In this work, microwaves and terahertz waves have performed a dual-frequency combined diagnosis in high-temperature, large-scale plasma. According to the attenuation and phase shift of electromagnetic waves in the plasma, the electron density and collision frequency of the plasma can be inversely calculated. However, when the plasma size is large and the electron density is high, the phase shift of the electromagnetic wave is large (multiple times 2π period). Due to the limitations of the test equipment, the true phase shift is difficult to test accurately or to recover reality. That is, there is a problem of phase integer ambiguity. In order to obtain a phase shift of less than 180°, a higher electromagnetic wave frequency (terahertz wave with 890 GHz) is used for diagnosis. However, the attenuation of the terahertz wave diagnosis is too small (less than 0.1 dB), only the electron density can be obtained, and the collision frequency cannot be accurately obtained. Therefore, a combined diagnosis was carried out by combining two frequencies (microwave with 36 GHz, terahertz wave with 890 GHz) to obtain electron density and collision frequency. The diagnosis result shows that the electron density is in the range of (0.65-1.5) ×10 19 m −3 , the collision frequency is in the range of 0.65-2 GHz, and the diagnostic accuracy is about 60%.

This paper explains the physical behavior of the electron density of high-power and large-volume plasma wind tunnel using a single channel hydrogen cyanide (HCN) laser interferometer. Based on the characteristics of inductively coupled plasma (ICP)-heated wind tunnel, the temperature and pressure distribution of the ICP-heated wind tunnel are obtained from numerical simulations, during which the influence of neutral particles is considered to calculate the accurate electron density. The typical electron density order of ICP-heated wind tunnel is [Formula: see text]. We discovered that there is a positive correlation between the electron density of argon plasma jet and mass flow rate, while that of air plasma jet decreases slightly. We also found that the peak of electron density appears when the argon is switched to air. Within the voltage range of 6–10 kV, the electron density of argon and air plasma increases slowly. However, when the voltage increases from 10 to 12 kV, the electron density of air plasma increases sharply with the mass flow rate of 15 g/s. Finally, the electron density of argon plasma is much higher than that of air plasma at the same mass flow rate and voltage.

A glow discharge plasma generator and diagnostic system has been developed to study the effects of rapidly variable plasmas on electromagnetic wave propagation, mimicking the plasma sheath conditions encountered in space vehicle reentry. The plasma chamber is 400 mm in diameter and 240 mm in length, with a 300-mm-diameter unobstructed clear aperture. Electron densities produced are in the mid 10 10 electrons/cm 3 . An 800 W radio frequency (RF) generator is capacitively coupled through an RF matcher to an internally cooled stainless steel electrode to form the plasma. The RF power is amplitude modulated by a waveform generator that operates at different frequencies. The resulting plasma contains electron density modulations caused by the varying power levels. A 10 GHz microwave horn antenna pair situated on opposite sides of the chamber serves as the source and detector of probe radiation. The microwave power feed to the source horn is split and one portion is sent directly to a high-speed recording oscilloscope. On mixing this with the signal from the pickup horn antenna, the plasma-induced phase shift between the two signals gives the path-integrated electron density with its complete time dependent variation. Care is taken to avoid microwave reflections and extensive shielding is in place to minimize electronic pickup. Data clearly show the low frequency modulation of the electron density as well as higher harmonics and plasma fluctuations. Published by AIP Publishing.

In the diagnosis of plasma microwave transmission, when the plasma size is large and the electron density is high, the microwave will cause a phase period ambiguity problem. In order to solve the phase ambiguity, a novel method of using the phase shift group delay (PSGD) to recover the true phase shift is proposed. The PSGD method is used to recover the simulated phase shift and diagnose the plasma. The diagnostic accuracy is all greater than 91%, which verifies the method. And the new method is used to diagnose the real inductively coupled plasma with high precision. Through analysis, the method of group delay recover phase shift is more suitable for stable plasma or slow-varying plasma.