Yu. P. Zakharov

Role of the hall flute instability in the interaction of laser and space plasmas with a magnetic field

The interaction of an expanding laser plasma with a uniform external magnetic field is studied over a wide range of experimental parameters (for a plasma energy of up to 300 J and a magnetic induction of up to 8 kG). By analyzing the data from these and other experiments, as well as the results of simulations with the use of a two-fluid Hall plasma model, it was found for the first time that the flute instability of the plasma boundary plays a decisive role in the process of the plasma cloud expansion. It is shown that, when the ion Larmor radius is sufficiently large, this instability can significantly affect the maximum radius of the diamagnetic cavity of the plasma cloud and the deceleration of its front by the magnetic field. A physical model based on the Hall effect is proposed to explain such influence. The model adequately describes data from one-dimensional simulations, as well as from experiments with quasi-spherical laser plasma clouds. The results obtained can be helpful in interpreting the data from active magnetospheric experiments with barium plasma clouds (such as AMPTE) and analyzing the plasma dynamics in future ICF reactors and propulsion systems with a magnetic field for direct conversion of fusion energy into electric energy.

A STUDY OF THE COLLISIONLESS INTERACTION OF INTERPENETRATING SUPER-ALFVEN PLASMA FLOWS

The authors attempt to obtain plasma parameters as required for effective magnetic laminar mechanism (MLM) interaction, while examining the collisionless interaction of an expanding laser-plasma cloud with a magnetized background for high mach numbers (M /sub A/ greater than or equal to 5) and the interaction parameter greater than 1. The experiments were preformed with the KI-1 apparatus for simulating astrophysical plasma processes. The main components were the 1-kJ CO/sub 2/ laser and a high vacuum chamber of diameter 1.2 m and length 5 m fitted with a pulsed background plasma source. Particular attention was given to eliminating the effects from ordinary Coulomb collisions, and various forms of collisional interaction were examined including Coulomb, screened nuclear repulsion, polarization-type attraction, and charge transfer.

Experimental study of a mini-magnetosphere

Magnetospheres at ion kinetic scales, or mini-magnetospheres, possess unusual features as predicted by numerical simulations. However, there are practically no data on the subject from space observations and the data which are available are far too incomplete. In this paper we describe the results of a laboratory experiment on the interaction of plasma flow with a magnetic dipole with parameters such that the ion inertial length is larger than the size of an observed magnetosphere. A detailed structure of the non-coplanar or out-of-plane component of the magnetic field has been obtained in the meridian plane. The independence of this component on dipole moment reversal, as was reported in a previous work, has been verified. In the tail distinct lobes and a central current sheet have been observed. It was found that lobe regions adjacent to boundary layer are dominated by a non-coplanar component of magnetic field. Tail-ward oriented electric current in the plasma associated with that component appears to be equal to the ion current in the upstream part of magnetosphere and in the tail current sheet implying that electrons are stationary in those regions while the ions flow by. The data obtained strongly support the proposed model of mini-magnetosphere based on two-fluid effects as described by the Hall term.

Laboratory simulation of field aligned currents in an experiment on laser-produced plasma interacting with a magnetic dipole

In an experiment on a magnetic dipole interacting with a laser-produced plasma the generation of an intense field aligned current (FAC) system was observed for the first time in a laboratory. The detailed measurements of the total value and local current density, of the magnetic field at the poles and in the equatorial magnetopause, and particular features of electron motion in the current channels revealed its similarity to the Region-1 current system in the Earth magnetosphere. Such currents were found to exist only if they can close via conductive cover of the dipole. Comparison of conductive and dielectric cases revealed specific magnetic features produced by FAC and their connection with electric potential generated in the equatorial part of the magnetopause. To interpret the data we consider a model of electric potential generation in the boundary layer which agrees with experiment and with measurements of the Earths transpolar potential in the absence of an interplanetary magnetic field as well. The results could be of importance for the investigation of Mercury as a magnetic disturbance due to FAC could be especially large because of the small size of the Hermean magnetosphere.

Experimental study of collisionless super-Alfvénic interaction of interpenetrating plasma flows

An experiment on the interaction between an expanding super-Alfvénic laser-produced plasma flow and a magnetized background plasma under conditions in which the ion gyroradius is comparable with the characteristic scale length of magnetic field displacement is described. The depletion of the background plasma in a substantial volume and the formation of a large-amplitude compression pulse propagating with a super-Alfvénic velocity are revealed. The efficiency of energy conversion into perturbations of the background plasma was found to be 25%. Combined data from magnetic, electric, and plasma measurements indicate that the interaction occurs via the magnetic laminar mechanism.

Laboratory simulation of artificial plasma releases in space

Abstract The properties of laser-produced plasma are similar in many features to the properties of various space and astrophysical plasma releases of explosive nature. It allows to investigate the processes of an interaction of those natural plasmas with background media by the means of laboratory simulation. In the presented paper such simulative experiments with quasi-spherical plasma clouds (from small pellet target) and with plasma streams (from solid ones) are described. This experiments are related to the dynamics of collisionless expansion of exploding space plasmas into surrounding magnetized media at various Alfven-Mach numbers MA. A non-uniform media properties has been simulated also. Results are analysed in connection with dynamics of Supernova remnants in interstellar medium, Barium or others releases in geoplasma and possible disturbances of the latter caused by high-energy explosions.

DIRECT CONVERSION OF THE ENERGY OF LASER AND FUSION PLASMA CLOUDS TO ELECTRICAL ENERGY DURING EXPANSION IN A MAGNETIC FIELD

The paper deals with the physical and electrotechnical principles of the promising method of direct conversion of the kinetic energy of an expanding plasma cloud to electrical energy by inductive generation of currents in short–circuited load coils that enclose the plasma and are oriented across the external magnetic field. An analysis of plasma deceleration by a magnetic field and transfer of plasma energy to an inductive load gave a solution of the problem in general form and the dimensionless parameters of the problem that determine the deceleration radius, the coil current, and the theoretical conversion efficiency. The role of the basic physical effects, including parasitic ones (plasma instabilities and Joule heating), influencing the real efficiency is assessed. A comparison of the results with data of experiments with laser–produced plasma clouds on a KI–1 facility and with available numerical results shows that in the optimized version of the method for conversion of inertial confinement fusion energy, a 30% efficiency can be achieved.

Laboratory experiment on region-1 field-aligned current and its origin in the low-latitude boundary layer

In previous experiments by the authors on a magnetic dipole interacting with a laser-produced plasma the generation of an intense field-aligned current (FAC) system on terrella poles was observed. In this paper the question of the origin of these currents in a low-latitude boundary layer of magnetosphere is investigated. Experimental evidence of such a link was obtained by measurements of the magnetic field generated by tangential drag and sheared stress. This specific azimuthal field was found to have quadruple symmetry and local maxima inside the magnetosphere adjacent to the boundary layer. Cases of metallic and dielectric dipole covers modeling good conductive and non-conductive ionosphere revealed that the presence or absence of FACs results in different amplitudes and spatial structures of the sheared field. The current associated with the azimuthal field flows upward at the dawnside, and toward the equator plane at the duskside. It was found to coincide by direction and to correspond by amplitude to a total cross-polar current measured independently. The results suggest that compressional and Alfven waves are responsible for FAC generation. The study is most relevant to FAC generation in the magnetospheres of Earth and Mercury following pressure jumps in solar wind.

Laser-produced plasmas interaction with high pulsed magnetic field

This work presents results of two experiments accomplished at the Institute of Plasma Physics and Laser Microfusion (IPPLM) in Warsaw: (1) investigations of the influence of magnetic field on a laser-produced plasma in the presence or absence of the background plasma, (2) investigations of dynamics of laser-produced plasma in a strong magnetic field. The aim of experiment 1 (performed in collaboration with the Institute of Laser Physics (ILP) RAS, Novosibirsk, Russia) was laboratory simulation of depolarization and deflection of plasma streams drifting across magnetic field in geoplasma background. An Nd:glass laser (5 ns, 2 J) was used to produce the plasma inside Helmholtz coils . The diagnostics for studying the interaction processes were: ion collectors, Langmuir and magnetic probes and an image converter camera. We present a comparison of the effects investigated in that experiment with some phenomena occurring in the geoplasma. In experiment 2 the plasma was produced by means of a Nd:glass laser (1 ns, 10 J) focused on a solid target located in a single-turn coil . Plasma expansion was investigated with an automated interferometer along the magnetic field lines and perpendicular to these lines. From the interferograms, it has been revealed that the plasma stream has a clear asymmetry for caused by an unmagnetized ion Rayleigh-Taylor instability.

Laboratory model of magnetosphere created by strong plasma perturbation with frozen-in magnetic field

Transient interactions of a magnetic dipole with plasma flow carrying a southward magnetic field has been studied in laboratory experiments. Flow with a transverse frozen-in field was generated by means of laser-produced plasma cross-field expansion into background plasma, which filled a vacuum chamber, along with the external application of a magnetic field prior to interaction. Probe measurements showed that at the plasma parameters realized, effective collisionless Larmor coupling takes place resulting in the formation of strong compressive perturbations, which propagate in the background with super-Alfvenic velocity and generate a frame-transverse electric field comparable in value to the expected induction one in the laboratory. Compression pulses with a southward field interact with a dipole and create a well-defined magnetosphere after a short propagation. Comparison of the magnetospheres created by laser-produced plasma expanding in a vacuum field and in a magnetized background revealed fundamental differences in the structure and behaviour of the electric potential in plasma. In the presence of a frozen-in southward field direct «sub-solar» penetration of outside electric potential deep inside of the magnetosphere was observed to take place, with a velocity close to the upstream Alfven speed.