Oscillating sound waves can exert a force known as acoustic radiation pressure. These forces can trap a bubble or levitate a particle. We have uncovered a term in the Navier-Stokes equation for gas/fluid dynamics which gives an acoustic radiation force that is enhanced by a density gradient. In a spherical geometry a standing wave in a plasma leads to an acoustic gravity that is over 1,000 times earth’s gravity ‘g’ in the lab. This enables ground-based observation of thermal convection in a spherical geometry that can also be subjected to rotation. Previous efforts took place in space because they were dependent on a fluid with a temperature dependent permittivity. With an applied electric field their effective gravity was less than g/10. Our plasma is excited in sulfur with microwave radiation. By pulsing the microwaves at the frequency of a spherical acoustic resonance the acoustic forces create a confined hot region near the center and a convecting region beyond the velocity antinode. The quality factor of the sound field is determined by a thermal boundary layer. The non-stationary nature of the convection is also representative of the plasma that forms on the surface of hypersonic vehicles and accounts for communications blackout. The Mach number of the spherical pulsation matches a Cepheid. We hope to modify the parameters of our system so that the plasma’s opacity causes it to become a natural oscillator. In this way continuous energy input will lead to high amplitude oscillations and confinement of the hot region.
Thanks to DARPA and the AFOSR and the Dean of Physical Sciences for Support!
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