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Strongly coupled plasmas (SCPs) are plasmas in which the average Coulomb potential energy of the interacting particles exceeds their average thermal kinetic energy. For electrons, the coupling parameter is defined as Γ = (e2 / 4πε0kBTe)(4πne / 3)1/3 where e, ε0, kB, Te, and ne are the electron charge, vacuum permittivity, Boltzmann constant, the electron temperature, and the electron density, respectively. Standard theories for an ideal plasma fail to describe strongly-correlated systems with Γ ≳ 1 [1, 2]. Unlike ideal plasmas, the Debye shielding assumption fails in SCPs. Furthermore, as Γ increases, the plasma differs from the gaseous behavior and exhibits liquid or solid characteristics. The short-range order interaction between particles makes the theoretical approach to SCP challenging. SCPs are encountered in many contexts such as galaxies, white dwarf stars, cores of Jovian planets, lightening in thick planetary atmospheres, and inertial confinement fusion experiments.
Figure 1 Plasma taxonomy retrieved from NRL Plasma Formulary (2016). The field of research in plasma physics is wide. The regime where the electron temperature or density is not extremely high or low has been traditionally studied. Strongly coupled plasmas are located in the top left corner of this phase diagram. Our experimental condition is shown with a green star.
In recent decades, theoretical and numerical studies have been pursued to characterize the equation of states and thermodynamic properties of SCPs, which are fundamentally different from those of weakly coupled plasmas. One of the essential research topics is energy transport by radiation or opacity. In particular, in a subcritical medium at a low temperature, condensation renders the medium inhomogeneous, which significantly affects the radiation transport or opacity.
However, no study has been conducted for opacity in inhomogeneous supercritical fluids (SCFs). Our recent study reveals that an inhomogeneous SCF with nanometer-sized clusters and micrometer-sized droplets can be prepared [3]. In this lab, we experimentally demonstrate that the emission timescale of an SCP in an inhomogeneous SCF is extended by up to 50% compared to that in a homogeneous SCF [4]. This implies that the inhomogeneity of the SCF significantly enhances the photon confinement. This result is expected to draw interest in the investigation of radiation transport or opacity in the inhomogeneous SCF.
Figure 2 Filtered images of the plasma discharges [4]. The first (last) two columns show the images with a bandpass filter 290 ± 13 nm (700 ± 25 nm). For each filter, the images are taken under two different medium conditions (inhomogeneous and homogeneous) and are respectively normalized with respect to the maximum intensity of the two cases at each time frame. Jet-like plasma shapes are similar in both medium conditions with their boundaries following the laser beam’s envelope (yellow dashed line). The plasma shape is attributed to the progressive backward expansion of the plasma absorbing the laser energy.
Figure 3 Blackbody analysis of the plasma radiation spectrum [4]. (a) Absolutely calibrated broadband spectra from the plasmas along with the corresponding blackbody fits. The spectra are fit with the blackbody radiation model within 230–640 nm, which excludes the highly broadened argon atomic line emissions that appear in the red and near-infrared region on top of the rising baseline. The emission is mostly continuum in the early stage, and line emissions gradually increase with time. While the Ar I peaks appear for both cases, the Si I peaks are only observed in the case of inhomogeneous medium. The radiations from plasmas in different medium conditions deviate increasingly with time. (b) Emissivity and temperature of the plasmas as a function of time at the origin (r = 0 and z = 0). The emissivities for both plasmas are identical when t ≤ 50 ns. However, after t > 50 ns, the emissivities start to deviate from each other, indicating the photon confinement of the plasma in the inhomogeneous medium exceeds that in the homogeneous medium, for identical temperature.
The relevant examples include lightening in a thick planetary atmosphere that is instantaneously cooled and locally condensed by storms, inertial confinement fusion discharges of targets with inevitable defects from the fabrication process, nanomaterial synthesis, and laser-produced plasma light sources for extreme ultraviolet (EUV) nanolithography.
Reference
[1] Ichimaru, S Theory of strongly correlated charged-particle systems: plasma turbulence and high-density electron liquids. Phys. Rev. A 15 744 (1977)
[2] Murillo, M. S. Strongly coupled plasma physics and high energy-density matter Phys. Plasmas 11 2964–71 (2004)
[3] Lee, S. et al. Quasi-equilibrium phase coexistence in single component supercritical fluids. Nat. Commun. 12 4630 (2021)
[4] Lee, S. et al. Characterization of strongly coupled plasmas produced in argon supercritical fluids. Plasma Phys. Control. Fusion 64 095010 (2022)
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