Inertially confined plasma in an imploding bubble

Journal name:
Nature Physics
Volume:
6,
Pages:
598–601
Year published:
DOI:
doi:10.1038/nphys1701
Received
Accepted
Published online

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 1021cm−3—comparable to the densities produced in laser-driven fusion experiments7—with effective plasma temperatures ranging from 7,000 to more than 16,000K. 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.

At a glance

Figures

  1. SBSL from sulphuric acid (85[thinsp]wt% H2SO4 containing Ar at 5% of saturation).
    Figure 1: SBSL from sulphuric acid (85wt% H2SO4 containing Ar at 5% of saturation).

    a, Photograph of a rapidly translating sonoluminescing bubble at the velocity node of a spherical quartz resonator. The driver piezoceramic is partially visible at the bottom of the image, and the microphone is to the right. The entire apparatus is rigidly clamped at the narrow neck of the quartz flask (top of image). b, A typical SBSL emission spectrum from a bubble driven with a relatively low acoustic driving pressure, Pa. The emission lines (~700–900nm) are due to electronic transitions between states within the 4p and 4s array of neutral Ar. Inset: A higher-resolution spectrum of SBSL Ar emission and a least-squares Lorentzian fit at a thermally equilibrated temperature of 10,000K.

  2. SBSL Ar emission line profiles as a function of the acoustic driving pressure, Pa.
    Figure 2: SBSL Ar emission line profiles as a function of the acoustic driving pressure, Pa.

    The solid lines, from the narrowest linewidth to the broadest, correspond to Pa of 2.7, 3.0, 3.3, 3.6 and 3.8 bar. The dotted line centred at 763.51nm is the same Ar line (4p2[3/2] to 4s2[3/2] states) from a low-pressure Hg(Ar) calibration lamp. Intensities have been normalized to the peak intensity.

  3. Line shape and deviation from a Lorentzian fit of the emission from Ar 4p2[3/2] to 4s2[3/2] states.
    Figure 3: Line shape and deviation from a Lorentzian fit of the emission from Ar 4p2[3/2] to 4s2[3/2] states.

    a, SBSL Ar emission line from a single bubble driven with a Pa of 3.6 bar compared to a least-squares-fit Lorentzian profile. b, The antisymmetric deviation of the Ar emission line from the Lorentzian fit shown in a. The per cent deviation is normalized to the peak intensity of the Lorentzian fit and is plotted as a function of the line peak centre offset in units of FWHM of the Lorentzian. The data are smoothed with a second-degree polynomial regression (Savitzky–Golay filter) using five points to the left and to the right of each point. The arrow at the peak negative deviation marks the deviation used in our analysis.

  4. Deviation from a Lorentzian fit as a function of the acoustic driving pressure, Pa, for the emission from Ar 4p2[3/2] to 4s2[3/2] states.
    Figure 4: Deviation from a Lorentzian fit as a function of the acoustic driving pressure, Pa, for the emission from Ar 4p2[3/2] to 4s2[3/2] states.

    a, Dependence of the deviation of the SBSL Ar line profile from a symmetric Lorentzian profile on the Pa used to drive the bubble. All data were smoothed with a second-degree polynomial regression. The deviations were normalized to the peak intensity of the least-squares-fit Lorentzian of each SBSL Ar line profile. b, The antisymmetric deviation function26 as a function Pa.

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  1. School of Chemical Sciences, Chemical and Life Sciences Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA

    • David J. Flannigan &
    • Kenneth S. Suslick

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All authors contributed extensively to the work presented in this paper.

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