Models of spherical supersonic bubble implosion in cavitating liquids predict that it could generate temperatures and densities sufficient to drive thermonuclear fusion1, 2. Convincing evidence for fusion is yet to be shown, but the transient conditions generated by acoustic cavitation are certainly extreme3, 4, 5. There is, however, a remarkable lack of observable data on the conditions created during bubble collapse. Only recently has strong evidence of plasma formation been obtained6. Here we determine the plasma electron density, ion-broadening parameter and degree of ionization during single-bubble sonoluminescence as a function of acoustic driving pressure. We find that the electron density can be controlled over four orders of magnitude and exceed 1021 cm−3—comparable to the densities produced in laser-driven fusion experiments7—with effective plasma temperatures ranging from 7,000 to more than 16,000 K. At the highest acoustic driving force, we find that neutral Ar emission lines no longer provide an accurate measure of the conditions in the plasma. By accounting for the temporal profile of the sonoluminescence pulse and the potential optical opacity of the plasma, our results suggest that the ultimate conditions generated inside a collapsing bubble may far exceed those determined from emission from the transparent outer region of the light-emitting volume.
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