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PLASMA DIAGNOSTICS WITH SHORT ELECTROMAGNETIC PULSES

Description: The propagation of short electromagnetic pulses in an ionized medium and the reflection of transient pulses from bounded ionized regions is discussed. Analyticai 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 that 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 afterflow 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 is 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 freguency of the ionized region, and its application for diagnostics of dense plasmas is indicated. (auth)
Date: July 1, 1963
Creator: Schmitt, H.J.
Partner: UNT Libraries Government Documents Department

Physical Limitations on the Measurement of Transient Fields in Air and in Dissipative Media Using Electric and Magnetic Probes

Description: The properties of electric and magnetic probes for the measurement of transient electromagnetic fields in air and in dissipative media are discussed briefly, It is shown that the effective height of an electrically small loop is independent of the ambient medium. This is also virtually true for a thin electrically short dipole (or monopole). If the open-circuit voltage of a magnetic probe can be measured accurately, it is possible (in principle) to reconstruct the time history of the incident magnetic field, even if the loop is immersed in dissipative media of unknown characteristics. The time function of the open-circuit voltage of an electric probe is essentially a replica of the time history of the incident electric field. In some schemes, the probes are lumped impedance loaded, and the voltage drop across the load impedances is measured. The source impedances of the probes are then involved in the equivalent circuits of the receiving antennas, and the leading terms in the expressions for these impedances depend on the properties of the environment. If an electrically short monopole is base-loaded by a capacitor divider, the voltage wave appearing across any capacitor is a faithful reproduction of the time sequence of the incident electric field provided the measurement is made in air or other dielectric. (auth)
Date: November 1, 1963
Creator: Harrison, C. W., Jr.
Partner: UNT Libraries Government Documents Department