Plasma Density Measurements by using a Modulated Submillimeter Laser Interferometer System

We have measured the electron-density, ne
e
, of the plasma confined by the magnetic field in a tokamak (in particular, the M.I.T. Alcator) by using a submillimeter interferometer. The interferometer, shown in Fig. 1, contains twin optically pumped waveguide lasers. The twin lasers operate at a wavelength of 118.8 micrometers (CH
3
OH) but are detuned with respect to each other by approximately one MHz for modulation purposes. This wavelength is appropriate for measurement of electron densities ≥ 10
¹⁴
cm
⁻³
.