Non-linear Interactions of Collective Oscillations and Waves with Particle Fluctuations
All non-linear interactions in a plasma occur while there are also present in the plasma natural particle fluctuations and the field produced by these fluctuations. One cannot avoid the non-linear interactions of the turbulent and regular wave fields with the fields of these fluctuations and with the fluctuations in the index of refraction produced by the particle fluctuations. To complete the discussion of the non-linear kinetics we must consider here the problems related to these non-linear interactions. We know already that the statistical particle fluctuations lead to collisions between particles and that those collisions are drastically different from the collisions between particles when there is no plasma present. It turns out that the non-linear interactions between waves and these particle fluctuations lead to the absorption or amplification of waves. However, this should then be a process connected with the emission by particles during collisions, which is nothing but bremsstrahlung. Since the cross-sections for collisions are significantly changed by collective processes in the plasma we may expect that the bremsstrahlung will also be significantly changed by collective plasma processes. A study of the non-linear interactions between the particle fluctuations and waves will thus enable us to find and investigate collective effects in bremsstrahlung. We shall see that the plasma drastically changes the bremsstrahlung processes and that processes which are very weak when there is no plasma present may become important in a plasma.
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