Detailed observations have been performed on the evolution of a viscous catenary, a rope of high-viscosity fluid suspended from two points falling under gravity. Stroboscopic imaging techniques are used to obtain the position and shape of the strand as a function of time. Depending on their initial thickness and profile, the filaments are observed to evolve into either a quasi-catenary, or other, more complex shapes. A conceptually simple, energy-based theory is developed and compared with observations. It is shown to describe reasonably, except for a scaling in the time scale, the catenary-like regime.

Spark discharges in high-pressure gas are known to emit a broadband spectrum during the first 10 s of nanoseconds. We present calibrated spectra of high-pressure discharges in xenon and show that the resulting plasma is optically thick. Laser transmission data show that such a body is opaque to visible light, as expected from Kirchoff's law of thermal radiation. Nanosecond framing images of the spark absorbing high-power laser light are presented. The sparks are ideal candidates for nanosecond, high-power laser switches. V C 2014 AIP Publishing LLC.

Sound can hold partially ionized sulfur at the center of a spherical bulb. We use the sulfur plasma itself to drive a 180 dB re 20 µPa sound wave by periodically heating it with microwave pulses at a frequency that matches the lowest order, spherically symmetric acoustic resonance of the bulb. To clarify the trapping mechanism, we generalize acoustic radiation pressure theory to include gaseous inhomogeneities and find an interaction of highamplitude sound with density gradients in the gas through which it propagates. This is the pycnoclinic acoustic force (PAF). Though generated by rapidly oscillating sound waves, it has a finite time average and manipulates the plasma through density gradients at its boundary. The PAF is essential for the description of the trap holding a plasma against its own buoyancy as well as understanding convection in the region outside the plasma. It has implications for pulse tubes, thermoacoustic engines, thermal vibrational convection in microgravity, combustion in the presence of sound, and the modeling of Cepheid variable stars

Acoustics is used to probe the temperature profile within a sulfur plasma lamp. A spherically symmetric temperature profile is assumed that drops with the square of the radius, consistent with a constant volumetric heating model. Acoustic resonance frequencies are calculated exactly in the case of an ideal gas. Experimental measurement of a few resonant frequencies allows determination of the temperature profile curvature. This technique can be viewed as an extension of ultrasonic resonant spectroscopy to systems that are highly non-uniform due to off-equilibrium energy flow.

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