arXiv subjects

Michael A. Raadu

Publications and source records attributed to Michael A. Raadu.

4 recordsLinked to original sources

arXiv2004

Potential of a moving test charge in a dusty plasma in the presence of grain size distribution and grain charging dynamics

It is well known that the form of grain size distribution strongly influences the linear dielectric response of a dusty plasma. In previous results [IEEE Trans. Plasma Sci. 29, 182 (2001)], it was shown that for a class of size distributions, there is an equivalence to a Lorentzian distribution of mono-sized particles. The electrostatic response to a slowly moving test charge, using a second order approximation can then be found [Phys. Lett. A 305, 79 (2002)]. It is also well known [Phys. Plasmas 10, 3484 (2003)] that the dynamical charging of grains in a dusty plasma enhances the shielding of a test charge. It seems natural at this stage to seek the combined effects of grain size distribution and grain charging dynamics to a test charge moving through the dusty plasma. Here we consider the effects of both grain size distribution and dynamical grain charging to a test charge moving slowly in a dusty plasma by expressing the plasma dielectric response as a function of both grain size distribution and grain charging dynamics. Both analytical as well as the numerical results are presented. It is interesting to note that the previous results can be retrieved by choosing appropriate values for different parameters. This kind of study is relevant for both laboratory and space plasmas.

physics.plasm-ph↗

arXiv2004

Electromagnetic radiation from double layers

The background motivation, and some preliminary results, are reported for a recently begun investigation of a potentially important mechanism for electromagnetic radiation from space, Double Layer Radiation (DL-radiation). This type of radiation is proposed to come from DL-associated, spatially localized high frequency (hf) spikes that are driven by the electron beam on the high-potential side of the double layer. It is known, but only qualitatively, from laboratory experiments that double layers radiate in the electromagnetic spectrum. In our experiment the spectrum has high amplitude close to the plasma frequency, several hundred MHz. No clear theoretical model exists today. The quantitative evaluation is complicated because measurements are made in the near field of the radiating structure, and in the vicinity of conducting laboratory hardware that distorts the field. Because the localized electrostatic wavelengths (approximately 1 cm) can be relatively small compared to the emitted electromagnetic wavelengths, the situation is further complicated. We discuss the mutual influence between the ion density profile and hf-spike formation, and propose that some kind of self-organization of the density profile through the ponderomotive force of the hf spike might be operating. First results regarding the electromagnetic radiation are reported: the frequency, the time variation of the amplitude, and the spatial distribution in the discharge vessel.

physics.plasm-ph↗

arXiv2004

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.

physics.plasm-ph↗

arXiv2004

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.

physics.plasm-ph↗