Trapping of plasma enabled by pycnoclinic acoustic force

Abstract

Sound can hold partially ionized sulfur at the center of a spherical bulb. We use the sulfur plasma itself to drive a 180 dB re 20 $\ensuremath{\mu}\mathrm{Pa}$ sound wave by periodically heating it with microwave pulses at a frequency that matches the lowest order, spherically symmetric, acoustic resonance of the bulb. To clarify the trapping mechanism, we generalize acoustic radiation pressure theory to include gaseous inhomogeneities and find an interaction of high-amplitude sound with density gradients in the gas through which it propagates. This is the pycnoclinic acoustic force (PAF). Though generated by rapidly oscillating sound waves, it has a finite time average and manipulates the plasma through density gradients at its boundary. The PAF is essential for the description of the trap holding a plasma against its own buoyancy as well as understanding convection in the region outside the plasma. It has implications for pulse tubes, thermoacoustic engines, thermal vibrational convection in microgravity, combustion in the presence of sound, and the modeling of Cepheid variable stars.