The conceptual design of high temporal resolution HCN interferometry for atmospheric pressure air plasmas

A heterodyne interferometer operating at the frequency f = 890 GHz has been designed for measuring the electron density of atmospheric pressure air plasmas, it's density range is from 1015 to 3×1019 m−3 and the pressure range is from 1 Pa to 20 kPa. The system is configured as a Mach\nobreakdash-Zehnder type interferometer. The light source is hydrogen cyanide (HCN) laser with a wavelength of 337 μm, which is suitable for the high density area, 1018 to 3×1019 m−3, of atmospheric pressure air plasmas. The HCN laser power is up to 100 mW. The intermediate frequency, shifted by 500 kHz, is generated by the Doppler shift with a high speed rotating grating (HSRG), which is installed in a vacuum vessel with gas pressure less than 1 Pa. With the HSRG, the temporal resolution of the HCN interferometry can reach to 2 μs. Beam path analysis based on Gaussian optics is used in the design of the optical system. The VDI planar-diode Integrated Conical Horn Fundamental Mixers optimized for high sensitivity—typical RF responsivity is around 900 V/W —will be used in the HCN interferometry. The detailed optical design and opto-mechanical design for the HCN interferometry are presented.