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 µPa 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 highamplitude 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

Convective instability in a stratified ideal gas containing an acoustic field

“…This theoretical work is guided by experimental observations of convection inside a spherical, stratified, rotating sulphur plasma bulb pictured in figure 2 that has been described by Koulakis et al. (2018 c ). The bulb contains a high-amplitude, spherical standing acoustic wave.…”

“…A preliminary understanding of the stability regions inside the bulb was given by Koulakis et al. (2018 c ), which proposed that the gas is stable to convection when the density gradient is parallel with the gradient of the time-averaged square of the acoustic velocity, and unstable otherwise. In this article, the arguments are expanded and applied to a stratified, two-dimensional (2-D) planar system containing an ideal gas, as drawn in figure 3.…”

Acoustic self-oscillation in a spherical microwave plasma

“…The current proposal is motivated by an observation made while studying acoustic plasma confinement [9,10]. As reported in [10], the 180 dB re 20 µPa sound wave that confines a lightly ionized collisional plasma causes luminosity oscillations in phase with its acoustic pressure.…”

“…The current proposal is motivated by an observation made while studying acoustic plasma confinement [9,10]. As reported in [10], the 180 dB re 20 µPa sound wave that confines a lightly ionized collisional plasma causes luminosity oscillations in phase with its acoustic pressure. It is well-known that there is a temperature oscillation associated with sound, and the amplitude quoted above corresponds to a temperature swing of more than 10 K. The opportunity here is that because the system sits at * sethpree@ucla.edu a temperature on the cusp of ionization, small changes in temperature lead to changes in electron density and microwave absorption that are in phase with acoustic compression.…”

“…In this letter, we present a type of acoustic selfoscillation that may occur in an acoustic cavity filled with partially ionized gas located within a microwave cavity as shown in Figure 1. For the present analysis, the configuration is assumed similar to that found in [9][10][11], where the acoustic cavity was a sealed quartz sphere with a radius around 2 cm, and the surrounding microwave cavity was a thin-walled metallic cavity with a resonance near 2.45 GHz. Future implementations may include a means to couple the sound out of the acoustic cavity, for example by attaching a horn shaped outlet.…”