Electron Temperature Perturbations Research Articles

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.

Alfvénic Propagation: A Key to Nonlocal Effects in Magnetized Plasmas

A long-standing puzzle in fusion research comes from experiments where a sudden peripheral electron temperature perturbation is accompanied by an almost simultaneous opposite change in central temperature, in a way incompatible with local transport models. This Letter shows that these experiments and similar ones are fairly well quantitatively reproduced, when induction effects are incorporated in the total plasma response, alongside standard local diffusive transport, as suggested in earlier work [Plasma Phys. Controlled Fusion 54, 124036 (2012).

Heat Transport During H-Mode in the HL-2A Tokamak

Perturbative experiments on electron heat transport have been successfully conducted on the HL-2A tokamak. The pulse propagation of the electron temperature is induced by the supersonic molecular beam injection (SMBI), which has characteristics of good localization and deep deposition. A model based on the electron heat transport in cylindrical geometry has been applied to reconstruct the measured amplitude and phase profiles of the electron temperature perturbation. The results show that the heat transport is significantly reduced near the pedestal region of the H-mode plasma. In the “profile stiffness/resilience" region, similar heat diffusivities have been observed in L-mode and H-mode plasmas, which verifies the gradient-driven transport physics in tokamaks.

HF waves heating ionosphere F-layer

To predict and evaluate the effects of ionospheric modification quantitatively, the electron temperature and density perturbations are analyzed at different heating conditions, based on a developed model including momentum equation, continuity equation and energy equation. The results show that (1) powerful HF radio waves can cause the electron temperature to increase markedly, and the largest perturbation is near the reflection layer, but the electron density decreases at this area; (2) the lower neutral gas density in the ionosphere, the larger disturbance of the electron temperature and density; the higher radio frequency, the lower increment of electron temperature; and the higher radio power, the larger disturbance of electron temperature and density are caused, but there are no linear relationships between the radio power and disturbance amplitude; (3) the perturbations at nighttime are larger than at daytime when heated by HF radio waves with the same powers; the electron temperature perturbations in the solar minimum are larger than that in solar maximum; and the electron temperature perturbations in spring and autumn are larger than in winter and summer; (4) compared with the low-latitude, the mid-latitude has the smaller perturbations, the maximum electron temperature perturbation is ∼30% (at daytime of winter), and the density is ∼5%.

Transport Studies Using Perturbative Experiments

By inducing a small electron temperature perturbation in a plasma in steady state one can in principle determine the conductive and convective components of the electron heat flux, and the associated thermal diffusivity and convection velocity. The same can be done for other plasma parameters, like density or ion temperature.In this paper experimental and analysis techniques are briefly reviewed. The fundamental question whether the fluxes are linear functions of the gradients or not is discussed. Experimental results are summarized, including so-called ‘non-local’ phenomena.

On the application of electron cyclotron emission imaging to the validation of theoretical models of magnetohydrodynamic activity

Two-dimensional (2D) imaging of electron temperature perturbations provides a powerful constraint for validating theoretical models describing magnetohydrodynamic plasma behavior. In observation of Alfvén wave induced temperature fluctuations, electron cyclotron emission imaging provides unambiguous determination of the 2D eigenmode structure. This has provided support for nonperturbative eigenmode solvers which predict symmetry breaking due to poloidal flows in the fast ion population. It is shown that for Alfvén eigenmodes, and in cases where convective flows or saturated perturbations lead to nonaxisymmetric equilibria, electron plasma displacements oriented parallel to a gradient in mean temperature are well defined. Furthermore, during highly dynamic behavior, such as the sawtooth crash, highly resolved 2D temperature behaviors yield valuable insight. In particular, addressing the role of adiabatic heating on time scales much shorter than the resistive diffusion time through the additional diagnosis of local electron density allows progress to be made toward a comprehensive understanding of fast reconnection in tokamak plasmas.

Fast Ion Induced Shearing of 2D Alfvén Eigenmodes Measured by Electron Cyclotron Emission Imaging

Two-dimensional images of electron temperature perturbations are obtained with electron cyclotron emission imaging (ECEI) on the DIII-D tokamak and compared to Alfvén eigenmode structures obtained by numerical modeling using both ideal MHD and hybrid MHD-gyrofluid codes. While many features of the observations are found to be in excellent agreement with simulations using an ideal MHD code (NOVA), other characteristics distinctly reveal the influence of fast ions on the mode structures. These features are found to be well described by the nonperturbative hybrid MHD-gyrofluid model TAEFL.

Transport Studies Using Perturbative Experiments

By inducing a small electron temperature perturbation in a plasma in steady state one can in principle determine the conductive and convective components of the electron heat flux, and the associated thermal diffusivity and convection velocity. The same can be done for other plasma parameters, like density or ion temperature.In this paper experimental and analysis techniques are briefly reviewed. The fundamental question whether the fluxes are linear functions of the gradients or not is discussed. Experimental results are summarized, including so-called ‘non-local’ phenomena.

Simulations of two-dimensional magnetic electron drift vortex mode turbulence in plasmas

AbstractSimulations are performed to investigate the turbulent properties of nonlinearly interacting two-dimensional magnetic electron drift vortex (MEDV) modes in a non-uniform unmagnetized plasma. The relevant nonlinear equations governing the dynamics of the MEDV modes are the wave magnetic field and electron temperature perturbations in the presence of the equilibrium density and temperature gradients. The important nonlinearities come from the advection of the electron fluid velocity perturbation and the electron temperature, as well as from the nonlinear electron Lorentz force. Computer simulations of the governing equations for the nonlinear MEDV modes reveal the generation of streamer-like electron flows, such that the corresponding gradients in the direction of the inhomogeneities tend to flatten out. In contrast, the gradients in an orthogonal direction vary rapidly. Consequently, the inertial range energy spectrum in decaying MEDV mode turbulence exhibits a much steeper anisotropic spectral index. The magnetic structures in the MEDV mode turbulence produce non-thermal electron transport in our non-uniform plasma.

Variations in deceleration of space vehicles in the upper ionosphere before strong earthquakes

Precursor effects of earthquakes at different atmospheric heights are pronounced in the variations of parameters of charged and neutral components [1, 2]. The authors of [2, 3] report that a few days before the earthquake, perturbations of electron density and temperature of charged particles were observed in the upper atmosphere. For example, according to the data in [4‐7] related to earthquakes in Japan with M > 5.5, in the daytime, the critical frequency of the F 2 ionospheric layer ( foF 2 ) increased 3‐7 days before the earthquakes and then decreased one to two days before the shock. Seismic ionospheric effects usually manifested themselves at distances up to 500 km from the epicenters. The ionospheric effects of earthquakes were found in the signals of GPS navigation space vehicles (SVs) during investigation of the total electron content (TEC) in the atmosphere. The electrons in the F- region of the ionosphere give the greatest contribution to the TEC. The authors of [8‐10] showed that a few percent increase in the variations of the TEC is observed 1‐ 7 days before strong earthquakes if the observation point is located in the region of earthquake preparation. The authors of [11, 12] estimated the variations in TEC over a region of seismic risk as tens of percent before a strong earthquake. The characteristics of the neutral atmosphere [13] also change before earthquakes, including the upper ionosphere, in which the orbits of SV for remote sensing of the Earth are located. Taking into account the differences in manifestation of ionospheric perturbations over the regions of seismic risk before earthquakes based on the data of land-based stations of ionospheric sounding and on the data of the atmosphere sounding by the signals of navigation SV we can suppose that atmospheric perturbations also exist above the F 2 ionospheric layer. These effects can also be pronounced in the characteristics of the orbital motion of SV in the upper ionosphere. We suggest searching for the forerunners of earthquakes in the upper ionosphere using the data of observations of SV deceleration on the orbit. Preliminary results of investigating the characteristics of SV orbital motion within the problem of lithospheric‐ionospheric relations were presented in [1]. The observations of deceleration characteristics of SVs for remote sensing of the Earth are carried out by land-based radio-technical complexes. The estimates of ballistic coefficient K b are calculated

Transport Studies Using Perturbative Experiments

By inducing a small electron temperature perturbation in a plasma in steady state one can in principle determine the conductive and convective components of the electron heat flux, and the associated thermal diffusivity and convection velocity. The same can be done for other plasma parameters, like density or ion temperature.In this paper experimental and analysis techniques are briefly reviewed. The fundamental question whether the fluxes are linear functions of the gradients or not is discussed. Experimental results are summarized, including so-called ‘non-local’ phenomena.

Nonlinear evolution of thermal self‐focusing instability in ionospheric modifications at high latitudes: Aspect angle dependence

A first simulation study for the nonlinear evolution of the thermal self‐focusing instability at high latitudes to examine the dependence of the nonlinear effects on the direction of the heater beam is presented. We demonstrate that due to a local heating of the F region anisotropic plasma, field‐aligned bunch‐scale structures with a strong enhancement of the electron temperature and negative density perturbations inside these structures are developed. The electron temperature increases up to more than 200% of the background when the incident beam is near the magnetic zenith, as observed in the heating experiment at the EISCAT high‐latitude facility (Rietveld et al., 2003). The values of the simulated electron temperature enhancements show a strong dependence on the heater beam direction that is in very good agreement with the EISCAT observations.

Transport Studies Using Perturbative Experiments

By inducing a small electron temperature perturbation in a plasma in steady state one can in principle determine the conductive and convective components of the electron heat flux, and the associated thermal diffusivity and convection velocity. The same can be done for other plasma parameters, like density or ion temperature.In this paper experimental and analysis techniques are briefly reviewed. The fundamental question whether the fluxes are linear functions of the gradients or not is discussed. Experimental results are summarized, including so-called ‘non-local’ phenomena.

Role of anomalous transport in onset and evolution of neoclassical tearing modes

A key role in the evolution of the neoclassical tearing modes (NTMs) belongs to the radial profiles of the perturbed plasma flow, temperature and density which are determined by the conjunction of the longitudinal and cross-field transport arising from thermal conduction, particle diffusion and viscosity. In a tokamak, the perpendicular transport of particles, heat and momentum is typically anomalous. In this paper the results of theoretical studies on the influence of anomalous perpendicular heat transport and anomalous ion perpendicular viscosity on early stages of NTM evolution are presented. Several parallel transport mechanisms competitive with anomalous cross-island heat transport in the formation of the perturbed electron and ion temperature profiles within the island are considered. The perturbed electron temperature profile is established in competition between anomalous perpendicular electron heat conductivity and parallel electron heat convection. The formation of the ion perturbed temperature profile was found to be dependent on the island rotation frequency. The perpendicular ion heat conductivity is balanced by the parallel transport associated with the ion inertia for an island rotating with subsonic frequency or with island rotation with respect to the plasma for supersonic islands. The partial contributions from the plasma electron and ion temperature perturbations in the bootstrap drive of the mode and magnetic curvature effect were taken into account in construction of a generalized transport threshold model of NTMs. This model gives more favourable predictions for NTM stability and qualitatively modifies the scaling law for βonset. The anomalous perpendicular ion viscosity is shown to modify the collisionality dependence of the polarization current effect, reducing it to the low collisionality limit. In its turn a viscous contribution to the bootstrap drive of NTMs is found to be of the same order as a conventional bootstrap drive for the islands of width close to the characteristic one of the transport threshold model. A viscous contribution to the perturbed bootstrap current is destabilizing for the island rotating in the ion diamagnetic drift direction. In this case, an alternative threshold mechanism should be considered.

Heater-induced altitude descent of the EISCAT UHF ion line enhancements: Observations and modelling

On 12 November 2001, artificial optical annuli were produced using the EISCAT high-frequency (HF) ionospheric heating facility. This unusual phenomenon was induced using O-mode transmissions at 5.423 MHz with 550 MW effective isotropic radiated power and the pump beam dipped 9° south of the zenith. The pump frequency corresponds to the fourth electron gyroharmonic frequency at 215 km altitude. The EISCAT UHF radar observed a persistent pump-induced enhancement in the ion line backscatter power near the HF reflection altitude. The optical and radar signatures of HF pumping started at ∼230 km and descended to ∼220 km within ∼60 s. This effect has been modelled using the solution to differential equations describing pump-induced electron temperature and density perturbations. The decrease in altitude of the ion line by ∼10 km and changes in electron density have been modelled. The results show that a maximum electron temperature enhancement of up to ∼5700 K can be achieved on average, which is not sufficient to explain the observed optical emissions.

Steady-state sustainment of high-β plasmas through stability control in Japan Atomic Energy Research Institute Tokamak-60 Upgrade

Recent results from steady-state sustainment of high-β plasma experiments in the Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U) tokamak [A. Kitsunezaki et al., Fusion Sci. Technol. 42, 179 (2002)] are described. Extension of discharge duration to 65s (formerly 15s) has enabled physics research with long time scale. In long-duration high-β research, the normalized beta βN=2.5, which is comparable to that in the steady-state operation in International Thermonuclear Experimental Reactor (ITER) [R. Aymar, P. Barabaschi, and Y. Shimomura, Plasma Phys. Controlled Fusion 44, 519 (2002)], has been sustained for about 15s with confinement enhancement factor H89PL above 2, where the duration is about 80 times energy confinement time and ∼10 times current diffusion time (τR). In the scenario aiming at longer duration with βN∼1.9, which is comparable to that in the ITER standard operation scenario, duration has been extended to 24s (∼15τR). Also, from the viewpoint of collisionality and Larmor radius of the plasmas, these results are obtained in the ITER-relevant regime with a few times larger than the ITER values. No serious effect of current diffusion on instabilities is observed in the region of βN≲2.5, and in fact neoclassical tearing modes (NTMs), which limit the achievable β in the stationary high-βp H-mode discharges, are suppressed throughout the discharge. In high-β research with the duration of several times τR, a high-β plasma with βN∼2.9–3 has been sustained for 5–6s with two scenarios for NTM suppression: (a) NTM avoidance by modification of pressure and current profiles, and (b) NTM stabilization with electron cyclotron current drive (ECCD)∕electron cyclotron heating (ECH). NTM stabilization with the second harmonic X-mode ECCD∕ECH has been performed, and it is found that EC current density comparable to bootstrap current density at the mode location is required for complete stabilization. Structure of a magnetic island associated with an m∕n=3∕2 NTM has been measured in detail (m and n are poloidal and toroidal mode numbers, respectively). By applying newly developed analysis method using motional Stark effect (MSE) diagnostic, where change in current density is directly evaluated from change in MSE pitch angle without equilibrium reconstruction, localized decrease∕increase in current density at the mode rational surface is observed for NTM growth∕suppression. In addition, it is found that characteristic structure of electron temperature perturbation profile is deformed during NTM stabilization. Hypothesis that temperature increase inside the magnetic island well explains the experimental observations. It is also found that the characteristic structure is not formed for the case of ECCD∕ECH before the mode, while the structure is seen for the case with ECCD∕ECH just after the mode onset, suggesting the stronger stabilization effect of the early EC wave injection.

Suppression of the Neoclassical Tearing Modes in Tokamaks under Anomalous Transverse Transport Conditions when the Magnetic Well Effect Predominates over the Bootstrap Drive

A study is made of the suppression of neoclassical tearing modes in tokamaks under anomalous transverse transport conditions when the magnetic well effect predominates over the bootstrap drive. It is stressed that the corresponding effect, which is called the compound suppression effect, depends strongly on the profiles of the electron and ion temperature perturbations. Account is taken of the fact that the temperature profile can be established as a result of the competition between anomalous transverse heat transport, on the one hand, and longitudinal collisional heat transport, longitudinal heat convection, longitudinal inertial transport, and transport due to the rotation of magnetic islands, on the other hand. The role of geodesic effects is discussed. The cases of competition just mentioned are described by the model sets of reduced transport equations, which are called, respectively, collisional, convective, inertial, and rotational plasmophysical models. The magnetic well is calculated with allowance for geodesic effects. It is shown that, for strong anomalous heat transport conditions, the contribution of the magnetic well to the generalized Rutherford equation for the island width W is independent of W not only in the collisional model (which has been investigated earlier) but also in the convective and inertial models and depends very weakly (logarithmically) on W in the rotational model. It is this weak dependence that gives rise to the compound effect, which is the subject of the present study. A criterion for the stabilization of neoclassical tearing modes by the compound effect at an arbitrary level of the transverse heat transport by electrons and ions is derived and is analyzed for two cases: when the electron heat transport and ion heat transport are both strong, and when the electron heat transport is strong and the ion heat transport is weak.

Transport Studies Using Perturbative Experiments

By inducing a small electron temperature perturbation in a plasma in steady state one can in principle determine the conductive and convective components of the electron heat ux, and the associated thermal diffusivity and convection velocity. The same can be done for other plasma parameters, like density or ion temperature.In this paper experimental and analysis techniques are briey reviewed. The fundamental question whether the uxes are linear functions of the gradients or not is discussed. Experimental results are summarized, including so-called ‘non-local’ phenomena.

Nonlinearly coupled upper-hybrid and magnetoacoustic waves in collisional magnetoplasmas.

The nonlinear interaction between large amplitude electrostatic upper-hybrid (UH) waves and magnetoacoustic perturbations in a collisional magnetoplasma is considered. For this purpose, we present a nonlinear Schrödinger equation for the UH wave envelope in the presence of the electron density, electron temperature, and compressional magnetic field perturbations of the dispersive magnetoacoustic waves, and derive an equation for the latter taking into account the combined influence of the UH ponderomotive force and the differential electron Joule heating in the UH wave electric field. The coupled equations are employed to investigate modulational instabilities of a constant amplitude UH wave, and the density profile modifications due to the UH ponderomotive and thermal forces. It is found that the thermal instability has its maximum at a wavelength half the one of the UH wave, and a numerical solution of the coupled time-dependent system of equations reveals that localized density cavities are also created with a spacing approximately half the wavelength of the pump wave.

On the mechanisms of formation of excited hydrogen atoms in a decaying helium-hydrogen plasma

We spectroscopically studied the population of the excited hydrogen atomic states with the principal quantum numbers n=3 and 4 in a decaying plasma produced by a pulsed discharge in a mixture of helium (p=40.4 Torr) with a small amount of hydrogen ([H2]≈1012 cm−3). Experiments on recording the response of the spectral line intensities to a short-duration electron temperature perturbation revealed the contribution of electron-ion recombination to the population of the H*(n=3) states in the early afterglow. The ions produced by collisions of hydrogen molecules with metastable He(23 S 1) atoms, whose density decreases relatively rapidly with time in the decaying plasma, were assumed to be involved in this process. No population of the H*(n=4) atomic levels due to electron-ion recombination was found. Our experimental results are consistent with the conclusions of previous studies that excitation transfer during collisions of metastable helium molecules with hydrogen molecules plays a major role in the population of the excited hydrogen atomic states both with n=3 and with n=4 during most of the afterglow.