When an intense ion beam propagates through a dense background plasma, its current is partially neutralized by the electron plasma return current. Due to the non-uniformity of the background plasma electrons longitudinal velocity profile (r), the flow can be unstable. The instability is similar to the Kelvin-Helmholz instability for the non-uniform flow of an incompressible neutral fluid, with the electrostatic potential playing the role of pressure. For the case of electron return current flow, the significant new feature is the presence of the partially self-neutralized magnetic field of the ion beam, which significantly affects the evolution of small-amplitude excitations. In this paper the stability properties of the flow of electrons making up the plasma return current is investigated using the macroscopic cold-fluid-Maxwell equations. It is shown that this flow may become unstable, but the instability growth rates are exponentially small. This unstable body mode is qualitatively different from previously studied surface-mode excitations of the electron plasma return current for an intense ion beam with a sharp radial boundary, which is found to be stable due to the stabilizing influence of the partially neutralized magnetic field of the ion beam.

Kα radiation from low charge chlorine is examined for cold dense plasma diagnostics. A relativistic atomic structure calculation shows that the transition energies of Kα lines are slightly shifted to a higher-energy side according to the degree of M-shell ionization. Total spectral shift from Cl+ Kα lines up to those with a fully stripped M-shell is about 10 eV. With an assumption that collisional-radiative-equilibrium is valid, spectral calculations were carried out for a C2H3Cl-plasma heated by an ion beam, and clear deformation among spectral line-shapes is found in the range of the electron temperatures of ≤~ 30 eV. Contribution to the composite spectra of Kα lines with an excited electron in the outer-shell is also briefly discussed. Cl-Kα lines with M-shell electrons can be expected to give us distinct understandings for energy deposition by an ion beam in cold dense plasma.

Newly developed erosion-plasma-source probe technique has been applied for virtually single shot recording of absorption/fluorescence spectra in the 190–510 nm spectral range of e-beam-excited Ar/Kr/NF3(F2 + N2) mixtures. The e-beam excitation rate of about 1 MW/cm3 is typical of large-volume rare-gas halide lasers. It is experimentally observed that, in Kr/F2 and Ar/F2 mixtures, fluorescence and absorption spectra of Rg2F species are shifted with respect to each other in the opposite direction. Continuous absorption spectrum of Ar2F excimer is reported, as far as we know, for the first time in the refereed literature. Strong overlapping between the fluorescence and absorption spectra of Ar2F is responsible for absence of lasing on Ar2F molecule. Absorption spectrum of Kr2F excimer is recorded in pure form using the mixture (Ne/Kr/F2) with no alternative broadband absorber. Minor additive of nitrogen to Ar/Kr/F2 mixture or use of NF3 instead of F2 has been found to result in broadband optical amplification centered at λ ~ 460 nm. The maximum optical gain is estimated as about 0.1 ± 0.05 m−1.