  Probe theories are only applicable in the regime where the probe's
perturbation of the plasma can be neglected. However, it is not always possible
to know, a priori, that a particular probe theory can be successfully applied,
especially in low density plasmas. This is especially difficult in the case of
transient, low density plasmas. Here, we applied probe diagnostics in
combination with a 2D particle-in-cell model, to an experiment with a pulsed
low density hydrogen plasma. The calculations took into account the full
chamber geometry, including the plasma probe as an electrode in the chamber. It
was found that the simulations reproduce the time evolution of the probe IV
characteristics with good accuracy. The disagreement between the simulated and
probe measured plasma density is attributed to the limited applicability of
probe theory to measurements of low density pulsed plasmas. Indeed, in the case
studied here, probe measurements would lead to a large overestimate of the
plasma density. In contrast, the simulations of the plasma evolution and the
probe characteristics do not suffer from such strict applicability limits.
These studies show that probe theory cannot be justified through probe
measurements.
