Electron Temperature Profiles Research Articles

Simulations of Evolution of Electron Temperature Profiles During Electron Cyclotron Heating of High-Temperature Plasma in Gas-Dynamic Magnetic Trap

Simulations of Evolution of Electron Temperature Profiles During Electron Cyclotron Heating of High-Temperature Plasma in Gas-Dynamic Magnetic Trap

Design of an electron cyclotron emission diagnostics suite for COMPASS Upgrade tokamak.

COMPASS Upgrade is a medium size and high field tokamak that is capable of addressing key challenges for reactor grade tokamaks, including power exhaust and advanced confinement scenarios. Electron cyclotron emission will be available among the first diagnostics to provide measurements of high spatial and temporal resolution of electron temperature profiles and electron temperature fluctuation profiles through a radial view. A separate oblique view at 12° from normal will be utilized to study non-thermal electrons. Both the radial and oblique views are envisioned to be located in a wide-angle midplane port, which has dimensions that enable simultaneous hosting of the front-end of their quasi-optical (QO) designs. Each QO design will have an in situ hot calibration source in the front-end to provide standalone and calibrated Te (R,t) measurements. The conceptual design for each QO system, the Gaussian beam analysis, and the details of the diagnostic channels are presented.

Physics of the electric field in the scrape-off layer in ASDEX Upgrade L-mode discharges and comparison to experiments

The electric field in the Scrape-Off Layer (SOL) of fusion plasmas is a key quantity affecting the local plasma transport and possibly also the overall plasma confinement. However, the physics determining the SOL electric field is experimentally not well investigated. In this study a systematic experimental analysis of the electric field in AUG L-mode discharges at various plasma currents, densities and heating powers is presented. In particular, the relation of the electric field to the divertor condition, as measured by Langmuir probes, is analyzed in detail by applying a simplified form of Ohm’s law. The analysis shows that the peak value of the radial electric field E r in the near SOL measured by Doppler reflectometry decreases from about 8 kV/m at the lowest densities to -2 kV/m at the highest densities, which is accompanied by a flattening of the outer divertor target electron temperature profiles. The electric field obtained by integrating Ohm’s law from the divertor target to the midplane agrees with E r in the near SOL within the uncertainties, evidencing the quantitative validity of the underlying model. Based on the findings about the relation between the electric field and the target conditions, a scaling to obtain the maximum of E r in the SOL in terms of upstream parameters is developed. • The radial electric field E r in the SOL of AUG L-mode discharges is analyzed. • It is shown that E r decreases with increasing Greenwald fraction. • A model for E r using outer target plasma measurements is presented. • A scaling for E r , based on the 2-point model, is introduced.

Towards explanation of ‘broad’ and ‘narrow’ ECRH regimes observed in the GDT experiment

In the experiments with combined ECRH and NBI plasma heating started at the GDT facility (Budker Institute) in 2014, two quite different scenarios of plasma discharges were realized. The first one was characterized by a broad radial distribution of the absorbed ECRH power and a stable plasma with improved energy confinement of fast ions, while in the other the electron temperature profile was highly peaked with local values of T e > 500 eV, but plasma was susceptible to low-frequency instabilities that had a net negative effect on confinement. We discuss the first theoretical explanation of these phenomena. Essentially different temperature profiles are considered as a result of self-consistent evolution in time after the ECRH is switched on. To explore this concept, we combine an advanced quasi-optical model for the calculation of microwave power absorption with a simple transport model describing electron energy balance in a gas-dynamic trap.

Simple model for tokamak outer midplane electron temperature profile in scrape-off-layer verified by SOLPS-ITER simulations

A simple analytical model for the estimation of scrape-off-layer (SOL) electron temperature profile in the equatorial outer midplane (OMP) is suggested. It is demonstrated that the OMP electron temperature profile in the SOL can be approximated by a power function of the radial coordinate for constant anomalous perpendicular transport coefficients. The obtained profile gives a slower radial decrease with respect to exponential fitting, which is widely used for OMP electron temperature estimation in the near-SOL region. It is shown that the analytical profiles are in good agreement with those obtained from SOLPS-ITER simulations for the semi-detached regimes of ITER, JET and ASDEX-Upgrade. They are also in reasonable agreement with experimental Thomson scattering profiles for JET and ASDEX-Upgrade. Analysis based on SOLPS-ITER simulation shows that account of more complicated effects such as energy exchange with ions, convective energy transport, etc, can change quantitatively by factors of the order of unity, but not qualitatively, the radial electron OMP temperature profiles.

Electron temperature and density measurement by Thomson scattering with a high repetition rate laser of 20 kHz on LHD

Thomson scattering measurements with a high-repetition-rate laser have commenced in the Large Helical Device. As an example of the fast phenomena captured by this diagnostic system, measurements at a 20 kHz repetition-rate in hydrogen pellet-injected plasmas are presented. Signal processing methods for this measurement have been developed and electron temperature profiles with almost 70 spatial points were evaluated at time intervals of 50 upmus. After Raman scattering calibration, electron density profiles were derived. Fast changes in the electron temperature and density profiles within 1 ms were observed.

A high-power long lifetime beam dump for the Thomson scattering diagnostic system in the XuanLong-50 experiment.

The Energy iNNovation's XuanLong-50 is a spherical torus experiment with up to 10s plasma operation duration. A 3 J/50Hz pulsed laser is used in the Thomson scattering diagnostic system that is developed to measure the time evolutions of plasma electron temperature and density profiles. The expected laser pulse number is about 7.5 × 106/year with a power load of 150W. To meet at least 1-year lifetime requirement, a Chevron type beam dump with polished molybdenum plates is designed and fabricated, which absorbs the laser beam energy in a 3D structure to reduce the laser fluence deposited on the material surface. To prevent the backscattered stray light from interfering with the Thomson scattering measurements, a 7.5m beam path with folding mirrors is set between the beam dump and the plasma scattering volumes. Details of the beam dump design procedure including the laser beam profile control, multi-pulse laser damage threshold, heat dissipation, Zemax modeling, folding mirror selection, and beam path enclosure are presented together with the testing results.

Electron temperature gradient driven transport model for tokamak plasmas

A new model for electron temperature gradient (ETG) modes is developed as a component of the multi-mode anomalous transport module [Rafiq et al., Phys Plasmas 20, 032506 (2013)] to predict a time-dependent electron temperature profile in conventional and low aspect ratio tokamaks. This model is based on two-fluid equations that govern the dynamics of low-frequency short- and long-wavelength electromagnetic toroidal ETG driven drift modes. A low collisionality NSTX discharge is used to scan the plasma parameter dependence on the ETG real frequency, growth rate, and electron thermal diffusivity. Electron thermal transport is discovered in the deep core region where modes are more electromagnetic in nature. Several previously reported gyrokinetic trends are reproduced, including the dependencies of density gradients, magnetic shear, β and gradient of β (β′), collisionality, safety factor, and toroidicity, where β is the ratio of the plasma pressure to the magnetic pressure. The electron heat diffusivity associated with the ETG mode is discovered to be on a scale consistent with the experimental diffusivity determined by power balance analysis.

Analysis of synthetic electron cyclotron emission from the high field side of HL-2M tokamak plasmas.

A synthetic electron cyclotron emission (ECE) diagnostic is used to interpret ECE signals from preset plasma equilibrium profiles, including magnetic field, electron density, and electron temperature. According to the simulation results, the electron temperature (Te) profile covering the harmonic overlap region can be obtained by receiving ECE signals at the high field side (HFS) of the HL-2M plasma. The third harmonic ECE at the low field side (LFS) cannot pass through the second harmonic resonance layer at the HFS unless the optical thickness (τ) of the second harmonic becomes gray (τ ≤ 2). In addition, the impact of the relativistic frequency down-shift has been evaluated and corrected. The measurable range of the HFS ECE has been calculated by scanning different parameters (electron density, temperature, and magnetic field). Higher plasma parameters allow a wider radial range of electron temperature measurements. The minimum inner measurable position can reach R = 120cm (r/a = -0.89) when the product of core temperature (Te0) and density (ne0) is greater than 35 × 1019keVm-3, which is extended by more than 30cm inward compared with that of the LFS measurement. The HFS ECE will greatly improve the diagnostic ability of ECE systems on the HL-2M tokamak.

Digital single-photon-avalanche-diode arrays for time-of-flight Thomson scattering diagnostics.

The collection optics of Thomson scattering systems for plasma devices are designed with maximum possible étendue to keep the required laser energy low. If the spatial resolution along the laser beam is performed by a time-of-flight method, then the detectors, in addition to a large sensitive area, must offer a high frequency bandwidth. Up until now, only microchannel-plate photomultipliers meet these requirements. Here, we investigate the potential use of digital avalanche photodiode arrays operated in the Geiger mode as alternative detectors. In this mode of operation, each array will serve as a fast, sensitive detector. The use of these detectors will lead to significant improvements of the Thomson scattering diagnostic. Most important of these will be a better spatial resolution, down to about 2cm without deconvolution. Furthermore, the lifetime of the detectors will be increased; the detectors will cover the whole blue wing of the scattered spectrum when using a single wavelength laser, and this will enable measurements of electron temperature and density profiles at kHz repetition rates.

Effects of ICRF waves to EAST LHCD discharges on intrinsic rotations

Ion cyclotron range of frequencies (ICRFs) have proven to efficiently serve as a toroidal rotation source in tokamak plasmas. Recent experiments on the experimental advanced superconducting tokamak show that the rotation profiles are remarkably modified when the internal inductance (l i) changed. The comparisons of the ion and electron temperature profiles among ICRF-heated plasmas suggest that the change in l i can remarkably affect the toroidal rotation velocity. The scaling of rotation velocity increments as a function of the change in l i also suggests that ICRF would serve as a reliable rotation source in plasmas with larger l i decrements.

An electron–phonon Monte Carlo study on thermal transport in GaN

Heat conduction in micro/nano-scale materials are well modeled by Boltzmann transport equation (BTE) and the Monte Carlo (MC) method is an effective computational tool for solving BTE. In conventional insulators and semiconductors, phonons are the majority heat carriers and contribution of electron–phonon interaction (EPI) is negligible. However, in polar semiconductors electron–phonon interaction and its contribution to thermal conductivity are significantly high. In this paper, we develop a novel MC scheme which combines phonon and electron transport effectively to address electron–phonon interaction (EPI). The method is applied in a case study, simulating the thermal transport in wurtzite Gallium Nitride (GaN), considering the EPI impact into account. Deformation potential as well as polar optical potential (POP) are used to characterize EPI. Individual scattering rates of electrons are first determined. Using them the net scattering rate and relaxation times are calculated. Both lattice temperature and the electron temperature profiles in the computational domain are estimated and compared. The final inference is that the lattice thermal conductivity of wurtzite GaN at room temperature is found to be reduced by 16%–22%, on incorporating EPI, for samples of varying thicknesses.

Ideal MHD Limited Electron Temperature in Spherical Tokamaks.

It is well documented that the central electron temperature in the national spherical torus experiment (NSTX) remains largely unchanged as the external heating power, and hence the normalized volume averaged plasma pressure β increases [Stutman, Phys. Rev. Lett. 102, 115002 (2009)PRLTAO0031-900710.1103/PhysRevLett.102.115002]. Here we present a hypothesis that low n, pressure driven ideal magnetohydrodynamic (MHD) instabilities that are nondisruptive, can break magnetic surfaces in the central region and thereby flatten the electron temperature profiles. We demonstrate this mechanism in a 3D resistive MHD simulation of a NSTX discharge. By varying the toroidal magnetic field strength, and/or the heating power, we show that there is a critical value of β, above which the central temperature profile no longer peaks on axis.

The structure of cluster merger shocks: turbulent width and the electron heating time-scale

ABSTRACT We present a new $2\rm \, Ms$Chandra observation of the cluster merger Abell 2146, which hosts two huge M ∼ 2 shock fronts each ${\sim }500\rm \, kpc$ across. For the first time, we resolve and measure the width of cluster merger shocks. The best-fitting width for the bow shock is $17\pm 1\rm \, kpc$ and for the upstream shock is $10.7\pm 0.3\rm \, kpc$. A narrow collisionless shock will appear broader in projection if its smooth shape is warped by local gas motions. We show that both shock widths are consistent with collisionless shocks blurred by local gas motions of $290\pm 30{\rm \, km\rm \, s^{-1}}$. The upstream shock forms later on in the merger than the bow shock and is therefore expected to be significantly narrower. From the electron temperature profile behind the bow shock, we measure the time-scale for the electrons and ions to come back into thermal equilibrium. We rule out rapid thermal equilibration of the electrons with the shock-heated ions at the 6σ level. The observed temperature profile instead favours collisional equilibration. For these cluster merger shocks, which have low sonic Mach numbers and propagate through a high β plasma, we find no evidence for electron heating over that produced by adiabatic compression. Our findings are expected to be valid for collisionless shocks with similar parameters in other environments and support the existing picture from the solar wind and supernova remnants. The upstream shock is consistent with this result but has a more complex structure, including a ${\sim}2\rm \, keV$ increase in temperature ${\sim }50\rm \, kpc$ ahead of the shock.

Dependence of the boundary heat flux width on core and edge profiles in Alcator C-Mod

This work presents new evidence that the heat flux width, λ q , in the Alcator C-Mod tokamak scales with the edge electron pressure, as observed in the ASDEX Upgrade (AUG) tokamak (Silvagni et al 2020 Plasma Phys. Control. Fusion 62 045015), but the scaling with volume-averaged pressure, , from the plasma stored energy, found by Brunner et al (2018 Nucl. Fusion 58 094002), is a better predictor of λ q in Alcator C-Mod than the edge electron pressure. These previous studies, which find that λ q decreases with increasing plasma pressure, imply that a high performance core at high pressure will lead to challenging heat and particle exhaust due to very small λ q . This concern has led to our significant enlargement of the C-Mod database with the electron density, temperature, and pressure profile data from the Thomson scattering and electron cyclotron emission diagnostics. Using the C-Mod database augmented with new profile data, we find that λ q decreases with increasing edge electron pressure as , similar to results from AUG, and showing the strength of cross-machine comparisons. We also find that , consistent with the original finding from C-Mod that the heat flux width scales as (Brunner et al 2018 Nucl. Fusion 58 094002). The scalings of λ q with separatrix pressure and gradient scale length are found to match the AUG results qualitatively. The C-Mod scalings with edge plasma quantities have more scatter than the scaling, and, importantly, show different trends for H-modes relative to L- and I-mode. Investigating the source of this discrepancy presents an opportunity for further study that may improve our ability to predict the heat flux width in different confinement scenarios in the pursuit of optimizing core-edge performance in future reactors.

Integrated data analysis on the electron temperature profile of HL-2A with the Bayesian probability inference method

Data analysis on tokamak plasmas is mainly based on various diagnostic systems, which are usually modularized and independent of each other. This leads to a large amount of data not being fully and effectively exploited so that it is not conducive to revealing the deep physical mechanism. In this work, Bayesian probability inference with machine learning methods have been applied to the electron cyclotron emission and Thomson scattering diagnostic systems on HL-2A/2M, and the effects of integrated data analysis (IDA) on the electron temperature of HL-2A with Bayesian probability inference are demonstrated. A program is developed to infer the whole electron temperature profile with a confidence interval, and the program can be applied in online analysis. The IDA results show that the full profile of the electron temperature can be obtained and the diagnostic information is more comprehensive and abundant with IDA. The inference models for electron temperature analysis are established and the developed programs will serve as an experimental data analysis tool for HL-2A/2M in the near future.

Electron temperature profile from optically grey X3-mode of electron cyclotron emission at Wendelstein 7-X using Bayesian analysis

The third harmonic extraordinary (X3) mode of electron cyclotron emission is investigated, aiming at its diagnostic capability towards the measurement of electron temperature, T e , of plasmas with higher densities exceeding the second harmonic extraordinary (X2) mode cutoff. A Bayesian data-analysis approach is adopted through the forward modeling of the X3 emission observations to extract the underlying T e profile. Bayesian analysis shows that the high field side of the X3 emission spectrum is sufficient to provide the T e profile. Additionally, for plasma control purposes, a correction factor is provided to track continuous core T e from the measured radiation temperature of the optically grey X3 emission.

Precipitating Electron Energy Spectra and Auroral Power Estimation by Incoherent Scatter Radar With High Temporal Resolution

AbstractThis study presents an improved method to estimate differential energy flux, auroral power and field‐aligned current of electron precipitation from incoherent scatter radar data. The method is based on a newly developed data analysis technique that uses Bayesian filtering to fit altitude profiles of electron density, electron temperature, and ion temperature to observed incoherent scatter spectra with high time and range resolutions. The electron energy spectra are inverted from the electron density profiles. Previous high‐time resolution fits have relied on the raw electron density, which is calculated from the backscattered power assuming that the ion and electron temperatures are equal. The improved technique is applied to one auroral event measured by the EISCAT UHF radar and it is demonstrated that the effect of electron heating on electron energy spectra, auroral power, and upward field‐aligned current can be significant at times. Using the fitted electron densities instead of the raw ones may lead to wider electron energy spectra and auroral power up to 75% larger. The largest differences take place for precipitation that produces enhanced electron heating in the upper E region, and in this study correspond to fluxes of electrons with peak energies from 3 to 5 keV. Finally, the auroral power estimates are verified by comparison to the 427.8 nm auroral emission intensity, which shows good correlation. The improved method makes it possible to calculate unbiased estimates of electron energy spectra with high time resolution and thereby to study rapidly varying aurora.

Progress in ITER ECE diagnostic design and integration

The ITER electron cyclotron emission (ECE) diagnostic system has primary roles in providing measurements of the core electron temperature profile and the electron temperature fluctuation associated with the neoclassical tearing modes. The ITER ECE system includes a radial and oblique line-of-sight. Four 43-meter long low-loss transmission lines (TLs) are designed to transmit millimeter wave power in the frequency range of 70–1000 GHz in both X- and O-mode polarization from the port plug to the ECE instrumentation room in the diagnostic building. The measurement instrumentation includes two Fourier transform spectrometer (FTS) systems and two radiometer systems. The Indian Domestic Agency (IN-DA) and United States Domestic Agency share the responsibility. The IN-DA scope excluding instrumentation and control has passed its preliminary design review and is progressing towards the final design review (FDR). In parallel, the diagnostic integration in different areas is ongoing. Several captive components for the TLs have passed FDR and will be manufactured for installation in the tokamak building soon. A peer review meeting has been held on the prototype hot calibration source, and its integration and new thermal analysis in the diagnostic shield module are continuing. A prototype TL is being tested. A prototype polarizing Martin-Puplett type FTS, operating in the frequency range 70–1000 GHz, features an in-vacuo fast scanning mechanism and a cryo-cooled dual-channel THz detector system. Its performance has been assessed in detail against ITER requirements.

Quantification of the effect of uncertainty on impurity migration in PISCES-A simulated with GITR

A Bayesian inference strategy has been used to estimate uncertain inputs to global impurity transport code (GITR) modeling predictions of tungsten erosion and migration in the linear plasma device, PISCES-A. This allows quantification of GITR output uncertainty based on the uncertainties in measured PISCES-A plasma electron density and temperature profiles (n e, T e) used as inputs to GITR. The technique has been applied for comparison to dedicated experiments performed for high (4 × 1022 m−2 s−1) and low (5 × 1021 m−2 s−1) flux 250 eV He–plasma exposed tungsten (W) targets designed to assess the net and gross erosion of tungsten, and corresponding W impurity transport. The W target design and orientation, impurity collector, and diagnostics, have been designed to eliminate complexities associated with tokamak divertor plasma exposures (inclined target, mixed plasma species, re-erosion, etc) to benchmark results against the trace impurity transport model simulated by GITR. The simulated results of the erosion, migration, and re-deposition of W during the experiment from the GITR code coupled to materials response models are presented. Specifically, the modeled and experimental W I emission spectroscopy data for a 429.4 nm line and net erosion through the target and collector mass difference measurements are compared. The methodology provides predictions of observable quantities of interest with quantified uncertainty, allowing estimation of moments, together with the sensitivities to plasma temperature and density.