Plasma diagnostics using microparticle motion in a dusty plasma under microgravity conditions

Abstract

Summary form only given. Diagnostic methods are developed to measure plasma electron temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> and ion density n <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> . We do this without making any direct observations of the electrons or ions. Instead, we observe motion of microparticles, or dust, levitated in the plasma. Microparticles immersed in an ionized gas form a dusty plasma (also known as a complex plasma). The microparticles develop a negative charge, and they can be confined electrically. When confined stably, their positions sometimes oscillate. In an experiment aboard the International Space Station, performed using the PK-3 Plus instrument, a glow-discharge plasma was generated by RF voltages at 13.56 MHz. Microparticles of 6.8-micron diameter were introduced into the plasma using a mechanical shaker. The microparticles settled in a main plasma region, not in a sheath, due to the microgravity conditions. Using video cameras and laser illumination, microparticles were imaged in situ, revealing a thin layer of particles at the edge of a central void. Tracking microparticle motion yields a resonance frequency, which along with a charging model allows an estimation of Q and T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> . (Unlike more common uses of charging models, here we use particle-motion measurements as inputs; plasma parameter values are our outputs, not our inputs.) The resonance frequency measurement can also be used with an ion drag model to estimate n <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> .