Electron Temperature Perturbations Research Articles

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.

Effect of intense laser irradiation on the thermal transport properties of metals

Ultrafast laser irradiation of metals can elevate the temperature of electrons to the same order as the Fermi temperature while the lattice remains cold. In this transient and highly nonequilibrium regime, the material properties can undergo drastic modifications, revealing insights into the physical phenomena of the condensed state that are otherwise not evident under equilibrium conditions. However, a clear description of these phenomena, even for elemental metals, remains greatly unexplored partly due to the limitations imposed by the phenomenological treatment of electron-phonon scattering with simplified assumptions for these highly nonequilibrium regimes. In this work, using recent advancements in first-principles calculations, we provide a detailed understanding of hot electron dynamics in a free-electron-like metal (aluminum) and a noble metal (gold) to demonstrate the important role played by the electronic structure in dictating their transport properties at elevated electron temperatures. By performing parameter-free density functional theory calculations of the electron-temperature-dependent heat capacities, electron-phonon coupling, electron mean-free paths, and thermal conductivities, we show that semiempirical and free-electron estimations can lead to erroneous predictions, especially for the case of gold where the excitation of the low-lying $d$ bands can drastically modify the transport properties. We find that the diffusive mean-free paths of electrons in gold can be increased from $\ensuremath{\sim}35\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ at ambient conditions to $\ensuremath{\sim}70\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ at electron temperatures of ${k}_{\mathrm{B}}{T}_{e}=6\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ as a result of drastically reduced electron-phonon scattering. In contrast, we find that the mean-free paths of electrons in aluminum are relatively insensitive to high electron temperature perturbations mainly resulting from the unchanged electron-phonon scattering even at elevated electron temperatures. This ultimately results in a much greater increase in the thermal conductivity of gold at electron temperatures of $\ensuremath{\sim}20000$ K, where it increases by more than two orders of magnitude. However, in aluminum, the thermal conductivity increase at the elevated electron temperatures is relatively not as pronounced (which increases by a factor of $\ensuremath{\sim}70$). Our results shed light on the microscopic understanding of hot electron dynamics in metals and is crucial for a plethora of applications such as in plasmonic devices.

An Unusually Large Electron Temperature Increase Over Arecibo Associated With an Intense Geomagnetic Storm

AbstractWe present an investigation of the F‐region electron temperature to an intense geomagnetic storm that occurred on 5 August 2011. The investigation is based on the incoherent scatter radar measurements at Arecibo Observatory, Puerto Rico (18.3°N, 66.7°W). The electron temperature exhibits a rapid and intensive enhancement after the commencement of the geomagnetic storm. The electron temperature increases by ∼800 K within an hour, which is seldomly reported at Arecibo. At the same time, a depletion of the electron density is also observed. The daytime perturbations of electron density and temperature are anticorrelated with the correlation coefficient, which is −0.88 and −0.91 on the day and the following day of the geomagnetic storm, respectively. According to the Global Ultraviolet Imager measurements, the ratio of atomic oxygen to molecular nitrogen concentration () decreases dramatically during the storm. Our analysis suggests that the enhancement of the electron temperature is due to the depletion of the electron density, which is likely associated with the decrease of . The reduction of maybe caused by a prompt upward plasma motion after the commencement of the geomagnetic storm.

Experimental signatures of electron cyclotron wave energy condensation in magnetic islands

Bifurcation of the electron temperature perturbation (ΔT e) in magnetic islands due to electron cyclotron current drive (ECCD) is reported for the first time in toroidal plasmas. In this DIII-D experiment ΔT e spontaneously increases by 50% in a growing 2, 1 island which in turn rapidly shrinks, consistent with improved stabilization by the ECCD due to the temperature bifurcation. This non-linearity of ΔT e maps out a hysteresis in the island evolution cycle, in qualitative agreement with the theoretically predicted radio frequency current condensation effect Reiman and Fisch 2018 (Phys. Rev. Lett. 121 225001). Therefore, this observed bifurcation can be critical for the avoidance of disruptions caused by tearing modes in tokamaks.

Electron temperature and ion density perturbations prior to the M6.8 Eastern Honshu, Japan earthquake of July 23, 2008

Electron temperature and ion density perturbations prior to the M6.8 Eastern Honshu, Japan earthquake of July 23, 2008

Observation of the electron thermal transport and temperature fluctuations for electron cyclotron resonance heated plasmas on J-TEXT

First dedicated experiments have been carried out on the J-TEXT tokamak for investigating the electron thermal transport and plasma fluctuations before and during electron cyclotron resonance heating (ECRH). The electron thermal diffusivities are calculated using the diffusive space-time evolution characteristic of the electron temperature perturbations produced by internal sawtooth collapse. It is found that the electron thermal diffusivity significantly increases as ECRH is turned on. The temperature fluctuations in the ECRH-assisted ohmic plasmas measured by the correlation electron cyclotron emission (CECE) diagnostic have a larger amplitude and broader bandwidth than that of pure ohmic plasmas. However, there is a slight change in the density fluctuations. The electron thermal diffusivities and the bandwidth of the electron temperature fluctuations decrease simultaneously as density increases in ohmic plasmas. However, the same trend is not found for ECRH-assisted ohmic plasmas with fixed ECRH power. The electron thermal diffusivities increase almost linearly with the ECRH power for the same discharge parameters, while the temperature fluctuations become broader at the same time. Linear stability analysis using the gyrokinetic electromagnetic numerical experiment code indicates that there is a turbulence transition from the mixed mode of ion temperature gradient mode and trapped electron mode (TEM) to the pure (broader) TEM when changing from pure ohmic to ECRH-assisted ohmic plasmas. It is consistent with the temperature fluctuations measured by CECE, which is sensitive to the long-wavelength modes. All these results suggest that the long-wavelength temperature fluctuations play an important role in electron thermal transport. In ECRH-assisted ohmic plasmas, electron thermal transport may become dominated by ETGs when TEMs are suppressed at high density.

Dependence of fishbone cycle on energetic particle intensity in EAST low-magnetic-shear plasmas

The dependence of fishbone cycle on energetic particle intensity has been investigated in EAST low-magnetic-shear plasmas. It is observed that the fishbone mode growth rate, saturation amplitude as well as fishbone cycle frequency clearly increase with increasing neutral beam injection (NBI) power. Moreover, enhanced electron density and temperature perturbations as well as energetic particle loss were observed with greater injected NBI power. Simulation results using M3D-K code show that as the NBI power increases, the resonant frequency and the energy of the resonant particles become higher, and the saturation amplitude of the mode also changes, due to the non-perturbative energetic particle contribution. The relationship between the calculated energetic particle pressure ratio and fishbone cycle frequency is obtained as ${f_{\textrm{FC}}} = 2.2{(1000{\beta _{\textrm{ep,calc}}} - 0.1)^{5.9 \pm 0.5}}$ . Results consistent with the experimental observations have been achieved based on a predator–prey model.

Simulation of dynamic characteristics for ELM filaments on EAST tokamak using BOUT++

To study the dynamic characteristics of filamentary structure during the edge-localized mode (ELM) burst on the experimental advanced superconducting tokamak, the BOUT++ six-field two-fluid model is used to simulate the temporal and spatial evolutions of the ELM filaments. In the peak gradient region at the outer mid-plane, the radial speed of filaments is changed frequently and varied from 0.11 km/s to 0.79 km/s, and the poloidal speed oscillates in the range of 0.61–0.85 km/s. The width of the ELM filaments is increased from 7.76 mm to 26.7 mm and grows rapidly to 41.99 mm from t = 0.2717 ms to 0.2831 ms after the start of the burst. After that, the width decreases gradually and stabilizes at 25–30 mm when it comes to the saturation phase. Further research indicates that the filament propagation speed is increased during their outward movement. The width gradually decreases when the filaments move outward, and it is proportional to electron temperature (Te) perturbation. All the results are consistent with the experimental data presented by Chen et al., IEEE Trans. Plasma Sci. 47, 799 (2019).

Plasma response to rotating resonant magnetic perturbations with a locked mode in the J-TEXT tokamak

Disruption caused by locked modes (LMs) is one of the most critical issues to be solved for tokamak fusion reactors. This paper aimed to understand the plasma response to external rotating resonant magnetic perturbations (RMPs), which are applied at a few kilohertz for controlling the pre-existing LMs. In the J-TEXT tokamak, the plasma response to a rotating RMP showed a feature of travelling wave, if no tearing mode (TM) existed. With a LM, the application of a rotating RMP led to unique features of plasma responses, which could be roughly described by the formula δbθ(θ, t) = 2Asin(mθ − mθO)cos(2πfRRMPt), where δbθ, A, m, θO and fRRMP are the perturbed poloidal magnetic field, the amplitude, the poloidal mode number, the poloidal location of the TM’s O-point and the frequency of the rotating RMP. These features are analogous to that of a standing wave in the TM rest frame, with the nodes locating at the O-/X- points. The perturbations of electron temperature, Te, due to plasma responses were zero or minimal inside the magnetic island or outside the magnetic island but at the same poloidal positions as the O-/X- points. Both δbθ and the Te perturbations increased linearly with the rotating RMP. A phenomenological model is proposed that the rotating RMP drives the phase oscillation of the TM by applying a periodical electromagnetic torque. Then the phase oscillation of TM, combined with the poloidal gradient of the plasma parameters, induces the plasma response measured experimentally. The nonlinear numerical modelling confirmed the forced phase oscillation of the TM, and qualitatively verified the amplitude and phase dependences among the plasma response, the phase oscillation of the TM and the rotating RMP, which have been predicted by the model.

Numerical simulation of oblique ionospheric heating by powerful radio waves

<p><strong>Abstract.</strong> In this paper, we investigate the ionospheric heating by oblique incidence of powerful high-frequency (HF) radio waves using three-dimensional numerical simulations. The ionospheric electron density and temperature perturbations are examined by incorporating the ionospheric electron transport equations and ray-tracing algorithm. The energy distribution of oblique incidence heating waves in the ionosphere is calculated by the three-dimensional ray-tracing algorithm. The calculation takes into consideration the electric field of heating waves in the caustic region by the plane wave spectral integral method. The simulation results show that the ionospheric electron density and temperature can be disturbed by oblique incidence of powerful radio waves, especially in the caustic region of heating waves. The oblique ionospheric heating with wave incidence parallel and perpendicular to the geomagnetic field in the mid-latitude ionosphere is explored by simulations, results of which indicate that the ionospheric modulation is more effective when the heating wave propagates along the magnetic field line. Ionospheric density and temperature striations in the caustic region due to thermal self-focusing instability are demonstrated, as well as the time evolution of the corresponding fluctuation spectra.</p>

Acoustic Disturbances in Galaxy Clusters

Abstract Galaxy cluster cores are pervaded by hot gas which radiates at far too high a rate to maintain any semblance of a steady state; this is referred to as the cooling flow problem. Of the many heating mechanisms that have been proposed to balance radiative cooling, one of the most attractive is the dissipation of acoustic waves generated by active galactic nuclei. Fabian et al. showed that if the waves are nearly adiabatic, wave damping due to heat conduction and viscosity must be well below standard Coulomb rates in order to allow the waves to propagate throughout the core. Because of the importance of this result, we have revisited wave dissipation under galaxy cluster conditions in a way that accounts for the self-limiting nature of dissipation by electron thermal conduction, allows the electron and ion temperature perturbations in the waves to evolve separately, and estimates kinetic effects by comparing to a semicollisionless theory. While these effects considerably enlarge the toolkit for analyzing observations of wavelike structures and developing a quantitative theory for wave heating, the drastic reduction of transport coefficients proposed in Fabian et al. remains the most viable path to acoustic wave heating of galaxy cluster cores.

Saturation effects of the lower ionosphere based on two‐dimensional HF heating model

AbstractThis paper presents a two‐dimensional (2‐D) theoretical model of the lower ionospheric heating by high‐frequency (HF) electromagnetic waves. Compared with the traditional one‐dimensional model, we develop the half‐power width of the heating beam into the theoretical formulations, which dictates the 2‐D distribution characteristics of electron temperature and density perturbations; we also introduce the Sodankylä Ion Chemistry (SIC) model. Based on this numerical model, the saturation effect of ionospheric electron temperature and density is investigated, and simulation is performed by heating at a wave frequency of 6 MHz. Results show that because of energy absorption, the electron temperature peaks at 74 km, and the relative increment of electron density peaks at 94 km in the height direction. Conversely, both the maximum perturbations of electron temperature and density appear on the beam center in the horizontal direction. Second, the saturation time of electron temperature initially increases slowly and then rapidly with increased height. Meanwhile, the saturation time of electron density decreases with increased height. In the horizontal aspect, both the saturation time of electron temperature and density are symmetrical at approximately the position of the beam center owing to the initial Gaussian distribution. Electron temperature far from the beam center results in a long saturation time. Conversely, with increased distance from the beam center, the saturation time of electron density decreases.

Low-noise heterodyne receiver for electron cyclotron emission imaging and microwave imaging reflectometry.

The critical component enabling electron cyclotron emission imaging (ECEI) and microwave imaging reflectometry (MIR) to resolve 2D and 3D electron temperature and density perturbations is the heterodyne imaging array that collects and downconverts radiated emission and/or reflected signals (50-150 GHz) to an intermediate frequency (IF) band (e.g. 0.1-18 GHz) that can be transmitted by a shielded coaxial cable for further filtering and detection. New circuitry has been developed for this task, integrating gallium arsenide (GaAs) monolithic microwave integrated circuits (MMICs) mounted on a liquid crystal polymer (LCP) substrate. The improved topology significantly increases electromagnetic shielding from out-of-band interference, leads to 10× improvement in the signal-to-noise ratio, and dramatic cost savings through integration. The current design, optimized for reflectometry and edge radiometry on mid-sized tokamaks, has demonstrated >20 dB conversion gain in upper V-band (60-75 GHz). Implementation of the circuit in a multi-channel electron cyclotron emission imaging (ECEI) array will improve the diagnosis of edge-localized modes and fluctuations of the high-confinement, or H-mode, pedestal.

Magneto-thermal reconnection of significance to space and astrophysics

Magnetic reconnection processes that can be excited in collisionless plasma regimes are of interest to space and astrophysics to the extent that the layers in which reconnection takes place are not rendered unrealistically small by their unfavorable dependence on relevant macroscopic distances. The equations describing new modes producing magnetic reconnection over relatively small but significant distances, unlike tearing types of mode, even when dealing with large macroscopic scale lengths, are given. The considered modes are associated with a finite electron temperature gradient and have a phase velocity in the direction of the electron diamagnetic velocity that can reverse to the opposite direction as relevant parameters are varied over a relatively wide range. The electron temperature perturbation has a primary role in the relevant theory. In particular, when referring to regimes in which the longitudinal (to the magnetic field) electron thermal conductivity is relatively large, the electron temperature perturbation becomes singular if the ratio of the transverse to the longitudinal electron thermal conductivity becomes negligible.

Temperature dynamics and velocity scaling laws for interchange driven, warm ion plasma filaments

The influence of electron and ion temperature dynamics on the radial convection of isolated structures in magnetically confined plasmas is investigated by means of numerical simulations. It is demonstrated that the maximum radial velocity of these plasma blobs roughly follows the inertial velocity scaling, which is proportional to the ion acoustic speed times the square root of the filament particle density times the sum of the electron and ion temperature perturbations. Only for small blobs the cross field convection does not follow this scaling. The influence of finite Larmor radius effects on the cross-field blob convection is shown not to depend strongly on the dynamical ion temperature field. The blob dynamics of constant finite and dynamical ion temperature blobs is similar. When the blob size is on the order of 10 times the ion Larmor radius the blobs stay coherent and decelerate slowly compared to larger blobs which dissipate faster due to fragmentation and turbulent mixing.

Transient thermal and nonthermal electron and phonon relaxation after short-pulsed laser heating of metals

Several dynamic thermal and nonthermal scattering processes affect ultrafast heat transfer in metals after short-pulsed laser heating. Even with decades of measurements of electron-phonon relaxation, the role of thermal vs. nonthermal electron and phonon scattering on overall electron energy transfer to the phonons remains unclear. In this work, we derive an analytical expression for the electron-phonon coupling factor in a metal that includes contributions from equilibrium and nonequilibrium distributions of electrons. While the contribution from the nonthermal electrons to electron-phonon coupling is non-negligible, the increase in the electron relaxation rates with increasing laser fluence measured by thermoreflectance techniques cannot be accounted for by only considering electron-phonon relaxations. We conclude that electron-electron scattering along with electron-phonon scattering have to be considered simultaneously to correctly predict the transient nature of electron relaxation during and after short-pulsed heating of metals at elevated electron temperatures. Furthermore, for high electron temperature perturbations achieved at high absorbed laser fluences, we show good agreement between our model, which accounts for d-band excitations, and previous experimental data. Our model can be extended to other free electron metals with the knowledge of the density of states of electrons in the metals and considering electronic excitations from non-Fermi surface states.

Synchronous oscillation prior to disruption caused by kink modes in HL-2A tokamak plasmas

A class of evident MHD activities prior to major disruption has been observed during recent radiation induced disruptions of the HL-2A tokamak discharges. It can be named SOD, synchronous oscillations prior to disruption, characterized by synchronous oscillation of electron cyclotron emission (ECE), core soft x-ray, Mirnov coil, and radiation signals at the divertor plate. The SOD activity is mostly observed in a parametric regime where the poloidal beta is low enough before disruption, typically corresponding to those radiation-induced disruptions. It has been found that the m/n = 2/1 mode is dominant during the SODs, and consequently it is the drop of the mode frequency and the final mode locking that lead to thermal quench. The mode frequency before the mode locking corresponds to the toroidal rotation frequency of the edge plasma. It is also found that during SODs, the location of the q = 2 surface is moving outward, and most of the plasma current is enclosed within the surface. This demonstrates that the current channel lies inside the rational surface during SOD, and thus the resistive kink mode is unstable. Further analysis of the electron temperature perturbation structure shows that the plasma is indeed dominated by the resistive kink mode, with kink-like perturbation in the core plasma region. It suggests that it is the nonlinear growth of the m/n = 2/1 resistive kink mode and its higher order harmonics, rather than the spontaneous overlapping of multiple neighboring islands, that ultimately triggered the disruption.

Radiative-condensation instability in gravitating strongly coupled dusty plasma with polarization force

The radiative-condensation instability (RCI) in self-gravitating strongly coupled dusty plasma (SCDP) is investigated considering the effects of dust thermal velocity and polarization force on the massive dust particulates. In particular, the outer core of the dense neutron star which is supposed to be strongly coupled in nature with temperature T∼107 K and number density n∼1.3×1030 cm−3 is analyzed. The modified generalized hydrodynamic (GH) equations and electron temperature perturbation equation with radiative effects are solved using the linear perturbation method. In the classical hydrodynamic limit, the modified condition of Jeans instability owing to radiative condensation, polarization force and dust thermal velocity is obtained. In the kinetic limit, velocity of compressional mode also modifies the condition of Jeans instability. The dust thermal velocity and viscoelastic effects have stabilizing whereas polarization force and radiative cooling have destabilizing influence on the growth rate of the Jeans instability. The radiative effects stabilize the growth rate of unstable radiative modes. In isobaric mode (short wavelength), the basic condition of radiative instability is obtained which is unaffected due to the presence of polarization force and viscoelastic effects. The radiative cooling time in the outer core of neutron star is estimated and compared with the gravitational free fall time, and it is found that the cooling takes place too fast for self-gravity to be important.

Impact of inward turbulence spreading on energy loss of edge-localized modes

Nonlinear two-fluid and gyrofluid simulations show that an edge localized modes (ELM) crash has two phases: fast initial crash of ion temperature perturbation on the Alfvén time scale and slow turbulence spreading. The turbulence transport phase is a slow encroachment of electron temperature perturbation due to the ELM event into pedestal region. Because of the inward turbulence spreading effect, the energy loss of an ELM decreases when density pedestal height increases. The Landau resonance yields the different cross phase-shift of ions and electrons. A 3 + 1 gyro-Landau-fluid model is implemented in BOUT++ framework. The gyrofluid simulations show that the kinetic effects have stabilizing effects on the ideal ballooning mode and the energy loss increases with the pedestal height.

Impact of relative phase shift on inward turbulent spreading

The relative cross-phase between density, temperature, and potential perturbations plays a major role in turbulent spreading and transport. Nonlinear Landau-Fluid simulations show that the electron wave-particle resonances provide a relatively strong parallel damping effect on the electron temperature perturbation and can induce a relative cross-phase shift of smaller than π∕2 angle between E × B velocity and the electron temperature perturbation for large electron temperature gradient, which yields a large spreading for electron. The relative phase for ions is about π∕2 and has no turbulent spreading effect on it. The inward turbulent spreading stops at the position where the radial turbulent correlation length is shorter than the magnetic surface spacing. The temperature pedestal height determines the energy loss due to the turbulent spreading.