No abstract

No abstract

The trillionfold concentration of sound energy by a trapped gas bubble, so as to emit picosecond flashes of ultraviolet light, is found to be extremely sensitive to doping with a noble gas. Increasing the noble gas content of a nitrogen bubble to about 1% dramatically stabilizes the bubble motion and increases the light emission by over an order of magnitude to a value that exceeds the sonoluminescence of either gas alone. The spectrum also strongly depends on the nature of the gas inside the bubble: Xenon yields a spectral peak at about 300 nanometers, whereas the helium spectrum is so strongly ultraviolet that its peak is obscured by the cutoff of water.

No abstract

No abstract

The resolution with which the synchronous picosecond flashes of acoustically generated light can be measured has been improved. The flash widths are now found to be considerably less than 50 ps and the jitter in the time between flashes can also be substantially less than 50 ps. The flashes of sonoluminescence appear to turn off very sharply without ringing or after pulsing. PACS numbers: 43.35.Sx, 43.35.EiSonoluminescence is the phenomenon whereby nonlinear effects cause the energy in a sound field to self-focus to such an extent that light is emitted. Since the photons must originate from a region of molecular dimensions and since photon energies are measured in eV whereas sound energies are measured in ergs/cc, sonoluminescence involves a spontaneous amplification that spans 12 orders of magnitude. Although sonoluminescence (SL) was discovered over 50 yr ago it is only recently that the dynamical properties of the acoustically generated flashes of light were measured. • In those experiments it was found that the light is emitted in spherically symmetric flashes that are less than 100 ps in width and which repeat with a rate given by the acoustic frequency. The jitter in the time between flashes was also found to be less than 200 ps. Each flash comprises over 105 photons and originates from a trapped cavity (bubble) at the pressure antinode.We have now used the fastest available microchannel plate photomultiplier tube (PMT) to further resolve SL flashes. We now find that the flash widths are less than 50 ps and that the jitter in the time between flashes is also less than 50 ps. Furthermore the SL flashes appear to turn off abruptly. This point is made clear by Fig. 1, which shows the voltage versus time curves that are measured at the output of the PMT for an SL flash as well as the flash that is generated by a 34-ps laser pulser (Hamamatsu PLP01 ). The laser operates at 410 nm, which matches the blue color of the SL. The overlay of these traces indicates that the SL flash turnsoff well before the laser flash. The cleaner turn off of SL is probably due to the absence of the after pulsing (or ringing) which characterizes the laser pulser.The experimental arrangement (Fig. 2) consists of a deciliter spherical pyrex flask that is filled with degassed distilled water and then driven at its fundamental acoustic resonance. 1 The light originates from a trapped cavity at the pressure antinode and is detected by a Hamamatsu R2809U two-stage microchannel plate PMT. The rated rise time of Department of Electrical Engineering. the tube is 170 ps. The output of the PMT is fed into a sampling oscilloscope (HP 54124T) which yields traces that become Fig. 1 after Fourier analysis is used to remove the scatter. The scope creates a trace by averaging (and piecing together) the digitized response that is obtained from many repetitive samplings of the signal (single shot oscilloscopes do not operate at 50 GHz). For this means of signal acquisition the raw trace shows a scatter or noise of about 5% around the line drawn in Fig. 1...

A derivation of acoustic streaming in a steady-state thermoacoustic device is presented in the case of zero second-order time-averaged mass flux across the resonator section (nonlooped device). This yields analytical expressions for the time-independent second-order velocity, pressure gradient, and time-averaged mass flux in a fluid supporting a temperature gradient and confined between widely to closely separated solid boundaries, both in the parallel plate and in the cylindrical tube geometries (two-dimensional problem). From this, streaming can be evaluated in a thermoacoustic stack, regenerator, pulse tube, main resonator of a thermoacoustic device, or in any closed tube that supports a mean temperature gradient, providing only that the acoustic pressure, the longitudinal derivative of the pressure, and the mean temperature variation are known.

It is well known that cavitation collapse can generate intense concentrations of mechanical energy, sufficient to erode even the hardest metals and to generate light emissions visible to the naked eye [sonoluminescence (SL)]. Considerable attention has been devoted to the phenomenon of "single bubble sonoluminescence" (SBSL) in which a single stable cavitation bubble radiates light flashes each and every acoustic cycle. Most of these studies involve acoustic resonators in which the ambient pressure is near 0.1 MPa (1 bar), and with acoustic driving pressures on the order of 0.1 MPa. This study describes a high-quality factor, spherical resonator capable of achieving acoustic cavitation at ambient pressures in excess of 30 MPa (300 bars). This system generates bursts of violent inertial cavitation events lasting only a few milliseconds (hundreds of acoustic cycles), in contrast with the repetitive cavitation events (lasting several minutes) observed in SBSL; accordingly, these events are described as "inertial transient cavitation." Cavitation observed in this high pressure resonator is characterized by flashes of light with intensities up to 1000 times brighter than SBSL flashes, as well as spherical shock waves with amplitudes exceeding 30 MPa at the resonator wall. Both SL and shock amplitudes increase with static pressure.