Power transfer efficiency and the power threshold for E–H mode transition in inductively coupled plasmas

The effects of gas pressure and gas component on the power transfer efficiency η and transition power threshold Pth during the E–H mode transition in inductively coupled plasmas are studied. The evolutions of η during the E–H mode transition in pure Ar and Ar/O2 discharges are similar, i.e., in E-mode discharge, η slightly increases with raising the applied power (η is below 30%), whereas, an abruptly upward jump of nearly two to four times for η occurs when the discharge transits to H-mode, and then η monotonously increases with increasing the applied power. In addition, as the pressure rises, η increases rapidly first and then slowly in pure Ar discharge. However, η in Ar/O2 mixture gases discharge shows a non-monotonic behavior. It reveals a tendency of first increase at low pressures and then decreases at high pressures, which is because the resistive component of the plasma varies with the electron density ne and the electron effective collision frequency νeff. Furthermore, a non-monotonic behavior of transition power threshold Pth with the pressure is found, i.e., with increasing the pressure, Pth first decreases and then increases. It may be caused by the threshold electron density, which is constant at low pressures (νeff/ω<1) but increases at high pressures (νeff/ω>1).