The Laser Plasma: Basic Phenomena and Laws

High power lasers when focused onto matter lead to extremely rapid ionization by direct photoeffect or, depending on wavelength and material, by multiphoton processes. When a sufficient number of free electrons is created the formation of a dense, highly ionized plasma is more efficiently continued by electron–neutrals and electron–ion impact ionization. In view of many important applications the generation of a homogeneous high density and, at the same time, very hot plasma would be most desirable. Unfortunately, at present high power lasers operate in the near infrared domain. As a consequence, direct interaction of the laser beam with matter is possible only below a limiting density, the so-called critical density which, at nonrelativistic intensities, is typically a hundred times lower than solid density. Only when the oscillatory velocity of the electrons becomes relativistic at laser intensities beyond 1018 Wcm−2 direct interaction with higher densities takes place. It is due to this cut-off that the plasma production process becomes a very dynamic interplay between laser beam stopping and plasma expansion and makes the plasmas created by lasers from overdense matter very inhomogeneous and short-living. Within certain limits efficient energy transfer from the laser to overdense plasma regions is made possible by electron thermal conduction. As there are physical limits inherent in this process also energy transfer to more dense matter is accomplished by shock wave heating and UV and X radiation from the moderately dense plasma. The dynamics of plasma formation and heating is best understood on the basis of elementary processes induced in atoms and on the electrons by the laser beam. Hence, first, elements of the motion of a single electron in the electromagnetic field and its collisions with atoms and ions are presented. The charged particles lead to collective fields which in turn act back on the single particles. These processes are described in the simplest way by the conservation equations of charge, momentum, and energy of the two-fluid plasma model. Owing to the high mobility of the electrons and ions under intense laser irradiation such a hydrodynamic description in terms of averaged quantities, density, flow velocity, temperature, averaged electric and magnetic fields, can never be the full truth. On the other hand there is its conceptual simplicity which makes of it a very powerful instrument for describing phenomena, even in regions where its validity is questionable. In the following sections of this chapter the basic concepts of collisional heating and quasineutrality are introduced and the basic building blocks of laser-plasma dynamics, linear plasma