"Anomalous" collisionality in low-pressure plasmas

Based on a theoretical argument from fundamental kinetic theory, by way of simple worked examples, and through the use of particle-in-cell simulations of capacitively coupled plasmas, we demonstrate that conventional methods for calculating the momentum transfer collision frequency in low-pressure plasmas can be seriously erroneous. This potentially plays an important and previously unconsidered role in many low-pressure discharges, and at least in part provides a possible explanation for anomalous behaviour often encountered in these plasmas.