Velocity Fluctuations and Transport in the JET Boundary Region

A new approach for turbulent fluxes and _E×B measurements in the bulk plasma is proposed. It is based in the measurement of fluctuations in the phase velocity of fluctuations. The structure of turbulence has been investigated in the JET plasma boundary region with a fast reciprocating Langmuir probe system. Fluctuations in the radial and poloidal phase velocity have been computed from floating potential and ion saturation current measurements. The correlation between density fluctuations and fluctuations in the radial velocity of fluctuations signals show a good agreement with the turbulent transport computed from the correlation between density and poloidal electric field fluctuations. These results suggest that turbulent transport might be computed in the plasma core from measurement of density fluctuations. E×B sheared flows, both constant and varying in time, are close to the critical value to trigger the transition to improve confinement regimes below the power threshold to trigger the formation of transport barriers.     

金刚石薄膜中二次电子输运的蒙特卡罗模拟

本文通过编写的二维MATLAB蒙特卡罗程序, 对倍增的二次电子在金刚石薄膜中的输运特性进行了初步模拟. 研究表明: 二次电子的迁移率对温度和外加电场的大小很敏感, 在杂质浓度比较低时(<10¹⁷/cm^-3)受杂质浓度的影响不大. 模拟得到的 二次电子的饱和速度为1.88×10⁷ cm/s, 无外加电场时的迁移率为3732 cm² /V.s. 同时, 通过对二次电子束团在金刚石薄膜中的整体输运特性的模拟, 证明了束团电荷密度在应用要求的范围内时, 空间电荷力的影响可以忽略不计.     

Velocity saturation in short channel field effect transistors

The high electric field transport of short channel MOS transistors has been investigated. Saturation of the electron velocity leads to strong inhomogeneous channel conditions which have to be accounted for by a self consistent analysis of the experimental results. It follows that the saturation value of the drift velocity in the inversion channel is close to the bulk value of 10⁷ cm sec^?1.     

新型二次电子倍增阴极构型设计与动力学过程

提出了一种可由脉冲功率驱动的新型二次电子倍增阴极构型,并对其进行了动力学过程的初步理论研究。首先,针对该二次电子倍增阴极,建立了动力学模型,获得了二次电子的位移和速度方程,讨论了电子初始出射速度对其轨迹、渡越时间和碰撞能量的影响,理论给出了渡越时间和碰撞能量的近似解析表达式。其次,通过动力学方程与Vaughan二次电子产额经验公式的耦合求解,获得了该二次电子倍增阴极的工作区间,并对其进行了细致讨论。结果表明:该新型二次电子倍增阴极二极管概念上是可行的,在涂敷高二次电子产额系数材料的圆柱形介质上施加合适的轴向和径向静电场(MV/m量级)以及轴向静磁场(T量级),可以达到电子沿阴极表面螺旋行进过程中实现二次电子倍增并最终获得电流沿轴向放大的设计目标。另外,讨论了正电荷沉积引发的二次电子倍增饱和现象,并对阴极发射电流密度进行了理论粗估,结果表明:阴极发射电流密度可达kA/cm2水平,具备强流发射特性;增加外加径向场强幅值可有效提升阴极发射电流密度。     

The Effect of Rotating Cylindrical Langmuir Probes in Magnetron Plasmas

Using a cylindrical Langmuir probe, the plasma properties (ion density, electron temperature, floating and plasma potentials) in a magnetron sputter source have been investigated, along one particular line‐of‐sight, but for different probe‐orientations with respect to the B‐field. The plasma in the region hosen for observation is haracterised by electrons, which are magnetised (Larmor radius r_le < both the electron mean‐free‐path λ_e, and plasma extension L) and ions, which are not (their Larmor radius r_I > L, λ_I). Through the development of a simple expression for the electron saturation current at different probe angles relative to the local B‐field, it is possible to correct for the diminished electron currents due to their restricted transport across the field. The results indicate that the measured ion density, electron temperature, floating and plasma potentials are unaffected by the probe orientation, however the electron saturation current is attenuated when the probe is aligned along the B‐field. A simple model for the collection of electrons indicates that classical electron diffusion may not operate in the magnetron, with cross‐field electron transport dominated by anomalous, possibly Bohm, diffusion.     

Dependence of the domain-wall velocity on the driving field in garnet films

The dependence of the domain-wall velocity on the amplitude of the driving magnetic field pulses is investigated in an iron garnet film of the (YSmCa)₃(FeGe)₅O₁₂ system with a (111) orientation. The results obtained are analyzed from the standpoint of existing theory. A maximum corresponding to the disruption of steady-state motion is observed on the dependence. Thereafter, the velocity at first decreases sharply and then increases. It is theorized that a process involving the periodic generation, propagation, and annihilation of horizontal Bloch lines occurs in the wall in this period. Data are obtained for the velocity saturation region, which confirm a previously proposed empirical formula and a theoretical model, according to which the saturation regime corresponds to a state of chaos. Fiz. Tverd. Tela (St. Petersburg) 39, 660–663 (April 1997)     

Kinetics of the formation of a phospholipid multilayer on a silica sol surface

The ordering of a multilayer consisting of DSPC bilayers on a silica sol substrate is studied within the modelindependent approach to the reconstruction of profiles of the electron density from X-ray reflectometry data. It is found that the electroporation of bilayers in the field of anion silica nanoparticles significantly accelerates the process of their saturation with Na⁺ and H₂O, which explains both a relatively small time of formation of the structure of the multilayer of 1–7×10⁵ s and ~13% excess of the electron density in it.     

A fluid model of the current-voltage characteristics of an electron emitting electrode immersed in a two electron temperature plasma

The current voltage characteristics of a negatively biased electron emitting electrode immersed in a two-electron temperature plasma are analyzed by a simple one dimensional fluid model. Based on the assumption that the electron density in the pre-sheath region obeys the Boltzmann law the Bohm criterion is derived in the form of a transcendental equation for the Mach number, which can have up to 3 solutions. According to these solutions the ion velocity at the sheath edge can be determined either by the hot or by the cool electron temperature. When it is determined by the cool electron temperature and the hot electron temperature is high enough the critical electron emission current from the collector can have a very pronounced local maximum and a minimum when regarded as a function of the electrode potential. Because of that the current voltage characteristics of the electrode may exhibit up to 3 different floating potentials. This result is in good agreement with the experimental observations reported in [J. Appl. Phys. 63, 5674 (1988)].     

Study of particle transport in ac plasmas on HT-7 tokamak

The particle transport coefficients were investigated in ac plasmas by density modulation method on HT-7 tokamak. It is observed that the particle diffusion coefficient (D) is almost the same in various cases and varies with the electron density. The convection velocity (V) is equal to the value of inward pinch velocity (VE×B) for low and high density discharges. The particle confinement time becomes much higher when the directions of plasma current and toroidal field are uniform.     

Theoretical assessment of electronic transport in InN

Among the group-III nitrides, InN displays markedly unusual electronic transport characteristics due to its smaller effective mass, high peak velocity and high background electron concentration. First, a non-local empirical pseudopotential band structure of InN is obtained in the light of recent experimental and first-principles results. This is utilized within an ensemble Monte Carlo framework to illuminate the interesting transport properties. It is observed that InN has a peak velocity which is about 75% higher than that of GaN while at higher fields its saturation velocity is lower than that of GaN. Because of the strongly degenerate regime brought about by the high background electron concentration, the electron–electron interaction is also investigated, but its effect on the steady-state and transient velocity–field characteristics is shown to be negligible. Finally, hot phonon generation due to excessive polar optical phonon production in the electron scattering and relaxation processes is accounted for. The main findings are the appreciable reduction in the saturation drift velocity and the slower recovery from the velocity overshoot regime. The time evolution of the hot phonon distribution is analysed in detail and it is observed to be extremely anisotropic, predominantly along the electric force direction.     

Ionization self-channeling of whistler waves in a collisional magnetized plasma

An investigation is made of the self-interaction of whistler waves (whistlers) involving the formation of waveguide channels in a collisional magnetoactive plasma as a result of its additional ionization by the field of the propagating wave. Simplified equations are derived to describe the behavior of the whistler field in a channel of enhanced plasma density in the presence of electron collisions. Self-consistent distributions of the field and the plasma corresponding to steady-state ionization self-channeling of whistlers are obtained by numerically solving the equations for the field together with balance equations for the electron density and energy. Our estimates indicate that this effect can be observed under laboratory conditions. Zh. éksp. Teor. Fiz. 112, 1285–1298 (October 1997)     

On the electric activity of superfluid helium at the excitation of first and second sound waves

We show that the electric activity of superfluid helium (HeII) observed in the experiments [3] during the excitation of standing second sound waves in an acoustic resonator can be described in terms of the phenomenological mechanism of the inertial polarization of atoms in a dielectric, in particular, in HeII, when the polarization field induced in the medium is proportional to the mechanical acceleration, by analogy with the Stewart-Tolman effect. The variable relative velocity w = v _n − v _s of the normal and superfluid HeII components that emerges in the second sound wave determines the mean group velocity of rotons, V _g ≈ w, with the density of the normal component related to their equilibrium number density in the temperature range 1.3 K ≤ T ≤ 2 K. Therefore, the acceleration of the 4He atoms involved in the formation of a roton excitation is proportional to the time derivative of the relative velocity.w. In this case, the linear local relations between the variable values of the electric induction, electric field strength, and polarization vector should be taken into account. As a result, the variable displacement current induced in the bulk of HeII and the corresponding potential difference do not depend on the anomalously low polarizability of liquid helium. This allows the ratio of the amplitudes of the temperature and potential oscillations in the second sound wave, which is almost independent of T in the above temperature range, consistent with experimental data to be obtained. At the same time, the absence of an electric response during the excitation of first sound waves in the linear regime is related to an insufficient power of the sound oscillations. Based on the experimental data on the excitation of first and second sounds, we have obtained estimates for the phenomenological coefficient of proportionality between the polarization vector and acceleration and for the drag coefficient of helium atoms by rotons in the second sound wave. We also show that the presence of a steady heat flow in HeII with nonzero longitudinal velocity and temperature gradients due to finite viscosity and thermal conductivity of the normal component leads to a change in the phase velocities of the first and second sound waves and to an exponential growth of their amplitudes with time, which should cause the amplitudes of the electric signals at the first and second sound frequencies to grow. This instability is analogous to the growth of the amplitude of long gravity waves on a shallow-water surface that propagate in the direction of decreasing basin depth.     

Electron spin beats in InGaAs/GaAs quantum dots

Time-resolved picosecond spectroscopy is used for the first time to study optical orientation and spin dynamics of carriers in self-organized In(Ga)As/GaAs quantum-dot (QD) arrays. Optical orientation of carriers created by 1.2 ps light pulses, both in the GaAs matrix and wetting layer, and captured by QDs is found to last a few hundreds of picosecond. The saturation of electron ground state at high-excitation-light intensity leads to electron polarization in excited states close to 100% and to its vanishing in ground state. Electron-spin quantum beats in a transverse magnetic field are observed for the first time in semiconductor QDs. We thus determine the quasi-zero-dimensional electron g factor in In_0.5Ga_0.5As/GaAs QDs to be: |g _⊥|=0.27±0.03. Fiz. Tverd. Tela (St. Petersburg) 41, 871–874 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

On Measuring and Modeling Local Average Particle Velocities for particle ensembles in classifying systems

In most multi-phase flow problems, the particulate phase is exposed to an external field which causes dispersion. Therefore, local velocity measurements of the disperse phase are no longer equivalent with respect to averaging in time and averaging in volume. While the local time-averaged velocity still characterizes the transport of the ensemble in the Eulerian sense, one has to be be careful in modeling this velocity average by considering the ensemble's composition. It is shown for different particle ensembles that the conventional particle velocity average M_1,0 calculated with respect to the dispersion relationship and a particle size number density distribution is far below the measured ensemble average; the deviation depends on the width of the particle size distribution. It is deduced that Eulerian particle velocity values referring to a certain time interval can be modeled by a ratio of velocity moments M_2,0/M_1,0 calculated with particle size number distributions referring to a certain probe volume. This relationship was confirmed by measurements.     

Formation of a shock wave in aerogel irradiated with a high-current pulsed electron beam

The propagation of pressure jumps excited by a high-current pulsed electron beam in SiO₂ aerogels with density ranging from 0.025 to 0.25 g/cm³ is investigated using a laser differential interferometer and optical methods. Spallation on the back side of the aerogel targets is observed and the velocity of the spallation fragments is measured. The expansion velocity of the aerogel in the direction of the electron beam is determined. The parameters of the shock adiabat are established in a wide range of aerogel porosities. The depth of the energy-deposition zone of the electron beam is determined experimentally as a function of the aerogel density in the range from 0.015 to 0.25 g/cm³. A model describing highly porous materials which reflects the fractal properties of highly porous aerogels is developed on the basis of the experimental data. Numerical calculations of the observed phenomena are performed. Zh. Tekh. Fiz. 69, 18–25 (December 1999)     

Electron transport simulation in bulk wurtzite ZnO and its n+–n–n+ diode,compared with GaN

We study the electron transport within bulk wurtzite ZnO and its n⁺–n–n⁺ diode by the Ensemble Monte Carlo method. In bulk ZnO we study the steady state and transient situation with three valley model for the conduction band and compare the results with GaN. Our results show that ZnO's threshold field occurs at a higher applied electric field than GaN. Also, velocity overshoot in ZnO occurs at higher electric fields, too. But the overshoot relaxation time is about 0.3 ps for both of them. As the results show, the role of the third valley is tiny, so for a diode we use two valley conduction bands. For anode voltage ranges from 0.25 to 3 V, we simulate the profiles of the electron density, electric field, potential, average electron velocity, and compare the results with that for GaN. Our results show, as we expect, electron velocity in active layer in the GaN diode is faster than in ZnO.     

Experimental Study of Planar Langmuir Probe Characteristics in Electron Current-Carrying Magnetized Plasma

Experimental study of planar Langmuir probe characteristics in a magnetized plasma with an electron current along the direction of the magnetic field shows that the usual procedure for determination of the electron temperature and plasma density, which is applicable in a current-free magnetized plasma, gives erroneous results for these plasma parameters. When this procedure is applied on the characteristics measured at two opposite orientations of the probe collecting surface with respect to the direction of the electron drift, different values of the electron temperature are obtained. These virtual electron temperatures and corresponding plasma densities calculated from the measured ion saturation currents are higher and/or smaller than the exact local electron temperature and plasma density. Calculation of particular averages of these quantities is proposed as a possible way to obtain correct results for the local electron temperature and plasma density. These averages are used in the approximate evaluation of the electron drift velocity from the electron saturation currents measured at the two orientations of the probe collecting surface.     

Längsfeldstärke,Elektronentemperatur, mittlere Neutralgasdichte,Ionenstromdichte auf die Wand und mittlere Entladungsstromdichte in der Freifallsäule bei hohem Ionisationsgrad

On the basis of a foregoing paper new theoretical results for the positive column at low pressure and strong ionization, especially for discharges in noble gas ion lasers, are given. The mean velocity v_n0 of the neutral atoms reemitted from the wall is taken into account. The electric conductivity is calculated for an argon plasma. The formulas connecting the electron temperature, the mean neutral gas density, and the electric field strength are derived. The electron temperature, the axial electric field intensity, the degree of ionization, the axial electron drift velocity, the ion flux to the wall, and the force density causing the main part of gas pumping along the column are calculated as functions of the product of the mean current density and the tube radius, and of v_n0 for argon. The axial drift velocity of the electrons is still smaller than the mean thermal electron velocity for high discharge currents, except at very low gas pressures. In general, the ion flux to the wall is not directly proportional to the discharge current. The factor for the determination of the charged particle density by means of probe measurements at the wall is discussed. The self-magnetic field affects the discharge only at high electron temperature, high degree of ionization, and relatively large tube radius, i.e. at high current density and low gas pressure in not too narrow discharge channels.     

Short Time Domain and Double Resonance Techniques in Electron Spin Resonance Spectroscopy

Abstract

In recent years improvements in technology have increased the amount of information obtainable from, and have broadened the areas of application of, electron spin resonance spectroscopy. Some of the techniques that have found wide use in recent years, such as pulse saturation relaxation time measurements, ENDOR and ELDOR, were originally discovered many years ago while others, such as saturation transfer spectroscopy and nanosecond time resolved techniques, are based upon more recent developments. In this review we will survey a number of these techniques, we will explain what they are and what we can learn from them, we will describe some of the spectrometer systems that are required for using them, and we will cite many representative articles which provide further details about them.     

Electrostatic Fluxes and Plasma Rotation in the Edge Region of EXTRAP-T2R

The EXTRAP-T2 reversed field pinch has undergone a significant reconstruction into the new T2R device. This paper reports the first measurements performed with Langmuir probes in the edge region of EXTRAP-T2R. The radial profiles of plasma parameters like electron density and temperature, plasma potential, electrical fields and electrostatic turbulence-driven particle flux are presented. These profiles are interpreted in a momentum balance model where finite Larmor radius losses occur over a distance of about two Larmor radii from the limiter position. The double shear layer of the E×B drift velocity is discussed in terms of the Biglari-Diamond-Terry theory of turbulence decorrelation.