Anomalous Electron Heating Research Articles

Artificial plasma cusp generated by upper hybrid instabilities in HF heating experiments at HAARP

AbstractThe High Frequency Active Auroral Research Program digisonde was operated in a fast mode to record ionospheric modifications by the HF heating wave. With the O mode heater of 3.2 MHz turned on for 2 min, significant virtual height spread was observed in the heater off ionograms, acquired beginning the moment the heater turned off. Moreover, there is a noticeable bump in the virtual height spread of the ionogram trace that appears next to the plasma frequency (~ 2.88 MHz) of the upper hybrid resonance layer of the HF heating wave. The enhanced spread and the bump disappear in the subsequent heater off ionograms recorded 1 min later. The height distribution of the ionosphere in the spread situation indicates that both electron density and temperature increases exceed 10% over a large altitude region (> 30 km) from below to above the upper hybrid resonance layer. This “mini cusp” (bump) is similar to the cusp occurring in daytime ionograms at the F1–F2 layer transition, indicating that there is a small ledge in the density profile reminiscent of F1–F2 layer transitions. Two parametric processes exciting upper hybrid waves as the sidebands by the HF heating waves are studied. Field‐aligned purely growing mode and lower hybrid wave are the respective decay modes. The excited upper hybrid and lower hybrid waves introduce the anomalous electron heating which results in the ionization enhancement and localized density ledge. The large‐scale density irregularities formed in the heat flow, together with the density irregularities formed through the parametric instability, give rise to the enhanced virtual height spread. The results of upper hybrid instability analysis are also applied to explain the descending feature in the development of the artificial ionization layers observed in electron cyclotron harmonic resonance heating experiments.

Kinetic simulations of 3‐D Farley‐Buneman turbulence and anomalous electron heating

Electric fields map from the magnetosphere to the E region ionosphere where they drive the intense currents of the auroral electrojet. Particularly during geomagnetic storms and substorms, these currents become sufficiently intense to develop Farley‐Buneman (FB) streaming instabilities and become turbulent. This leads to anomalous electron heating which can raise the electron temperature from 300 K to as much as 4000 K and, also, modifies auroral conductivities. This paper describes the first fully kinetic 3‐D simulations of electric field‐driven turbulence in the electrojet and compares the results with 2‐D simulations and observations. These simulations show that 3‐D turbulence can dramatically elevate electron temperatures, enough to explain the observed heating. They also show the saturated amplitude of the waves; coupling between linearly growing modes and damped modes; the propagation of the dominant modes at phase velocities near the acoustic velocity, slower than in 2‐D simulations; and anomalous cross‐field electron transport, leading to a greatly increased E region Pedersen conductivity. These simulations provide information useful in accurately modeling FB turbulence and represent significant progress in understanding the electrojet.

Magnetosphere-ionosphere coupling throughEregion turbulence: 1. Energy budget

During periods of intense geomagnetic activity, strong electric fields and currents penetrate from the magnetosphere into high-latitude ionosphere where they dissipate energy, form electrojets, and excite plasma instabilities in the E-region ionosphere. These instabilities give rise to plasma turbulence which induces non-linear currents and strong anomalous electron heating (AEH) as observed by radars. These two effects can increase the global ionospheric conductances. This paper analyzes the energy budget in the electrojet, while the companion paper applies this analysis to develop a model of anomalous conductivity and frictional heating useful in large-scale simulations and models of the geospace environment. Employing first principles, this paper proves for the general case an earlier conjecture that the source of energy for plasma turbulence and anomalous heating equals the work by external field on the non-linear current. Using a two-fluid model of an arbitrarily magnetized plasma and the quasilinear approximation, this paper describes the energy conversion process, calculates the partial sources of anomalous heating, and reconciles the apparent contradiction between the inherently 2-D non-linear current and the 3-D nature of AEH.

Magnetosphere-ionosphere coupling throughEregion turbulence: 2. Anomalous conductivities and frictional heating

[1] Global magnetospheric MHD codes using ionospheric conductances based on laminar models systematically overestimate the cross–polar cap potential during storm time by up to a factor of 2. At these times, strong DC electric fields penetrate to the E region and drive plasma instabilities that create turbulence. This plasma density turbulence induces nonlinear currents, while associated electrostatic field fluctuations result in strong anomalous electron heating. These two effects will increase the global ionospheric conductance. On the basis of the theory of nonlinear currents developed by Dimant and Oppenheim [2011], this paper derives the correction factors describing turbulent conductivities and calculates turbulent frictional heating rates. Estimates show that during strong geomagnetic storms the inclusion of anomalous conductivity can double the total Pedersen conductance. This may help explain the overestimation of the cross–polar cap potentials by existing MHD codes. The turbulent conductivities and frictional heating presented in this paper should be included in global magnetospheric codes developed for predictive modeling of space weather.

Observation of reduced core electron temperature fluctuations and intermediate wavenumber density fluctuations in H-mode plasmas

In this paper, we report observations of reduced core electron temperature and intermediate-scale density fluctuations in H-mode. Electron temperature fluctuation levels are observed to decrease from L-mode levels ( for kθρs < 0.5 as measured by correlation electron cyclotron emission radiometry) by at least a factor of four in H- and quiescent H-mode regimes in the DIII-D tokamak (r/a = 0.7). Linear stability calculations (using the trapped gyro-Landau fluid (TGLF) code) indicate that the observed temperature fluctuations are associated with L-mode ITG turbulence which is shear-stabilized at the L- to H-mode transition. Recent results from DIII-D provide the first experimental evidence that, in addition, intermediate-scale turbulence (0.5 < kθρs ⩽ 3) is reduced at the L–H transition. A 30–40% prompt reduction (r/a ⩾ 0.7) has been found at the L–H transition in co-injected medium density plasmas, with a larger decrease (⩾75%) observed near the pedestal top. Experimental results and TGLF calculations indicate that intermediate/small scale turbulence persists in H-mode at a reduced amplitude (0.6 ⩽ r/a ⩽ 1) and may substantially contribute to the residual anomalous H-mode electron heat transport.

Effects of impurities on electron temperature gradient modes

The persistence of large anomalous electron heat transport in regions of improved ion confinement sometimes observed in tokamak plasmas suggests that the electron-temperature-gradient-driven drift waves may be the antecedents of the small-scale turbulence and associated energy flow. Whereas most previous investigations of these modes were carried out for a two-species plasma, in this study we retain impurities and examine their effects using gyrokinetic simulations supported by interpretations based on a simple analytic model. The inclusion of impurities is found to have a strong stabilizing effect on the electron temperature gradient modes.

Electron heat transport comparison in the Large Helical Device and TJ-II

The electron heat transport in the Large Helical Device (LHD) [K. Ida, T. Shimozuma, H. Funaba et al., Phys. Rev. Lett. 91, 085003 (2003)] and TJ-II [F. Castejón, V. Tribaldos, I. García-Cortés, E. de la Luna, J. Herranz, I. Pastor, T. Estrada, and TJ-II Team, Nucl. Fusion 42, 271 (2002)] is analyzed by means of the TOTAL [K. Yamazaki and T. Amano, Nucl. Fusion 32, 4 (1992)] and PRETOR-Stellarator [J. Dies, F. Castejon, J. M. Fontdecaba, J. Fontanet, J. Izquierdo, G. Cortes, and C. Alejaldre, Proceedings of the 29th European Physical Society Conference on Plasma Physics and Controlled Fusion, Montreux, 2002, Europhysics Conference Abstracts, 2004, Vol. 26B, P-5.027] plasma simulation codes and assuming a global transport model mixing GyroBohm-like drift wave model and other drift wave model with shorter wavelength. The stabilization of the GyroBohm-like model by the E×B shear has been also taken into account. Results show how such kind of electron heat transport can simulate experimental evidence in both devices, leading to the electron internal transport barrier (eITB) formation in the LHD and to the so-called “enhanced heat confinement regimes” in TJ-II when electron density is low enough. Therefore, two sources for the anomalous electron heat transport can coexist in plasmas with eITB; however, for each device the relative importance of anomalous and neoclassical transport can be different.

Anomalous electron heating and its effect on the electron density in the auroral electrojet

Measurements made with the Sondrestrom incoherent scatter radar during the April 17, 2002 magnetic storm are presented. The magnetic storm produced strong convection electric fields with no particle precipitation at Sondrestrom. The observed increase in the E‐region electron temperature was caused by the anomalous electron heating in the altitude range of 100–120 km, where Farley‐Buneman turbulence is localized. An model of anomalous electron heating was validated by comparison with the ISR data. Estimates of electron density perturbations during the strong electric fields show a good agreement with the model that accounts for suppression of the electron/ion recombination rate due to anomalous electron heating. The observed increase in the electron density leads to modification of the ionospheric conductivity. This effect is of importance for ionosphere–magnetosphere coupling, and should be included in global models that are developed for space weather forecast.

Nonlinear electron resonance heating in capacitive radio frequency discharges

Technological processing plasmas are frequently operated at relatively low gas pressure (&amp;lt;10Pa). A characteristic feature of this regime is that collisions of the electrons with the atoms or molecules of the neutral background are relatively rare, and the so-called collisional or Ohmic heating ceases to be an effective mechanism of energy deposition into the plasma. Experiments indicate that at low pressure an alternative mechanism of electron heating exists which can sustain the plasma. Despite 30years of intense research, the exact nature of this “anomalous” heating mechanism is still under discussion. The two standard models are known as “stochastic heating” and “pressure heating,” respectively. This work proposes a third explanation of anomalous electron heating and suggests that, in the last analysis, all three mechanisms may contribute to the observed effect.

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.

Effect of anomalous electron heating on the transpolar potential in the LFM global MHD model

The tendency of global MHD models to overestimate the transpolar potential in simulations of strong geomagnetic storms and evidence of an adverse feedback of the ionospheric conductance on the potential suggest that these models lack important physics leading to the conductance enhancement. Farley‐Buneman instability in the auroral ionosphere provides this lacking physics. This instability is believed to cause strong anomalous electron heating which affects the ionospheric conductivity. We use an earlier developed model of anomalous electron heating to estimate the ionospheric conductance disturbance as a function of the local electric field. This result is used to modify the ionospheric conductance in the LFM model to study its effect on the simulated transpolar potential. An idealized and a real‐case simulations are accomplished. In both cases a considerable drop in the simulated transpolar potential is found. The latter is in a good agreement with AMIE model and DMSP data.

Novel Mechanism of Anomalous Electron Heat Conductivity and Thermal Crashes during Alfvénic Activity in the Wendelstein 7-AS Stellarator

Enhanced plasma heat conductivity in the presence of kinetic Alfvén waves (KAW) is predicted theoretically. The enhancement is shown to be strongest when the electron collision frequency exceeds the particle transit frequency in the wave field. Alfvén waves (both KAW and ideal MHD Alfvén eigenmodes generating the KAW) are studied in a shot of the Wendelstein 7-AS stellarator. On the basis of these results, strong thermal crashes observed during bursting Alfvénic activity in the mentioned shot are explained.

Microislands in tokamaks

Analytical treatment of magnetic islands with high poloidal mode numbers and widths smaller than ion Larmor radius (microislands) is presented. The profile functions and rotation frequencies of microislands in the scope of the standard two-fluid magnetohydrodynamics are investigated. The effects of anomalous perpendicular resistivity and anomalous perpendicular electron heat conductivity are included. It is shown that, in such a problem statement, the microislands are described in terms of two profile functions, one of which characterizes the electric field and perturbed plasma density (the microisland profile function) and second—perturbed electron temperature (the electron temperature profile function). Analytical expressions for these profile functions in the case of stationary microislands are derived. Contribution of the oscillatory parallel electric current (the polarization current) into the generalized Rutherford equation for the stationary island width is calculated. The problem of rotation frequencies of microislands is discussed. It is shown that, as in the case of large-scale magnetic islands, for calculation of these frequencies, it is necessary to take into account nonstationarities of profile functions in the near-separatrix region. These nonstationarities are due to discontinuities of the derivatives of profile functions with respect to island magnetic flux. It is shown that the contribution of these nonstationarities into the equation for the rotation frequency does not depend on this frequency. This leads to the conclusion that taking into account only the electron dissipation is insufficient for calculation of the rotation frequency. It is suggested that for such a calculation the ion dissipation should also be involved.

Model of anomalous electron heating in the E region: 1. Basic theory

Numerous radar observations demonstrate anomalously strong electron heating in polar electrojets during magnetic storms. The effect often correlates with the onset of the Farley‐Buneman instability caused by strong convection electric field. Anomalous heating of electrons is usually attributed to turbulent electric fields developing in the E region due to that instability. We propose a theoretical model of the effect based on heuristic assumptions on nonlinearly saturated turbulence and equations of the electron and ion energy budget. The model has been quantitatively tested in the companion paper by Milikh and Dimant [2003]. Good agreement with observations supports the implied physical mechanism.

Kinetic model of electron heating by turbulent electric field in the E region

Numerous radar observations demonstrate anomalously strong electron heating in polar electrojets during magnetic storms. The effect correlates with strong convection electric field. Anomalous heating of electrons is caused by turbulent electric fields developing in the E region due to the Farley‐Buneman instability. A quantitative model of the effect is proposed. Numerical computations are based on a kinetic code. A good agreement of the theoretical results with the existing observations supports the physical ideas underlying the model.

Role of collisionless trapped electron mode in tokamak plasmas with electron internal transport barrier

A self-sustained turbulence model for collisionless trapped electron mode is presented to examine the role of dynamics of trapped electrons in forming electron internal transport barrier (ITB) in tokamak plasmas with negative magnetic shear. Results show that strong negative magnetic shear as well as large Shafranov shift can lead to the precession drift reversal of trapped electrons and large reduction of anomalous electron heat conductivity; it may further trigger the formation of electron ITB. The complete precession drift reversal seems to be in good agreement with the onset condition of electron ITB observed in JT-60U discharges.

On nonlocal heating in inductively coupled plasmas

A simple analytic model describing nonlocal (anomalous) electron heating ininductively coupled plasmas (ICPs) is developed. The model takes into accountboth collisionless (nonlocal) and collisional effects, and is thus valid forall collisionality regimes. The spatial profile of power absorption and thetotal power absorption of the wave are found. It is shown that the regionsof the negative power absorption exist even with a given exponential profileof the electric field in plasma. The results of our simple model are inqualitative agreement with more rigorous self-consistent calculations andwith experimental data.

Observation of isotropic electron temperature in the turbulent E region

Abstract. Using EISCAT radar data, we find that electrons are strongly heated in the magnetic field-line direction during high electric field events. The remote site data show that the electron temperature increases in almost the same way in the field-perpendicular direction; electron heating by E region plasma turbulence is isotropic. We discuss the implications of our observation for the "plasmon"-electron as well as the wave Joule heating models of the anomalous electron heating in the E region.Key words. Ionosphere (auroral ionosphere; plasma temperature and density; plasma waves and instabilities)

Stationary Fluctuation Spectra and Anomalous Transport in a Three-Species Plasma

Low-frequency electromagnetic plasma excitations in the presence of an extra specie, are investigated. The stationary spectra of the low-frequency electromagnetic excitations and the relevant transport coefficients are found. It is conjectured that intrusion of an adiabatically distributed component decreases the level and additional nonlinear broadening of the convective spectrum. The anomalous electron heat conductivity associated with the short-wave motions, is reduced due to the presence of a third specie. The dope also affects anomalous diffusion relevant to the long-wave perturbations in the plasma periphery.

Ion-sound parametric turbulence and anomalous electron heating with application to helicon plasma sources

Parametric excitation by a helicon pumping wave of the ion-sound plasma turbulence and subsequent absorption of ion-sound wave energy by electrons are shown to form a strong mechanism of helicon absorption. Level of ion-sound parametric turbulence, electron heating rate and effective collision frequency are determined and numerical estimates of these quantities are given.