Observation of dominant Ohmic electron power absorption in capacitively coupled radio frequency argon discharges at low pressure

We present a spatio-temporally resolved analysis of electron power absorption in capacitively coupled argon plasmas at low pressures (1-10 Pa), based on the 1D momentum balance equation embedded into 1d3v particle-in-cell/Monte Carlo collisions simulations. In contrast to the predictions of theoretical models we find 'Ohmic heating' to be the dominant electron power absorption mechanism on time average at the lowest pressures, and not 'stochastic' or 'Pressure heating'. The cause for this is identified to be the attenuation of electron power absorption due to electron acceleration by the 'ambipolar' electric field on time average at low pressure, which is a consequence of the collisionless transit of energetic beam electrons generated during sheath expansion at one electrode to the opposite electrode. At such conditions, these energetic electrons arrive during the local sheath collapse and can be lost to the surface, thereby reducing the plasma density and creating a temporally more symmetric electron temperature within the radio frequency (RF) period compared to that in discharges operated at higher pressures. The more symmetric temperature profile causes a reduction of 'Pressure heating' on time average. The latter is reduced further, even to negative values, by the attenuation of the 'ambipolar' electric field at each electrode during the local sheath collapse, which is a consequence of the temporal modulation of the electron density profile within the RF period, observed at the lowest pressures studied.