Measurement of plasma electron density by the laser self-heterodyne

This paper proposes a method of measuring the electron density of a magnetized plasma by incorporating the plasma in a laser cavity. The two refractive indices of the magnetized plasma force each lasing mode to oscillate at two closely spaced frequencies. These two frequencies can be made to beat with 100% modulation by passing the laser radiation through a polarizer and this beat frequency is independent of the plasma temperature and to first order also independent of mode pattern and mirror vibrations. A full analysis is carried out for the high-gain lasing transitions of neon and carbon dioxide which lase at wavelengths of 3·39 and 10·6 μm respectively. The lowest electron density that can be measured is determined by frequency locking and a tentative figure of 50 Hz is suggested. At a wavelength of 10·6 μm this beat frequency corresponds to an electron density of 4×10 10 cm −3 for a plasma containing an axial field of 10 kG. For smaller fields the density is increased pro rata for the same beat frequency. However, the magnetic field must lie above a certain density dependent limit set by the onset of collisional depolarization. Finally a steady-state experiment is described using a magnetized block of fused quartz inside a laser cavity operated at 3·39 μm wavelength. This experiment shows that the heterodyne technique is perfectly stable and reproducible and also enables the Verdet constant of the dielectric to be accurately measured.