On the plasma confinement by acoustic resonance

Abstract

In an applied research project on the development of a pulsed microwave sulfur lamp prototype of 1 kW, we have discovered an amazing phenomenon in which the plasma forms a ball staying at the center of the bulb despite gravity, thus protecting the glass from melting. In this paper, it is shown that this results from an acoustic resonance in a spherical mode. Measurements of the plasma response to short pulses are presented showing beats at the spherical resonance. It is demonstrated that the beats could result from the simultaneous excitation of two normal modes with a frequency difference of approximately 1%. One of the two frequencies matches precisely the microwave pulses repetition, a little below 30 kHz. Thus this one is due to a forced oscillation, whereas the other one is due to a free oscillation. The phase velocity of sound was calculated as a function of temperature in order to find the series of temperatures at which a resonance would occur if the bulb were an isothermal solid sphere. The mean temperature inside the actual bulb was determined from the only doublet of this series, that has characteristic frequencies close enough to cause the observed beats. In addition, one of these two modes has a spherical symmetry that can explain the plasma ball formation. The obtained mean temperature is consistent with the direct measurements on the bulb surface as well as with the temperature in the core of a similar plasma found in the literature. We have also proposed a model of the resonance onset based on the acoustic dispersion and the sound amplification due to electromagnetic coupling. Key words. molecular plasma-pulsed microwave plasma-plasma confinement-acoustic resonanceacoustic dispersion-sound amplification due to electromagnetic coupling-plasma of sulfur-electrodeless light bulb-high-pressure lamps-light sources

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.…”

“…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.…”

Sound Waves Mimic Gravity

“…In 2017, research on high-powered sulfur lamps revealed that sound waves could drive hot gas to ball up in the center of the bulbs [2]. The surprising phenomenon caught the attention of Seth Putterman's acoustic group at the University of California, Los Angeles.…”

Magnetron Coupling to Sulfur Plasma Bulb

“…al. [8][9][10] have recently observed the existence of acoustic modes inside the bulb that hold the hottest region of the plasma centrally, away from the glass. Their interest in these modes is motivated by a need to replace the mechanical rotation systems used in these lamps to distribute heat and suppress the formation of hotspots.…”