Controlling the radiation frequency of a plasma relativistic microwave oscillator during a nanosecond pulse

It is shown experimentally and by numerical simulation that the radiation frequency of a 50-MW plasma relativistic microwave oscillator can be varied within 15% during a 60-ns-wide pulse by varying the plasma concentration. The plasma is generated by pre-ionization of a low-pressure gas. When the degree of ionization increases in a microwave field, the radiation frequency rises. Conversely, when plasma electrons are forced out by the electrostatic field of a high-current relativistic electron beam, the radiation frequency declines. By appropriately selecting the initial gas pressure and degree of gas ionization, one can control both trends and thereby the radiation frequency.     

Effects of magnetic field and temperature on the nonrelativistic bremsstrahlung process in magnetized anisotropic plasmas

The magnetic field and thermal effects on the nonrelativistic electron-ion bremsstrahlung process are investigated in magnetized anisotropic plasmas. The effective electron-ion interaction potential is obtained in the presence of an external magnetic field. Using the Born approximation for the initial and final states of the projectile electron, the bremsstrahlung radiation cross section and bremsstrahlung emission rate are obtained as functions of the electron energy, radiation photon energy, magnetic field strength, plasma temperature, and Debye length. It is shown that the effects of the magnetic field enhance the bremsstrahlung radiation cross section for low plasma temperatures and, however, suppress the bremsstrahlung cross section for high plasma temperatures. It is also shown that the magnetic field effects diminish the bremsstrahlung emission rate in magnetized high temperature plasmas.     

基于MULTI2D-Z程序的Z箍缩动态黑腔形成过程模拟

对辐射流体力学程序MULTI-2D进行改造,增加磁场演化方程程序模块,自洽地在运动方程模块中增加洛伦兹力,在能量方程模块中增加欧姆加热,将它改造成辐射磁流体力学程序MULTI2D-Z.验证了新增磁场程序模块的可靠性,并发现温度和密度的增大会抑制磁场的扩散,负径向速度梯度的流体对流也会抑制磁场的扩散.利用改造好的MULTI2D-Z程序模拟了峰值为8 MA的脉冲电流驱动的钨丝阵Z箍缩动态黑腔形成过程.得到了X光功率(约30 TW)和能量(约300 k J)、泡沫辐射温度(约120 eV)、箍缩轨迹等模拟结果.在动态黑腔形成过程中,磁场主要分布在钨主体等离子体中;辐射向内传播,烧蚀泡沫柱而使它膨胀;辐射热波在被撞击的泡沫柱中传播,其传播速度比物质温度传播得快,当辐射热波传播到中心轴时泡沫柱中的辐射场变得比较均匀,并且除了冲击波处外辐射温度与物质温度基本上没有分离.这些模拟结果可增强人们对磁场扩散和对流规律以及动态黑腔形成机制的理解,同时表明了MULTI2D-Z程序可成为Z箍缩及其应用的新的程序模拟工具.     

Excitation of Nonsymmetric Waves by Given Sources in a Magnetoplasma in the Presence of a Cylindrical Plasma Channel

We consider the problem of excitation of electromagnetic field by spatially bounded, arbitrary given sources in a magnetoplasma in the presence of a cylindrical plasma channel aligned with an external magnetic field. We obtain a rigorous solution for the total field comprising both the discrete and continuous parts of the spatial spectrum of excited waves. Expressions for the radiation pattern and total radiation power of given sources are analyzed. For the whistler range, we calculate the radiation power of a ring electric current located in a channel with enhanced plasma density. It is shown that in this range, the presence of such a channel can lead to a significant increase in the radiation power of ring currents as compared with the case where the considered sources are immersed in a uniform background magnetoplasma, regardless of their orientation.     

真空二极管辐射微波的机理分析

在研究真空开关的过程中,发现真空二极管能辐射出宽带微波.这种器件只由带触发装置的阴极和平板阳极组成,不存在金属波纹慢波结构,所以真空二极管的辐射机理与等离子体填充微波器件不同,不能直接套用等离子体填充微波器件的相关理论.本文描述了真空二极管产生辐射的物理过程,建立了真空二极管辐射的数学模型,通过求解波动方程得到产生辐射的色散关系,并绘制出了色散曲线.将理论分析得到的色散曲线与已经测得的微波辐射进行比较,两者能很好地符合.理论分析和实验结果表明,电子束和磁化等离子体的相互作用是真空二极管产生微波辐射的原因.     

Z箍缩Al等离子体X辐射谱线的分离及电子温度的提取

以"强光一号"Z箍缩装置10174发次光谱诊断实验结果为例,描述了一种对Z箍缩等离子体X辐射光谱分离提纯、诊断的方法.对连续辐射谱和特征辐射线谱进行分离,并从连续辐射谱和特征辐射线谱中提取了等离子体电子温度信息.结果显示:等离子体连续谱主要由等离子体中心的高温区(Te=290.7 eV±1.2 eV)和温度较低的壳层区域(Te=95.3 eV±8.3 eV)两部分叠加而成;特征辐射线谱主要反映了等离子体中心的高温区信息,根据非局域热动平衡模型计算提取的电子温度约为299—313 eV,与连续谱诊断结果基本符合.     

Theoretical study of two-element array of equilateral triangular patch microstrip antenna on ferrite substrate

The radiation characteristics of a two-element array of equilateral triangular patch microstrip antenna on a ferrite substrate are studied theoretically by considering the presence of bias magnetic field in the direction of propagation of electromagnetic waves. It is found that the natural modes of propagation in the direction of magnetic field are left- and right-circularly polarized waves and these modes have different propagation constants. In loss-less isotropic warm plasma, this array antenna geometry excites both electromagnetic (EM) and electroacoustic plasma (P) waves in addition to a nonradiating surface wave. In the absence of an external magnetic field, the EM- and P-waves can be decoupled into two independent modes, the electroacoustic mode is longitudinal while the electromagnetic mode is transverse. The far-zone EM-mode and P-mode radiation fields are derived using vector wave function techniques and pattern multiplication approaches. The results are obtained in both plasma medium and free space. Some important antenna parameters such as radiation conductance, directivity and quality factor are plotted for different values of plasma-to-source frequency.     

Radiation characteristics of dipole antenna in a grounded composite magnetized semiconductor plasma

The radiation characteristics of dipole antenna in a grounded nonreciprocal semiconductor plasma are examined using the spectral domain-exponential matrix technique, and such solid plasma is taken as GaAs material. The effects of constitutive and geometrical parameters of the semiconductor plasma on the radiation patterns are demonstrated in detail, and the results show that the far-field behaviour of dipole antenna can be changed by adjusting both the operating frequency and the applied magnetic field, et al.     

Plasma radiation spectra in the presence of static electric and high-frequency radiation fields

Harmonics generation of high-frequency radiation in a plasma embedded in a constant electric field is investigated theoretically. It is shown that the electron directed motion due to the static electric field yields the appearance in the plasma emission spectrum of high-frequency radiation even harmonics. The conditions are established when the even harmonics generation is as effective as that of the odd ones. At variance with the odd harmonics, the even harmonics polarization plane is found to rotate with respect to that of the fundamental field. The basic dependencies concerning the rotation angle and the generation efficiency on the plasma and field parameters are established.Received: 3 May 2004, Published online: 14 September 2004PACS: 52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.) - 52.38.Dx Laser light absorption in plasmas (collisional, parametric, etc.)     

Effect of Longitudinal Magnetic Field on the Spread of Erosion Laser Plasma in Vacuum and Generation of Plasma Magnetic Fields

Dynamics of erosion laser plasma in vacuum and generation of magnetic field by moving plasma (in particular, in the presence of external static magnetic field oriented along the direction of plasma motion) are experimentally studied. Radial confinement of the spread of plasma, a decrease in the electrification of target upon plasma formation, and an increase in the induction of the plasma magnetic field by a factor of 10–15 are revealed at an induction of the external magnetic field of about 0.35 T. Dependences of the induction of the plasma magnetic field on the power density of the laser radiation are determined for the above regimes.     

脉冲等离子体辐射微波机理的初步研究

 在分析了等离子体辐射微波的电流波形和喇叭天线接收到的微波信号波形的基础上,对脉冲等离子体辐射微波的机理进行了理论分析：阴阳极间存在TEM和TM两种模式的波;流经等离子体的电流产生的磁场使等离子体中的波成为慢波;高频场的激励因素有两个,一是作加速运动的电子,二是阳极斑点溅射时阳极电位突降;同一时刻具有不同速度的电子将能量转换给相应相速的波是其产生超宽带辐射的原因。     

Qualitative model of a plasma photoelectric converter

A converter of focused optical radiation into electric current is considered on the basis of the photovoltaic effect in plasmas. The converter model is based on analysis of asymmetric spatial distributions of charge particle number density and ambipolar potential in the photoplasma produced by external optical radiation focused in a heat pipe filled with a mixture of alkali vapor and a heavy inert gas. Energy balance in the plasma photoelectric converter is analyzed. The conditions in which the external radiation energy is effectively absorbed in the converter are indicated. The plasma parameters for which the energy of absorbed optical radiation is mainly spent on sustaining the ambipolar field in the plasma are determined. It is shown that the plasma photoelectric converter makes it possible to attain a high conversion efficiency for focused solar radiation.     

Radiation from a small spheroidal antenna with plasma shell

We construct and study an analytical solution of the boundary-value problem for the radiation field of a small spheroidal antenna located in free space and surrounded by a thin shell of cold homogeneous isotropic plasma. Conditions for a resonant increase in the field in free space as a function of the plasma-shell thickness with the variation in the spheroidal-antenna shape are studied. It is shown that the plasma shell has the largest effect on the radiation field of a strongly prolate spheroidal antenna. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 3, pp. 246–257, March 2006.     

Nonlinear analysis of relativistic harmonic generation by intenselasers in plasmas

A linearly polarized, ultraintense laser field induces transverse plasma currents which are highly relativistic and nonlinear, resulting in the generation of coherent harmonic radiation in the forward direction (i.e., copropagating with the incident laser field). A nonlinear cold fluid model, valid for ultrahigh intensities, is formulated and used to analyze relativistic harmonic generation. The plasma density response is included self-consistently and is shown to significantly reduce the current driving the harmonic radiation. Phase detuning severely limits the growth of the harmonic radiation. The effects of diffraction are considered in the mildly relativistic limit. No third-harmonic signal emerges from a uniform plasma of near-infinite extent. A finite third-harmonic signal requires the use of a semi-infinite or finite slab plasma. For an initially uniform plasma, no second-harmonic radiation is generated. Generation of even harmonics requires transverse gradients in the initial plasma density profile     

Observation of magnetically induced transparency in a classical magnetized plasma

We report the first demonstration of magnetically induced transmission in an opaque magnetized plasma. Magnetically induced transmission in a plasma is a classical analog to the electromagnetically induced transparency in atomic systems. The transmission of radiation through an axially magnetized plasma is obtained by applying an additional one dimensional transverse spatial periodic magnetic field. The transverse-periodic magnetic field uncouples the right-hand electromagnetic wave from interacting with plasma electrons, rendering the plasma band-stop transparent. This provides means to control the extent of absorption of electromagnetic radiation in magnetized plasma.     

Spontaneous emission from a semi-bounded plasma

The electromagnetic radiation field of sources arbitrarily distributed throughout space is found for the case of a semi-bounded plasma when the charged plasma particles are specularly and randomly scattered by the plasma interface. The spectral distributions of the thermal radiation emitted by a nonisothermal semi-bounded plasma into an external medium having a nonzero temperature are calculated.Institute of Theoretical Physics, Ukrainian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, No. 10, pp. 1237–1261, October, 1994.     

Low-Frequency Radiation from an Axisymmetric Current in an Ionospheric Plasma

We consider the radiation of low-frequency electromagnetic waves from an axisymmetric current and propagation of these waves in a homogeneous ionospheric plasma with Hall and Pedersen conductivities. We obtain an analytical expressions for the fields excited by pulsed radial and ring currents. It is shown that electromagnetic radiation propagates as a wave inside a cone along the magnetic field. The field propagation outside the cone is described by a diffusion law. The spatio-temporal characteristics of the signal are determined by the ionospheric conductivities.     

Scattering of polarized electromagnetic radiation by turbulent fluctuations of a high-temperature plasma

An investigation was made of the process of Raman scattering of polarized electromagnetic radiation by turbulent fluctuations in a plasma with a magnetic field. It was found that a turbulent plasma with a magnetic field scatters left- and right-handed circularly polarized radiation in different ways. The possibility is analyzed of using the results obtained in order to provide diagnostics for different plasma characteristics in thermonuclear fusion devices employing magnetic plasma confinement. Institute of Solar and Terrestrial Physics, Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 102–107, April, 1998.     

Kinetic theory of the plasma-dynamical modes and the transport coefficients of a relativistic plasma

The kinetic equation of an inhomogeneous relativistic plasma, consisting of an electron gas and a radiation field, is studied with particular regard to its eigenvalues in the hydrodynamical limit. The treatment is classical for the particles and quantum-mechanical for the field oscillators.After a suitable regularization, the eigenvalues are obtained by a perturbation theory through second order in the strength of the gradients. It is shown that these eigenvalues are in exact correspondence with the macroscopic relativistic plasma-dynamical modes. The important role played by the Vlassov operator in building up the peculiar structure of these modes is underlined. From a comparison of the macroscopic and microscopic eigenvalues we obtain general expressions for the thermal conductivity, the shear viscosity and the bulk viscosity of a relativistic plasma. The contribution of the radiation field to these quantities is a noteworthy feature of these expressions.     

Generation of low-frequency radiation by dense hot plasma under pondermotive action of a short laser pulse

The theory of the generation of low-frequency radiation under the pondermotive action of a femtosecond laser pulse on dense hot plasma is developed. It is shown that, at fairly high plasma temperatures, when electron-electron collisions are rare and the low-frequency field is excited under conditions of the anomalous skin effect, the generation efficiency can be close to maximal. The optimal generation conditions are achieved if the carrier frequency of the laser pulse is close to the plasma frequency and the pulse is tightly focused. Under irradiation by pulses with durations of tens to hundreds femtoseconds, terahertz radiation is generated in a broad angular range.