Tuning of AC Sheath Thickness by Varying Plasma Excitation Frequency*

It is known that increasing the magnitude of discharge current, either DC or RF, in abnormal glow discharge regime results in shrinking of sheath layer. Since plasma sheath behaves mainly as a capacitor due to its low electron density, this property was successfully employed to make a current-controlled tunable RF capacitor\footnote{A. Semnani et al. IEEE Trans. Plasma Sci., 44, 1396-1404 (2016).}. On the other hand, RF sheath thickness also depends on the frequency of plasma excitation field. In this work and to prove this concept, an $ LC $ resonator with resonant frequency in the range of 100s of MHz was fabricated and measured. In this resonator, a gas discharge tube (GDT) ignited by a kHz-range electric field performs as a variable capacitor. By changing the frequency of the plasma excitation signal in the range of 1-1200 kHz, the measured resonant frequency of the $ LC $ resonator tuned in the range of 410 MHz to 300 MHz. This measurement clearly shows the possibility of achieving frequency-controlled sheath thickness in AC abnormal glow discharge regime.