Ultrafast laser filamentation is a nonlinear process that occurs when a high-power laser pulse propagates through air or another transparent medium, forming a thin plasma channel known as a filament. The filament results from self-focusing and beam collapse, which can occur over distances ranging from meters to kilometers, depending on the laser properties and propagation medium. These filaments have applications in standoff plasma generation for gas, aerosol, and solid material probing, laser and supercontinuum light guiding, and controlled electrical discharge [1], [2]. To study filament phenomena in a space-constrained laboratory environment, the pulse collapse is typically facilitated by pre-focusing the beam with a lens. However, the properties of the resulting filament can differ from those formed through unassisted nonlinear self-focusing. Lower f-number lens focusing produces strong plasma generation, but geometric focusing dominates light propagation, suppressing the energy-dependent self-focusing and intensity clamping effects characteristic of unassisted filamentation. In contrast, large f-number focusing reduces these effects, and increases propagation distance. The transition between linear and nonlinear regimes-shaped by the interplay among strong plasma generation, defocusing due to geometric effects, and Kerr self-focusing-introduces noticeable changes to filament characteristics.
Nonthermal atmospheric pressure plasma jets (APPJs) are of interest due to the reactive plasma species including electric fields and reactive oxygen and nitrogen species (RONS), which can modulate cellular function and are of growing interest for biomedical applications. These factors can be influenced by the presence of biological materials as part of the electric circuit [1]. In this work, the temporal and spatial distribution of the electric field in a nanosecond pulsed APPJ (ns-APPJ) impinging on different biomaterials was investigated by the picosecond electric field induced second harmonic (E-FISH) method. Driven by 200 ns, 7 kV pulses at 1 kHz, a helium plasma jet was generated between the hollow needle and the biomaterial. The spatiotemporally resolved measurements were performed in a needle-to-plate electrode configuration with ultrapure water, phosphate-buffered saline (PBS), and pig skin as the grounded targets by scanning a fs-duration laser sheet across the interelectrode gap. At ~40 ns after the onset of the discharge current, the streamer head arrived at the surface of the biomaterials, placed 1 cm away from the HV needle tip, and the electric field of the streamer head reached the peak values of 15, 12, and 18 kV/cm for ultrapure water, PBS, and pig skin, respectively. The radially symmetric electric field profile was observed relaxing with the distance away from the needle tip.
Dusty or complex plasma provides an ideal environment for studying nonlinear wave phenomena, as it enables direct visualization of particle dynamics by tracking individual micron-sized grains. Shock waves in such systems have previously been generated using various external perturbations, most commonly through the application of a square pulse that produces a sudden potential or density disturbance propagating through the medium at characteristic speeds. However, shock formation resulting from continuous mechanical forcing-analogous to the classical piston problem in fluid dynamics [1]–remains comparatively less explored in complex plasma systems. The piston shock problem represents a prototypical example of strongly nonlinear compressible flow, widely used to study extreme fluid behavior, nonlinear steepening, and shock structure formation. In the present work, we experimentally investigate the piston-driven shock problem in a dusty plasma medium. The motion of an object through the strongly coupled dust component at sufficiently high velocity generates a compressive disturbance that evolves nonlinearly and steepens into a shock front as it propagates. By directly imaging individual particles, we resolve the detailed spatiotemporal evolution of density, velocity, and structural rearrangements within the shock region. The experiments reveal rich nonlinear behavior, including shock front steepening, shock splitting under specific parameter regimes, and the formation of oscillatory shock structures consistent with dispersive effects in strongly coupled media. To interpret these experimental observations, we carried out complementary numerical simulations.[2] The simulation framework developed in this work closely reproduces the experimental conditions and provides a comprehensive physical picture of shock generation, steepening, and propagation in a complex plasma medium. Achieving strong agreement between simulations and experiments represents a significant advancement. It enhances our understanding of nonlinear shock dynamics in strongly coupled plasmas and establishes a reliable modeling framework applicable to a broad range of systems, from plasma processing technologies to astrophysical shock phenomena.
The challenges of achieving reliable ignition and maintaining stable flames under ultra-lean combustion still remain. Transient plasma ignition (TPI) utilizing a surface discharge under repetitive nanosecond voltage pulses was shown to improve combustion efficiency by generating a sufficiently large ignition kernel without requiring an impractically high peak voltage in extremely lean-fuel mixtures [1]. This study examines the spatiotemporal distribution of transient electric field in surface discharge prior to the discharge breakdown or development of ignition kernel in a lean $(\phi=0.4)$ CH4 / air mixture at 1 bar using the electric field-induced second harmonic generation (EFISH) method. The electric field reaches its maximum during the rising phase of the voltage pulse and drops quickly as the current starts increasing rapidly. The peak electric field for a high-impedance streamer discharge excited at 6 kV was found to be 38 kV/cm. At higher voltages, the streamer transitions to a transient spark. A minimum peak electric field of 55 kV/cm was determined to initiate this mode transition at single pulse of 10 kV. The effect of the pulse repetition frequency on the electric field was studied by employing two-pulse sequences and determining the field of the $2^{\text {nd }}$ pulse.
Opacity describes how a material absorbs incident photons and transfers radiation through a plasma system. Wide-ranging and accurate opacity models are critical for informing simulations of high-energy density (HED) plasma systems like stars and inertial fusion targets. Benchmark opacity experiments that provide detailed absorption data as a function of photon energy from well-characterized samples enable rigorous evaluation of atomic-scale opacity models. This talk will present highlights of recent experimental opacity efforts at Sandia's Z facility, including new measurements of oxygen [1] and iron opacities as well as advances in data analysis [2]. The talk will also summarize theoretical and modeling efforts aimed at resolving the longstanding discrepancies [3] between modeled and measured opacities of HED iron.
