Low-frequency (2.72-3.70 Hz) relaxation oscillations at 100m Tort at higher absorbed power were observed from time-varying optical emission of the main discharge chamber and the periphery.We interpret the low frequency oscillations using an electromagnetic model of the slot impedance with parallel connection variational peripheral capacitance,coupled to a circuit analysis of the system including the matching network.The model results are in general agreement with the experimental observations,and indicate a variety of bchaviours dependent on the matching conditions.