A microwave reflection method is applied to the measurement of very high electron densities in plasmas produced by shock waves. The electron density can be determined from the phase angle of a reflection coefficient at the shap boundary of the plasma which exists at the end of the shock tube. The electron densities measured by an X-band frequency (9000 Mc) range from 1013 to 1015 cm−3. The plasma is produced by shock waves of Mach number 8 to 15 in argon at a pressure of 0.1 mm Hg. The electron densities are compared with values predicted by theoretical calculations under thermal equilibrium. The maximum density detectable reaches 1×1016 cm−3. The afterglow plasma which continues after the shock wave has passed is investigated. The loss mechanism of electrons during a few hundred microseconds seems not to depend on diffusion but on recombination. The temperature of electrons which are almost in thermal equilibrium with ions and neutral atoms in this afterglow plasma is sustained for a relatively long time.
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