ASTROPHYSICAL AND LABORATORY EXPERIMENTS AND THEORIES ON HIGH-ENERGY PLASMAS

The effort to produce plasmas that can approach the conditions where ignition by fusion reactions can take place has generated a series of new experiments and theories on high energy plasmas whose results are relevant to the understanding of important phenomena in astrophysics. In particular, magnetic reconnection processes that occur in collisionless regimes where the effects of finite electrical resistivity are no longer significant are observed in magnetically confined plasmas where the peak electron temperatures approach or exceed 10 keV. In addition, means have been devised to control the onset of reconnection, for instance by injecting or producing high energy particle populations in regions of the plasma column where the factor which drives reconnection - the pressure gradient - resides. The relationship of these results to space physics regimes such as those characteristic of the Solar Corona or of planetary magnetospheres is discussed. Another important process is that of the so-called anomalous viscosity: the rate of angular momentum transfer which is higher than that expected from the effects of (discrete) ion-ion collisions. To explain the observation of accretion on compact objects (e.g. neutron stars or black holes) it is, in fact, necessary to assume an effective viscosity with a rather specific spatial profile. Toroidal laboratory experiments where plasma rotation is induced by the injection of energetic neutral beams show definite evidence of anomalous viscosity which theoretical models can explain. As for the spatial profile, the support laboratory experiments have provided to the so-called ''Principle of Profile Consistency'' of the electron temperature, that translates into a specific class of relevant transport coefficients, can be considered as a starting basis to deal with the problem of the viscosity characterizing accretion disks. Finally, the scattering of high energy particle populations produced within thermal plasmas by collective modes such as lower hybrid modes (in the so-called slide-away regime), magnetosonic and whistler modes (as observed in deuterium-tritium plasmas where 3.5 MeV alpha-particles are produced) and bursting processes associated, for instance, with the so-called ''fishbone'' modes are described. The applicability is pointed out to such diverse issues in space physics as the observation of ion-conics in the Earth's magnetosphere and the process by which the transverse energy level of electrons emitting synchrotron radiation are repopulated.