Research on plasma electron density distribution based on microwave diffraction

Abstract

In this paper, a non-contact plasma microwave diffraction measurement method is proposed, which can obtain the electron density at different diameters of the cylindrical plasma. There is a lot of diffraction when a non-focused antenna is used to transmit plasma. As we all know, when the frequency of the incident microwave is lower than the characteristic frequency of the plasma, the microwave cannot be transmitted through the plasma, so this interface can be regarded as a metal. According to the microwave diffraction of the plasma, the size of the plasma correspond-ing to the characteristic frequency can be obtained. Furthermore, by sweeping the incident elec-tromagnetic wave, the size of plasma with different characteristic frequencies can be obtained, and the distribution of electron density can be obtained. To verify the method, a cylindrical plasma was measured by microwave diffraction, in which the electron density of the plasma column gradually decreased along the increase in radius. According to the diffraction of the plasma column at different frequencies, the distribution of the electron density along the diame-ter is obtained. And compared with the transmission diagnosis method, the validity and accuracy of this method are verified. In non-uniform high-temperature plasma, the diffraction method greatly improves the accuracy of spatial diagnosis compared with traditional transmission diag-nosis.

Microwave method based on curve fitting method for high-precision collision frequency diagnosis

“…In the plasma generating device, the plasma is generated by inductive coupling, and is ejected into the vacuum microwave anechoic chamber through a circular nozzle with a diameter of 200 mm [26,27]. The plasma microwave transmission diagnosis system mainly includes a vector network analyzer, focusing lens antennas, coaxial cables, sealing flanges and antenna brackets.…”

Diagnosing ratio of electron density to collision frequency of plasma surrounding scaled model in a shock tube using low-frequency alternating magnetic field phase shift

“…The microwave interferometry and the transmission methods [20][21][22][23][24] obtain the average electron density on the propagation path and have the advantages of not interfering with the flow field, a wide dynamic range, and a precise time resolution. Among these methods, microwave interferometry measures the phase difference between the signal transmitted through the plasma and the reference signal without plasma.…”

“…By combining the attenuation and phase shift with the characteristics of the plasma medium, it is possible to obtain the electron density and collision frequency separately. [21] However, when using the microwave transmission method to diagnose the plasma surrounding a model, the accuracy of the diagnostic method heavily relies on the main beam alignment of transceiver antennas. This requires the transceiver antenna to have a narrow beam and strong directivity.…”

The multi-peak point phenomenon of broadband microwave reflection caused by inhomogeneous plasma

“…The electromagnetic wave propagation within the plasma along the z-direction, exhibiting attenuated TEM wave behavior, is analogous to the propagation characteristics in lossy microwave transmission lines. Therefore, the propagation process of the electromagnetic wave in the direction perpendicular to the aircraft surface in the plasma sheath can be equivalent to the cascading of transmission lines [33] with different characteristic wave impedances, as shown in figure 4.…”