Fixed Electron Temperature Research Articles

Weibel-like instability in magnetohydrodynamics

In magnetohydrodynamics (MHD), a density perturbation perpendicular to an electron temperature gradient generates a magnetic field around itself that acts to increase the perturbation, which can lead to instability. An MHD dispersion relation is obtained for perturbations perpendicular to a fixed electron temperature gradient with an initial in-plane magnetic field, including resistivity, viscosity, and the electrothermal coefficient. Instability occurs for sufficiently small electron temperature-gradient scale lengths determined by the ion collisionless skin depth. Both viscosity and resistivity are required to prevent growth at arbitrarily small spatial scales and to give a physical result for the fastest growing mode. The perpendicular electrothermal coefficient is only significant for a narrow range of low electron Hall parameters, causing a modest reduction in magnetic field growth and modifying the criteria for instability in the presence of viscosity. If the definition of the Weibel instability [E. S. Weibel, Phys. Rev. Lett. 2, 83 (1959)] is extended to include all instabilities due to anisotropy in the electron velocity distribution, then this is a Weibel-like instability because an electron temperature gradient implies an anisotropic electron velocity distribution. The implications for the formation of filaments in laser-produced plasmas and for the verification of MHD codes are considered.

Role of ion thermal velocity in the formation and dynamics of electrostatic solitary waves in plasmas

We perform fluid simulations to examine the effect of ion thermal velocity on the formation and dynamics of solitary waves in an unmagnetized two-component plasma consisting of ions and electrons. Based on the linear and nonlinear fluid theories, some of the previous studies have reported that the plasma with the electron temperature greater than the ion temperature (i.e., Te > Ti) supports ion acoustic solitary waves (IASWs), whereas the plasma with Te ≪ Ti supports electron acoustic waves (EASWs). In this paper, we have considered a wide range of ion temperatures (with fixed electron temperature) to examine the criteria of temperature and thermal velocities in the generation of EASWs and IASWs in plasmas. Our simulation shows that the plasma with Ti > Te possesses two wave modes depending on the ratio of its thermal velocities. When the ratio of electron to ion thermal velocities R = Vthe/Vthi > 1, the system supports the generation of IASWs, whereas for R < 1, it supports the generation of EASWs. The analysis of characteristics like the amplitude, width, and phase speed of these solitary waves implies that the EASWs have a negative potential, whereas the IASWs have the positive potential. The transition from IASWs to EASWs occurs when the phase speed of the solitary wave exceeds the limiting value of 3Vthe. This simulation study presents the detailed investigation of the evolution of EASWs and IASWs generated in plasmas having Ti > Te, which will have implications in modeling such waves in space and laboratory plasmas.

Statistical study of Saturn's auroral electron properties with Cassini/UVIS FUV spectral images

About 2000 FUV spectra of different regions of Saturn's aurora, obtained with Cassini/UVIS from December 2007 to October 2014 have been examined. Two methods have been employed to determine the mean energy 〈E〉 of the precipitating electrons. The first is based on the absorption of the auroral emission by hydrocarbons and the second uses the ratio between the brightness of the Lyman-α line and the H2 total UV emission (Lyα/H2), which is directly related to 〈E〉 via a radiative transfer formalism. In addition, two atmospheric models obtained recently from UVIS polar occultations have been employed for the first time. It is found that the atmospheric model related to North observations near 70° latitude provides the results most consistent with constraints previously published.On a global point of view, the two methods provide comparable results, with 〈E〉 mostly in the 7–17keV range with the hydrocarbon method and 〈E〉 in the 1–11keV range with the Lyα/H2 method. Since hydrocarbons have been detected on ∼20% of the auroral spectra, the Lyα/H2 technique is more effective to describe the primary auroral electrons, as it is applicable to all spectra and allows an access to the lowest range of energies (≤5keV), unreachable by the hydrocarbon method. The distribution of 〈E〉 is found fully compatible with independent HST/ACS constraints (emission peak in the 840–1450km range) and FUSE findings (emission peaking at pressure level ≤0.2µbar). In addition, 〈E〉 exhibits enhancements in the 3LT–10LT sector, consistent with SKR intensity measurements.An energy flux–electron energy diagram built from all the data points strongly suggests that acceleration by field-aligned potentials as described by Knight's theory is a main mechanism responsible for electron precipitation creating the aurora. Assuming a fixed electron temperature of 0.1keV, a best-fit equatorial electron source population density of 3 ×103m−3 is derived, which matches very well to the plasma properties observed with Cassini MAG and CAPS/ELS instruments. However, several auroral regions are characterized by relatively high 〈E〉 and low energy flux, suggesting that additional processes such as plasma injections or magnetic reconnections must be accounted for to explain the emission in these regions.The Lyα/H2 ratio technique can be used to build maps of 〈E〉 from single spectral images. As expected, preliminary results show that the spatial distribution of 〈E〉 is not uniform, as seen on Jupiter.Our study reveals that a fraction of the aurora is due to very low energy electrons (<1keV). Even in this case, comparisons between observed and modeled spectra show that 100eV is a suitable value to represent the average energy of the secondary electrons.

Characteristics of north jovian aurora from STIS FUV spectral images

We analyzed two observations obtained in Jan. 2013, consisting of spatial scans of the jovian north ultraviolet aurora with the HST Space Telescope Imaging Spectrograph (STIS) in the spectroscopic mode. The color ratio (CR) method, which relates the wavelength-dependent absorption of the FUV spectra to the mean energy of the precipitating electrons, allowed us to determine important characteristics of the entire auroral region. The results show that the spatial distribution of the precipitating electron energy is far from uniform. The morning main emission arc is associated with mean energies of around 265keV, the afternoon main emission (kink region) has energies near 105keV, while the ‘flare’ emissions poleward of the main oval are characterized by electrons in the 50–85keV range. A small scale structure observed in the discontinuity region is related to electrons of 232keV and the Ganymede footprint shows energies of 157keV. Interestingly, each specific region shows very similar behavior for the two separate observations.The Io footprint shows a weak but undeniable hydrocarbon absorption, which is not consistent with altitudes of the Io emission profiles (∼900km relative to the 1bar level) determined from HST-ACS observations. An upward shift of the hydrocarbon homopause of at least 100km is required to reconcile the high altitude of the emission and hydrocarbon absorption.The relationship between the energy fluxes and the electron energies has been compared to curves obtained from Knight’s theory of field-aligned currents. Assuming a fixed electron temperature of 2.5keV, an electron source population density of ∼800m−3 and ∼2400m−3 is obtained for the morning main emission and kink regions, respectively. Magnetospheric electron densities are lowered for the morning main emission (∼600m−3) if the relativistic version of Knight’s theory is applied.Lyman and Werner H2 emission profiles, resulting from secondary electrons produced by precipitation of heavy ions in the 1–2MeV/u range, have been applied to our model. The low CR obtained from this emission suggests that heavy ions, presumably the main source of the X-ray aurora, do not significantly contribute to typical UV high latitude emission.

Using X‐shooter to measure the extinction in long GRB host galaxies from emission lines

AbstractThe spectra of the host galaxies of gamma‐ray bursts at low redshift generally show strong hydrogen lines of the Balmer, Paschen and Brackett series, in addition to strong nebular metal lines. In special cases the hosts can be resolved in separate star forming regions, and spatially resolved spectroscopy can be obtained. Generally, the three strongest Balmer lines are used to derive the reddening experienced by the emission lines of the host gas, assuming a Milky Way extinction curve, case B recombination and a fixed electron temperature. We demonstrate how the wide wavelength range of X‐shooter, in combination with a rigorous calibration strategy, can be used to fit explicitly for RV, Te, and AV simultaneously using a large number of H and He I recombination lines, explicitly corrected for stellar atmosphere absorption. This increases our understanding of extinction and absorption in starforming regions in GRB hosts. We use two GRB hosts as examples of the methods, outlining the advantages of using X‐shooter over other instruments (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

TWO-TEMPERATURE COMBINED DIFFUSION COEFFICIENTS IN ARGON-HELIUM THERMAL PLASMAS

From the derivation of two-temperature diffusion number fluxes of species proposed by Rat et al and that of ambipolar diffusion coefficients, combined diffusion coefficients introduced by Murphy at equilibrium are extended to non-equilibrium thermal plasmas, where the kinetic temperature of electrons Te is different from that of heavy species Th. Two-temperature ordinary and thermal combined diffusion coefficients are calculated in an argon-helium plasma at atmospheric pressure up to 30,000 К. Plasma composition is obtained using a non-equilibrium constant method. The dependence on the electron temperature of combined diffusion coefficients is analyzed for different mole percentages of argon in the mixture and for different values of the non-equilibrium parameter q = Te/Th Similar behaviors to those obtained at equilibrium are highlighted with a decrease in the combined diffusion coefficients, at fixed electron temperature, as q increases.

Investigation of carbon chemical erosion with increasing plasma flux and density

The possible reduction of the chemical erosion yield of carbon Ychem withincreasing hydrogenic ion flux is a critical issue for the use ofcarbon based plasma facing materials in future high flux confinementdevices. The PISCES linear plasma device is used to assess carbonYchem through nearly an order of magnitude scan in flux (⩽ 1023 s-1 m-2) and plasma density, while controlling other exposureparameters that are known to affect Ychem; namely, incident ion energyand surface temperature. Two independent techniques are used tomeasure Ychem, CD band emission spectroscopy and sample massloss. Both techniques are carefully benchmarked using a combination ofmethane injection experiments and modelling. The key results of thisstudy are that the CD molecular band photon efficiency is sensitive tothe plasma density at fixed electron temperature and that theredeposition efficiency of hydrocarbons increases with density,thereby decreasing the measured net erosion. After taking these trendsinto account, no measurable flux dependence of Ychem is found fromeither of the two measurement techniques, with Ychem ∼ 3-5% for anincident deuterium ion energy of 30 eV. Independent scans of incidentenergy and surface temperature confirm previously measured (by ionbeams with relatively low fluxes) Ychem dependences on theseparameters. These results together suggest that apparent reductions ofYchem in plasma devices with increasing flux are more likely to beattributed to either increasing redeposition efficiency with density,changing dissociation/excitation/transport properties of thehydrocarbons near the surface, decreasing incident energy or somecombination thereof.

Heat transfer from a rarefied plasma flow to a metallic particle with high surface temperature

Analytical results are presented concerning heat transfer to a metallic spherical particle immersed in a plasma flow for the extreme case of free-molecule regime and thin plasma sheath. The results show that as the surface temperature of the heated particle is comparatively high, additional heat transfer mechanisms due to radiation heat loss and thermionic emission from the particle surface should be all taken into account in the calculation of heat transfer to the particle. Radiation heat loss becomes appreciable at wall temperatures lower than those at which significant thermionic emission occurs. As the plasma temperature is comparatively low, radiation heat loss even restricts the surface temperature of the particle to such a low value that the effect of thermionic emission on heat transfer can be ignored. The heat flux to the particle increases with increasing gas pressure or speed ratio, and decreases with increasing electron/heavy-particle temperature ratio for a fixed electron temperature of two-temperature plasmas.

Simulation of High Ion Temperature Mode in JT-60

The high ion temperature mode in JT-60, where T i above 10 keV were obtained by hydrogen beam heating of 20 MW into hydrogen limiter plasmas with I p =1∼2 MA, is investigated by using the one dimensional transport code and experimental data of electron density and temperature. It is found that, in a low density plasma with high Z eff , the dilution of thermal ions due to impurities and fast ions produced by neutral beam injection causes rapid increase in the ion temperature. On the other hand, in the steady state, the dilution has little effect on the central ion temperature for fixed electron temperature because the convection loss is dominant.

Temperature anisotropy effect on the toroidal ion temperature gradient mode

Using the local kinetic and fluid approaches, the effects of the anisotropies in the ion temperature and in the ion temperature gradient on the toroidal ion temperature gradient driven mode are investigated. A study of the effect of the increasing ion temperature relative to the fixed electron temperature in the peaked density limit is also made. Comparisons are given between the new toroidal results and the previous slab results.

Rovibrationally enhanced dissociative electron attachment to molecular lithium.

We have investigated the role played by initial rovibrational excitation of ${\mathrm{Li}}_{2}$ on the cross sections and rates for dissociative electron attachment to the molecule. For a given internal energy, the vibrational excitation enhances the attachment cross section more than the rotational excitation. The attachment cross sections and the attachment rates reach their maximum values when the process of dissociative attachment to rovibrationally excited molecules is still endoergic and, furthermore, these quantities stay close to their maximum values even when the process changes from being endoergic to exoergic. The upper bounds on the cross sections and the rates for dissociative electron attachment to ${\mathrm{Li}}_{2}$ are 12.8 A${\mathrm{\r{}}}^{2}$ and 1.25\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. At a fixed electron temperature, the kinetic energy of the negative ion formed by this process increases as the vibrational quantum number of the initial neutral molecule increases; the maximum kinetic energy of the ${\mathrm{Li}}^{\mathrm{\ensuremath{-}}}$ ion formed by attachment to the v=12 level of ${\mathrm{Li}}_{2}$ is 0.153 eV.

Modeling of an inductively coupled plasma at reduced pressures

Plasma sintering experiments in this laboratory at reduced pressures revealed efficient heating of the ceramic sample due to recombination of dissociated and/or ionized species on the surface. For establishing a model for this plasma sintering process, it is necessary to first consider the plasma itself. Therefore, a suitable model for an RF inductively coupled plasma has been developed considering reduced pressures. As the pressure decreases, the electron density also decreases at a fixed electron temperature, causing substantial deviations from chemical equilibrium. Due to the poor collisional coupling between electrons and heavy particles at reduced pressures, large deviations from kinetic equilibrium have also to be expected. The model is based on a rotationally symmetric plasma contained in a quartz tube. The power level ranges from 1.5 to 3 kW and the operating pressure is varied from 1 to 0.01 atm. Both deviations from chemical and kinetic equilibrium are included in this model. Thermodynamic and transport properties for two-temperature plasmas are used for this modeling work. The results indicate that for pressures below 0.1 atm, there is a strong ambipolar flux of charge carriers to the confining walls, leading to significant variations of the temperature across the tube. The electron temperature increases rapidly as the pressure decreases, whereas the heavy-particle temperature decreases.

Ion Sampling Considerations for a Discharge Plasma of Nitrogen

A quadrupole mass-spectrometric sampling probe is employed for the study of ion extraction and plasma sheath formation in a discharge plasma of nitrogen at low pressures (9 to 27 μ). The dependence of the specific ion current ratio i(N+)/i(N2+) on the potential of the sampling probe is presented for various pressures and a fixed electron temperature. Kinetic energy distribution curves for individual ionic species are obtained under a variety of conditions of sampling probe potential, pressure, and electron temperature using a retarding potential field applied at the entrance grid of a quadrupole mass filter. Interpretations of the results indicate that ion generation processes and ion-molecule collisions occurring in the sheath may strongly influence the sampling process.

Departures from the Saha equation in an optically thin nitrogen gas

The populations of the excited and ionized states in an ionized nitrogen gas are calculated for various steady state non-local thermodynamic equilibrium (NLTE) situations. The non-equilibrium conditions basically result from the low electron densities (10 11−10 18 e/ cm 3) considered. A model consisting of 26 energy levels is assumed for the ions N I-N VIII. The optically thin gas is assumed to be maintained at a fixed electron temperature and is irradiated by an external Planckian field weakened by a dilution factor, W. Specific densities and temperatures for which LTE holds (for the particle occupations) in N are presented. The electrons are assumed to have a Maxwellian distribution. The populations are presented for electron temperatures between 10 4 and 10 6 °K for various W's.