A single-channel far-infrared (FIR) hydrogen cyanide (HCN) laser interferometer will be constructed aiming to provide the line integrated electron density on the Space Plasma Environment Research Facility (SPERF) . A continuous glow discharge HCN laser with a cavity length of 3 m is used as the laser source with a wavelength of 337 μm and an output power about 10 mW . The interferometer system is a Mach-Zehnder type. The temporal resolution of the system is 0.1 ms since the phase modulation is achieved by a rotating grating with a modulation frequency of 10 kHz. The beat signal is detected by AlGaN/GaN high electron mobility transistors (HEMT) detector. It has a noise equivalent power of 10 pW/Hz−1/2. Excellent port access (1500×150 mm) and high laser power would promise a profile measurement across the whole plasma section with good signal quality. The design and structure of the single-channel FIR HCN laser interferometer in SPERF is described in detail.
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