Eur. Phys. J. D 63, 431 (2011)
Determination of sheath parameters by test particles upon local electrode bias and plasma switching
highlighted in EPJ D
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Determination of sheath parameters by test particles upon local electrode bias and plasma switching
In 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. Experiments for which the reported findings might be of relevance range from dusty plasmas to numerous technological plasma processing environments. Arcing and sudden surface charging have been observed in plasma enhanced chemical vapor deposition processes (PECVD) or magnetron sputtering. Similar conditions are important in plasma ion immersion implantation (PIII) for surface treatment, where an ion matrix sheath is created by an abrupt negative high-voltage pulse that is used to accelerate the ions for implantation. In both cases, the results of our studies will apply to microparticles (dust, flakes, powders etc.) present in those discharges.
Figure caption: PIC simulation of the electron and ion densities (left) and electric field (right) along the symmetry axis of the discharge at different distances (z) from the electrode for various pixel bias voltages (UAE). The electric field declines and the electron increases with increasing negative bias, whereas the ion density is large unaffected by a change in the bias voltage.
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