A Narrow Band Far Infrared Interferometer for Electron Density Measurements

An interferometer has been constructed with reflection optics and lamellar gratings to operate in the wavelength range of 175–400 μ. The gratings serve not only as beam splitters and recombiners but also as filters to separate out a narrow band of radiation. In this way a simple wide band Hg arc lamp can be used as the source. Because of the required response time for afterglow plasmas, a Putley detector measures the radiation. An application of the interferometer has been made to measure electron densities in repetitively pulsed plasma with λ = 200 μ and a time response of 3 μsec. The electron line density sensitivity is 3×1013 cm−2 and line densities have been measured up to 1015 cm−2, still substantially below the instrumental limit.