Abstract
Plasma has been of great interest to engineers and scientists during the past few decades due to its wide applications. Besides, the plasma-sheath-caused lose of communication (i.e. re-entry blackout) that happens when a spacecraft re-enters the earth atmosphere is still a problem to be solved. The microwave characterisation of shock tube excited plasma has been an important method for exploring the transmission and reflection of microwave signals in plasma. The existing frequency sweep or multi-frequency technologies are not desirable for the characterisation of high-speed time-varying plasma generated in shock tubes. Hence, in this paper a novel signal-frequency approach is proposed to measure both electron density and collision frequency of plasma in shock tube. As frequency sweep is not required in this method, it is extremely suitable for characterising the shock tube excited high-speed time-varying plasma. The genetic algorithm is applied to extract electron density and collision frequency from the reflection coefficient. Simulation results demonstrate excellent accuracy for electron density within 1010∼1012 cm−3 and collision frequency within 5×1010∼1012 Hz. This work paves the way for a fast and compact microwave reflection measurement of shock tube generated plasma.
Highlights
As the fourth fundamental state of matter, plasma has attracted a great deal of attention due to its unique physical and electromagnetic (EM) properties
Solving the re-entry blackout caused by plasma cloud around a returning spacecraft requires proper understanding of transmission/reflection of EM waves in plasma.[8,9]
The probe techniques[13,14,15] have been successfully implemented to measure the electron density of the plasma in shock tube, the contact-less methods based on either transmission or reflection of microwave signals are more desirable and can aElectronic mail: ybjingtian@gmail.com bTo whom correspondence should be addressed. putang@uestc.edu.cn
Summary
As the fourth fundamental state of matter, plasma has attracted a great deal of attention due to its unique physical and electromagnetic (EM) properties. The microwave transmission diagnostics[12,23] can measure both electron density (ne) and collision frequency ( f ) of plasma, providing more useful data for plasma EM property investigation. To extract the electron density and collision frequency, a frequency sweep utilising at least two different frequencies are needed. This results in a sweep time on the scale of 5∼10 μs, not desirable for the measurement of high-speed time-varying plasma in shock tube. A single-frequency microwave reflection approach is proposed to measure both ne and f of the high-speed time-varying plasma in shock tube. The effects of magnitude and phase error of S11 are investigated and the retrieved results are compared with original ne[0] and f 0 for validation
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