The self-excitation of plasma series resonance (PSR) oscillations is a
prominent feature in the current of low pressure capacitive radio frequency
(RF) discharges. This resonance leads to high frequency oscillations of the
charge in the sheaths and enhances electron heating. Up to now, the phenomenon
has only been observed in asymmetric discharges. There, the nonlinearity in the
voltage balance, which is necessary for the self-excitation of resonance
oscillations with frequencies above the applied frequencies, is caused
predominantly by the quadratic contribution to the charge-voltage relation of
the plasma sheaths. Using PIC/MCC simulations of single- and multi- frequency
capacitive discharges and an equivalent circuit model, we demonstrate that
other mechanisms such as a cubic contribution to the charge-voltage relation of
the plasma sheaths and the time dependent bulk electron plasma frequency can
cause the self-excitation of PSR oscillations, as well. These mechanisms have
been neglected in previous models, but are important for the theoretical
description of the current in symmetric or weakly asymmetric discharges
