Plasma Diagnostics with Short Electromagnetic Pulses

The propagation of short electromagnetic pulses in an ionized medium and the reflection of transient pulses from bounded ionized regions is discussed. Analytical expressions for transient oscillations in reflected and propagated electromagnetic fields excited by a unidirectional pulse are derived for certain simple cases. The transient oscillations show a periodicity which is directly related to the plasma frequency and are useful for nearly instantaneous plasma diagnostics. An experiment is described in which the reflection and transmission of unidirectional pulses of approximately 1 nanosecond duration is observed in the afterglow of a coaxial discharge tube. In addition to the dispersion of the quasi TEM mode in the coaxial test tube a transient oscillation with much lower periodicity is also observed and interpreted as arising from a surface-wave propagating near the interface between glass walls and the annular ionized region. This surface-wave propagates only at frequencies below the plasma frequency of the ionized region and its application for diagnostics of dense plasma is indicated.