A 15hyphen;channel farhyphen;infrared (FIR) interferometer system has been constructed to measure the electron densities in the Advanced Toroidal Facility (ATF) device. The system consists of a pair of cw 214 mgr;m difluoromethane (CH2F2) lasers, optically pumped by separate CO2lasers. The large number of channels is achieved by the use of reflective beam expansion optics to create a beam of 2 cmtimes;45 cm. After passing through the plasma discharge, the beam is dissected by an array of 15 offhyphen;axis paraboloid reflectors, each of which illuminates a single Schottkyhyphen;diode detector. The system can also be operated at a wavelength of 119 mgr;m for high density experiments by changing the gas in FIR laser cavities to methanol. Details of the system are described and the recent results of the measurements are presented.
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