Focus: Plasma Extremes Seen through Gas Bubble
A gas bubble in a liquid becomes a plasma and emits intense flashes of light when squeezed by sound waves, a process called sonoluminescence. Researchers continue to puzzle over the phenomenon, but now a team reporting in Physical Review Letters shows that a sonoluminescing bubble can serve as a test bed for theories of dense plasmas in astrophysical settings and nuclear fusion experiments. The team measured the bubble’s response to lasers at two wavelengths in order to distinguish among plasma theories and to measure the free electron density. The results confirmed that a bubble in a room-temperature liquid can produce a plasma that is nearly as dense as those used in fusion research.
Sonoluminescence has been studied for years and is now well-controlled in laboratory settings. A gas bubble surrounded by liquid and pounded by sound waves produces a flash of light during each sound-wave cycle. At the point of maximum compression, the gas in the bubble reaches a temperature of around kelvin, which is hot enough to ionize the gas, at least partially. The freed electrons deflect off of atoms or ions, causing them to emit photons in a picosecond-long flash of light (a process called thermal bremsstrahlung).
The bubble collapse, which concentrates the energy of the sound waves a trillion-fold, resembles the laser-induced implosions of fuel pellets in inertial confinement fusion (ICF). Experiments in ICF produce much hotter plasmas (millions of degrees), but they suffer from instabilities that allow energy to leak out. Sonoluminescence, by contrast, creates spherically symmetric plasmas thousands of times per second that are stable for days at a time, offering an opportunity to study a manageable system at “the jumping off point” to fusion, says Seth Putterman of the University of California, Los Angeles. So in addition to studying the sonoluminescence effect for its own sake, Putterman and his colleagues have now begun to probe the plasma inside the bubble to better understand other plasmas in a similar range of density and temperature.
Putterman and his colleagues have developed a technique for focusing laser pulses onto a sonoluminescing bubble and recording both the scattered laser light and the bubble flash. In a previous experiment investigating -micrometer-wide bubbles in phosphoric acid, the team measured strong absorption by the bubbles, implying a high density of free electrons , but they could only set a lower limit on the density. The researchers have now adjusted their system to use two wavelengths of laser light, and they’ve switched to water, which produces smaller ( -micrometer), denser bubbles than acid.
Using standard techniques, the researchers generated a xenon gas bubble in the center of a water-filled cylinder outfitted with actuators that produced a standing sound wave at a frequency of kilohertz. The team synchronized laser pulses to hit the bubble at the moment in the sound wave cycle when the plasma formed. Essential information came from sensors that detected the bubble’s flashes—the brighter the flash, the more laser energy had been absorbed by the bubble.
Measuring absorption at two different laser wavelengths ( and nanometers) allowed the team to test models of dense, highly ionized plasmas. One critical issue is the speed that energy is transported within the plasma via collisions among particles. Two classes of models have different predictions for the particle interactions, differing by a factor of in the collision frequency. The team found that the model with a higher collision rate was a better fit to their laser-plasma interaction data. Within this model, they calculated an electron density of electrons per cubic centimeter, which agrees with a previous sonoluminescence experiment in acid that used a different technique . By comparison, ICF researchers aim for around electrons per cubic centimeter to ignite fusion. Putterman believes sonoluminescence research might offer insights into techniques to confine dense plasmas for long periods of time.
The experiments represent “an exciting step forward,” says Jerome Daligaut of Los Alamos National Laboratory in New Mexico. He says that researchers generally rely on simulations for even the most basic properties of dense plasmas because of the experimental challenges. So these “ingenious” experiments could eventually help test theories with a low-cost alternative to large and expensive plasma facilities, he says.
Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.
- S. Khalid, B. Kappus, K. Weninger, and S. Putterman, “Opacity and Transport Measurements Reveal That Dilute Plasma Models of Sonoluminescence Are Not Valid,” Phys. Rev. Lett. 108, 104302 (2012)
- D. J. Flannigan and K. S. Suslick, “Inertially Confined Plasma in an Imploding Bubble,” Nature Phys. 6, 598 (2010)