The Fingerprints of Periodic Electric Fields on Line Shapes Emitted in Plasmas

Abstract

Periodic electric fields are found in many kinds of plasmas and result from the presence of collective fields amplified by plasma instabilities, or they are created by external sources such as microwave generators or lasers. The spectral lines emitted by atoms or ions in a plasma exhibit a frequency profile characteristic of plasma conditions, such as the temperature and density of charged particles. The fingerprints of periodic electric fields appear clearly on the line shape for a large range of frequencies and magnitudes of the oscillating electric field. Satellite structures appear near to multiples of the oscillation frequency and redistribute the intensity of the line far from the line center. The modeling of the simultaneous effects of the plasma microfield and of a periodic electric field has been active since the seventies, but it remains difficult to be conducted accurately since the quantum emitter is submitted to several time-dependent electric fields, each with their own characteristic time. We describe here a numerical approach which couples a simulation of the motion of charged plasma particles with an integration of the emitter Schrödinger equation. Resulting hydrogen line shapes are presented for different plasmas and periodic fields encountered in laboratory and astrophysical plasmas.

General Aspects of Line Shapes in Plasmas in the Presence of External Electric Fields

“…Specifially, it was shown that the usual formula employed in calculations without an external field, which assumes stationarity, does not recover the correct Blokhintsev limit. For small fields, this does not make much of a difference [29], but it can be important for larger fields. It was also shown that dressing either electrons or ions alone is theoretically suspect, that random phases of the oscillatory electric fields do not alter the satellite positions and intensities-something that is also numerically convenient-but they do in principle affect the width and that stiffness may be an issue, which may be handled by solving once and for all by computing the atomic plus oscillatory field time evolution.…”