Observation of Shell Structure, Electronic Screening, and Energetic Limiting in Sparks.

We study the formation of micron-sized spark discharges in high-pressure xenon on the nanosecond time scale. The spark's energy per length is measured through the expansion dynamics of the generated shock wave, and is observed to scale linearly with the spark radius. At the same time, the surface temperature of the spark channel remains constant. Together, these observations allow us to conclude that the spark channel, up to 40 µm in overall radius, is actually an energetically hollow shell about 20 µm thick. Further, the energy per nucleus in the shell is about 15 eV, independent of size and density. To reconcile these findings with the opacity to visible light, we appeal to collective screening processes that dramatically lower the effective ionization potential, allowing a much higher electron density than is otherwise expected. Thus, nanosecond measurements of sparks provide access to the thermodynamics and kinetics of strongly correlated plasmas.

Blackbody emission from laser breakdown in high-pressure gases.

Laser induced breakdown of pressurized gases is used to generate plasmas under conditions where the atomic density and temperature are similar to those found in sonoluminescing bubbles. Calibrated streak spectroscopy reveals that a blackbody persists well after the exciting femtosecond laser pulse has turned off. Deviation from Saha's equation of state and an accompanying large reduction in ionization potential are observed at unexpectedly low atomic densities-in parallel with sonoluminescence. In laser breakdown, energy input proceeds via excitation of electrons whereas in sonoluminescence it is initiated via the atoms. The similar responses indicate that these systems are revealing the thermodynamics and transport of a strongly coupled plasma.

Collision time measurements in a sonoluminescing microplasma with a large plasma parameter.

The plasma which forms inside of a micron-sized sonoluminescing bubble in water for under a nanosecond has been probed with 3 ns long laser pulses. A comparison of the response to 532 and 1064 nm light indicates that the plasma number density is about 2×10(21) cm(-3) and that transport properties are dominated by strong screening and correlation effects. The spherical shape, well-defined atomic density, and blackbody temperature make the sonoluminescing plasma a test bed for theories of strongly coupled plasmas. The plasma in this experiment distinguishes between competing theories of strong, intermediate, and weak effective screening.

Energy balance for a sonoluminescence bubble yields a measure of ionization potential lowering.

Application of energy conservation between input sound and the microplasma which forms at the moment of sonoluminescence places bounds on the process, whereby the gas is ionized. Detailed pulsed Mie scattering measurements of the radius versus time for a xenon bubble in sulfuric acid provide a complete characterization of the hydrodynamics and minimum radius. For a range of emission intensities, the blackbody spectrum emitted during collapse matches the minimum bubble radius, implying opaque conditions are attained. This requires a degree of ionization >36%. Analysis reveals only 2.1±0.6 eV/atom of energy available during light emission. In order to unbind enough charge, collective processes must therefore reduce the ionization potential by at least 75%. We interpret this as evidence that a phase transition to a highly ionized plasma is occurring during sonoluminescence.

Stable charged-particle acceleration and focusing in a laser accelerator using spatial harmonics.

Regarding the laser-driven acceleration of charged particles in photonic systems, a central unmet challenge is the achievement of simultaneous transverse and longitudinal stability at nonultrarelativistic energies. At such energies, Earnshaw's theorem [S. Earnshaw, Trans. Cambridge Philos. Soc. 7, 97 (1842)] indicates that a synchronous accelerating wave gives a defocusing effect. We present a scheme in which particles are accelerated by interaction with a resonant spatial harmonic and are focused by strong ponderomotive interaction with nonresonant spatial harmonics. We show that this scheme exhibits net transverse focusing and longitudinal stability, and we discuss its use in a compact laser accelerator.

Upper bound for neutron emission from sonoluminescing bubbles in deuterated acetone.

An experimental search for nuclear fusion inside imploding bubbles of degassed deuterated acetone at 0 degrees C driven by a 15 atm sound field and seeded with a neutron generator reveals an upper bound that is a factor of 10 000 less than the signal reported by Taleyarkhan et al. The strength of our upper bound is limited by the weakness of sonoluminescence, which we ascribe to the relatively high vapor pressure of acetone.

Neutron production from feedback controlled thermal cycling of a pyroelectric crystal.

The LLNL Crystal Driven Neutron Source is operational and has produced record ion currents of approximately 10 nA and neutron output of 1.9(+/-0.3)x10(5) per thermal cycle using a crystal heating rate of 0.2 degrees C/s from 10 to 110 degrees C. A 3 cm diameter by 1 cm thick LiTaO(3) crystal with a socket secured field emitter tip is thermally cycled with feedback control for ionization and acceleration of deuterons onto a deuterated target to produce D-D fusion neutrons. The entire crystal and temperature system is mounted on a bellows which allows movement of the crystal along the beam axis and is completely contained on a single small vacuum flange. The modular crystal assembly permitted experimental flexibility. Operationally, flashover breakdowns along the side of the crystal and poor emitter tip characteristics can limit the neutron source. The experimental neutron results extend earlier published work by increasing the ion current and pulse length significantly to achieve a factor-of-two higher neutron output per thermal cycle. These findings are reviewed along with details of the instrument.

Observation of nuclear fusion driven by a pyroelectric crystal.

While progress in fusion research continues with magnetic and inertial confinement, alternative approaches--such as Coulomb explosions of deuterium clusters and ultrafast laser-plasma interactions--also provide insight into basic processes and technological applications. However, attempts to produce fusion in a room temperature solid-state setting, including 'cold' fusion and 'bubble' fusion, have met with deep scepticism. Here we report that gently heating a pyroelectric crystal in a deuterated atmosphere can generate fusion under desktop conditions. The electrostatic field of the crystal is used to generate and accelerate a deuteron beam (> 100 keV and >4 nA), which, upon striking a deuterated target, produces a neutron flux over 400 times the background level. The presence of neutrons from the reaction D + D --> 3He (820 keV) + n (2.45 MeV) within the target is confirmed by pulse shape analysis and proton recoil spectroscopy. As further evidence for this fusion reaction, we use a novel time-of-flight technique to demonstrate the delayed coincidence between the outgoing alpha-particle and the neutron. Although the reported fusion is not useful in the power-producing sense, we anticipate that the system will find application as a simple palm-sized neutron generator.

Comparison of obstetric outcomes in twin pregnancies after in vitro fertilization, ovarian stimulation and spontaneous conception.

OBJECTIVE: To compare the outcomes of liveborn twin gestations conceived after in vitro fertilization (IVF) or ovarian stimulation with spontaneously conceived twin pregnancies. METHODS: A review of all twin gestations delivered at Winthrop-University Hospital from 1 January 1999 to 31 December 2000. Women who underwent fetal reduction or had a demise of one twin were excluded. Maternal demographics, antepartum complications, mode of delivery and perinatal outcome were compared. RESULTS: Sixty pregnancies were conceived after IVF, 34 were conceived by ovarian stimulation and 101 were spontaneously conceived. Women in the IVF group were older (p < 0.001), were more often 35 years or older (p < 0.001) and primiparous (p = 0.005). More women in the ovarian stimulation group had a poor obstetric history (p = 0.04). Spontaneous gestations had a higher incidence of monochorionic placentations (p = 0.002). There were no differences in gestational age at delivery, antepartum complications, or mode of delivery. There were fewer low-birth-weight neonates in the IVF group (odds ratio 0.59, 95% confidence interval 0.35-0.98; p = 0.03) than in the spontaneous group, but the difference disappeared when only the dichorionic pregnancies were compared. Other neonatal outcomes studied were the same among groups. CONCLUSION: Twin gestations conceived following IVF and ovarian stimulation appear to have similar outcomes to spontaneously conceived twin gestations.

Blackbody spectra for sonoluminescing hydrogen bubbles.

The dynamical motion of sonoluminescing bubbles formed from a mixture of water and hydrogen gas indicates that these bubbles contain hydrogen. Their spectrum is well matched by an ideal 6000 K blackbody radiating from a surface with a radius less than 1/4 microm. According to this model, the state of matter inside the collapsed bubble is so stressed that the photon mean free path is much smaller than 1 microm. Implications for various theories of the light-emitting mechanism and the role of chemical reactions are discussed.

Comment on "Mie scattering from a sonoluminescing bubble with high spatial and temporal resolution".

A key parameter underlying the existence of sonoluminescence (or SL) is the time dependence of the radius R(t) of the collapsing bubble from which SL originates. With regard to the use of light scattering to measure this quantity, we wish to note that we disagree with the statement of Gompf and Pecha-highly compressed water causes the minimum in scattered light to occur 700 ps before SL-and that this effect leads to an overestimate of the bubble wall velocity. We discuss potential artifacts in their experimental arrangement and reply to their criticisms of our experiments on Mie scattering.

Observation of bubble dynamics within luminescent cavitation clouds: Sonoluminescence at the nano-scale.

Measurements of acoustically driven cavitation luminescence indicate that this phenomenon is robust over a huge parameter space ranging from 10 kHz to >10 MHz. The minimum bubble radius achieved is an upper bound for the size of the light-emitting region and ranges from about 1 microm at 15 kHz to tens of nm at 11 MHz. Although lines can be discerned in the spectra of some cavitation clouds, they sit on top of a broadband continuum which can have greater spectral density in the ultraviolet than is observed for resonantly driven sonoluminescence from a single bubble.

Sonoluminescence: nature's smallest blackbody.

The transduction of sound into light through the implosion of a bubble of gas leads to a flash of light whose duration is delineated in picoseconds. Combined measurements of spectral irradiance, Mie scattering, and flash width (as determined by time-correlated single-photon counting) suggest that sonoluminescence from hydrogen and noble-gas bubbles is radiation from a blackbody with temperatures ranging from 6000 K (H(2)) to 20,000 K (He) and a surface of emission whose radius ranges from 0.1 microm (He) to 0.4 microm (Xe) . The state of matter that would admit photon-matter equilibrium under such conditions is a mystery.

Temperature and pressure dependence of sonoluminescence.

The dependence of sonoluminescence on ambient pressure and temperature is measured. As water is cooled, there occurs a 100-fold increase in light emission which can be accompanied by only slight changes in the ambient radius of the pulsating bubble. This suggests that water vapor trapped in the collapsing bubble is a key parameter for this system. For fixed concentration of gases in water, the maximum intensity of sonoluminescence decreases as the ambient pressure is lowered below 1 atm.

Physical acoustics of ultrasound-assisted lipoplasty.

The acoustic fields generated by probes and cannulas used for ultrasound-assisted lipoplasty (UAL) are sources of cavitation and sonoluminescence. The localized stress fields and heating caused by cavitation are strong enough to lyse cells and, in the authors' opinion, constitute the means of therapeutic action of UAL. The spectrum of sonoluminescence extends into the ultraviolet. Various devices have been calibrated and various issues relating to health risks are discussed.

Effect of noble gas doping in single-bubble sonoluminescence.

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.