The structure of plasma sheath is a non-static one due to the inclusion of weak but finite electron inertia. This electronic inertia is responsible for imperfect screening of ion-acoustic wave potential thereby rendering the sheath structure unstable. It is equivalent to a resonating series LCR circuit with all the electrical elements dependent explicitly on diversified plasma parameters. The further details may be found at the following:

[1] C. D. Child, Phys. Rev. 32, 492 (1911).

[2] C. B. Dwivedi, U. Deka and A. Sarma, Phys. Scr., 69, 108 (2004).

[3] U. Deka, Ram Prakash, A. Sarma and C. B. Dwivedi, Jpn. J. Appl. Phys., 43, 2704 (2004).

The rf sheath( its thickness) forms the the capacitor.The equivalent resistance is just that resistance which accounts for the power lost due to the ion current flowing across this rf sheath. The diode is due to the dc voltage which prevents more electrons than ions current crossing the sheath. Another way of looking at this is that the ions are much more massive than the electrons.

The plasma its self is so much more conductive that one does not normally include it except for the inductive sheath in an ICP and other heating effects. For more details see : J.H. Keller and W. B. Pennebaker, "Electrical Properties of RF Sputtering Systems", IBM Journal of research and development, Vol. 23, No.1, Jan 1979

thank you ....and in bulk plasma why inductor is in series with capacitor ?

I do not know if I understand your question If you mean the sheath capacitor and the bulk electrons it is because they are in series. However You would have to go at least near GHz to see much inductance impedance.