Theoretical and experimental study of a finite cylindrical antenna in a plasma column

A numerical technique has been used to solve the integral equation determining the current distribution on a finite cylindrical antenna in a long plasma column; a "cold" isotropic plasma model with a relative dielectric constant
\epsilon_{r} = 1 - \omega_{p}^{2}/\omega^{2}
is used in the analysis. Collision losses and inhomogeneities in the plasma have been ignored for the sake of mathematical simplicity. It is found that when
0 < \epsilon_{r} < 1
, the plasma column tends to behave like a dielectric and reduces the effective electrical length of the antenna. The current distribution is still somewhat sinusoidal but with a longer wavelength. In the range
-1 < \epsilon_{r} < 0
, the antenna characteristics undergo a drastic change; the current decays very rapidly along the antenna and, for antennas longer than a quarter wavelength, the input admittance remains almost constant and independent of the length. Extensive experimental measurements have been made to verify the theory. The plasma sheath surrounding the antenna was produced by an active, hot.cathode, helium discharge; the plasma diagnostic measurements were made by using Langmuir probes and cavity perturbation techniques. The theoretical and experimental results show a significant degree of qualitative agreement.