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.     

Energetic-ion excited internal kink modes with weak magnetic shear in q0> 1 tokamak plasmas

The energetic particle driven internal kink mode is investigated in this paper for q0 1 tokamak plasma with weak magnetic shear. With the effect of energetic particles, the m/n = 1/1 internal mode structure in tokamak plasma does not appear as a rigid step-function when safety factor passes through q = 1 rational surface. It is found that even when the rational surface is removed, the mode may be still unstable under the low magnetic shear condition if the energetic particle drive is strong enough; with the low shear region of safety factor profile widening, the mode becomes more unstable with its growth-rate increasing. Furthermore, we find that the existence of the q = 1 rational surface does not have a significant effect on the stability of the plasma if energetic particles are present, which is very different from the scenarios of the ideal-MHD modes.     

Generalized model for the nonlinear dynamics of plasma waves driven unstable by energetic ions near the stability threshold

In this paper, an attempt to develop the original single mode theory describing the nonlinear dynamics of a linearly unstable plasma wave, excited by the resonant interaction with energetic ions near the stability threshold to the case of n interacting plasma modes has been made. The effects of an energetic ion source and classical collisional processes represented by the Krook, diffusion and dynamical friction (drag) collision operators are included in the model. For numerical purposes, the problem has been reduced to ten nonlinearly coupled integro-differential equations. In comparison to the previous papers, the system revealed similar (the steady-state, oscillation, and blow-up solutions), as well as quite new types of the amplitudes behaviour, i.e. different levels of competition between the modes.     

Electromagnetic Forces on Plasma Particles in the Presence of Resonant and Nonresonant Plasma Turbulence

Nonlinear forces on plasma particles in the presence of a test nonresonant wave and resonant plasma wave turbulence are calculated. The important feature of the considered nonlinear effect is that the forces due to the nonresonant test wave act on the plasma particles in the absence of linear and nonlinear resonances between the wave and the particles. Although in a closed plasma-wave system the process is balanced by the quasilinear interaction between the plasma resonant turbulence and plasma particles (leading to nonstationarity and inhomogeneity of the system), in open systems the effect can be significant.     

Hot Electron Ring Formation in ECR Heated Plasma

Energetic particles can be .generated by electron cyclotron resonant heating (ECRH) in tokamak plasma, forming electron ring. The bounce frequency is taken into account in resonance condition. The electron energy is limited by detaching of the adjacent overlapped islands. An analytical criterion for transition is obtained. The critical potential obtained is in good agreement with numerical calculation. Experiment for observing the trapped energetic electrons is suggested.     

Observation of pulsed fast electron precipitations and the cyclotron generation mechanism of burst activity in a decaying ECR discharge plasma

We have detected and investigated quasi-periodic series of pulsed energetic electron precipitations in the decaying plasma of a pulsed ECR discharge in a mirror axisymmetric magnetic trap. The observed particle ejections from the trap are interpreted as the result of resonant interaction between energetic electrons and a slow extraordinary wave propagating in the rarefied plasma across the external magnetic field. We have been able to explain the generation mechanism of the sequences of pulsed precipitations at the nonlinear instability growth phase in terms of a cyclotron maser model in which the instability threshold is exceeded through a reduction in electromagnetic energy losses characteristic of the plasma decay.     

聚变等离子体中高能量粒子驱动的鱼骨模不稳定性研究

在考虑高能量粒子密度的空间分布及箍缩角分布的条件下,建立了研究鱼骨模的色散关系,并作了数值研究。结果表明：当高能量粒子的密度高于某个阈值时,鱼骨模会在高能量粒子密度梯度最大处被激发,其频率与高能量粒子的环向进动频率一致。在高β_h 区间,高能量勉强通行粒子将驱动鱼骨模进入第二稳定区。高能量捕获粒子能激发非共振鱼骨模,与勉强通行粒子激发的鱼骨模类似,在高β_h 区存在第二稳定区。     

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A general dispersion relation for fishbone mode is obtained when spatial density profile and pitch angle profile are taken into account. The fishbone modes can be driven into the second stable regime by barely passing energetic particles in high β_h region. The nonresonant fishbone mode can be induced by trapped energetic particles. Similar to the resonant fishbone modes, the nonresonant fishbone modes can also be driven into the second stable regime by trapped energetic particles and the real frequency linearly increases with increasing β_h.     

Simulation of laser–plasma interactions and fast-electron transport in inhomogeneous plasma

A new framework is introduced for kinetic simulation of laser–plasma interactions in an inhomogeneous plasma motivated by the goal of performing integrated kinetic simulations of fast-ignition laser fusion. The algorithm addresses the propagation and absorption of an intense electromagnetic wave in an ionized plasma leading to the generation and transport of an energetic electron component. The energetic electrons propagate farther into the plasma to much higher densities where Coulomb collisions become important. The high-density plasma supports an energetic electron current, return currents, self-consistent electric fields associated with maintaining quasi-neutrality, and self-consistent magnetic fields due to the currents. Collisions of the electrons and ions are calculated accurately to track the energetic electrons and model their interactions with the background plasma. Up to a density well above critical density, where the laser electromagnetic field is evanescent, Maxwell’s equations are solved with a conventional particle-based, finite-difference scheme. In the higher-density plasma, Maxwell’s equations are solved using an Ohm’s law neglecting the inertia of the background electrons with the option of omitting the displacement current in Ampere’s law. Particle equations of motion with binary collisions are solved for all electrons and ions throughout the system using weighted particles to resolve the density gradient efficiently. The algorithm is analyzed and demonstrated in simulation examples. The simulation scheme introduced here achieves significantly improved efficiencies.     

Langmuir probe study in the nonresonant current drive regime of helicon discharge

Characterization of the current drive regime is done for helicon wave-generated plasma in a torus, at a very high operating frequency. A radiofrequency-compensated Langmuir probe is designed and used for the measurement of plasma parameters along with the electron energy distributions in radial scans of the plasma. The electron energy distribution patterns obtained in the operational regime suggest that Landau damping cannot be responsible for the efficient helicon discharge in the present study. A typical peaked radial density profile, high plasma temperature and absence of an appreciable amount of energetic electrons for resonant wave-particle interactions, suggest that the chosen operational regime is suitable for the study of nonresonant current drive by helicon wave. Successful and significant current drive achieved in our device clearly demonstrates the capability of nonresonant current drive by helicon waves in the present operational regime.      

Alfvén Waves in Non-Stationary and Non-Uniform Media: An Exactly Solvable Model

The modulation of Alfvén waves interacting with a non-uniform and non-stationary plasma is considered. The waveforms are allowed to change rapidly. We examine our phenomena by means of exact analytical solutions of the MHD equations in the presence of large amplitude disturbances of the magnetic field and plasma density. In contrast to the WKB approach, we do not have to use limiting assumptions regarding the variations of the background medium. We show that the large amplitude time and space disturbances lead to a new cut-off frequency for Alfvén wave propagation. A rapid reshaping of the Alfvén waveform can also obstruct the resonant interactions between the waves and the plasma particles.     

Thermonuclear Instabilities and Fueling in a Tokamak Reactor

The problem of fueling is of primary importance in the conceptual design of a fusion reactor. We consider a possible mechanism of supplying fresh fuel from a cold-plasma layer at the surface of the plasma. The existence of an energetic component of ions, viz, the alpha particles, may excite unstable collective oscillations of the plasma (called the thermonuclear instabilities). Such instabilities could give rise to microscopic processes which, in principle, would allow influx of fresh fuel while helping efflux of reaction products. A three-regime model is used to understand the nature of such fueling mechanisms and the possibility of using a cold-plasma layer as a surface fueling source.     

Propagation instabilities of high-intensity laser-produced electron beams

Measurements of energetic electron beams generated from ultrahigh intensity laser interactions (I>10(19) W/cm(2)) with dense plasmas are discussed. These interactions have been shown to produce very directional beams, although with a broad energy spectrum. In the regime where the beam density approaches the density of the background plasma, we show that these beams are unstable to filamentation and "hosing" instabilities. Particle-in-cell simulations also indicate the development of such instabilities. This is a regime of particular interest for inertial confinement fusion applications of these beams (i.e., "fast ignition").     

Statistical simulation of kinetic processes in a low-temperature molecular gas plasma

Despite the absence of a single unified theory of gas discharge, it is possible to describe the kinetics of particle interaction in a low-temperature plasma by statistical simulation using the Monte-Carlo method. This present study considers the interactions between atoms, molecules, and radicals in a gas discharge in a mixture of oxygen and hydrogen. Certain principles are clarified governing the behavior of unstable short-lived particles and their role in molecular-compound formation, which cannot be obtained by other methods.     

Zero-point oscillations and radiative-resonant interactions

We discuss new results describing the relationship between quantum radiation caused by zero-point oscillations and radiative resonant wave-particle interactions. We show that the formation of a power-law spectrum of fast particles produced by radiative resonant interactions [V.N. Tsytovich, Phys. Rep. 178 (1989) 261; Physica 210 (1981) 136; Physica Scripta 52 (1982) 54] is related to the interaction of zero-point oscillations with resonant particle acceleration. Possible experiments to measure the electromagnetic radiation produced by this process are suggested.     

Modulational instability of relativistic ion-acoustic waves in aplasma with trapped electrons

The association between the modified Korteweg-de Vries solitary wave and the modulationally unstable envelope solitary wave in a weakly relativistic unmagnetized plasma with trapped electrons is discussed. The effect of trapped electrons modifies the nonlinearity of the nonlinear Schrodinger equation and gives rise to the propagation of the modulationally unstable ion-acoustic solitary wave. The amplitude of the envelope solitary wave increases while the number of trapped electrons decreases. The velocity of the solitary wave decreases with increasing ionic temperature and increasing particle velocities. The ion oscillation mode, which satisfies the nonlinear dispersion relation, is also derived. The theory is applied to explain space observations of the solar energetic flows in interplanetary space and of the energetic particle events in the Earth's magnetosphere     

高能飞行粒子对托卡马克中内扭曲模的稳定效应

用普遍能量原理分析了托卡马克中具有各向异性高能飞行粒子成分等离子体对m=1,n=1内扭曲模的稳定性,发现刚刚飞行的高能粒子对此模能提供稳定作用,并比较了高能俘获粒子和高能飞行粒子对内扭曲模稳定作用的异同及其物理图象。 关键词：     

Nonlinear Theory of the Instability of a Modulated Electron Beam of Low Density in a Plasma IV. Excitation of Nonresonant Waves

A system of nonlinear equations derived in a previous paper which describes the evolution of nonresonant waves in beam-plasma systems is solved numerically. It is given a physical interpretation of essential features of the nonresonant beam-plasma instability. The significant influence of higher harmonics on the evolution of an electron beam in the plasma which is modulated on a nonresonant unstable frequency, is investigated. The interaction of unstable nonresonant and resonant waves in beam-plasma systems is examined. It is shown that the evolution of resonant waves can be essentially influenced by an initial modulation of the beam on an unstable non-resonant frequency. The role of an initial velocity modulation of the beam for the influence of the wave spectrum is demonstrated.     

基于试验粒子模拟的电离层人工调制激发的极低频和甚低频波对磁层高能电子的散射效应

电离层调制加热能够有效激发极低频和甚低频(ELF/VLF)波,其中向上传播进入磁层的ELF/VLF波能够与高能电子发生共振相互作用,具有人工沉降高能电子、消除辐射带等潜在实际用途.本文综合运用射线追踪和试验粒子方法模拟电离层人工激发的单频ELF/VLF波在电离层和磁层的传播,以及在外辐射带层与高能电子的共振相互作用过程,通过投掷角和能量散射系数评估人工ELF/VLF波对磁层高能电子的共振散射效应.研究表明,电离层人工ELF/VLF波传播到磁层后呈现高倾斜性,传播所能跨域的空间范围主要取决于加热的纬度位置和调制频率.在内辐射带,与～100 keV到几个MeV高能电子发生一阶共振相互作用的为10 kHz的VLF波段;在外辐射带,为几百Hz到1 kHz的ELF波段.对于L=4.5的外辐射带,试验粒子模拟结果显示,单个粒子在人工ELF波作用下投掷角和能量(α,E)的改变具有随机性,而所有试验粒子平均化的?α2和?E2随时间呈现出近似线性的增大,说明波粒共振散射过程体现出整体性.基于试验粒子模拟得到的共振散射系数表明,幅度为10 pT的人工ELF波可在外辐射带的磁赤道局地对1 MeV电子产生较强的投掷角散射效应,进而影响高能电子的损失、沉降等动力学过程.当人工ELF/VLF波在传播过程中变得高度倾斜,不仅最基本的一阶共振十分重要,高阶共振散射也具有较大效应.这些定量分析结果表明,通过电离层加热激发人工ELF/VLF哨声波来沉降、消除辐射带高能电子具有可行性.