On the origin of the coherent x-ray radiation from plasma focus

In the experiments with plasma focus a highly monochromatic radiation at λ∼13 angstrom was observed at a small angle to the direction of the plasma discharge. This radiation was attributed to the following features. Firstly, during the development of a plasma focus the short-period (T approx-lt 1μm) modulation of the plasma density due to the increasing instabilities takes place along the discharge over many periods. Secondly, a definite part of electrons is accelerated up to MeV energies due the cyclotron instability and the increasing diffusion of the magnetic field in the pinch region. In the present report in order to explain the experimental results, we discuss possible mechanisms of coherent X-ray radiation in plasma focus, including the transition radiation from relativistic electrons in the medium with periodically modulated dielectric permittivity, undulator radiation in periodic electric field in the medium, and Cherenkov radiation from plasma in the presence of a strong magnetic field. The calculations of the spectral and angular distributions of X-rays are presented for the various types of radiation under discussion and estimates of the radiation power are made. We also discuss the possibility of the stimulated radiation from plasma focus