The flute instability of a plasma driven by the electrostatic high-frequency eigenmodes

The dispersion equation of the low-freguency flute instability driven by the high-frequency electrostatic eigenmode excited in a plasma slab is studied. If the amplitude of h.f. mode is small the equation is solved and the explicit formula for the maximum growth rate is obtained. As an example we investigate this problem numerically for a slab with the parabolic density profile. In our model we are able to consider the heating only in the frequency band stretching from the lower hybrid frequency to the electron cyclotron frequency. For this case it follows from our calculation that the danger of the low-frequency disruption of a plasma in the process ofheating is serious.     

弱相对论电子等离子体的宏观非寻常模稳定性

本文利用宏观冷流体模型及麦克斯韦方程研究了磁约束在圆柱形导体容器内的弱相对论电子等离子体的宏观非寻常模稳定性，寻出了槽纹扰动下（ｋｚ＝０）的普遍本征方程，分析中包含平衡自洽场。在低频扰动情况下，简化了本征方程，并仔细研究了电子等离子体在矩形分布下的稳定性质。     

Nonlinear interaction of a modulated beam with plasma

Nonlinear evolution of h. f. instabilities excited by a premodulated electron beam in a magnetized plasma is investigated both theoretically and experimentally. Only the erenkov type of excitation (k_¦|v_b) of the upper branch of the electron plasma oscillations has been observed. The dynamics of excitation of the quasistationary fundamental wave (having the frequency of modulation) has been determined numerically. The wave absorption has been included into the theoretical model through an effective collision frequency. Numerical results agree well with the measured evolution of the amplitude and of the phase velocity of the fundamental wave along the system, as well as with the beam distribution functions. The observed bunching of beam electrons and characteristic features of plasma electrons heating are also reported. Broadening of the frequency spectrum and the occurrence of satellite waves with lower phase velocities have been observed downstream. Physical mechanism of spantaneous excitation of these satellites is suggested and their role in the process of the beam relaxation is discussed. More detailed studies of the nonlinearly excited waves have been performed by modulating the beam at two frequencies.It is a pleasure for us to thank Dr .Körbel who has kindly performed the numerical calculations.     

Velocity gradient driven flute instabilities in plasmas

The effect of velocity gradient across the magnetic field on the low frequency flute modes is examined in detail, using the normal mode analysis. It is shown that some new type of instabilities driven primarily by the velocitygradient arise and these excited modes eventually attain the convective saturation. The onset of plasma turbulence due to these instabilities may possibly be one of the major contributors for anomalous heating process and enhanced plasma resistivity.     

Modal propagation characteristics of EM waves in ternary one-dimensional plasma photonic crystals

We have theoretically studied the modal dispersion characteristics, group velocity, and effective group as well as phase index of refraction of ternary one-dimensional (1D) plasma photonic band gap (PBG) structure having periodic multilayers of three different materials in one unit cell. The dispersion characteristics related for such structure is derived by solving Maxwell wave equation based on principle of Kronig-Penny model. From the computed results we observe that the dispersion characteristics of such structure also show the frequency gap and cutoffs as found in (binary) one-dimensional plasma photonic crystal. The frequency gap is shown to become larger with the increase of plasma frequency as well as plasma width. It is seen that such structure provide additional degree of freedom to control dispersion characteristic, group velocity and effective index of refraction compared to conventional one-dimensional plasma photonic crystal.     

Interference simulation in a cold collisionless moving magnetized plasma slab and (free surface of plasma slab)

Abstract

The pattern of intensity due to the interference in a cold collisionless magnetized moving plasma slab is investigated. Theoretically, it is assumed the mentioned layer has been located as a thin layer in an etalon Fabry–Perot interferometer surrounded by vacuum. The direction of external magnetic field is normal to the plasma surface and the plasma slab moves parallel with external constant magnetic field. By taking into account the relativistic considerations, the functions of transmitted intensity are presented coincident with the Airy function form in laboratory and plasma slab frames, respectively. The effects of plasma frequency, cyclotron frequency, thickness of plasma slab, and velocity of the plasma slab on band width, finesse factor, and visibility are simulated. Finally with the assumption that there are two wavelengths near together in incident electromagnetic beam the power resolution for this configuration are analyzed. All studies mentioned above have been done for S-polarized and P-polarized electromagnetic beams separately.     

Electromagnetic Plasma Modes at ?pe

The problem of the electromagnetic plasma radiation near the electron plasma frequency ?pe is studied in the absence of an external magnetic field. Maxwell's equations together with the fluid equations, which include thermal effects, are solved for an infinite geometry as well as for a finite geometry, and the problem of the eigenmodes at a plasma frequency ? ?pe is studied. The excitation of these modes by a small density relativistic beam is discussed. It is shown that the presence of a small density beam with finite ?o/c (where ?o is the beam velocity and c the velocity of light in a vacuum) in a plasma of finite thermal velocity, can couple linearly the plasma oscillations excited by the beam with the electromagnetic plasma mode at ?pe. It is also shown that surface waves at a frequency ? ?pe can be excited by the beam.     

The Polarization Response Function and the Dielectric Permittivity of a Plasma

We give a simple direct derivation of the polarization response function h for linear electrostatic excitations of a plasma (without magnetic field) considering the effect of a percussion on the electrons. The physical meaning of the procedure is discussed, thus bringing into light basic facts of the plasma dielectric behavior. The result h = ?p2f0(x/t) (where f0 is the electron distribution function in velocity space and ?p the plasma frequency) is obtained without passing through the Vlasov-Poisson equations as in the standard theory. We show that the equivalence between the present method and the classic Landau analysis rests on properties of the Fourier transform applied on velocity space.     

Simplified models of flue instruments: influence of mouth geometry on the sound source

Flue instruments such as the recorder flute and the transverse flute have different mouth geometries and acoustical response. The effect of the mouth geometry is studied by considering the aeroacoustical response of a simple whistle. The labium of a transverse flute has a large edge angle (60 degrees) compared to that of a recorder flute (15 degrees). Furthermore, the ratio W/h of the mouth width W to the jet thickness h can be varied in the transverse flute (lips of the musician) while it is fixed to a value W/h approximately 4 in a recorder flute. A systematic experimental study of the steady oscillation behavior has been carried out. Results of acoustical pressure measurements and flow visualization are presented. The sharp edge of the recorder provides a sound source which is rich in harmonics at the cost of stability. The larger angle of the labium of the flute seems to be motivated by a better stability of the oscillations for thick jets but could also be motivated by a reduction of broadband turbulence noise. We propose two simplified sound source models which could be used for sound synthesis: a jet-drive model for W/h>2 and a discrete-vortex model for W/h<2.     

纯电子等离子体的宏观非寻常模稳定性

本文利用冷流体模型及麦克斯韦方程研究了磁约束在圆柱形导体容器内的纯电子等离子体的宏观非寻常模稳定性,导出了电磁性槽纹扰动(k_Z=0)下的普遍本征方程。数值研究了电子密度为矩形分布和钟形分布下等离子体的稳定性。     

Behavior of an electron plasma in a thin metallic slab placed in a variable electric field

The response of an electron plasma in a thin metallic slab to a variable electric field applied normally to the slab surface is studied in the framework of the kinetic theory. The radiation power absorbed by a unit surface area is calculated under the assumption that the electric field frequency is small compared with the plasma frequency and that electron scattering by the metal surface is of a purely diffuse character.     

Plasma acceleration in a wave with varying frequency

The averaged velocity of a test particle and the averaged velocity of a plasma in an electro-magnetic wave packet with varying frequency (e.g., a radiation pulse from pulsar) is derived. The total momentum left by the wave packet in regions of plasma inhomogeneity is found. If the plasma concentration is changing due to ionization, the plasma may be accelerated parallelly or antiparallelly to the direction of the wave packet propagation, which is relevant for a laser induced breakdown in gas.The author thanks R.Klíma for numerous discussions.     

On the containment of plasma in stationary magnetic and high-frequency fields I

The stationary state of a collisionless plasma in a magnetic mirror is analyzed provided that a strong h.f. field is present. The h.f. field frequency is somewhat higher than the electron cyclotron frequency. The pressure acting on the plasma and the height of the h.f. potential barrier are derived in particular. The h.f. potential yields the possibility of eliminating the loss cone in the velocities distribution. The upper limits, which the electron temperature must not exceed, are derived. If an appropriate stationary magnetic field is used, the required incident wave power can be diminished and the allowable electron temperature can be increased. Some new results concerning the case of a homogeneous magnetic field are presented.     

Adaptive finite elements method for the solution of the Maxwell equations in inhomogeneous magnetized plasma

The wave propagation in magnetized cold plasma inhomogeneous in one direction (the slab plasma model) involves solution of the set of the second order ODE’s. Several problems have to be treated in the numerical solution. An initial value approach cannot be used for the boundary problem where exponentially growing solution exists. The equations are stiff (the O and X modes have different wavelengths) and there is also, in the cold plasma model, a singularity of the solution in the upper hybrid resonance. For the boundary problem of this type, the finite elements method fits well. To remove the singularity at UHR, we introduce small ad hoc collisions. In warm plasma, the O and X waves are converted in the UHR region to the electron Bernstein waves. It is well known that the power of the O and X waves absorbed in UHR region in a weakly collisional cold plasma represents the power converted to EBW. To minimize the computational requirements of the algorithm, an adaptive mesh, based on the local error estimation, is constructed. The code is used to analyze electron cyclotron emission (ECE) from the spherical tokamak MAST.     

Surface instability of a current carrying plasma under the influence of a high frequency electric field

Surface instability of a collisionless semi-infinite current carrying plasma is studied. The semi-infinite plasma bounded by a plane surface is under the influence of a high frequency (hf) field. There are two classes of surface modes. One is a normal extension of zero high frequency field and the other due entirely to the presence ofhf field. As expected, with the increase in thehf field, the growth rates of the surface instabilities decrease. There are regions defined by the electron drift velocityu where the unstable surface and bulk regions overlap. The interesting result is that unlike the bulk plasma, there is a stable region on theu-axis flanked by two unstable regions. The width of this stable region increases with the increase in the field strength.     

Excitation of slow EM waves in a waveguide having a thin, annularplasma sheet

A thin, annular plasma sheet in a waveguide slows down the phase velocity of electromagnetic modes in a plasma density window for frequencies below the electron cyclotron frequency via Cerenkov interaction. At lower densities, the plasma is not effective enough to reduce the phase velocity of the EM wave below c. At higher densities the plasma expels the radiation field, leading to enhancement of phase velocity. The lowest mode having azimuthal number l=0 is most unstable. The radial mode having amplitude maximum at r=a, viz the plasma boundary, has the largest growth rate     

On the fitting of displaced maxwellians to laser-produced plasma ion velocity distributions

The empirical fitting of displaced maxwellians to ion velocity distributions measured at some distance from the target in laser-produced plasma studies is shown to lack a theoretical basis. A similar ion velocity spectrum to a displaced maxwellian is obtained if a simple self-similar plasma expansion is assumed. The discussion is illustrated by experimental ion velocity distributions measured at 0.8 m from a slab carbon target irradiated by a neodymium laser pulse of duration 4 ns focussed to an intensity of ≈10¹² W cm^-2.     

б�����Ų��ڷǾ�����ײ���������е�����ͷ�������

提出碰撞吸收型等离子体的等效折射指数,重新计算了折射角,并与利用常规方法得到的折射角进行了比较。分析了等离子体密度和碰撞频率对折射角的影响。考察了斜入射电磁波在吸收型等离子体中的折射特性,并以一维非均匀碰撞吸收型等离子体为例,对斜入射电磁波的反射特性进行了考察。结果表明,等效折射指数的概念涵盖了入射角度的影响,在考察斜入射时更准确、便捷。     

磁化碰撞等离子体对雷达波的共振吸收

使用平板几何对雷达工作频段的电磁波在磁化碰撞等离子体中的传播作了数值计算,计算结果表明,在均匀等离子体中,当等离子体碰撞频率f∞=0.1,0.5,1GHz及电磁波频率接近高混杂频率时,电磁波衰减和被吸收功率出现最大峰值,即出现共振吸收;当fen=1,10GHz时,电磁波衰减、被吸收功率和透射功率曲线变得很平坦;衰减和吸收功率随等离子体密度的增大而增大,在n=10^11cm^-3时,衰减可达100dB,吸收比可达80％。在非均匀等离子体中,电磁波的反射功率峰值比在均匀等离子体中大。因此,磁化均匀等郭了体更有利于等离子体隐身。     

Measurement of the flow and its vibration in Japanese traditional bamboo flute using the dynamic PIV

All wind instruments produce sound due to the vibration of air inside of the instrument. In the case of a trumpet or a clarinet, the mouth or a reed helps to generate variable tones. In the case of a flute, there is no mechanical vibration. Additional detail about the flow and the sound vibration inside and outside of the flute are investigated in order to understand the mechanism of the wind instrument and to aid in the manufacture of quality instruments. In this report, a traditional Japanese bamboo flute was investigated experimentally. The dynamic PIV technique was applied to measure the vibration. Two kinds of experiments were performed. Argon-gas flow containing an oil mist as tracer particles both inside and outside the bamboo flute was measured using a high frequency pulse laser. The periodical flow near a hole of the bamboo flute was successfully measured. The flow was found to go into and out from the flute and the balance of a mass flow rate and the averaged velocity were almost zero at the hole. Then, the flow in the bamboo flute was visualized when a human played the instrument, using a CW-laser and water-mist as the tracer. It was discovered that the two instructors had unique methods for playing the flue instrument.