Electron thermal diffusivity due to the electron temperature gradient mode

Charge neutrality breaks down in the short wavelength toroidal electron temperature gradient mode. In contrast to the ion temperature gradient mode, the wave number normalized by the Debye wave number, k/k(De), appears as a natural scale parameter, rather than the finite Larmor radius parameter k( perpendicular )rho(e). The growth rate and consequent mixing length estimate yields an electron thermal diffusivity large enough to be relevant to tokamaks.     

Solitary modes of bacterial culture in a temperature gradient

We study the behavior of a bacterial culture in a one-dimensional temperature gradient. The bacteria first accumulate near their natural temperature due to thermotaxis. The maximum of the bacterial density profile then drifts to lower temperature with a velocity proportional to the initial concentration of bacteria (typical velocity 0.5 microm/sec). Above a critical concentration of 10(8) cells/cm(3), a new mode develops from the initial accumulation in the form of a sharp pulse moving at a faster velocity ( approximately 3.5 microm/sec). The time of development of this mode diverges as the concentration approaches its critical value. This mode is a result of a positive feedback mechanism provided by interbacterial communication. A theoretical model shows good agreement with the experimental results.     

Density gradient stabilization of electron temperature gradient driven turbulence in a spherical tokamak

In this Letter we report the first clear experimental observation of density gradient stabilization of electron temperature gradient driven turbulence in a fusion plasma. It is observed that longer wavelength modes, k(⊥)ρ(s) ? 10, are most stabilized by density gradient, and the stabilization is accompanied by about a factor of 2 decrease in the plasma effective thermal diffusivity.     

Short wavelength temperature gradient driven modes in tokamak plasmas

New unstable temperature gradient driven modes in an inhomogeneous tokamak plasma are identified. These modes represent temperature gradient (ion and electron) driven modes destabilized in the short wavelength regions with k( perpendicular )rho(i,e)>1, respectively. The instability occurs due to a specific plasma response that significantly deviates from Boltzmann distribution in the regions k( perpendicular )rho(i,e)>1.     

Measurements of thermal electron attachment rate coefficients to molecules using an electron swarm technique

An existing electron swarm apparatus has been redesigned and upgraded. In particular, the new design incorporates a novel planar radioactive foil to form an integral part of the drift tube, allowing us to overcome inherent problems present in our earlier system which used a cylindrical radioactive source. In addition to this, substantial upgrades have been made to improve the gating and amplification electronics and the data acquisition system. This has resulted in a much greater signal to noise ratio and improved accuracy. This paper describes the upgraded apparatus and its use in obtaining thermal (300 K) attachment rate coefficients to a number of molecules. The quality of the measurements and data are illustrated through the measurement of the thermal attachment rate coefficient for SF₆ (k_th(SF₆) = (2.38 ±0.15 ) ×10 ^-7 cm³ s^-1). Thermal electron attachment rate coefficients for four other molecules are presented, namely for two derivatives of SF₆, SF₅CF₃ and SF₅Cl, and two perfluorocarbons, c-C₄F₈ and 2-C₄F₈.     

Experimental evidence for a reduction in electron thermal diffusion due to trapped particles

New high time resolution measurements of the electron thermal diffusion χ(e) throughout the sawtooth cycle of the Madison Symmetric Torus reversed-field pinch have been made by utilizing the enhanced capabilities of the upgraded multipoint, multipulse Thomson scattering system. These measurements are compared to the χ(e) due to magnetic diffusion predicted by using information from a new high spectral resolution zero-β nonlinear resistive magnetohydrodynamic simulation performed, for the first time, at the Lundquist number of high current Madison Symmetric Torus plasmas (S≈4×10(6)). Agreement between the measured and predicted values is found only if the reduction in thermal diffusion due to trapped particles is taken into account.     

Controlling the spectrum of a two-photon field: Inhomogeneous broadening due to a temperature gradient

A method is proposed and implemented for controlling the spectrum of spontaneous parametric down-conversion. The method is based on generating a temperature gradient along a nonlinear crystal in which biphotons are generated. In this case, the phase-synchronism conditions are modified due to the temperature dependence of the refractive indices. As a result, the spectral composition of the emitted two-photon field is changed.     

Experimental evidence for gradient length-driven electron transport in tokamaks

Energy transport by the electrons in a tokamak is examined in steady-state and power modulation experiments using electron cyclotron heating. The results are consistent with the assumption that temperature profiles are limited by a critical gradient length, leading to "stiff" profiles. The modulation experiments show that the stiffness factor increases with temperature. They strongly suggest that turbulence driven by the electron temperature gradient may be a dominant mechanism of electron transport. Although possibly not universal, these results are valid under various plasma conditions.     

Crystallization of a eutectic Pb-Sn melt in the thermal field of a temperature gradient

The crystallization of a eutectic Pb-Sn melt is investigated under nonuniform conditions in vacuum, in air, and in water. It is found that the structure of the solid-state systems formed depends on the cooling conditions. A structural inhomogeneity in volume and a correlation between the microstructure and the coordinate in a thermal field are revealed. Results of technological, electron-microscopic, and statistical investigations are reported. It is shown that the experiment is consistent with the crystallization theory of eutectic melts.     

Ballistic and diffuse modes of electron transport in nanocontacts of magnetics

The transition from the ballistic mode of electron transport to the diffuse one has been experimentally observed in Ni nanocontacts of various sizes where the magnetic phase transition has been investigated. It is shown that the voltage U _C required for the Joule heating of the near-contact region to the critical temperature is independent of the contact size only in the diffuse mode. It is demonstrated that a decrease in the electron transport mean free path due to the heating of the near-contact region should be taken into account when determining the electron transport mode in nanocontacts.     

Measurements of electron transport in foils irradiated with a picosecond time scale laser pulse

The heating of solid foils by a picosecond time scale laser pulse has been studied by using x-ray emission spectroscopy. The target material was plastic foil with a buried layer of a spectroscopic tracer material. The laser pulse length was either 0.5 or 2 ps, which resulted in a laser irradiance that varied over the range 10(16)-10(19) W/cm(2). Time-resolved measurements of the buried layer emission spectra using an ultrafast x-ray streak camera were used to infer the density and temperature conditions as a function of laser parameters and depth of the buried layer. Comparison of the data to different models of electron transport showed that they are consistent with a model of electron transport that predicts the bulk of the target heating is due to return currents.     

Single-particle thermal diffusion of charged colloids: Double-layer theory in a temperature gradient

The double-layer contribution to the single-particle thermal diffusion coefficient of charged, spherical colloids with arbitrary double-layer thickness is calculated and compared to experiments. The calculation is based on an extension of the Debye-Hückel theory for the double-layer structure that includes a small temperature gradient. There are three forces that constitute the total thermophoretic force on a charged colloidal sphere due to the presence of its double layer: i) the force F _W that results from the temperature dependence of the internal electrostatic energy W of the double layer, ii) the electric force F _el with which the temperature-induced non-spherically symmetric double-layer potential acts on the surface charges of the colloidal sphere and iii) the solvent-friction force F _sol on the surface of the colloidal sphere due to the solvent flow that is induced in the double layer because of its asymmetry. The force F _W will be shown to reproduce predictions based on irreversible-thermodynamics considerations. The other two forces F _el and F _sol depend on the details of the temperature-gradient-induced asymmetry of the double-layer structure which cannot be included in an irreversible-thermodynamics treatment. Explicit expressions for the thermal diffusion coefficient are derived for arbitrary double-layer thickness, which complement the irreversible-thermodynamics result through the inclusion of the thermophoretic velocity resulting from the electric- and solvent-friction force.     

Hydrodynamical field fluctuations in a non-equilibrium quasi-stationary state due to a temperature gradient: I. General theory

We consider a system in a state of non-equilibrium due to the introduction of a temperature gradient. The essential problem of this paper is to determine, in their most general form, the fluctuation correlation functions for hydrodynamical fields. We use a notation which enables us to distinguish the contributions from the individual mechanisms: contributions from fluctuations in number density, momentum and energy; contributions appearing due to having taken into account fluctuations in sound velocity and the heat conductivity coefficient; and ones related with the presence of the temperature gradient.     

Hydrodynamical field fluctuations in a non-equilibrium quasi-stationary state due to a temperature gradient : II. Application to light scattering

Within the general theory of hydrodynamical field fluctuations in a system at non-equilibrium due to a temperature gradient, the problem of light scattering on fluctuations in number density is discussed. The effect of the gradient as well as fluctuations in sound velocity and heat conductivity on the Rayleigh-Brillouin spectrum is analyzed. The presence of the temperature gradient and fluctuations modifies the scattered-light spectrum leading to asymmetry in the heights of the Brillouin lines.     

Transport of electron thermal energy in high temperature plasmas

The nature of the anomalous transport of electron thermal energy in existing experiments on magnetically confined toroidal plasmas is discussed and a new form of the relevant electron thermal conductivity, that is consistent with the observed temperature profiles, is presented.     

温度传感器在测定不良导体导热系数实验中的应用

将单片计算机与温度传感器结合用于导热系数测定仪中测量温度,改进后的导热系数测定仪结构紧凑,操作简单、读数方便.     

Non-equilibrium transport in NbSe3: Effects of a temperature gradient

We have measured the dc response characteristics of NbSe₃ in the charge density wave (CDW) state, in the presence of an applied temperature gradient. The threshold field E_T for the onset of nonlinear conduction remains sharp, and is determined by the average temperature of the specimen. No change is observed in the amplitude or quality factor of the coherent current oscillations in the nonlinear conductivity region, other than that expected from a change in average temperature. We interpret our results as evidence for macroscopic dynamical coherence throughout the specimen.