Session CO8: Dusty Plasmas, Sources, and Diagnostics

Experimental measurement of shock thickness in a strongly coupled dusty plasma

Dusty plasma is a system of highly charged micron-size particles immersed in an ionized gas. Dusty plasma has much in common with warm dense matter (WDM): it can support shock waves, and it has a strongly coupled component. These common features allow us to use dusty plasma to study shock phenomena that also occur in WDM. An experiment we performed in a two-dimensional dusty plasma reveals that the shock width is as small as two interparticle distances. We were able to make this measurement at the particle level because dusty plasma allows the direct observation of individual particles.   

Experimental measurement of the confinement potential between a two-particlevertical dust chain

A dust particle in a 2D crystal structure or 1D chain structure is confined by forces from the
environment, including the interaction force between neighboring particles. This force can be
simplified by modeling it as a linear recovering force, f = kdx , which results from a parabolic
confinement potential kΔx²/2 . The force constant k = mω₀² depends on the experimental
conditions with m the dust mass and ω₀ the dust resonance frequency. Properties of the dusty
plasma system, such as the ion focusing effect downstream of particles in a flowing plasma, alter
the shape of the confining potential. In this case, accurately modeling the force between the
particles requires including higher-order, nonlinear terms. This presentation will discuss
employing probability distribution function and mean square displacement techniques to obtain
the non-linear force terms by analyzing dust particle position fluctuations. As an application, we
will analyze the linear thermal expansion coefficient of a two-particle vertical chain.   

Thermally Excited Dust Lattice Waves in PK-4 Complex Plasmas

In the PK-4 experiment onboard the International Space Station, the dust cloud can form systems of multiple chains within the DC trap. The study of the dispersion relations of the Dust Lattice Waves (DLWs) along and perpendicular to the chain direction can potentially reveal important information on intra- and inter-chain interactions. In this research, the phonon spectra for thermally excited DLWs are obtained from the spontaneous motion of particles in a single chain within a multi-chain system. Direct fitting of the experimental spectra to previous theoretical dispersion relations [1] gives a general scale of the intrachain interaction strength between particles. Using the measured particle charge and Debye length [2], the thermal Mach numbers found from fitting the dispersion relations are in agreement with prediction from kinetic theory for the ions [3]. These results are compared to simulations of dust chains in this environment, and the potential of using dispersion relations to determine interchain interactions discussed.   

Ionization Waves of Arbitrary Velocity

Flying focus is a technique that uses a chirped laser beam focused by a highly chromatic lens to produce an extended focal region within which the peak laser intensity can propagate at any velocity [1]. When that intensity is high enough to ionize a background gas, an ionization wave will track the intensity isosurface corresponding to the ionization threshold [2]. We report on the demonstration of such ionization waves of arbitrary velocity (IWAV’s) [3]. Subluminal and superluminal ionization fronts were produced that propagated both forward and backward relative to the ionizing laser. All backward and all superluminal cases mitigated the issue of ionization-induced refraction that typically inhibits the formation of long, contiguous plasma channels. IWAV’s can be used to optimize plasma-based laser amplifiers [4], photon accelerators, and many other applications.

Ultrafast Thomson Scattering and the Effects of Collisions on the Electron Plasma Wave Feature

Thomson-scattering spectra from collisional electron plasma waves were measured in an underdense (∼10¹⁹ cm^–3) H₂ plasma irradiated by a 60-ps, 1053-nm laser pulse with an intensity of ∼2  × 10¹⁴ W/cm². The picosecond-resolved spectra were obtained with a novel pulse-front–tilt compensated streaked optical spectrometer. The Thomson scattering spectrum provides a measure of the electron temperature and density and show that the temperature heats from 5 eV to 20 eV over the first 20 ps. Over this time the density increases from 0.8 × 10¹⁹ cm^–3 to its plateau at 1.1 × 10¹⁹ cm^–3. The temperature plateaus after 40 ps at ∼100 eV. The Thomson spectra have been compared to a collisionless Thomson-scattering model, a Bhatnagar–Gross–Krook model, and a solution of the Landau kinetic equation that includes collision integrals.