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F. Cap University of Innsbruck, Innsbruck, Austria

ABSTRACT After a presentation of the concept "plasma" its physical properties and characteristic quantities are discussed. Typical plasmas like the intergalactic plasma, the Solar corona, a Q-machine plasma, the stellar interior ect are considered. The theories modelling different plasma states are presented and waves and instabilities in plasmas are discussed. The different types of instabilities like macroscopic, microscopic, convective, absolute instabilities are presented. Finally, this lecture mentions plasma applications in physics and technology.

F. Cap (ed.), Waves and Instabilities in Plasmas © Springer-Verlag Wien 1994

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F. Cap

1. INTRODUCTION

"Plasma" is a state of matter in which electromagnetic interactions between charged particles determine the material properties. A plasma may be a solid state plasma in which electrons and positively charged holes move around in the ion lattice, a strong electrolyte exhibits typical plasma properties and fully or partially ionized gases behave as plasmas. Originally plasmas have been defined to be quasi-neutral, that means that the number of negatively and positively charged particles per unit of volume is the same. Since recent years one speaks, however, also of charge excess plasmas. A little later on we will discuss the physical conditions determining if a lump of matter can be regarded as a plasma or not. Waves are perturbations of physical quantities like pressure, temperature or electromagnetic fields, which are standing or propagating. Normally waves are dissipative, that means that their energy is disspated into heat and their amplit.ude decreases during propagation. As in optics, the propagation speed of waves depends on the wave-frequency. The mathematical relation is given by a socalled dispersion relation. If the amplitude of a wave increases we speak of a growing wave or of an instability. Waves propagate in a medium due to the interaction between the particles of which the medium consists. In a fluid consisting of electrically not charged particles the interactions are mediated by the collisions between the particles. In a plasma containing charged particles, electromagnetic forces and collisions give rise to interactions. A plasma can be homogeneous or more often inhomogeneous, in which case its physical properties are functions of the spatial coordinates, it is often anisotropic, which means that the plasma properties depend on a specified direction defined by an external magnetic field. Plasmas are very often dispersive and dissipative so that the wave propagation velocity depends on the wave frequency and that the wave energy may be converted into heat. Plasmas are electrically and heat conducting and may be viscous, so the mechanical flow energy is dissipated into heat. Interaction between charged particles and therefore collective effects in an ensemble of particles can occur only if the average interparticle distance 1 = n- 113 is smaller than the effective range of the force interacting