Interpretation of harmonic structure in solar type II radio bursts

Spectrographic and partly imaging observations of three Type II solar radio bursts displaying three drifting bands with frequencies related as 1¬2¬3 are discussed. The radio data of two of these events were simultaneousely recorded by the digital radiospectrograph of the Observatory of Solar Radioastronomy in Potsdam-Tremsdorf and the radioheliograph of the Paris-Meudon Observatory in Nançay. The data allow the brightness temperatures of radio emission in the three frequency bands to be determined. The second harmonic is traditionally explained as a result of coalescence of two plasma waves into an electromagnetic wave at twice the plasma frequency. Two nonlinear merging processes—the coalescence of three plasma waves, and of a plasma wave and an electromagnetic wave at twice the plasma frequency—are considered to explain the occurrence of the third harmonic on Type II dynamic spectra. The analysis shows that both processes can fit the observed brightness temperatures of the second and third harmonic. The first process acts preferably at low phase velocities of plasma waves and sharp electron density gradients in the source, and the second process dominates in the case of high plasma wave phase velocities. It is shown that the occurrence of the third harmonic in type II bursts due to nonlinear processes in the coronal plasma indicates not only a powerful event but also some specific conditions in the shock or foreshock region. Finally, we propose a method to distinguish between the two invoked nonlinear processes by a statistical investigation of Type II burst data.     

激光等离子体对反射波频移影响的实验研究

 研究了微波被激光等离子体反射时反射波的频率变化。实验中微波发生器产生的微波入射到激光等离子体并被等离子体反射,反射波的频率由频谱分析仪测量,发现反射波的频率与入射波的频率明显不同,分析了在激光等离子体膨胀和熄灭两种情况下激光功率密度和入射波频率对反射波频移影响的原因。结果表明：在等离子体膨胀和熄灭过程中,反射波频移的最大值随激光功率密度的增加而增加;随入射微波频率的增加而减小。     

Inductive plasma sources for plasma implantation and deposition

External and reentrant radio frequency inductive plasma sources are developed for plasma ion implantation and deposition processes in a 1.8 m³ vacuum vessel. Plasma densities in the range 10¹⁶ -10¹⁷ m^-3 desirable for the above processes. External plasma sources could not yield the required plasma densities because of high particle losses in the transition region between the source and the main vessel. The particle losses are clarified through experiments and analysis, with and without multipole magnetic confinement. Reentrant plasma sources eliminate transmission losses and yield high plasma densities with good spatial uniformity     

双频容性耦合等离子体相分辨发射光谱诊断

采用相分辨发射光谱法, 对双频容性耦合纯Ar和不同含O₂量的Ar-O₂混合气体放电等离子体的鞘层激发模式进行了探究. 在射频耦合电源上极板的鞘层区域处观察到两种电子激发模式: 鞘层扩张引起的电子碰撞激发模式和二次电子引起的电子碰撞激发模式; 并发现这两种激发模式均受到低频射频电源周期的调制. 在纯Ar放电等离子体中, 两种激发模式的激发轮廓相似; 而在Ar-O₂混合气放电等离子体中, 随着含O₂量的增加, 二次电子的激发轮廓变弱. 此外, 利用相分辨发射光谱法对不同含O₂量的Ar-O₂混合气放电下Ar的 750.4 nm谱线的平均低频电源周期轴向分布进行了研究, 得到了距耦合电源上极板约3.8 mm处为双频容性耦合射频等离子体的鞘层边界. 关键词： 双频容性耦合等离子体 等离子体鞘层 发射光谱     

Probe Measurements on Parametric Decay and Density Threshold in a Linear Microwave Discharge

Flat Langmuir probes, a flush mounted probe and an insulated antenna were used to investigate the correlation between the plasma density and the occurrence of radio frequency‐ and X‐band‐noise as a fingerprint for parametric decay processes in a magnetized linear plasma device. The decay products of the driving microwave may influence the entire scenario of electron and ion heating in the plasma.     

Microwave-induced thermal escape in Josephson junctions

We investigate, by experiments and numerical simulations, thermal activation processes of Josephson tunnel junctions in the presence of microwave radiation. When the applied signal resonates with the Josephson plasma frequency oscillations, the switching current may become multivalued in a temperature range far exceeding the classical to quantum crossover temperature. Plots of the switching currents traced as a function of the applied signal frequency show very good agreement with the functional forms expected from Josephson plasma frequency dependencies on the bias current. Throughout, numerical simulations of the corresponding thermally driven classical Josephson junction model show very good agreement with the experimental data.     

Interpretation of harmonic structure in solar type II radio bursts

Spectrographic and partly imaging observations of three Type II solar radio bursts displaying three drifting bands with frequencies related as 1?2?3 are discussed. The radio data of two of these events were simultaneousely recorded by the digital radiospectrograph of the Observatory of Solar Radioastronomy in Potsdam-Tremsdorf and the radioheliograph of the Paris-Meudon Observatory in Nan?ay. The data allow the brightness temperatures of radio emission in the three frequency bands to be determined. The second harmonic is traditionally explained as a result of coalescence of two plasma waves into an electromagnetic wave at twice the plasma frequency. Two nonlinear merging processes—the coalescence of three plasma waves, and of a plasma wave and an electromagnetic wave at twice the plasma frequency—are considered to explain the occurrence of the third harmonic on Type II dynamic spectra. The analysis shows that both processes can fit the observed brightness temperatures of the second and third harmonic. The first process acts preferably at low phase velocities of plasma waves and sharp electron density gradients in the source, and the second process dominates in the case of high plasma wave phase velocities. It is shown that the occurrence of the third harmonic in type II bursts due to nonlinear processes in the coronal plasma indicates not only a powerful event but also some specific conditions in the shock or foreshock region. Finally, we propose a method to distinguish between the two invoked nonlinear processes by a statistical investigation of Type II burst data. Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia; Astrophysical Institute, Potsdam, Germany; Paris-Meudon Observatory, France. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 1, pp. 61–83, January, 1998.     

Plasma Processes Driven by Current Sheets and Their Relevance to the Auroral Plasma

Satellite and rocket observations have revealed a host of auroral plasma processes, including large dc perpendicular electric fields (E?) associated with electrostatic shocks, relatively weak parallel electric fields (E?) associated with double layers, upflowing ions in the form of beams and conics, downflowing and upflowing accelerated electron beams, several wave modes such as the electrostatic ion-cyclotron (EIC), lower hybrid (LH), very low frequency (VLF), extremely low frequency (ELF), and high-frequency waves and associated nonlinear phenomena. Recently, we have attempted to simulate the various processes using a two-dimensional particle-in-cell code in which the plasma is driven by current sheets of a finite thickness. Striking similarities between the observed auroral plasma processes and those seen in the simulations are found. In this paper we give a review of the plasma processes dealing with dc and ac electric fields, formation of ion beams and conics, and electron acceleration. Electrostatic shock-type electric fields (E?e) occur near the current sheet edges. Such fields arise because of the contact between the high-and low-density plasmas inside and outside the sheet, respectively. Double layers having upward electric fields form inside the sheet and they are distinguishable from the large perpendicular electric fields (E?e) only in wide sheets with thicknesses l >> ?i, the ion Larmor radius. Double layers with a reverse polarity form outside the sheet where downward currents flow. The most energetic ions are found to have pitch angles near 90°, implying a large perpendicular acceleration of the ions.     

Plasma-Maser Instability of Bernstein Mode in Presence of Magnetohydrodynamic Turbulence

A theoretical study is made on the amplification mechanism of electrostatic Bernstein mode wave in presence of kinetic Alfven wave turbulence in a magnetized plasma on the basis of plasma-maser interaction. It is shown that a test high frequency electrostativ Bernstein mode wave is unstable in the presence of low frequency kinetic Alfven wave turbulence. The growth rate of the Bernstein wave vanishes only in an unmagnetized plasma. Because of the universal existence of the kinetic Alfven waves in large scale plasmas, the results have potential importance in space and astrophysical radiation processes.     

Angular momentum transport in thin accretion disks and intermittent accretion

The plasma modes, transporting angular momentum in accretion disks, under minimally restrictive conditions when the magnetic energy density is significant relative to the thermal energy density, are shown to be singular if the ideal MHD approximation is adopted. A similarity with the modes producing magnetic reconnection in current carrying plasmas is established. The combined effects of finite plasma temperature, of plasma compressibility, of the gradient of the rotation frequency, and of appropriate transport processes (outside ideal MHD) are involved in the onset of these nonaxisymmetric and locally corotating modes.     

等离子体刻蚀工艺的物理基础

介绍了等离子体刻蚀工艺背景以及有关等离子体刻蚀机理的研究进展，综述了等离子体刻蚀机理的研究方法，着重阐述了电容耦合放电和电感耦合放电等离子体物理特性，特别是双频电容耦合放电等离子体和等离子体鞘层研究中的关键问题。     

Cathode Sheath Instability at Frequencies Near the Ion Plasma Frequency

We consider steady-state and nonstationary processes in a near-cathode region. Equations describing the plasma dynamics near a cathode at frequencies close to the ion plasma frequency are derived, and solutions of these equations for various zones of a discharge gap are found. A piecewise-uniform model of a cathode sheath is developed, which points out the possibility of an instability at a frequency slightly less than the near-cathode ion plasma frequency. The gas pressure effect on the instability threshold with respect to the discharge current is considered. The obtained results are in good agreement with the data of experimental studies of the cathode sheath in a hollow-cathode discharge.     

The influence of electron plasma formation on superbroadening in light filaments

Avalanche ionization limits the maximum field strength in small scale filaments, produced by laser pulses in solids and liquids. The temporal and spatial variations in the index of refraction due to electron plasma formation contribute significantly to the processes of self-phase-modulation. A new interpretation of previous experimental results on frequency superbroadening is presented.     

Sun-Earth connection: Boundary layer waves and auroras

Boundary layers are the sites where energy and momentum are exchanged between two distinct plasmas. Boundary layers occurring in space plasmas can support a wide spectrum of plasma waves spanning a frequency range of a few mHz to 100 kHz and beyond. The main characteristics of the broadband plasma waves (with frequencies >1 Hz) observed in the magnetopause, polar cap, and plasma sheet boundary layers are described. The rapid pitch angle scattering of energetic particles via cyclotron resonant interactions with the waves can provide sufficient precipitated energy flux to the ionosphere to create the diffused auroral oval. The broadband plasma waves may also play an important role in the processes of local heating/acceleration of the boundary layer plasma.     

Thermal plasmas

Although many thermal plasma processes have been developed for industrial applications, the wide acceptance as a manufacturing technology is prevented due to economical and competitive reasons, and/or reproducibility and reliability aspects. This paper is devoted to an assessment of the present knowledge in the following topics: (1) plasma torch and performance of blown arc (dc or ac), transferred arc and radio frequency torches; (2) established industrial applications with special emphasis on cutting, welding, spraying, transferred arc reclamation, reheating and purification, reheating metal melts, smelting reduction, chemical operations, and waste destruction; (3) recent developments in the knowledge of fundamental processes in plasma torches with power sources, cathodes (hot and cold), anodes (static and dynamic behavior), and torch components; (4) modeling-thermodynamic and transport properties, plasma flow with and without the Maxwell's equations; (5) measurement techniques including emission and absorption spectroscopy, laser scattering, enthalpy probes, video cameras, spectral analysis, shadowgraphy, and particle diagnostics either in flight with statistical measurements and those giving characteristics of a single particle upon flattening on a substrate; and (6) plasma-processing development in the presently used industrial processes and also in prospective processes with surface hardening, ultrafine powder production, plasma-assisted CVD, and plasma-fluidized or spouted bed reactors     

Amplitude modulation and demodulation of an electromagnetic wave in magnetised acousto-optic diffusive semiconductor plasmas: Hot carrier effects

In communication processes, amplitude modulation is very helpful to save power using a single band transmission. Using the hydrodynamical model of semiconductor plasma analytical investigations are made for the amplitude modulation as well as demodulation of an electromagnetic wave incorporating carrier heating (CH) effects in acousto-optic magnetised semiconductor plasma. The CH effects add new dimensions in the present analysis. Analysis are made under different wave number regions over a wide range of cyclotron frequencies. It is found that incorporation of CH effects modifies the amplitude modulation and demodulation processes effectively. Numerical estimations are made for III–V semiconductor crystal irradiated by pump wave of frequency 1.6×10¹³ s^?1. Complete absorption of the waves takes place in all the possible wavelength regimes when the cyclotron frequency ω_c becomes nearly equal to ω₀, the pump frequency on neglecting the collision term in modulation/ demodulation indices.     

Parametrically generated spectra and optical breakdown in H2O and NaC1

Picosecond light pulses of high intensity have shown to generate broad light spectra by parametric four photon interaction. The threshold for plasma formation was investigated in order to assess the contribution to the frequency broadening by free electrons in the breakdown and prebreakdown region. It could be confirmed that our previously observed parametric spectra were not effected by prebreakdown processes.     

Interference stabilization of atoms in a strong laser field for obtaining inversion and lasing in the visible and VUV frequency ranges

The interference stabilization of Rydberg atoms in strong laser fields is proposed for producing a plasma channel with the inverse population. Inversion between a group of Rydberg levels and low-lying excited levels and the ground state permits amplification and lasing in the IR, visible, and VUV frequency ranges. The lasing and light amplification processes in the plasma channel are analyzed using rate equations and the efficiency of this method is compared with that in the usual method for high harmonic generation during rescattering of electrons by a parent ion.     

Linear and Nonlinear Wavetransformation and Absorption in the Inhomogeneous Plasma Capacitor

The parametric decay process in inhomogeneous layers existing near the plasma boundaries or in front of antennas and probes in a plasma has been investigated. The linear enhancement of the pump field near ω = ω_p, the threshold fieldstrength, the wavenumber selection rules and the influence of spontaneous low frequency fluctuations are discussed in detail using a one-dimensional model of the inhomogeneous plasma capacitor. According to this model the instabilities appear in the layers with maximum linear transformation and (linear) absorption. In addition, a strong nonlinear part of absorption in the presence of the instability has been observed. The level of the spontaneous low frequency fluctuations influences strongly the spectrum of the parametrically excited ion waves. The experiments show a redistribution of the transferred ion acoustic wave energy over the whole wave continuum up to ω_pi, if a sufficient strong spontaneous fluctuation level exists in the plasma. It is impossible, however, to excite ion acoustic turbulence by the decay of the high frequency pump field under the present conditions. The conditions for the linear field enhancement are disturbed by the action of the ponderomotive forces changing the density profile near the critical point before reaching the strong pump amplitude being necessary for the excitation of a cascade of decay processes.