Researchers from the University of York in UK and scientists from the Tata Institute of Fundamental Research in Mumbai were examining the interaction of an ultra-intense laser with a plasma target when they saw something unexpected.
In the trillionth of a second after the laser strikes, plasma flowed rapidly from areas of high density to more stagnant regions of low density, in such a way that it created something like a traffic jam, researchers said.
Plasma piled up at the interface between the high and low density regions, generating a series of pressure pulses: a sound wave.
With a frequency of nearly a trillion hertz, the sound generated was not only unexpected, but was also close to the highest frequency possible in such a material – six million times higher than that which can be heard by any mammal, researchers said.
“One of the few locations in nature where we believe this effect would occur is at the surface of stars,” said Dr John Pasley, of the York Plasma Institute in the Department of Physics at York.
The technique used to observe the sound waves in the lab works very much like a police speed camera. It allows the scientists to accurately measure how fluid is moving at the point that is struck by the laser on timescales of less than a trillionth of a second.
“It was initially hard to determine the origin of the acoustic signals, but our model produced results that compared favourably with the wavelength shifts observed in the experiment,” Robinson said.
“This showed that we had discovered a new way of generating sound from fluid flows. Similar situations could occur in plasma flowing around stars,” he said.