Experiments and modeling of dust particle heating resulting from changes in polarity switching in the PK-4 microgravity laboratory

In the presence of gravity, the micrometer-sized charged dust particles in a complex (dusty) plasma are compressed into thin layers. However, under the microgravity conditions of the Plasma Kristall-4 (PK-4) experiment on the International Space Station (ISS), the particles fill the plasma, allowing us to investigate the properties of a three-dimensional multi-particle system. This paper examines the change in the spatial ordering and thermal state of the particle system created when dust particles are stopped by periodic oscillations of the electric field, known as polarity switching, in a dc glow discharge plasma. Data from the ISS are compared against experiments performed using a ground-based reference version of PK-4 and numerical simulations. Initial results show substantive differences in the velocity distribution functions between experiments on the ground and in microgravity. There are also differences in the motion of the dust cloud; in microgravity, there is an expansion of the dust cloud at the application of polarity switching, which is not seen in the ground-based experiments. It is proposed that the dust cloud in microgravity gains thermal energy at the application of polarity switching due to this expansion. Simulation results suggest that this may be due to a modification in the effective screening length at the onset of polarity switching, which allows the dust particles to utilize energy from the potential energy in the configuration of the dust cloud. Experimental measurements and simulations show that an extended time (much greater than the Epstein drag decay) is required to dissipate this energy.