Microwave Reflection Measurements of Electron Densities in Electromagnetically Driven Shock-Produced Plasmas

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Abstract
The electron densities from 1013 to 1017 cm−3 in plasmas produced by electromagnetically driven, propagating, or reflected shock waves are measured by a microwave reflection method. The Mach number of the shock waves is varied from 9 to 25 in argon, air, and helium at initial pressures from 0.4 to 2.0 Torr. A 35‐GHz microwave reflection probe is inserted into a T‐type shock tube perpendicular to its axis, or is placed at the tube end parallel with the axis. Despite some perturbation on the gas flow by the inserted probe, surprisingly good agreements are found between the microwave and spectroscopic measurements. The measured electron densities are compared with predicted values calculated from the Rankine‐Hugoniot and Saha equations. For Mach numbers higher than 20, the maximum values of the electron densities measured behind the shock fronts agree well with the theoretically predicted values. For low Mach numbers, however, the measured maximum electron densities are lower than the theoretically predicted values. Reasons for the discrepancy between the theoretical and experimental values in low Mach numbers are discussed. In addition, radial and axial density distributions behind the shock fronts are measured for various Mach numbers.