Investigation of a Molecular Plasma From Its Acoustic Response

In developing a pulsed microwave lamp of 1 kW, fitted with a static electrodeless spherical bulb of glass, we discovered a phenomenon in which the plasma forms a ball at the bulb's center, despite the effect of gravity. The light is produced by plasma of sulfur, which is so hot that it might melt the bulb if it remained in contact with the glass, although it is fused quartz. In a preceding publication, we reported on photometric measurements showing the plasma response to the modulation of the microwaves. We have shown that the ball formation results from an acoustic resonance in a spherical mode. From a measurement of the resonance frequency, we have assessed the average value of the pressure inside the bulb. The plasma ball formation has been reproduced by another team of researchers with a similar setup but with a spinning bulb in order to stabilize the ball. From measurements of the oscillations of the ball diameter, they have shown that the acoustic amplitude inside the bulb can reach 180 dB (re20 μPa). In our setup, we have not measured this feature. However, improving our model of the plasma modulation, we have also been able to assess the acoustic amplitude, and we have found results in an agreement, as shown in this article. In addition, our analysis suggests that stimulated emission is involved in the observed fast time scale modulation of the light emission.