Session HR1: Basic Phenomena: Waves and Instabilties

Physical Regimes of Electrostatic Wave-Wave nonlinear interactions generated by an Electron Beam Propagation in Background Plasma

The interaction of energetic electron beams with a cold plasma is important for a wide range of applications, such as in the hollow cathode and in electron beam-generated plasmas. A crucial outstanding problem is to determine how quickly the beam heats cold electrons for different experimental conditions as well as the physical processes in the systems relevant for industrial applications, such as plasma processing reactors that use electron beams, or hollow cathodes for electrical propulsions and plasma switches. By performing a large number of high resolution two-dimensional (2D) particle-in-cell (PIC) simulations with analytical theory, we extensively studied the collective processes of a mono-energetic electron beam emitted from a thermionic cathode propagating through a cold plasma. We confirm that an initial two-stream instability between the beam and background cold electrons is saturated by wave trapping. Further evolution then occurs due to strong wave-wave nonlinear processes. We show that the beam-plasma interaction can be classified into four different physical regimes in the parameter space for beam energy to plasma temperature versus beam density to plasma density . We found a new regime, in which a local Langmuir wave packet grows, faster than the ion frequency, is found. In this new regime, electron beam-plasma interaction occurs as a periodic burst (intermittent) process. The beams are strongly scattered, and the Langmuir wave spectrum is significantly broadened, which gives rise to the strong heating of bulk electrons. We propose scaling laws for the boundary of the unstable regimes and methods to suppress the instabilities, which could be used to guide future operations of low-pressure large-scale plasma devices.   

Electron-Field Instability: Excitation of electron plasma waves by an ambipolar electric field

Electric fields are common in plasmas, especially ambipolar fields in quasineutral regions, and affect transport by driving currents and, in some cases, instabilities. The condition for instability in collisionless plasmas is understood via the Penrose criterion, which quantifies the relative drift between different populations of particles that must be present for wave amplification via inverse Landau damping. For example, electric fields drive drifts between electrons and ions that can excite the ion-acoustic instability. Here, we use particle-in-cell simulations and linear stability analysis to show that electric fields can drive a fundamentally different type of kinetic instability, named the electron-field instability. This instability excites electron plasma waves with wavelengths >∼ 30λDe, has a growth rate that is proportional to the electric field strength, and does not depend on the relative drift between electrons and ions, violating the Penrose criterion. Fluctuations formed by the instability oscillate near the electron plasma frequency, further distinguishing it from the ion-acoustic instability, which oscillates near the ion plasma frequency. The ubiquity of electric fields in plasmas suggests that this instability is possible in a host of systems, including low-temperature and space plasmas. In fact, damping from neutral collisions in such systems is often not enough to completely damp the instability, adding to the robustness of the instability across plasma conditions.   

Ponderomotive interactions between a laser pulse and preformed micro-scale plasma

The study of ponderomotive interactions between a laser pulse and plasma are important for many applications. These include stimulated Raman/Brillouin scattering processes and Terahertz (THz)/microwave emission from femtosecond filaments. Our goal is to study the ponderomotive interaction between a propagating laser pulse and a preformed micro-plasma. As the laser propagates across the micro-plasma, ponderomotive forces perturb the electron distribution and generate a current. To model this process, a beam propagation model describing the pulse propagation dynamics is coupled to a kinetic model that describes the response of the plasma via the laser-induced ponderomotive force. The forces acting on the electrons consist of the electrostatic force, ponderomotive and radiation pressure forces. The electron distribution function is determined by solving the nonstationary one-dimensional Boltzmann equation in the Bhatnagar-Gross-Krook (BGK) approximation. The electrostatic field is determined using the Poisson equation. Self-focusing (Kerr effect) and plasma defocusing are accounted for in order to capture the impact of these processes on the forces driving the electron dynamics. Preliminary results will be presented that show how the current resulting from the ponderomotive interaction is impacted by relevant parameters (laser intensity, plasma density, etc.). Implications of these results for emission at THz and microwave wavelengths will be discussed.   

Three-dimensional particle-in-cell simulations of the spoke-like activity in a Penning discharge

Hall plasmas, confined by crossed fields (E×B) with magnetized electrons and unmagnetized ions, exhibit high plasma density at low pressures and have been employed in advanced plasma sources for industry, such as for Hall thrusters, magnetrons and Penning discharges. In certain applications, such as Penning discharges of particular interest in this study, the spoke-like activity refers to the widely reported azimuthally rotating structure with increased plasma density. However, the underlying mechanism responsible for the formation of this structure has not been studied numerically in a 3D kinetic sense. Therefore, in this study, we aim to investigate the spoke-like activity within a Penning discharge by employing the LTP-PIC (Low Temperature Plasma Particle-In-Cell) code. Our simulations reveal that, at steady state, the plasmas behave as a rigid body experiencing low-frequency precession with a tilt angle along the axis, resembling the structure of a spoke. Analysis indicates that the initial displacements of electron and ion centroids are caused by the electron-hose instability. A centroid theory that incorporates both electron and ion motions is proposed to predict the rotation frequency.   

Obliquely Propagating Nonlinear Electrostatic Waves With (r,q) Distributed Electrons in Space Plasmas

Electron velocity distributions (EVDs) in space plasmas, such as solar wind, magnetosheath and Earth’s magnetosphere, are frequently observed with flat tops at the low energies and/or enhanced tails at high energies. Such observed distributions can only be fitted with the two spectral indices generalized (r,q) distribution function and neither Maxwellian nor kappa distributions fit the observed EVDs at low energies as well at high energy part simultaneously. In the limiting cases, r >0, q→∞, r =0, (κ+1) and r =0, q→∞ the (r,q) distribution reduces to Druyvesteyn-Davydov, kappa and Maxwellian distributions, respectively. Thus by employing the (r,q) distribution function for electrons and adopting fully nonlinear Sagdeev potential technique, we derive the Sagdeev potential for electrostatic waves. Then by using the values of the spectral indices r and q which fit the observed distributions, we numerically studied the propagation characteristics of nonlinear structures. We found that propagation characteristics of such nonlinear waves alter significantly from the Maxwellian or kappa distributed plasmas.