Dips in spectral line profiles and their applications in plasma physics and atomic physics

Abstract

We review studies of two kinds of dips in spectral line profiles emitted by plasmas – the dips that have been predicted theoretically and observed experimentally: Langmuir-wave-caused dips (L-dips) and charge-exchange-caused dips (X-dips). There is a principal difference with respect to positions of L-dips and X-dips relative to the unperturbed wavelength of a spectral line: positions of L-dips scale with the electron density Ne roughly as Ne1/2, while positions of X-dips are almost independent of Ne (the dependence is much weaker than for L-dips). L-dips and X-dips phenomena are important both fundamentally and practically. The fundamental importance is due to a rich physics behind each of these phenomena. As for important practical applications, they are as follows. Observations of L-dips constitute a very accurate method to measure the electron density in plasmas – the method that does not require the knowledge of the electron temperature. L-dips also allow measuring the amplitude of the electric field of Langmuir waves – the only one spectroscopic method available for this purpose. Observations of X-dips provide an opportunity to determine rate coefficient of charge exchange between multi-charged ions. This is an important reference data virtually inaccessible by other experimental methods. The rate coefficients of charge exchange are important for magnetic fusion in Tokamaks, for population inversion in the soft x-ray and VUV ranges, for ion storage devices, as well as for astrophysics (e.g., for the solar plasma and for determining the physical state of planetary nebulae).