Inertially confined plasma in an imploding bubble

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 extreme3–5. 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. A bubble acoustically driven into nonlinear radial oscillation can focus the diffuse energy of the sound field by many orders of magnitude8. The energy focusing is such that broadband light emission is observed (sonoluminescence)4 and molecular bonds are broken (sonochemistry)9. Measurement of the bubble dynamics of a single sonoluminescing bubble (single-bubble sonoluminescence (SBSL)) has shown the implosion velocity to be greater than the speed of sound with enormous acceleration near maximum collapse10. The bubble dynamics and the properties of the emitted light suggest the generation of extreme intracavity conditions. Indeed, recent molecular dynamics simulations predict temperatures approaching 108 K but lasting for only a few hundred femtoseconds2. The extreme conditions generated during SBSL arise from quasi-adiabatic compression of the bubble contents. One measure of the intensity of bubble implosion is the ratio of maximum tominimumbubble volume (that is, compression ratio). The value of the compression ratio, and hence the bubble kinetic energy, increases with increasing acoustic pressure (Pa; ref. 11). Thus, at high Pa there is more energy available to be transferred to the bubble contents, which should ultimately produce more extreme intracavity conditions. Recently, Taleyarkhan and co-workers claimed to observe neutrons during acoustic cavitation in deuterated acetone12,13 resulting from intracavity fusion reactions (that is, ‘sonofusion’). These reports were met with immediate skepticism, and serious issues with the validity of the claims arose14,15. Indeed, subsequent studies in several independent laboratories have not succeeded in reproducing the original report16–18. Despite the controversy