Inductively coupled plasmas at low driving frequencies

We discuss the peculiarities of Inductively Coupled Plasma (ICP) at low driving frequencies. The ratio of electric to magnetic field, |E/(cB)|, decreases with decreasing frequency according to Faraday's law - higher magnetic fields are required to induce the same electric field at lower frequencies. We point out that the ratio of |E/(cB)| can be non-uniform in space depending on primary coil configuration and the presence of ferromagnetic materials. In this paper, we consider examples of low-frequency ICPs with negligibly small magnetic fields in plasma. The disparity of time scales for ion transport and the electron energy relaxation results in nonlinear plasma dynamics at low frequencies. Numerical simulations demonstrate that at low frequencies, the presence of plasma has very little effect on spatial distributions of the electric and magnetic fields, which are determined solely by the coil geometry and by the presence of ferromagnetic cores. Simulations of plasma dynamics in ICP over a wide range of driving frequencies and gas pressures illustrate high-frequency, quasi-static and dynamic regimes of discharge operation and explain some trends observed in experiments.