Kinetic simulation of mode transitions and hysteresis effects in low pressure capacitive discharges

At very low gas pressures around 1 Pa the electron heating in capacitive radio frequency discharges is dominated by stochastic heating. In this regime electrons are accelerated by the oscillating sheaths, traverse through the plasma bulk and interact with the opposite sheath. Depending on the driving frequency the energetic electrons can enter the sheaths at different phases. These are i) the collapsing phase when the electrons are decelerated, ii) the expanding phase when they are accelerated, and iii) the instant of time, when the sheath width has its minimum and energetic electrons may reach the electrode and be lost. This work analyzes the resulting complex discharge dynamics by means of Particle-In-Cell simulation. It is shown that at certain frequencies the discharge switches abruptly from a low-density mode in a high-density mode. The inverse transition is also abrupt, but shows a significant hysteresis. This phenomenon is explained by the complex interaction of the bulk and the sheath.