Strong effect of outer plasma layers of femtosecond laser-induced plasma-channel antenna and its metallic counterpart on their radiation

Abstract

The effect of various radially inhomogeneous distributions of plasma conductivity and dielectric permittivity on the far-field radiation characteristics are studied for the femtosecond laser-induced plasma-channel Vee antenna and compared to that of the radially homogeneous conductivity distribution, frequently used as a rough approximation. The radiation characteristics of the antenna are affected mainly by the outer boundary plasma layer due to skin effect and are fairly different from that of the cross-sectional averaged homogeneous conductivity distribution, and it is preferable to modulate a laser beam intensity to have radially increasing or annular distribution. The inner part of an antenna channel beyond the skin depth does not have an essential effect on its radiation characteristics, even though the inner is a highly conductive metal, as the current can hardly flow through it. The outer plasma layer can produce a complicated phenomenon, such as the signal attenuation or blackout, when it is formed around the solid metal antenna in a sufficiently fast-flow field. The simultaneous action of the radiation absorption and current flow benefiting to the radiation in the plasma layer can vary the antenna radiation characteristics in a complicated manner. The effect of plasma sheath existing between the metal antenna surface and the plasma layer on the radiation characteristics is also considered. It is also shown that the negativity of plasma permittivity does not have a great effect on the far-field radiation characteristics due to its insignificance in the radiation absorption and the irrelevance to the current.