Abstract

Semiconductor diode lasers represent an important spectroscopic tool for performing high resolution measurements of spectral lines, from which plasma state parameters can be deduced. It is easy to obtain a fast scanning of the emission frequency of the laser so that the kinetics of the plasma can be followed with a high temporal resolution. The plasma that we want to analyse is produced in a mag- netoplasmadynamic (MPD) thruster, which consists of a coaxial cylindrical Lorentz force accelerator. A pulse of Argon (4 gr/sec) of the length of few msec is injected in the thruster and excited by a current pulse, roughly rectangular, with a typical duration of about 1 msec and an intensity of several KA. A diffusion pump keeps the vacuum at a value of the order of 10⁻⁵ torr. The experimental apparatus is shown in Fig. 1. The used diode laser was a Mitsubishi ML64114R. Operational wavelength, corresponding to transition starting from metastable level, was chosen by tuning the temperature of the diode through a Peltier module and the injection current. The laser emission frequency was quickly scanned, every 26 µs over the transition line. The laser output was sent into the vacuum chamber using an optical fiber (200 µm of core). The radiation was collimated into a parallel beam of about 1 cm of diameter through the plasma and finally focused on a fast silicon detector. A beamsplitter sent a fraction of the laser radiation into a low pressure, room temperature d.c. discharge in argon, whose transmission was simultaneously recorded and used as a reference. A typical recording is shown in Fig. 2. A sequence of single frequency sweeps, alternatively forwards and backwards, scanning the transmission profiles of the 1s₅−2p₈ transition (801.479 nm), are simultaneously observed through the plasma (orthogonal to the thruster axis) and a low pressure argon discharge. We extract from the experimental data the frequency dependent optical thickness. The lineshapes were fit to a Gauss, Lorentz, or Voigt profile to infer atomic density, temperature, and velocity. Two types of measurement were carried out in different points and in two directions (orthogonal and transversal). Figure 3 shows atomic density evolution at 12 cm from thruster anode. Further measurements are in progress. Two crossing beams will be used to obtain the bidimensional velocity profile.

© 1994 IEEE