The use of a 0.185-mm far-infrared (FIR) laser interferometer on MTX required the design and development of optical components which are nonmagnetic and nonconducting. This arose from the requirement that the path length of the FIR interferometer not change by more than (1)/(50) of a wavelength during the half-second plasma pulse. The time-dependent stray magnetic fields, 7000–400 G over the optical table area, generate strong eddy current forces which would produce path length changes greater than the design criteria if the components were metallic. Thus, the optical tower which supports the optical tables, one above and one below the tokamak, is a composite structure of 304 stainless steel and G-10 fiberglass. Also, the optical table and components are made of G-10 with a minimum of 304 stainless-steel parts to avoid eddy current forces and maintain structural rigidity. The design criteria of the optical components are discussed with a comparison of suitable materials. A discussion of the process which was developed to fabricate plastic parabolic beam expanding and focusing mirrors with a metallized reflective surface is presented. The mixer and electronics enclosure design minimizes eddy current forces by use of printed circuit board material while maintaining excellent EMI/RFI shielding.
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Burns et al. (1988) studied this question.