Determination of sheath parameters by test particles upon local electrode bias and plasma switchingIn this paper, we report how microparticles trapped in the plasma sheath above a grounded or weakly biased electrode can be used to determine changes in the operating conditions of the plasma. We focus on two specific scenarios, (i) switching off the plasma while retaining the biasing of the electrode and (ii) changing the bias voltage during operation of the plasma. Above a critical bias, the microparticle is subject to an additional downward acceleration that cannot be explained solely by gravity and ion drag, but is attributed to a positive charging of the particle induced by extreme out-of-equilibrium conditions of the surrounding plasma sheath: locally, the plasma sheath can be completely deprived of electrons because of the bias. The observed particle dynamics in such a locally tailored plasma sheath environment directly monitors changes in electric field structures and plasma density profiles (see figure). The experimental results are discussed in the context of a theoretical model that uses basic equations with input data from particle-in-cell (PIC) simulations.