Nonlocal ion transport in a weakly ionized nonequilibrium plasma

Nonlocal ion transport in a weakly ionized plasma with a strong electric field is analyzed. It is assumed that charge-exchange interactions are the main mechanism of ion scattering. Ion density and drift velocity are determined for nonuniform time varying electric field by using both the direct solution of the kinetic equation and the Chapman-Enskog-type approach. The ion mean velocity is given by an integro-differential operator applied to the electric field. Ion density and drift velocity exhibit resonant behaviour when ω≃kW0, which corresponds to the resonance between ions moving with average velocity W₀ and wave traveling with the phase velocity ω/k     

Modeling of Ag<Superscript>+</Superscript> mobility in AgI by space charge depolarization process

A mathematical modeling of mobility of Ag⁺ ion in AgI is presented. In the model, regime is space charge polarized initially by applying a fixed d.c. field of ∼0.5 V across the sample, sandwiched between two electronically conducting graphite electrodes. The depolarization potential is recorded at various isothermal conditions in the temperature range 300–535 K. By considering open-circuit condition (where the sum of all current densities, i.e., drift current density, trapped current density, and displacement current density, is vanished), ionic drift- and trap-modulated mobilities are modeled. Result obtained through this model is compared with the ionic mobility, measured by conventional transient ionic current technique in the same temperature range.     

Acoustic double layers in multispecies plasma

Particle simulation in a one-dimensional bounded system is used to examine the formation of acoustic double layers in the presence of two ion species. Double-layer formation depends critically on the details of the distribution functions of the supporting ion populations, and their relative drifts with respect to the electrons. The effect of having two ion components, an H⁺ and an O⁺ beam, on double-layer evolution from ion acoustic turbulence driven by an electron drift relative to the H⁺ beam of ≈0.5u _e, where uu_e is the electron thermal speed, is examined. The ratio of ion drifts is taken to be consistent with acceleration by a quasi-static auroral potential drop (i.e. V _H/V_O=√M_O/ M_H=4.0). Acoustic double layers form in either ion species on the time scale τ≈100ω_ps^-1, where ω_ps is the ion plasma frequency for species `s' and s=H or O, and for drifts relative to the electrons lower than that required for double layer formation in simulations of single ion component plasma     

Effect of Resonant Electrons on Propagation of Nonlinear Electrostatic Drift Waves

A modified Korteweg-de Vries equation in a two-dimensional space comprising the externla magnetic field and the direction normal to a density gradient is derived for a plasma consisting of warm ion fluid (?i = 3) and nonisothermal electrons (?e > 1). The effect of ionic temperature and resonant electrons (both trapped and free electrons) on the amplitude and the width of the drift solitary wave is examined.     

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.     

Nonresonant Parametric Decay of a Finite Extent Lower Hybrid Wave

The decay of lower hybrid waves into nonresonant quasimodes inside the lower hybrid resonance cone is observed. Sideband frequency shifts of up to 30 percent from the pump frequency occur. The low-frequency mode is identified as a nonresonant quasi-mode satisfying the approximate conditions ?/k? ? ?e. The sideband wave-number and the sideband and quasi-mode growth rates were measured. The transition between ion-acoustic mode and quasi-mode decay spectra is observed as plasma density or pump frequency is varied.     

Critical Review of Plasma Spectroscopic Diagnostics via MTE

The often used absolute, ratio, and off-axis peaking methods of LTE spectroscopy are examined using the Multithermal Equilibrium (MTE) multifluid model. The partial MTE model distinguishes between the upper level and total excitation temperatures of atom and ion (Tex?a or Tex?i and Texa or Texi, respectively) as well as different valued electron (Te) and heavy particle (Ta = Ti) temperatures. In complete MTE, all excitation temperatures are equal valued but may differ from kinetic temperature values. In this paper, the particular temperature approximated by common diagnostic methods is identified. The probable errors in the temperature values are presented in a figure as a function of probable errors in data, temperature and level energy difference. The results indicate that the ratio methods (neutral-neutral, ion-neutral, and continuumneutral) yield upper level excitation temperature values; the absolute neutral line and absolute continuum (at low Texa) yield total atom excitation temperatures; and the off-axis peaking method for the neutral line yields the total excitation temperature. Application of the methods are more complex than in LTE spectroscopy. Probable errors are smallest for the absolute and off-axis peaking methods. The electron temperature may be determined from the continuum-neutral ratio at very high excitation temperatures (Texa >> E?/10k). The paper also comments on MTE diagnostic methods.     

Kinetic Instability of Anisotropic Drift Wave Accompanied by Field Aligned Currents in Solar Coronal Loop

Solar coronal loops are frequently accompanied by the field-aligned currents, which drive instabilities if the drift velocity u₀> v_A the Alfvén velocity. For our choice of parameters, the critical threshold value of u₀/v_A is~3.0 for growth and the corresponding current filling factor~10^-3-10^-4. Below this value we are no longer in the kinetic regime. The coronal loops also have short-scale density gradients within each loop. The electron resonance in the presence of density gradient causes the drift mode to grow. We study the effect of these two free energy sources, the electron drift and the density gradient, in the presence of temperature anisotropy T_⊥α > T_||α. These effects simultaneously exist in the coronae. Using gyrokinetic theory, we investigate the influence of these effects, examine how they interplay with each other and study the consequent growth of the magnetosonic wave. We observe that kinetic instability driven by density gradient can be suppressed by field-aligned currents. The temperature anisotropy with chosen signatures causes further stabilizing effect. The results may prove useful to study the heating mechanism of solar coronal loops, acceleration of particles and confinement of particles in the thermonuclear reactors.     

压强梯度对托卡马克等离子体有理面上由磁场曲率驱动的漂移模的稳定效应

在流体模型内，我们导出了一个托卡马克等离子体内环形漂移模的色散关系，该色散关系包含了离子压强梯度、磁场梯度与曲率。一方面，磁场梯度与曲率驱动有理面上的不稳定性。另一方面，压强梯度使这些不稳定性稳定。通过解析和数值两种方法，我们获得了把不稳定性增长率压缩到零的临界压强梯度阈值。     

Rectification of terahertz radiation in semiconductor superlattices in the absence of domains

We study theoretically the dynamical rectification of a terahertz AC?electric field, i.e.?the DC?current and voltage response to the incident radiation, in strongly coupled semiconductor superlattices. We address the problem of stability against electric field domains: a spontaneous DC?voltage is known to appear exactly for parameters for which a spatially homogeneous electron distribution is unstable. We show that by applying a weak direct current bias the rectifier can be switched from a state with zero DC?voltage to one with a finite voltage in full absence of domains. The switching occurs near the conditions of dynamical symmetry breaking of an unbiased semiconductor superlattice. Therefore our scheme allows for the generation of DC?voltages that would otherwise be unreachable due to domain instabilities. Furthermore, for realistic, highly doped wide miniband superlattices at room temperature, the generated DC?field can be nearly quantized, that is, be approximately proportional to an integer multiple of ?ω/ea where a is the superlattice period and ω is the AC?field frequency.     

Drift ion acoustic solitons in an inhomogeneous 2-D quantum magnetoplasma

Linear and nonlinear propagation characteristics of quantum drift ion acoustic waves are investigated in an inhomogeneous two-dimensional plasma employing the quantum hydrodynamic (QHD) model. In this regard, the dispersion relation of the drift ion acoustic waves is derived and limiting cases are discussed. In order to study the drift ion acoustic solitons, nonlinear quantum Kadomstev-Petviashvilli (KP) equation in an inhomogeneous quantum plasma is derived using the drift approximation. The solution of quantum KP equation using the tangent hyperbolic (tanh) method is also presented. The variation of the soliton with the quantum Bohm potential, the ratio of drift to soliton velocity in the co-moving frame, , and the increasing magnetic field are also investigated. It is found that the increasing number density decreases the amplitude of the soliton. It is also shown that the fast drift soliton (i.e., v_*>u) decreases whereas the slow drift soliton (i.e., v_*<u) increases the amplitude of the soliton. Finally, it is shown that the increasing magnetic field increases the amplitude of the quantum drift ion acoustic soliton. The stability of the quantum KP equation is also investigated. The relevance of the present investigation in dense astrophysical environments is also pointed out.     

New interior solution describing relativistic fluid sphere

Nonlinear low-frequency electrostatic waves in a magnetized, three-component plasma consisting of hot electrons, hot positrons and warm ions have been investigated. The electrons and positrons are assumed to have Boltzmann density distributions while the motion of the ions are governed by fluid equations. The system is closed with the Poisson equation. This set of equations is numerically solved for the electric field. The effects of the driving electric field, ion temperature, positron density, ion drift, Mach number and propagation angle are investigated. It is shown that depending on the driving electric field, ion temperature, positron density, ion drift, Mach number and propagation angle, the numerical solutions exhibit waveforms that are sinusoidal, sawtooth and spiky. The introduction of the Poisson equation increased the Mach number required to generate the waveforms but the driving electric field E ₀ was reduced. The results are compared with satellite observations.     

Analytical solution for capacitive RF sheath

A self-consistent solution for the dynamics of a high voltage, capacitive radio frequency (RF) sheath driven by a sinusoidal current source is obtained under the assumptions of time-independent, collisionless ion motion and inertialess electrons. Expressions are obtained for the time-average ion and electron densities, electric field and potential within the sheath. The nonlinear oscillation motion of the electron sheath boundary and the nonlinear oscillating sheath voltage are also obtained. The effective sheath capacitance and conductance are also determined. It was found that: (1) the ion-sheath thickness S _m is √50/27 larger than a Child's law sheath for the DC voltage and ion current density; (2) the sheath capacitance per unit area for the fundamental voltage harmonic is 2.452 ϵ₀ /S_m, where ϵ₀ is the free space permittivity; (3) the ratio of the DC to peak value of the oscillating voltage is 54/125; (4) the second and third voltage harmonics are, respectively, 12.3 and 4.2% of the fundamental; and (5) the conductance per unit area for stochastic heating by the oscillating sheath is 2.98 (λ_D/S_m)^2/3 (e ²n₀/mu_e), where n ₀ is the ion density, λ_D is the Debye length at the plasma-sheath edge, and u_e is the mean electron speed     

Onset of turbulence and profile resilience in the Helimak configuration

An experimental study of the onset of drift wave and flute interchange instabilities in the Helimak configuration is presented. It is shown that the Helimak offers the opportunity to separate the regions where these instabilities are active and to assess their relative role in cross-field anomalous transport and in the self-organization of exponential plasma density profiles with resilient scale length. Some results indicating a period doubling route to turbulence are also presented.     

原子过程对极向CXRS测量影响的数值模拟

考虑到原子过程的影响，对极向CXRS测量过程进行了数值模拟.计算结果表明由于受辐射率 、回旋运动以及离子辐射寿命的影响，测量的结果会在离子逆磁漂移方向高估了离子的旋转 速度，而低估了离子的温度.这种偏差会随着离子温度的升高、磁场的增加而增大.相对与原 子过程的影响来说，由于等离子体非均匀性所引起的速度和温度的偏离很小，可以忽略不计 . 关键词： 原子过程 数值模拟 等离子体     

Current induced domain wall motion in GaMnAs close to the Curie temperature

Domain wall dynamics produced by spin transfer torques is investigated in (Ga, Mn)As ferromagnetic semiconducting tracks with perpendicular anisotropy, close to the Curie temperature. The domain wall velocities are found to follow a linear flow regime which only slightly varies with temperature. Using the D?ring inequality, boundaries of the spin polarization of the current are deduced. A comparison with the predictions of the mean field k·p theory leads to an estimation of the carrier density whose value is compatible with results published in the literature. The spin polarization of the current and the magnetization of the magnetic atoms present similar temperature variations. This leads to a weak temperature dependence of the spin drift velocity and thus of the domain wall velocity. A combined study of field- and current-driven motion and deformation of magnetic domains reveals a motion of domain walls in the steady state regime without transition to the precessional regime. The ratio between the non-adiabatic torque β and the Gilbert damping factor α is shown to remain close to unity.     

Effects of Ion Temperature and Inertia on Kinetic Alfvén Waves

Kinetic Alfvén wave (KAW) has been an interesting topic for discussion extensively in the fields of labora-tory, space, and astrophysical plasmas. A general dispersion equation is derived from the exact two-fluid model in thisambient magnetic field. For the short wavelength cases of kλi >> 1, where λi = vA/ωci and ωci are the ion inertial lengthand gyrofrequency, respectively, our dispersion relations are appropriate for discussing effects of the ion temperatureand inertia on KAWs. The present results show that both the ion temperature and inertia can affect considerably thebehaviors of KAWs in propagation, resonance, and polarization. In particular, our results may be a great help to un-derstanding some salient features of the low-frequency (in comparison with the ion gyrofrequency ωci) electromagneticfluctuations frequently observed by the FREJA and FAST satellites in the auroral zone of the Earth's ionosphere andmagnetosphere.     

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.     

Effective activation energy Ue(T,J,H) in textured Bi1.84Pb0.4Sr2Ca2Cu3Oy silver-clamped thick films

We have measured the resistivity of textured Bi_1.84Pb_0.4Sr₂Ca₂Cu₃O_y silver-clamped thick films as a function of temperature, current density ranging from 10 to 1×10³ A/cm² and magnetic field up to 0.3 T. We find that the effective activation energy U_e follows U_e(T,J,H)=U₀(1−T/T_p)^mln(J_c0/J)H⁻ with m=1.75 for Hab-plane and 2.5 for Hc-axis and =0.76 for Hab and 0.97 for Hc, for the current density regime above 100 A/cm², where T_p is a function of applied magnetic field and current density. This result suggests the effective activation energy U_e be correlated with the temperature, current density and magnetic field. The possible dissipative mechanisms responsible for the temperature, current density and magnetic field dependence of the effective activation energy are discussed.     

Compressional Effects on Drift-Resistive Instabilities in General Toroidal Plasma

The drift-resistive modes in general toroidal geometry are studied analytically and numerically. The study includes the effects from ion acoustic couplings, ion polarization drift, and perpendicular resistivity. These effects can completely stabilize the drift-resistive modes. The perpendicular resistivity is effective in stabilizing mainly the drift interchange modes, while the ion acoustic couplings are the dominant mechanism for the stabilization of the drift-tearing modes. From the ion polarization drift effects of the perpendicular compression, the critical value of magnetic energy A, saturates for a moderate diamagnetic drift frequency region. The favorable average curvature is a stabilizing factor for the drift-tearing modes with the criterion of ?' < ?c, but an instability from unfavorable curvature even with ?' < 0 exists in the semicollisional region.