John P. Koulakis

Nanosecond high-power dense microplasma switch for visible light

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 Kirchoffs 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.

Comment on “Early stage time evolution of a dense nanosecond microdischarge used in fast optical switching applications” [Phys. Plasmas 22, 123518 (2015)]

In an effort to understand the opaque nature of high-voltage nanosecond microdischarges and their ability to block 532 nm nanosecond laser pulses, as demonstrated by Bataller et al. [Appl. Phys. Lett. 105, 223501 (2014)], Levko and Raja have published a paper [Phys. Plasmas 22, 123518 (2015)] which simulates spark discharges in 1 bar of xenon gas. At this ambient pressure, Levko and Raja simulate final electron densities much too low to explain the observed opacity and conclude the probing laser generates the requisite ionization for self-blocking. However, the experimental findings of Bataller et al. showed opacity being reached at 10 bar, an order of magnitude larger than the pressure simulated by Levko and Raja. Furthermore, Bataller et al. showed the spark remained opaque for very low laser intensities, insufficient to cause any significant ionization. Although the simulation performed by Levko and Raja could have merit at 1 bar, it is not an appropriate comparison to the regime studied by Bataller et al.

Pycnoclinic acoustic force

The interaction of high amplitude sound with density gradients in the background gas through which the sound propagates gives rise to the pycnoclinic acoustic force (PAF). This force is a generalization of acoustic radiation pressure for non-isentropic systems and is large compared to the known second-order pressure associated with sound when there is a large density change over a distance that is shorter than a wavelength. The PAF can squeeze pockets of low density gas or pull dense gas into regions of lower density. It is needed for a full understanding of Rijke and Sondhauss tubes, combustion in the presence of sound, and acoustic mixing of different density gases. A mathematical derivation is given and photographs in the literature provide evidence for its existence. The authors demonstrate an acoustic plasma trap based on these principles.The interaction of high amplitude sound with density gradients in the background gas through which the sound propagates gives rise to the pycnoclinic acoustic force (PAF). This force is a generalization of acoustic radiation pressure for non-isentropic systems and is large compared to the known second-order pressure associated with sound when there is a large density change over a distance that is shorter than a wavelength. The PAF can squeeze pockets of low density gas or pull dense gas into regions of lower density. It is needed for a full understanding of Rijke and Sondhauss tubes, combustion in the presence of sound, and acoustic mixing of different density gases. A mathematical derivation is given and photographs in the literature provide evidence for its existence. The authors demonstrate an acoustic plasma trap based on these principles.

Magnetron Coupling to Sulfur Plasma Bulb

Sulfur plasma lamps are a convenient, table-top system for the study of acoustics in dense, weakly ionized plasmas. Herein, we describe the construction and tuning of a passive waveguide circuit capable of igniting and sustaining the sulfur plasma and exciting acoustic modes within it.

Interstitial Matrix Prevents Therapeutic Ultrasound From Causing Inertial Cavitation in Tumescent Subcutaneous Tissue

We search for cavitation in tumescent subcutaneous tissue of a live pig under application of pulsed, 1-MHz ultrasound at 8 W cm-2 spatial peak and pulse-averaged intensity. We find no evidence of broadband acoustic emission indicative of inertial cavitation. These acoustic parameters are representative of those used in external-ultrasound-assisted lipoplasty and in physical therapy and our null result brings into question the role of cavitation in those applications. A comparison of broadband acoustic emission from a suspension of ultrasound contrast agent in bulk water with a suspension injected subcutaneously indicates that the interstitial matrix suppresses cavitation and provides an additional mechanism behind the apparent lack of in-vivo cavitation to supplement the absence of nuclei explanation offered in the literature. We also find a short-lived cavitation signal in normal, non-tumesced tissue that disappears after the first pulse, consistent with cavitation nuclei depletion in vivo.

Acousto-convective relaxation oscillation in plasma lamp

Periodic instability in sulfur plasma bulbs driven at their acoustic resonant frequency leads to behavior similar to a relaxation oscillation. This instability, which develops over the course of more than 500 acoustic periods manifests as both an oscillation in the total luminosity of the lamp and a frequency modulation of the resonator. We present evidence that the cause of this oscillation cycle stems from periodic eruptions of plasma from within a region near the acoustic velocity antinode. We propose that these oscillations and eruptions indicate a coupling between high amplitude acoustic waves and interface waves and consider whether this system might provide a platform with which to study turbulent heat transport.Periodic instability in sulfur plasma bulbs driven at their acoustic resonant frequency leads to behavior similar to a relaxation oscillation. This instability, which develops over the course of more than 500 acoustic periods manifests as both an oscillation in the total luminosity of the lamp and a frequency modulation of the resonator. We present evidence that the cause of this oscillation cycle stems from periodic eruptions of plasma from within a region near the acoustic velocity antinode. We propose that these oscillations and eruptions indicate a coupling between high amplitude acoustic waves and interface waves and consider whether this system might provide a platform with which to study turbulent heat transport.

Comment on “Fluid modeling of a high-voltage nanosecond pulsed xenon microdischarge” [Phys. Plasmas 23, 073513 (2016)]

Simulations of sparks in 10 atmosphere Xenon gas by Levko and Raja [Phys. Plasmas 23, 073513 (2016)] are unable to reproduce the experimental fact of their opacity to visible light [Bataller et al., Appl. Phys. Lett. 105, 223501 (2014)]. Levko and Raja have argued the discrepancy is due to enhanced ionization from the probing laser radiation and/or cathode field emission. Having observed comparable opacity in similar systems without probing lasers and without electrodes, we instead argue that the enhanced ionization is a thermodynamic result of dense plasma screening effects that lower the effective ionization potential. Levko and Raja do not adequately address these density effects in their spark discharge simulations.