Understanding the Force Low-Temperature Plasmas Exert on Solid Boundaries

Low-temperature plasmas exert such little pressures on their solid boundaries that no much attention has been paid to this effect so far. However, recent force measurements, where small test surfaces were brought in contact with a plasma, have shown that the “plasma pressure” is in the order of magnitude of the electron pressure (order of magnitude of 1 mPa, or
$\mu \mathrm{N}$
cm
⁻²
) [1], [2], where measured forces range from significantly smaller forces up to a few times the electron pressure force. Until now, only qualitative descriptions of the underlying mechanisms for this behavior could be presented. A one-dimensional fluid model (including spherically symmetric geometry) was applied in a general treatment of the problem, and it was concluded that the pressure at the wall should equal the electron pressure in the center of the discharge [3]. However, the found systematic deviations remained unexplained. In this study, we provide a short summary of the experiments performed in Kiel so far with force measurements in capacitively-coupled radio-frequency discharges and microwave-generated plasmas, and we report on our latest attempts to understand the observed features of the force.