Microwave reflection probe

A microwave probe is proposed, which consists of a waveguide or coaxial cable terminated by a thin insulating material. The probe can be applied to unbounded plasmas whose electron plasma frequencies, ω_p, are higher than the applied microwave frequency ω. The principle is based on the fact that the phase angle of the reflection coefficient is a direct measure of the electron density of the plasmas. The electron densities measured by a waveguide probe are compared for a range of ω_p²/ω²<10 with those obtained also by a reflection method which has been developed already by the authors for plasmas bounded in the waveguide. The electron density measured by the probe showed a slightly larger value than the real value for the measured range, and it is shown that this difference is reasonable if the effect of the fringing field outside the probe is taken into considerations. The difference is expected to disappear for the range of ω_p²/ω²>>1. Although an electron density range from 5 × 10¹¹ to 6 × 10¹² cm^-3 are measured by a X-band (9 Gc) waveguide probe in the present experiment, a K-band (35 Gc) probe which is scaled down to a smaller dimension is recommended for higher densities. Finally it is pointed out that the proposed probe is suitable for high density measurement has a good time and space resolutions, insensitive to the magnetic field, needs no auxiliary information of the electron or ion temperatures, and is very simple in treatment and calculation. (author)