Characterizing and modeling the statistics associated with the initiation of gas breakdown has proven to be difficult due to a variety of rather unexplored phenomena involved. Experimental conditions for high power microwave window breakdown for pressures on the order of 100 to several 100 torr are complex: there are little to no naturally occurring free electrons in the breakdown region. The initial electron generation rate, from an external source, for example, is time dependent and so is the charge carrier amplification in the increasing radio frequency (RF) field amplitude with a rise time of 50 ns, which can be on the same order as the breakdown delay time. The probability of reaching a critical electron density within a given time period is composed of the statistical waiting time for the appearance of initiating electrons in the high-field region and the build-up of an avalanche with an inherent statistical distribution of the electron number. High power microwave breakdown and its delay time is of critical importance, since it limits the transmission through necessary windows, especially for high power, high altitude, low pressure applications. The delay time distribution of pulsed high power microwave surface flashover has been examined for nitrogen and argon as test gases for pressures ranging from 60 to 400 torr, with and without external UV illumination. A model has been developed for predicting the discharge delay time for these conditions. The results provide indications that field induced electron generation, other than standard field emission, plays a dominant role, which might be valid for other gas discharge types as well. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3534823]
The simplest way to describe a plasma is by following the trajectories of individual particles in given electric and magnetic fields. This single-particle model describes various drift-motions in inhomogeneous, curved or time-dependent fields. Electrons and ions perform gyro-orbits, which can be considered as small diamagnetic current loops, which can be trapped by inhomogeneous magnetic field in “mirror” geometries. The magnetic confinement of plasmas is described for tokamak and stellarator geometries.
The plasma state emerges from ionization of neutral atoms. In thermodynamic equilibrium, free particles attain a Maxwell distribution, atomic states are populated according to a Boltzmann distribution, and the ionization equilibrium is described by the Saha equation. The most important property of a plasma is its quasineutrality. Ideal plasmas show collective behavior in terms of Debye shielding or plasma oscillations. Plasmas are found over ten decades in temperature and 25 decades in plasma density. Non-ideal plasmas can be determined by strong-coupling effects and by quantum effects.
The heliopause is the boundary between the hot heliospheric (solar wind) plasma and the relatively cold interstellar plasma. Pressure balance considerations show that there should be a large (factor of 20 to 50) density increase across the heliopause. Here we report electron density measurements from the Voyager 1 and 2 plasma wave instruments near and beyond the heliopause. The plasma density in the outer heliosphere is typically about 0.002 cm −3 . The first electron density measured by the Voyager 2 plasma wave instrument in the interstellar medium, 0.039 cm −3 ± 15%, was on 30 January 2019 at a heliocentric radial distance of 119.7 au. The density jump, about a factor of 20, confirms that Voyager 2 crossed the heliopause. The new density is very similar to the first density measured in the interstellar medium by the Voyager 1 plasma wave instrument, 0.055 cm −3 , on 23 October 2013 at a radial distance of 122.6 au. These small differences in the densities and radial distances are probably due to the relative locations of the spacecraft in the boundary layer that forms in the interstellar plasma just beyond the heliopause.
The description of the hot gas of electrons and ions forming a plasma involves a number of non-deterministic or stochastic processes that require a statistical description. The plasma constituents have a wide spread of velocities and perform collisions between the charged particles, or with the gas atoms of the parent gas. The behavior of the plasma as a whole can no longer be reduced to the deterministic motion of individual particles in prescribe felds. Rather, the large number of particles introduces uncertainties that force us to describe the plasma by average quantities. For example, the average motion depends on macroscopic quantities like temperature and density gradients, which generate particle fluxes or electric currents. This Section discusses the stochastic motion of particles and introduces simple statistical concepts to describe typical transport processes in gas discharges. To illustrate the concepts, typical applications are given in gas discharges, in ion thrusters design for spacecrafts, or in the heat balance for nuclear fusion.
In the single-particle model (Chap. 3) the motion of the particles was derived from fixed external electric and magnetic fields. This approach is very useful to obtain a first insight into the richness of plasma motion, which results in a host of particle drifts. The major drawback of this model is the neglect of the modification of the fields by the electric currents represented by these drifts. The present chapter on fluid models attempts to overcome this weakness.
A plasma separates itself from metallic or dielectric surfaces by forming a boundary layer, which appears darker than the bulk plasma itself. This is a first hint that the boundary layer is depleted of electrons that are needed to excite the neutral atoms producing the glow of an electric discharge. It was Langmuir who identified these dark spaces as regions that are not electrically neutral but are governed by a net (positive) space charge. The particle motion is determined by physical mechanisms that are different from those discussed for the quasineutral part of the plasma. The interaction of an ion with the electric field from the space charge of all the other ions is a new type of many-body interaction that is characteristic for the collective behavior of a plasma.
Lightnings and technical plasmas are generated by an electric breakdown in a gas. The ignition process leads to a subsequent current flow that generates an electrical discharge. Depending on the power source that feeds the plasma, we distinguish direct current (dc), low-frequency alternating current (ac), and radio-frequency (rf) discharges. This chapter gives a brief introduction into the most common types of discharges and the associated plasma processes with emphasis on the how-questions rather than giving answers to all why-questions.
Non-linear MHD simulations of edge localized modes (ELMs) show features in qualitative agreement with the experimental observations such as the formation and speed of filaments, features in the radial profiles and the fine structure observed in the power deposition profiles at the divertor target. The density perturbation predominantly follows the ballooning mode convection cells leading to density filaments. The temperature perturbation, due to the large parallel conduction, follows the magnetic field perturbation. Simulations of pellets injected in the H-mode pedestal show that the high pressure in the high density plasmoid can become large enough to drive ballooning type modes forming a single helical structure located at the pellet (plasmoid) position.
The analytic and numerical approaches to the investigation of the two-dimensional steady-state plasma flows are analyzed and compared with reference to a plasma accelerator channel in the presence of a longitudinal magnetic field. The present study continues a cycle of research into the plasma flows in the coaxial channels with the traditional azimuthal magnetic field. The additional longitudinal field opens new possibilities for controlling the dynamic processes and achieving the transonic flows. The research is based on the magnetohydrodynamic equations.
