nonlinear effects, such as the reduction of local E_rtimes B shear and turbulence spreading, may play an important role in governing turbulence enhancement and expansion. These results imply that the turbulence and its interaction with the island facilitate the stochasticity of the magnetic flux and formation of the cold bubble, and hence, the plasma disruption.

Mechanism for Collective Energy Transfer from Neutral Beam-Injected Ions to Fusion-Born Alpha Particles on Cyclotron Timescales in a Plasma

Helium ash alpha particles at ∼100 keV in magnetically confined fusion plasmas may have the same Larmor radius, as well as cyclotron frequency, as the energetic beam-injected deuterons that heat the plasma. While the velocity-space distribution of the helium ash is monotonically decreasing, that of the energetic deuterons is a delta function in the edge plasma. Here we identify, by means of first principles particle-in-cell computations, a new physical process by which Larmor radius matching enables collective gyroresonant energy transfer between these two colocated minority energetic ion populations, embedded in majority thermal plasma. This newly identified nonlinear phenomenon rests on similar underlying physics to widely observed ion cyclotron emission from suprathermal minority ion populations.

Optical design and synthetic analysis of the electron cyclotron emission imaging diagnostic of HL-2M tokamak

The electron cyclotron emission imaging (ECEI) diagnostic is a powerful tool to study the MHD and turbulent transport in magnetically confined fusion plasmas. In this work, the optical system including the local oscillator (LO) coupling and radio frequency (RF) receiving optics, has been designed and analyzed for the HL-2M ECEI diagnostic. The LO optics can illuminate the antenna array and drive the mixer diode to work efficiently, with more than 36% of the beam intensity for the channels which probe the plasma edge relative to those of channels which probe the plasma core. The RF optics aims at guiding the plasma emission signal to the antenna array. To meet different physical requirements, three types of field of view have been achieved for the plasma imaging, with zoom factors of around 1, 1.5 and 2, respectively. The focal surfaces are almost flat, with a maximum off-mapping (defined as the radial distance between the beam waists of the lower-/uppermost antennas and electron cyclotron emission layer) less than 2.5 cm, which can match the electron cyclotron emission layer quite well and suppress the image distortion in the plasma edge region. The impact of possible vibrations and installation error on the focal plane has been evaluated. In addition, the predefined MHD and turbulence perturbations are well reproduced by combining the optical simulation results and synthetic ECEI modeling, which further verifies the good performance of the RF optics.

Development of the diagnostic tools for the COMPASS-U tokamak and plans for the first plasma

The COMPASS-U tokamak (R = 0.894 m, a = 0.27 m, Bt = 5 T, Ip = 2 MA) is a new medium-size device with fully metallic plasma facing components, currently under construction at the Institute of Plasma Physics of the Czech Academy of Sciences in Prague. It features a unique combination of parameters, such as a high temperature of the tokamak walls up to 500 °C allowing a high recycling regime, a high magnetic field connected with a high plasma density above 1020m−3 and with a high heat flux (perpendicular to divertor targets) density at the outer strike-point up to 90 MW/m2 in attached conditions. These parameters of the device pose strict constraints and requirements on the design of individual diagnostic systems. Strategy and present status of the development of the diagnostic systems for COMPASS-U are provided. Plans for a diagnostic set for the first plasma are reviewed. The review of the diagnostics systems involves the high-temperature compatible slow (up to 20 kHz) and fast (up to several MHz) inductive and non-inductive magnetic sensors (including Thick Printed Copper coils and Hall sensors), the sub-millimetre interferometer with an unambiguous channel, Electron Cyclotron Emission, the interlock and overview cameras, high resolution Thomson scattering, radiation diagnostics (neutron diagnostics, soft and hard X-ray diagnostics, bolometers, impurity monitors, effective ion charge), probe diagnostics (including rail probes) and manipulators.

Excitation of toroidal Alfvén eigenmodes with counter-current NBI in the TCV tokamak

In Tokamak á Configuration Variable (TCV), unstable modes excited by resonant interaction between the shear Alfvèn waves in continuum gaps and energetic particles have been observed in scenarios with neutral beam injection (NBI). TCV is a middle-size device () equipped with a , tangential neutral beam injector. In this paper the phenomenology of modes excited with on-axis NBI is presented. The Alfvènic nature of the modes has been confirmed investigating their sensitivity against plasma parameters such as NBI energy, toroidal magnetic field, and cross-checking with the predictions from linear kinetic stability code. The mode radial profile is estimated using electron cyclotron emission measurement and agrees well with modelling results. In addition, the fast particle distribution function has been modelled using TRANSP/NUBEAM code. Even with counter-current NBI (leading to higher losses), the drive from the resonant particles is sufficient for the mode excitation. An ad-hoc additional diffusion model allows to estimate the fast particle transport, modifying the fast particle gradient at the mode location and matching the neutron rates.

Role of magnetohydrodynamic activity in sawtooth induced heat pulse propagation in ADITYA tokamak

Fast propagation of sawtooth induced heat pulse is observed in the magnetohydrodynamic (MHD) active plasmas of ADITYA tokamak. The sawtooth crash deposits heat beyond the inversion radius, which gets transported to the edge region of the plasma, and is reflected as inverted sawtooth modulation in the edge channels of electron cyclotron emission (ECE) and spectral line emission. The time-lag analysis on ECE signal reveals the propagation time from plasma core to edge of ∼150 µs. From this analysis, the estimated transient electron heat diffusivity is found to be ∼50–60 m2 s−1, which is ten times higher than that of power balance heat diffusivity , in the MHD active discharges of ADITYA. It has been observed that the presence of MHD (m/n = 2/1, 3/1) activity in the intermediate region between the q = 1 and the edge radii, significantly influences the heat transport from the plasma core to the edge region. Stochastic magnetic field region formation with overlapping m/n = 2/1 and 3/1 MHD islands facilitates the fast heat-pulse propagation during a sawtooth crash in ADITYA tokamak. The disparity between the measured and the power-balance estimated diffusivity is significantly reduced by considering the electron heat diffusivity due to stochastization of magnetic field in the intermediate region.

Non-monotonic radial structures of fluctuating temperatures and densities associated with fishbone activities in KSTAR

General characteristics of a fishbone mode in KSTAR are investigated. Fishbone activities are observed with a Mirnov coil, an electron cyclotron emission radiometer (from the core to the edge of plasmas) and an beam emission spectroscopy system (core or edge plasmas) which are measuring fluctuations of poloidal magnetic fields, electron temperatures, and densities, respectively. Temporal evolutions of these fluctuations are similar to the observations from other tokamaks. An interesting and notable feature found in KSTAR fishbone modes is that radial coherence structures of electron temperature and density with respect to magnetic fluctuations are non-monotonic that they have a local minimum at r/a∼0.7 and a maximum at r/a∼0.9 in addition to the usual global peak near the q = 1 surface, where r/a is the normalized minor radius and q is the safety factor. Furthermore, the associated temporal evolution of the electron temperatures in slow-time scale, i.e., less than 1 kHz, with the fishbone activities show that there exist a drop in temperature or increase in temperature depending on inside or outside the q = 1 surface, respectively, from the core to the edge plasmas except that there are almost no temperature changes in the intermediate region which seems to be correlated with the non-monotonic coherence profile. Such a non-monotonic structure and the slow temporal evolution of temperatures are explained with barely trapped resonating fast ions with the banana orbit widths of the order of the minor radius, so that they transit the core and the edge regions simultaneously without trespassing the mid-plane intermediate region.

Terahertz cyclotron emission from two-dimensional Dirac fermions

Since the emergence of graphene, we have seen several proposals for the realization of Landau lasers tunable over the terahertz frequency range. The hope was that the non-equidistance of the Landau levels from Dirac fermions would suppress the harmful non-radiative Auger recombination. Unfortunately, even with this non-equidistance an unfavorable non-radiative process persists in Landau-quantized graphene, and so far no cyclotron emission from Dirac fermions has been reported. One way to eliminate this last non-radiative process is to sufficiently modify the dispersion of the Landau levels by opening a small gap in the linear band structure. A proven example of such gapped graphene-like materials are HgTe quantum wells close to the topological phase transition. In this work, we experimentally demonstrate Landau emission from Dirac fermions in such HgTe quantum wells, where the emission is tunable by both the magnetic field and the carrier concentration. Consequently, these results represent an advance in the realization of terahertz Landau lasers tunable by magnetic field and gate-voltage.

Physics Studies for Assessment of Requirement of Displacement Compensation System for ITER ECE Diagnostic

The Electron Cyclotron Emission (ECE) diagnostic has the key function of measuring the core electron temperature profile and electron temperature fluctuation, from the intensity of electron cyclotron radiation emitted from the plasma along the major radius. The ECE diagnostic consists of three main systems: (1) front-end optics, which collects the radiation from the plasma, (2) transmission lines including polarizer splitter unit, which transports the ordinary and extraordinary ECE emission modes separately from the front-end and distributes it to the instrumentation, and (3) detection and analysis instrumentation which is housed at a distance from the tokamak, in the diagnostics building [1]. With its high electron temperatures and harsh environment, ITER presents various challenges for the diagnostic system. One of the most insidious is the misalignment between the in-vessel front-end optics and the ex-vessel transmission line which is caused by vibration of the vacuum vessel during operational and baking phases. Since the electron temperature is inferred from the intensity of the ECE, transient misalignment may lead to poor accuracy in this critical measurement. These displacements are expected to be ~ 15 mm in vertical (z) and horizontal (x) directions, and ~ 5 mm in the toroidal (y) direction. It is important to minimize the effect of these displacements, so that the system maintains alignment during operation, and reliable temperature information is attained. Our objective is to first study the coupling losses due to imperfect coupling of Gaussian beams owing to port plug displacements. Measurements are done to determine the power loss due to coupling of offset beams experimentally. The measured value for coupling loss is ~2 dB at 120 GHz, which is quite high, and it is therefore concluded that a mechanism is needed to compensate for the displacements.

Investigation ofTemeasurements discrepancies between ECE and Thomson diagnostics in high-performance plasmas in JET

For high-temperature JET and TFTR discharges, electron cyclotron emission (ECE) measurements of central electron temperature were systematically found to be up to 20% higher than those taken with Thomson scattering. In recent high-performance JET discharges, centralTemeasurements, performed with LIDAR Thomson scattering and the X-mode ECE interferometer, have been studied in a large database, including deuterium (DD), and deuterium-tritium plasmas (DT). Discrepancies betweenTemeasurements have been observed outside of the experimental uncertainties. ECE measurements, at highTe, have been found to be higher or lower than those of LIDAR, depending on the specific plasma scenario. In addition, discrepancies between the peaks of the second and third harmonic ranges of the ECE spectrum have been interpreted as evidence for the presence of non-Maxwellian features in the electron distribution function. These comparisons seem to suggest that such features can be found in most of the high-performance scenarios selected in this JET database.

Database study of turbulent electron temperature fluctuation measurements at ASDEX Upgrade

In this work, an automated method for the analysis of data from the correlation electron cyclotron emission (CECE) diagnostic is applied to discharges in the ASDEX Upgrade (AUG) tokamak. This recently developed, automated method provides an efficient means of accurately analysing large quantities of experimental turbulence data, enabling the development of the largest database of CECE measurements of tokamak plasmas to-date. The turbulence database provides the opportunity to search for large-scale trends in experimental data to improve our understanding of transport-relevant plasma turbulence. The results of physics-based investigations utilizing this turbulence database will be reported on separately from this work.

Electron temperature fluctuation levels of the quasi-coherent mode across the plasma radius

EDA H-mode is an ELM-free regime in which the edge quasi-coherent mode (QCM) replaces the ELMs. The estimated location of the quasi-coherent mode is in a partly optically thin region of steep gradients localized between ρpol = 0.96 -1. Relative fluctuations of radiation temperature between 15 and 80 kHz are about 7% with significant density contribution. In the electron cyclotron emission (ECE) channels with resonances in the plasma core, a mode with the same frequency as the quasi-coherent mode is measured. The peak amplitude of both core and edge modes matches the strongest electron temperature gradient in the core and the edge, respectively. The ECE core and edge signals are out of phase. The radiation transport forward model (ECRad) shows that the refraction explains the phase relation between the edge and the core ECE channels. The phase correlates with the sign of the core Te. The amplitude of the fluctuations in the core decreases with decreasing gradients, which is the trend seen in the experiment. The amplitude ratio of the core and edge fluctuation is a factor of five in the experiment; this ratio remains a factor of a hundred in the modeling.

Design of Stray Radiation Sensor for ITER ECE Diagnostic

The Electron Cyclotron Emission (ECE) diagnostic has a primary role in the measurement of electron temperature profile and electron temperature fluctuations in ITER. This diagnostic shall be exposed to significant power due to unabsorbed Electron Cyclotron Heating (ECH) power in the plasma. The expected stray power loads could be a few tens of watts, and therefore, the protection of millimetre wave components is one of the design challenges of ITER ECE diagnostic. This protection system includes sensors, a band stop notch filter, and a shutter to stop the RF stray radiation from being incident on the sensitive components. The sensors will be positioned along the ECE transmission line, and shall be used for real-time power monitoring of the stray radiation. Here, we describe a novel design of a sensor for monitoring the stray radiation power. This sensor is a Schottky Diode rectenna, known for high-power and high-speed millimetre wave detection capability. It consists of a 2x2 microstrip patch antenna array, a matching circuit, a diode, and a low pass filter. The antenna array is designed analytically and optimized in CST Microwave Studio, for wide reception angle, high gain, and low side lobe levels. Furthermore, the rectifying circuit is optimized using Agilent Advanced Design System (ADS) software to get better rectification and impedance matching of the signal, thereby improving its detection sensitivity. The ADS simulation results show that the detection sensitivity is about 1000V/W for input power of -30 dBm at 170 GHz, thereby achieving the required performance of the sensor.

Electron temperature fluctuation measurements with Correlation Electron Cyclotron Emission in L-mode and I-mode plasmas at ASDEX Upgrade

The Correlation Electron Cyclotron Emission (CECE) diagnostic at ASDEX Upgrade (AUG) is used to investigate the features of outer core and pedestal (ρpol = 0.85-1.0) turbulence across confinement regime transitions. The I-mode confinement regime is a promising operational scenario for future fusion reactors because it features high energy confinement without high particle confinement, but the nature of the edge and pedestal turbulence in I-mode plasmas is still under investigation. The edge Weakly Coherent Mode (WCM) appears in the I-mode pedestal and may play a role in transport. In this work we explore electron temperature (Te) fluctuations in the plasma outer core and pedestal using a 24-channel high radial resolution CECE radiometer. CECE measurements provide turbulence information including the Te fluctuation amplitude, turbulent spectra, and radial localization of turbulent features. With CECE measurements we show that the WCM is localized in the pedestal region in both L-mode and I-mode and is measured in optically thick plasmas with a Te fluctuation amplitude of 2.3%. Broadband drift wave turbulence is measured in the outer core with a Te fluctuation amplitude of <1%. A second CECE system recently installed at AUG allowed for non-standard fluctuation measurements during L-mode and I-mode experiments. The second CECE system was toroidally separated from the primary system, allowing measurements of the long-range toroidal correlation of the WCM indicating its low toroidal mode number. A reflectometer sharing a line of sight with the second CECE system enabled density-temperature cross-phase (αne Te ) measurements. The WCM αne Te changes between L-mode and I-mode as the Te gradient steepens.

Parametric Decay Instabilities during Electron Cyclotron Resonance Heating of Fusion Plasmas, Problems and Possibilities

We review parametric decay instabilities (PDIs) expected in connection with electron cyclotron resonance heating (ECRH) of magnetically confined fusion plasmas, with a specific focus on conditions relevant for the ITER tokamak. PDIs involving upper hybrid (UH) waves are likely to occur in O-mode ECRH scenarios at ITER if electron density profiles allowing trapping of UH waves near the ECRH frequency are present. Such PDIs may occur near the plasma center in ITER full-field scenarios heated by 170 GHz O-mode ECRH and on the high-field side of half-field ITER plasmas heated by 110 GHz or 104 GHz O-mode ECRH. Additionally, 110 GHz O-mode ECRH of half-field ITER scenarios may have low ECRH absorption, due to the electron cyclotron resonance being located on the high-field side of the main plasma. This potentially allows PDIs driven by a significant amount of ECRH radiation reaching the UH resonance in X-mode to occur, as X-mode radiation can be generated by reflection of unabsorbed O-mode radiation from the high-field side wall. The occurrence of PDIs during ECRH may damage microwave diagnostics, such as the electron cyclotron emission and low-field side reflectometer systems at ITER, as well as complicate the calculation of heating and current drive characteristics. However, if PDIs are induced in a controlled manner, they may provide novel diagnostic tools and allow the generation of a moderate fast ion population in plasmas heated only by ECRH.

Current status of ECE system on EAST tokamak

The electron cyclotron emission (ECE) diagnostic on the experimental advanced superconducting tokamak (EAST) has had a major upgrade since 2020, when EAST heating system also went through a significant upgrade, including one NBI system changed from counter-current to co-current (moving from port F to port D), and the antenna and the installation port of LHW and ICRF system have also been changed. The quasi-optical (QO) antenna of P port ECE system has been redesigned, the main purpose of which is to add one oblique ECE view. The angle with respect to perpendicular to the magnetic field is about 10°, which will facillitate measurement of the electron velocity distribution altered by LHW system. The ellipsoidal mirror has also been moved close to the plasma, about 70 cm away from the plasma center, and the poloidal beam waist radius in the plasma has been optimised to be less than 3 cm. The CECE system has also been moved from port G to port C. The frequency coverage of the CECE system has been upgraded to 104-132 GHz by adding one radio frequency (RF) module. Also in the intermediate frequency (IF) module, 8 narrow-band filters have been added to improve the spacial coverage of the system. On port F, a new superheterodyne radiometer with narrow-band filters in IF module has been installed. It consists of eight channels, the radial coverage is about 8 cm, the main purpose of this new system is to study the fine structure of magnetic island.

Radial localization of electron temperature pedestal and ELM-like events using ECE measurements at Wendelstein 7-X

A magnetic configuration scan was performed at Wendelstein 7-X stellarator by varying the rotational transform to analyze the plasma confinement for magnetic configurations with different edge magnetic island locations and sizes. For the magnetic configurations, where the 5/5 island chain was moved inside the last closed flux surface, it was observed with electron cyclotron emission measurements that an electron temperature, Te, pedestal develops in the plasma buildup phase and followed by the edge localized mode (ELM)-like crashes. From the mapping of the island to the plasma radius from HINT equilibrium, it was found that the Te pedestal is formed at the island location on the high field side of the plasma. The ELM-like crashes occur at the location of the pedestal and the transport barrier is broken typically with an energy loss of 3-4% during a single ELM-like event. The frequency and the amplitude of the ELM-like crashes were observed to be changing with island size, plasma heating power and density. Additionally during the plasma decay, after the heating was switched-off, a transition to degraded plasma confinement state was observed with changed Te profile gradients, faster decay rate of diamagnetic energy, and increased H-alpha levels.

A model of non-Maxwellian electron distribution function for the analysis of ECE data in JET discharges

Recent experiments performed in JET at high level of plasma heating, in preparation of, and during the DT campaign have shown significant discrepancies between electron temperature measurements by Thomson Scattering (TS) and Electron Cyclotron Emission (ECE). In order to perform a systematic analysis of this phenomenon, a simple model of bipolar distortion of the electron distribution function has been developed, allowing analytic calculation of the EC emission and absorption coefficients. Extensive comparisons of the modelled ECE spectra (at both the 2nd and the 3rd harmonic extraordinary mode) with experimental measurements display good agreement when bulk electron distribution distortions around 1-2 times the electron thermal velocity are used and prove useful for a first level of analysis of this effect.

Upgrade of the relative calibration methods and Bayesian inference processing for electron cyclotron emission radiometry

An upgraded local oscillator (LO) hopping calibration method based on a blackbody hot source and a perturbation analysis of the magnetic field difference method are introduced in this work. The blackbody hot source is used to evaluate the difference in the relative coefficients between the two LO hopping frequencies in the same channels. Then the coefficients are obtained by multiplying the LO hopping frequencies coefficients by LO hopping calibration coefficients. In this way, it is more flexible and stable than the in-situ calibration. The magnetic field difference method provides another calibration method to obtain the relative calibration coefficients of the electron cyclotron emission radiometers (ECE). In general, the magnetic field difference method needs two similar shots but with a difference of 2.1% (for HL-2M) in the magnetic field. Meanwhile, there are some errors because of the deviation of detection positions in the same channels between the two shots. For evaluating the calibration errors, the impact of the displacement, Te perturbation of the core region, and magnetic field difference has been discussed. The result shows that a larger magnetic field difference can improve the accuracy of the calibration. In the end, Bayesian inference has been utilized to evaluate the calibration coefficients and get the most probable calibration coefficients along with its the confidence interval.

DEVELOPMENT OF RENEWED WAWEGUIDE SYSTEM FOR MICROWAVE DIAGNOSTICS IN URAGAN-2M STELLARATOR

This paper presents initial proposal for the development of quasi-optical (QO) microwave technology for Electron Cyclotron Emission (ECE) and reflectometry diagnostics in the Uragan-2M (U-2M) stellarator. For the existed ECE radiometer systems and for the operational plasma parameters of U-2M new quasi-optical beam separation dichroic filter is designed. For such filter the mechanical parameters and performed attenuation characteristic are calculated. Frontend antenna/splitter system for the combined ECE/reflectometry radiation detection is presented. As an example, QO beam pattern for the ECE receiving antenna is numerically calculated.