Plasma sparks from a hot gas bubble

By I. Peterson

When intense, high-frequency sound waves bombard a gas bubble in water, they can induce the bubble to generate brief, brilliant flashes of visible and ultraviolet light. Known as sonoluminescence, this conversion of sound into light occurs during the rapid, violent contraction of a bubble as it oscillates in step with the sound wave. Now, William C. Moss and his colleagues at the Lawrence Livermore (Calif.) National Laboratory have applied techniques from nuclear fusion research to model the behavior of the gas inside a luminescing bubble. The researchers report their findings in the May 30 Science.

The team assumed that a collapsing bubble generates an intense shock wave, which compresses and heats the gas to create a partially ionized, light-emitting plasma of ions and electrons. Computer simulations of the behavior of such a plasma reveal that accelerated electrons produce the flash, and rapid changes in the plasma's transparency limit its duration. Moreover, both the duration and the spectra of flashes are very sensitive to the maximum bubble radius, which could explain the puzzling variability of experimental results. The team's calculations also show that only the argon component of the gas is involved in the process.

"Although it remains to be confirmed experimentally that shock waves or plasmas are present in a bubble undergoing [sonoluminescence], no other model of which we are aware has been able to explain such a broad array of experimental data," Moss and his coworkers conclude.

The results also suggest that finding a way to increase the maximum bubble radius significantly might lead to thermonuclear fusion inside an oscillating bubble, say Lawrence A. Crum and Thomas J. Matula of the University of Washington in Seattle.

References:

Crum, L.A., and T.J. Matula. 1997. Shocking revelations. Science 276(May 30):1348-1349.

Moss, W.C., D.B. Clarke, and D.A. Young. 1997. Calculated pulse widths and spectra of a single sonoluminescing bubble. Science 276(May 30):1398-1401.

From Science News, Vol. 151, No. 25, June 21, 1997, p. 391.