The first results of the hydrogen cyanide (HCN) interferometer measuring experimental research apparatus for electromagnetic science (ERAES) for hypersonic vehicle plasma in near space.

This article focuses on the development and application of a heterodyne hydrogen cyanide (HCN) laser interferometer designed to measure electron densities in the plasma sheath simulated by the Experimental Research Apparatus for Electromagnetic Science (ERAES). The ERAES device replicates plasma conditions encountered by high-speed vehicles during atmospheric entry, with electron densities ranging from 1 × 10⁹ to 3 × 10¹³ cm⁻³ and pressures between 50 and 1500 Pa. The HCN interferometer, operating at a wavelength of 337 µm and configured as a Mach–Zehnder interferometer with Doppler-shift modulation, achieves a temporal resolution of 100 µs and a spatial resolution of approximately 14 mm, enabling precise, non-contact measurement of plasma electron density. Initial experimental results demonstrate the interferometer's capability to detect electron density variations under different gas flow and power conditions, with a phase noise corresponding to a minimum measurable density of 8.8 × 10¹¹ cm⁻³. Future work includes enhancing temporal resolution to 1 µs and expanding measurement channels for multi-point diagnostics.