The penetration of plasma clouds across magnetic boundaries : the role of high frequency oscillations

Experiments are reported where a collisionfree plasma cloud penetrates a magnetic barrier by self-polarization. We here focus on the resulting anomalous magnetic field diffusion into the plasma cloud, two orders of magnitude faster than classical, which is one important aspect of the plasma cloud penetration mechanism. Without such fast magnetic diffusion, clouds with kinetic beta below unity would not be able to penetrate magnetic barriers at all. Tailor-made diagnostics has been used for measurements in the parameter range with the kinetic beta ? 0.5 to 10, and with normalized width w/r(gi) of the order of unity. Experimental data on hf fluctuations in density and in electric field has been combined to yield the effective anomalous transverse resistivity eta(EFF). It is concluded that they are both dominated by highly nonlinear oscillations in the lower hybrid range, driven by a strong diamagnetic current loop that is set up in the plasma in the penetration process. The anomalous magnetic diffusion rate, calculated from the resistivity eta(EFF), is consistent with single-shot multi-probe array measurements of the diamagnetic cavity and the associated quasi-dc electric structure. An interpretation of the instability measurements in terms of the resistive term in the generalized (low frequency) Ohm's law is given.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France

Conditions for plasmoid penetration across magnetic barriers

The penetration of plasma clouds, or plasmoids, across abrupt magnetic barriers (of the scale less than a few ion gyro radii, using the plasmoid directed velocity) is studied. The insight gained earlier, from experimental and computer simulation investigations of a case study, is generalised into other parameter regimes. It is concluded for what parameters a plasmoid should be expected to penetrate the magnetic barrier through self-polarization, penetrate through magnetic expulsion, or be rejected from the barrier. The scaling parameters are n(e), v(0), B(perp), m(i), T(i), and the width w of the plasmoid. The scaling is based on a model for strongly driven, nonlinear magnetic field diffusion into a plasma, which is a generalization of the laboratory findings. The results are applied to experiments earlier reported in the literature, and also to the proposed application of impulsive penetration of plasmoids from the solar wind into the Earth's magnetosphere.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France

Single-shot 3D imaging of hydroxyl radicals in the vicinity of a gliding arc discharge

Chemical processing by plasma is utilized in many applications. Plasma-related studies, however, are challenging to carry out due to plasmas' transient and unpredictable behavior, excessive luminosity emission, 3D complexity and aggressive chemistry and physiochemical interactions that are easily affected by external probing. Laser-induced fluorescence is a robust technique for non-intrusive investigations of plasma-produced species. The hydroxyl radical (OH) is an interesting molecule to target, as it is easily produced by plasmas in humid air. In this letter, we present 3D distributions of ground state OH radicals in the vicinity of a glow-type gliding arc plasma. Such radical distributions, with minimal plasma emission, are captured instantaneously in one single camera acquisition by combining structured laser illumination and a lock-in based imaging analysis method called FRAME. The orientation of the plasma discharge can be reconstructed from the 3D data matrix, which can then be used to calculate 2D distributions of ground state OH radicals in a plane perpendicular to the orientation of the plasma channel. Our results indicate that OH distributions around a gliding arc are strongly affected by gas dynamics. We believe that the ability to instantaneously capture 3D transient molecular distributions in a plasma discharge, with minimal plasma emission interference, will have a strong impact on the plasma community for in-situ investigations of plasma-induced chemistry and physics

Interaction Between Solid Copper Jets and Powerful Electrical Current Pulses

The interaction between a solid copper jet and an electric current pulse is studied. Coppe

Dynamics of the Current-Voltage Characteristic and the Potential in a Triple Plasma Machine

A model for the static current voltage characteristic of a thin collisionless plasma inwich an electrical double layer is present is used for simulating the behaviour of anelectrical circuit in wich the plasma is a component. The numerical results thusobtained are compared to experimental results and found to be quite accurate forlow frequencies and amplitudes. The discrepancy between simulations andexperiments are found to be caused mainly by effects of hysteresis in the plasma.The hysteresis is then shown to be caused by 'slow ions'. The hysteresis is dividedinto three typical cases and a method to predict wich case to expect for a certaincombination of applied potential and frequeny is presented. The formation of thedouble layer in time and space is also studied by means of Langmuir probe measurements. Comments are also made concerning the internal plasma potential for different frequencies of the applied potential

Performance Evaluation of KauNet in Physical and Virtual Emulation Environments

Evaluation of applications and protocols in the context of computer networking is often necessary to determine the efficiency and level of service they can provide. In practical testing, three different options are available for the evaluation; using a physical network as a testbed, using an emulator to simplify the infrastructure, or using a simulator to remove reliance on infrastructure entirely. As a real network is costly and difficult or even impossible to create for every scenario, emulation and simulation is often used to approximate the behavior of a network with considerably less resources required. However, while a simulator is limited only by the time required to perform the simulation, an emulator is also limited by the hardware and software used. It is therefore important to evaluate the performance of the emulator itself, to determine its ability to emulate the desired network topologies. The focus of this document is the KauNet emulator, an extension of Dummynet that adds several new features, primarily deterministic emulation of various network characteristics through the use of pre-generated patterns. A series of tests were per- formed using a testbed with KauNet in both physical and virtual environments, as well as a hybrid environment with both physical and virtual machines. While virtualization greatly increases the flexbility and utilization of resources compared to a pure physical setup, it may also reduce the overall performance and accuracy of the emulation. From the results achieved, KauNet performs well in a physical environment, with a high degree of accuracy even at high traffic loads. Virtualization on the other hand, clearly introduces several issues with both processing and packet loss that may make it undesirable for use in experiments, although it may still be sufficient for scenarios where the requirements for accuracy are lower. The hybrid environment represents a compromise, with both performance and flexibility midway between the physical and fully virtualized testbed.

Laminar burning velocity of lean methane/air flames under pulsed microwave irradiation

Laminar burning velocity of lean methane/air flames exposed to pulsed microwave irradiation is determined experimentally as part of an effort to accurately quantify the enhancement resulting from exposure of the flame to pulsed microwaves. The experimental setup consists of a heat flux burner mounted in a microwave cavity, where the microwave has an average power of up to 250 W at an E-field in the range of 350–380 kV/m. Laminar burning velocities for the investigated methane/air flames increase from 1.8 to 12.7% when exposed to microwaves. The magnitude of the enhancement is dependent on pulse sequence (duration and frequency) and the strength of the electric field. From the investigated pulse sequences, and at a constant E-field and average power, the largest effect on the flame is obtained for the longest pulse, namely 50 μs. The results presented in this work are, to the knowledge of the authors, the first direct determination of laminar burning velocity on a laminar stretch-free flame exposed to pulsed microwaves

Numerical simulation of microwave-enhanced methane-air flames I : Modeling and one-dimensional premixed laminar flames

This study considers the numerical modeling of sub-breakdown microwave-enhanced combustion in one-dimensional laminar methane-air flames over a range of stoichiometries at standard temperature and pressure. The effect of uniform microwave fields on the laminar flame speed, post-flame temperature and composition is of primary interest here. A model for the non-thermal plasma generated by the microwave field in such flames is constructed based on fitting of electron properties and electron impact reaction rates to pre-computed solutions of the Boltzmann equation, combined with ambipolar diffusion transport for charged species. This model is used with a selection of existing and novel methane-air kinetics schemes to compute laminar flame profiles at a range of microwave field strengths and varying stoichiometry. The trends for laminar flame strength and temperature, peak- and outlet species fractions are compared over this parameter space. While all schemes demonstrate increasing flame speed and temperature with higher field strength and richer flames, significant variations in the form and magnitude of this increase are observed for the various kinetics schemes considered. The laminar flame-speed increase at sub-breakdown electric field strengths is found to be due primarily to electron heating of the gases in both the pre-heat region and across the flame. Analysis of the electron energy transfer shows that the majority of the energy at moderate field strengths is transferred via excitement of vibrational states of nitrogen, followed by water and carbon monoxide. In the pre-heat regime, vibrational excitement of methane is also seen to play an important role for fuel-rich flames. Particular attention is paid to the modification of the flame speed due to the relaxation time of the vibrational N2 states, which is demonstrated to result in a moderate reduction of the flame speed for lean and stoichiometric flames. A driving concern here is the development of models suitable for use within large-scale three-dimensional numerical simulations and the knowledge gained in comparing the existing schemes is used to derive a range of simplified mechanisms more suited for this purpose. The ability of these reduced models to capture the flame enhancement is demonstrated over a range of flame stoichiometries and sub-breakdown field strengths