Effect Of Electron Temperature Anisotropy Research Articles

Effect of electron temperature anisotropy on TEM in reversed-field-pinch plasmas

For the first time in the reversed-field-pinch configuration, trapped electron mode (TEM) with anisotropies of electron temperature and its gradient is studied by solving the gyrokinetic integral eigenmode equation. Detailed numerical analyses indicate that TEM is enhanced by the anisotropy with temperature in the direction perpendicular to the magnetic field that is higher than that in the direction parallel to the magnetic field when the latter is kept constant. However, the enhancement is limited, such that TEM is weakened and even stabilized when the anisotropy is higher than a critical value, due to strong Landau damping. In comparison with the isotropic case, the lower Landau damping with the higher parallel electron temperature makes TEM instability easier to excite, which expands the TEM unstable region in the diagram of density and temperature scale lengths. In addition, it is found that the electron temperature gradient in the perpendicular direction offers a stronger driving force on TEM instability than that in the parallel direction. The overall effects of the temperature gradients of electrons and ions, magnetic shear, safety factor and density gradient on TEM in the presence of the anisotropies are presented in detail.

Effects of Electron Temperature Anisotropy on Proton-beam Instability in the Solar Wind

Abstract Solar wind observations often show that the drift velocity of a proton beam relative to a background proton decreases with the heliocentric distance. Proton-beam instability has been suggested to play an important role in the deceleration of the proton-beam velocity; the effects of electron temperature anisotropy on the proton-beam instability have not been examined. Based on a general kinetic dispersion relation solver for magnetized plasma (PDRK), we investigate the effects of electron temperature anisotropy on the oblique Alfvén/ion-cyclotron (A/IC) and parallel magnetosonic/whistler (M/W) instabilities driven by proton beams in the solar wind. The results show that the growth rates, real frequencies, and threshold conditions for both instabilities are sensitive to the electron temperature anisotropy T e⊥/T e∣∣ and the parallel electron beta β e∣∣. In the low-beta regime with , where is a critical plasma beta for which the threshold velocities of both instabilities are equal, the growth rate of the oblique A/IC instability is weakly dependent on the electron temperature anisotropy. In the high-beta regime with , the growth rate of the parallel M/W instability increases with decreasing T e⊥/T e∣∣. Moreover, the threshold velocities of both instabilities are shifted to lower values as T e⊥/T e∣∣ decreases, especially for the parallel M/W instability in the regime with β e∣∣ ≥ 1. The theoretical results for the threshold velocity together with the observed parallel electron beta and/or electron temperature anisotropy are used to explain the observed proton-beam drift velocity in the solar wind.

Comment on “Effects of electron temperature anisotropy on proton mirror instability evolution” by Ahmadi et al. (2016)

AbstractIn a recent paper, Ahmadi et al. (2016) analyze the effect of electron temperature anisotropy on proton mirror instability. They find that the electron whistler instability grows faster and consumes all the available electron free energy so that no anisotropy is left to fuel the proton mirror mode growth. In this comment we present both observational and theoretical arguments about why we think this is incorrect.

Reply to comment by Remya et al. on “Effects of electron temperature anisotropy on proton mirror instability evolution”

AbstractIn the comment to our paper, Remya et al. (2017) state that we conclude that their theory is incorrect; however, no such conclusion is in our paper. In fact, as stated in their paper, we agree with their theory that shows the impact of heavy ions and electron temperature anisotropy on the competition of the ion anisotropy instabilities. While their linear theory is correct, our paper focused on the nonlinear evolution, where one needs to be careful in assuming a given electron anisotropy, because electrons themselves can be unstable to the electron whistler instability, which quickly lowers the anisotropy to levels where, in the absence of heavy ions, it is not sufficient to significantly change the balance between proton cyclotron and mirror mode. We agree that the electron whistler instability will not lead to complete isotropization of the electrons but only lower it to the instability threshold. In the parameter regime addressed, this limited isotropization will still eliminate the dominance of mirror mode and restore the usual dominance of the proton cyclotron mode, so our point still stands. Our simulations showed an isotropization of the electrons beyond the electron whistler threshold. In this reply, we will show that there are two contributing reasons: The nonlinear evolution of the mirror instability affects the electron anisotropy, as does unphysical numerical heating due to the limited resolution of a particle‐in‐cell simulation. We further discuss the coexistence of electron whistler instability and mirror instability, and we agree that both instabilities can be present in the magnetosheath.

Effect of electron temperature anisotropy on near-wall conductivity in Hall thrusters

The electron velocity distribution in Hall thrusters is anisotropic, which not only makes the sheath oscillate in time, but also causes the sheath to oscillate in space under the condition of low electron temperatures. The spatial oscillation sheath has a significant effect on near-wall transport current. In this Letter, the method of particle-in-cell (2D + 3 V) was adopted to simulate the effect of anisotropic electron temperatures on near-wall conductivity in a Hall thruster. Results show that the electron-wall collision frequency is within the same order in magnitude for both anisotropic and isotropic electron temperatures. The near-wall transport current produced by collisions between the electrons and the walls is much smaller than experimental measurements. However, under the condition of anisotropic electron temperatures, the non-collision transport current produced by slow electrons which reflected by the spatial oscillation sheath is much larger and closes to measurements.

Effect of electron temperature anisotropy on plasma-wall interaction in Hall thruster

To further reveal the physical mechanism of the saturated electron temperature which is about 50-60 eV in the discharge channel of Hall thruster, the effect of electron temperature anisotropy (ETA) on plasma-wall interaction in Hall thruster is studied by using a 2D3V particle-in-cell sheath dynamic model. Some important physical parameters such as electron-wall collision frequency, electron energy deposition at wall and the cooling effect of near-wall sheath on channel electron are calculated. Numerical results indicate that the influence of ETA on plasma-wall interaction is neglectable when electron temperature is low. However, when Te>24 eV, the ETA can significantly reduce electron-wall collision frequency, thereby reducing the electron energy deposition at wall and weakening the cooling effect of near-wall sheath on channel electron. It suggests that the anisotropy of electron temperature tends to increase the saturated electron temperature in the discharge channel of Hall thruster through remarkably weakening the interaction between channel electron and wall.

Effect of electron temperature anisotropy on BN dielectric wall sheath characteristics in Hall thrusters

The effect of electron temperature anisotropy on BN dielectric wall sheath characteristics in Hall thruster plasma is studied by using a one-dimensional fluid sheath model with the help of emitted electron velocity distribution and multi-species mixed ion effects. Analytic results show that, in comparison with that of a pure univalent xenon plasma, the sheath potential drop and the critical secondary electron emission coefficient are decreased in mixed valence xenon plasmas, while the primary electron flux at the wall is increased. The electron temperature anisotropy in Hall thrusters thus significantly enhances the electron energy emission coefficient, and further reduces the sheath potential drop while intensifies the electron-wall interaction. Numerical results also indicate that the electron temperature anisotropy influences the potential distribution of space charge saturated sheath remarkably.

The effect of thermal anisotropy on the propagation of whistler waves in mixed hot-cold electron plasmas

The first-order CGL fluid equations for electrons including the first-order heat fluxes are applied to the propagation of whistler waves. The dispersion relation of whistler waves is derived for two types of equilibrium electron distribution functions with cold and hot components. The effect of electron temperature anisotropy and the existence of cold electrons on the field-aligned propagation of whistler waves is analysed. It is shown that the electron temperature anisotropy intensifies the tendency of whistler waves to follow the lines of force of static magnetic field, that the existence of cold electrons in an anisotropic plasma further intensifies this tendency, and that under certain conditions the waves propagate only along the static magnetic field.

The effect of electron temperature anisotropy on the propagation of whistler waves

The first-order Chew, Goldberger & Low (GGL) equations for electrons including the effect of finite Larmor radius are applied to the whistler wave. The zerothorder velocity distribution function for electrons in the GGL expansion is assumed to be an anisotropic Maxwellian. The effect of electron thermal motion on the propagation of whistler waves is analysed by use of a dispersion relation and properties of the refractive index surface. It is shown that the electron thermal motion intensifies the tendency of whistler waves to follow the lines of force of the earth's magnetic field at appropriate values of electron temperature anisotropy.

The Effect of Electron Temperature Anisotropy on the Field-Aligned Propagation of Whistler Waves

The first order CGL equations of electrons are applied to the propagation of whistler waves. It is shown that the electron thermal motion intensifies the tendency of whistler waves to follow the lines of force of the static magnetic field at appropriate values of electron temperature anisotropy.