Antenna impedance measurements and sheath effects in an RF generated plasma

A device using an RF discharge quasi-homogeneous plasma to measure antenna impedance is described. The plasma diagnosis is done by using a new probe made of a transparent resonant cavity. The electron densities (
2 \times 10^{8}
to
6 \times 10^{9}
particles/cm
³
) and collision frequencies (
6 \times 10^{7}
to
1.5 \times 10^{8}
per second) are controlled by the discharge power. The density distribution measurements indicate a quasi-homogeneous region of 20 by 20 by 14 cm. Impedance measurement results are given for a a thick unipole between 100 and 750 MHz. The values of
X = (\omega_{p}/\omega)^{2}
range from 0 to 44. They agree fairly well with Deschamps model theory applied to scaled frequency measurements as well as to a quasistatic simple analytical formula. Sheath effects enhanced by a dc bias are observed especially at low frequencies and the sheath thickness increase is calculated and found consistent with Pavkovich's parabolic potential assumption. The possibility of electronic tuning by a suitable bias is suggested.