Low-frequency current oscillations in a linear hot-cathode discharge

Low-frequency (ω ⪡ ω_pi) plasma oscillations in the transition regime between the high and the low current mode of a thermionic hot-cathode discharge are investigated experimentally. This type of current oscillation often shows chaotic dynamics. The current oscillations are related to nonlinear short wavelength potential structures which are identified as ion bunches formed by a fluctuating ionization front. These ion bunches are separated by ion holes and move at ion thermal speed rather than ion acoustic speed. By entering the negative space charge region of the cathode sheath, the ion bunches trigger electron current fluctuations that provide the required feedback mechanism for the observed wave train formation.     

关于等离子体中的瞬变过程

利用Spitzer的广义欧姆定律分析等离子体中由垂直于磁场的压强梯度所引起的宏观瞬变性质。在垂直于磁场方向的初始流动速度为零的条件下,得出在恒稳磁场中的瞬时扩散系数(当ω_ceτ?1)为 D= (2km_ec²)/e² T/(B²τ)+(kc)/eT/Be^-m_e/(miτ)tsinω_cit,式中τ为电子离子的平均碰撞时间间隔。     

Microwave heating of electrons of a dense plasma column at frequencies higher than electron cyclotron frequency

In the overdense collisionless plasma column inserted through the narrow sides of a rectangular waveguide, the excited electron cyclotron harmonic waves (CHWs) are studied by means of two movable probes and a phase interferometer in the range of parameters of 2 >ω/ω _ce >1; 0·5 < (ω _pe /ω)² < 15. Two kinds of CH waves have been found in the dipole mode:

1. Stable backward CH waves with the phase velocity in the direction from the axis to the periphery.
2. Unstable backward CH waves with the phase velocity in the opposite direction.

From the correlation measurements and amplitude distribution of the CH waves at the applied frequency and its second harmonic frequency it has been found that in the range of 2 >ω/ω _ce > >1·5 efficient nonlinear resonant interaction of CH waves takes place. The position of the loci of resonant interaction inside the plasma column has been found both experimentally and by a simple new graphical procedure for the resonant conditions of the formω ₂=2ω ₁; k₂=2¦k₁¦. In absence of this condition, no effective generation of the second harmonic frequency has been found. The resonant interaction of CHWs atω/ω _ce =1·85 is the cause of self-trapping of CH waves between the zones of resonant interaction in radial direction and of the anomalous heating rate of electrons.     

High frequency Brillouin scattering in metals and gaseous plasmas

Spontaneous and stimulated Brillouin scattering are studied in metals and gaseous plasmas, for an incident laser frequencyω ₁ greater than the corresponding plasma frequencyω _plin the medium. The calculation of threshold powers for the stimulated scattering in aluminium metal and non-degenerate Al-plasmas shows that their values become quite small asω ₁ approachesω _plFor the case of backward wave scattering we also estimate the critical power above which a temporal instability sets in such media. It is argued that this instability may be one of the factors for anomalously large absorption of high power laser beams in laser-induced plasmas.     

Ion sound wave excitation in a plasma under a Langmuir turbulence regime

Ion sound wave excitation in a warm non-relativistic (W_b ≤ 400 eV) electron beam unmagnetized plasma system is studied experimentally. The spectrum of these waves shows two peaks at frequencies of 70 and 230 kHz respectively. The origin of these waves is connected with modulational instability and cavity collapse. We show that the energy of bulk accelerated electrons can explain the measured value of kr_De for high-frequency sound waves. The energy of ion sound waves is not high enough to have an influence on the Langmuir turbulence dynamics.     

Current neutralization and plasma polarization for intense ion beams propagating through magnetized background plasmas in a two-dimensional slab approximation

A two-dimensional electromagnetic Particle-in-Cell （PIC） simulation model is proposed to study the propagation of intense, ion beams with beaIn width wb small compared to the electron skin depth c/wpe through background plaslnas in tile presence of external applied magnetic fields. The effcctive eleetron gyrora,dius wge is found to be an important parameter for ion beam transport in the presence of magnetic fields,In the bealn regions, the background plasmas respond differently to the ion beanl of width wb〈wge and wb 〉 wge for the given magnetic field and beaan encrgy, For the case of beam width wb 〈 wge with relative weak external magnetic fiehts, the rotalion effects of plasma electrons a,re found to be signifieant and contributes to the signitica,nt enilaneeinent of the self electric and seif-magnetic fields. While for the ease of beam width wb 〉 wge with relative strong external magnetic fields, the rotation effects of plasma cleetrons are strongly inhibited and a, well neutraliza, tion of ion beam current can be found. Finally, the influences of diftiuent beam widths. beam energies and magnetic fields on the neutralization of ion bc, anl eurrellt are summarized for the eases of wb 〈 wge〈 c/wpe,wge 〈wb〈c/wbp and wb〈c/wpe〈wge.     

Self-focusing of electromagnetic waves in a magnetoplasma

The steady state self-focusing of a Gaussian electromagnetic beam in a magneto-plasma has been studied. On a short time scale, a non-linearity in the dielectric constant of a plasma appears due to the ponderomotive force. This force in the case of the extraordinary mode has opposite signs forω>ω _c andω<ω _c, whereω _c is the electron cyclotron frequency. The self-focusing due to this effect is predicted at frequencies except forω _c /2<ω<ω _c . The focusing of the ordinary mode is adversely affected by the magnetic field. On a larger time scale, the non-uniform heating of electrons by the beam and the resulting redistribution of the electron density is a source of non-linearity. This non-local non-linearity is several orders of magnitude higher than the ponderomotive non-linearity. We predict self-focusing of the extraordinary mode only above the gyroresonance (ω>ω _c ), while the ordinary mode can be focused at all frequencies.     

Phase analysis of non-coherent processes in a moving CDW

Earlier it has been found that the signal produced by a moving CDW contains along with fundamental of narrow band frequency (ω₁) and its harmonics (ω_n) a non-coherent part—a lot of non-coherent oscillations. Due to a rather broad frequency range the power associated with non-coherent oscillations is comparable to or even greater than that for fundamental frequency, and hence, can play an essential role in CDW dynamics.     

Investigation of the diocotron instability of an infinitely wide sheet electron beam by using the macroscopic cold-fluid model theory

This paper investigates the diocotron instability of an infinitely wide relativistic sheet electron beam in conducting walls propagating through a uniform magnetic field by using the macroscopic cold-fluid model theory. Assuming low-frequency perturbations with long axial wavelengths, the eigenvalue equation and the dispersion relation are acquired for a sheet electron beam with sharp boundary profile and uniform density. The results presented in this paper has developed the use of the macroscopic cold-fluid model theory by extending the parameter of the electron cyclotron frequency ω_c to a wider usage range, which is restricted to be much larger than the plasma frequency ω_p in the previous research work. Theoretical analyses and numerical calculations indicate that the transport of the sheet electron beam will be completely stabilized by augmenting the normalized beam thickness to a conductor gap larger than a threshold λ_b, which is greatly dependent on the parameter ω_c/ω_p. The larger ω_c/ω_p is, the smaller λ_b will be needed. Moreover, the system parameters, including the wave number k_x of the perturbations and the relativistic mass factor γ_b, will also influence the growth rate of diocotron instability obviously.     

Plasmons in semiconductors and insulators: A simple formula

Based on the dielectric function of an excitonic system, a simple model dielectric function is constructed. The high energy zero of this function yields the plasma frequency, and is given by: (hω_x)² = (hω_f)² + E_x² where ω_f is the free electron plasma frequency and E_x the lowest exciton energy. It is argued that this formula is valid for both, Frenkel and Wannier excitons, and comparison is made with experiments on a variety of crystals, ranging from InSb to Ar. In all cases, surprisingly good agreement is found.     

Parametric excitation of relativistic plasma oscillations by strong electromagnetic waves

Parametric interaction of strong waves ($\text{?=}\text{eE}{}_{\mathrm{<mtext>o</mtext>}}\text{m}{}_{\mathrm{<mtext>e</mtext>}}\text{cω}{}_{\mathrm{<mtext>o</mtext>}}\text{～ 1}$) with a cold, two-fluid (ion and electron) plasma is studied in the limits of high frequency (ω_o?ω_Le) and resonance (ω_o ≈ ω_Le). Unstable oscillations of both electrons and ions are found in the resonance limit.     

Geodesic mode instability driven by the electron current in tokamak plasmas

Effect of the parallel electron current on Geodesic Acoustic Modes (GAM) in a tokamak is analyzed by kinetic theory taking into the account the ion Landau damping and diamagnetic drifts. It is shown that the electron current modeled by shifted Maxwell distribution may overcome the phase velocity threshold and ion Landau damping thus resulting in the GAM instability when the parallel electron current velocity is larger than the effective parallel GAM phase velocity Rqω. The instability occurs due to its cross term of the current with the ion diamagnetic drift. Possible applications to tokamak experiments are discussed.     

(Ni, Zn, Sn) Ru and (Ni, Sn) Sn substituted barium ferrite prepared by mechanical alloying

NiRu, ZnRu, SnRu and SnSn mixtures considerably improved the saturation magnetization, M_s with low substitution values; diminishing quickly at the same times the coercivity, H _ci to suitable values for high-density magnetic recording applications. On the other hand, the NiSn mixture also decreased the coercivity rapidly however without enhancing the saturation magnetization. The shown differences on magnetic properties were mainly due both to magnetic nature of divalent ion and to secondary phase apparitions. The mixtures with Sn^2?+? as partner ion diminished markedly to T _c. The tetravalent Ru^4?+? ion has a special effect on magnetic properties of hexagonal ferrites (increases M_s and diminishes fast H _ci with low substitutions).     

The ion streaming effect on Weibel instability in the relativistic dense plasma

The Weibel instability plays an important role in stopping hot electrons and energy deposition mechanism in fast ignition of inertial fusion process. In this paper, the ion Weibel instability in counter propagating electron‐ion plasmas is investigate. The obtained results show that the growth rate of Weibel instability will be decreased about 40% with the anisotropy velocity as v_xe = 2v_ze = 20; the ion density ratio, b = n _{0i 1}/n _{0i 2}, and density gradient, are increasing 50 and 90% respectively. The ion streaming in density gradient of dense plasma leads to increasing the Weibel instability growth rate and its amplification through ion streaming in the large wavenumber. The maximum unstable wavenumber has been decreased with decreasing the ion beam density ratio. For fixed ion density ratio, increasing 90% of the density gradient in the near of fuel plasma corona leads to reducing growth rate and unstable wavenumber about 43 and 42% respectively.     

束-离子通道电磁模式的边界效应分析

考虑束-离子通道边界上等离子体电子可能发生的扰动，导出了TM模本征方程的理论式.通过对理论结果的数值模拟计算，在阶跃边界情况下将束-离子通道与一般介质波导的电磁关系进行了比较，发现束-离子通道可以通过改变等离子体频率来控制其工作模式.分析对照扰动电荷边界与阶跃边界对束-离子通道电磁模式的影响，观察到在扰动电荷边界情况下，束-离子通道在低频区域(ω＜ω_p,ω_p为等离子体频率)内截止频率显著提高，并在高频区域(ω>ω_p)内出现了新的电磁模式.研究结果对离子通道激光(ICL)和离子通道电子回旋脉塞(ICECM)的设计提供重要的理论依据. 关键词： 束-离子通道 阶跃边界 扰动电荷边界 电磁模式     

Experimental Evidence of Parametric Instabilities in an Unmagnetized Plasma Subjected to a Strong H.F. Electric Field

Experimental evidence of parametric excitation, by an intense external H.F. field, of an electron surface mode and an ion wave is presented. The pumping electromagnetic energy density is equal to or slightly larger than the thermal energy density of the electrons. The value of f_pc/f₀ (electron plasma frequency/external field frequency) is that for an electron surface wave. Depending on the pressure and field intensity, this decay instability can lead to three types of low frequency oscillations, with frequencies close to the ion plasma frequency. Two of these are described by Aliev and Silin's intense field theory: one is the volume ion plasma oscillation and the other a surface ion plasma oscillation. The third corresponds to no known ion eigenmode. Several other features of the theory by Aliev and co-workers are also confirmed experimentally, such as the harmonic excitation of the instability (nf₀ ≈ f_pe/√2, where n is an integer), the instability amplitude as a function of f_pe/f₀ (above threshold conditions), the value of the mismatch parameter as a function of field strength and ion mass, and the existence of a fine structure corresponding to the symmetric and antisymmetric electron surface oscillations. Even at high pump field strengths, the decay products are nearly monochromatic i.e. the plasma does not become turbulent.     

Screening of the dust-particle charge in a plasma with an external ionization source

An asymptotic theory of the screening of the dust-particle charge in a plasma with an external ionization source has been developed. It has been shown analytically that the screening of the charge of a dust particle adsorbing the charge of charged plasma particles that fall on it is not generally described by the Debye theory. The screening radius is determined by the relation between the coefficients β_ei and β_L = 4πek _i (k _i is the ion mobility) of the electron-ion and Langevin recombinations, respectively. When β_L ? β _ei , the screening radius is much larger than the electron Debye radius. It has been shown that the contribution of the ion component of an isothermal plasma to screening is equal to the electron contribution if the coefficient of the electron-ion recombination is twice or more larger than the Langevin coefficient of the ion recombination, β_ei ≥ 2β_L.     

The large polaron in a magnetic field: Possible existence of a phase transition

It is shown that the ground state energy of a polaron in a magnetic field (calculated in the Feynman approximation) exhibits a discontinuity in the first derivative with respect to α (the electron-phonon coupling constant) if ω_c/ω_o (cyclotron frequency to LO-phonon frequency) is larger than 2.24. This discontinuity has the characteristics of a first order phase transition if α is interpreted as an inverse temperature. For ω_c/ω_o = 2.24 the transition is second order. We found that below the transition point the phonon cloud cannot follow the electron in its motion in the magnetic field.     

Structural,dielectric and photoluminescence properties of co-precipitated Zn-doped SnO2 nanoparticles

Zn-doped SnO₂ nanoparticles were prepared by the chemical co-precipitation route. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses of these prepared nanoparticles were carried out for structural and morphological studies. All the samples have been found to have tetragonal rutile structure of the polycrystalline SnO₂ having crystallite size in the range 13–25 nm. TEM micrographs show agglomeration of nanoparticles in all the samples. At a particular temperature, the dielectric constant of all the samples has been found to decrease with increasing frequencies which may be due to rapid polarization processes occurring in SnO₂ nanoparticles. The ac conductivity, σ (ω), has been found to vary with frequency according to the relation σ (ω) ∝ ω^S. The value of S has been found to be temperature dependent, decreasing with increasing frequency which suggests that a hopping process is the most likely conduction mechanism in these nanoparticles. The room temperature photoluminescence (PL) spectra of the undoped and Zn-doped SnO₂ nanoparticles consist of the near band-edge ultraviolet (UV) emission and the defect related visible emissions. The origin of emission peaks in the visible region is attributed to oxygen-related defects that are introduced during growth.     

Low-frequency fluctuations in a pure toroidal magnetized plasma

A magnetized, low-β plasma in pure toroidal configuration is formed and extensively studied with ion mass as control parameter. Xenon, krypton and argon plasmas are formed at a fixed toroidal magnetic field of 0.024 T, with a peak density of ∼10¹¹ cm⁻³, ∼4 × 10¹⁰ cm ⁻³ and ∼2 × 10¹⁰ cm ⁻³ respectively. The experimental investigation of time-averaged plasma parameter reveals that their profiles remain insensitive to ion mass and suggests that saturated slab equilibrium is obtained. Low-frequency (LF) coherent fluctuations (ω < ω _ci) are observed and identified as flute modes. Here ω _ci represents ion cyclotron frequency. Our results indicate that these modes get reduced with ion mass. The frequency of the fluctuating mode decreases with increase in the ion mass. Further, an attempt has been made to discuss the theory of flute modes to understand the relevance of some of our experimental observations.