On the hysteresis in E to H and H to E transitions and the multistep ionization in inductively coupled plasma

Plasma densities, E to H and H to E transition coil currents, and electron energy distribution functions (EEDFs) are measured at various argon pressures in an inductively coupled plasma. The measured plasma density versus coil current shows that the hysteresis during the E - H transition is clearly observed only when the pressure is sufficiently high. At low gas pressures the hysteresis is not obvious. The measured EEDFs show that when the hysteresis is obvious (high pressures), electrons whose energy is not sufficient for excitation or ionization of the ground state atom are strongly depleted in the H mode. This depletion may be caused by multistep ionization. However, for the case where the hysteresis is not obvious (low pressures), the depletion due to multistep ionization is also not present. These experimental results show that the multistep ionization is a dominant factor in the E - H transition hysteresis.