Plasma dynamics, instabilities and OH generation in a pulsed atmospheric pressure plasma with liquid cathode: a diagnostic study

While plasma-liquid interactions have been an important focus in the plasma research community, the impact of the strong coupling between plasma and liquid on plasma properties and processes remains not fully understood. In this work, we report on the impact of the applied voltage, pulse width and liquid conductivity on the plasma morphology and the OH generation for a positive pulsed DC atmospheric pressure plasma jet with He-0.1% H2O mixture interacting with a liquid cathode. We adopted diagnostic techniques of fast imaging, 2D laser induced fluorescence (LIF) of OH and Thomson scattering spectroscopy. We show that plasma instabilities and enhanced evaporation occur and have a significant impact on the OH generation. At elevated plasma energies, it is found that the plasma contracts due to a thermal instability through Ohmic heating and the contraction coincides with a depletion in the OH density in the core due to electron impact dissociation. For lower plasma energies, the instability is suppressed/delayed by the equivalent series resistor of the liquid electrode. An estimation of the energy flux from the plasma to the liquid shows that the energy flux of the ions released into the liquid by positive ion hydration is dominant, and significantly larger than the energy needed to evaporate sufficient amount of water to account for the measured H2O concentration increase near the plasma-liquid interface.

Cross-Beam energy transfer saturation: Ion heating and pump depletion

Cross-beam energy transfer (CBET) was measured in two regimes where the energy transfer saturation mechanism was determined by the plasma and laser beam conditions. Linear kinetic CBET theory agreed well with the measured energy transfer in all experiment configurations and at all probe beam intensities when accounting for pump depletion and the plasma conditions measured using Thomson-scattering. Simultaneous CBET and Thomson-scattering measurements enabled uncertainties in the plasma conditions to be isolated from CBET theory, which allowed the saturation mechanisms to be identified. In the perpendicular-beam configuration the saturation mode was through ion heating, which resulted from ion trapping in the driven waves and subsequent ion-ion collisional heating. In the co-propagating beam configuration there was minimal ion heating and the saturation mode was through pump depletion.

Nonlinear Thomson scattering from a tightly focused circularly polarized laser with varied initial phases

Within the frame of classical electrodynamics, nonlinear Thomson scattering by an electron of a tightly focused circularly polarized laser has been investigated. The electron motion and spatial radiation characteristics are studied numerically when the electron is initially stationary. The numerical analysis shows that the direction of the maximum radiation power is in linear with the initial phase of the laser pulse. Furthermore, we generalize the rule to the case of arbitrary beam waist, peak amplitude and pulse width. Then the radiation distribution is studied when the electron propagates in the opposite sense with respect to the laser pulse and the linear relationship still holds true. Last we pointed out the limitation of the single electron model in this paper.

Assessment of shutdown dose rates in the ITER Collective Thomson Scattering system and in equatorial port plug 12

The ITER Collective Thomson Scattering (CTS) system will be the main diagnostic responsible for measuring the velocity distribution function of fusion-born alpha particles in the plasma. As the CTS diagnostic is integrated in the equatorial port plug 12 (drawer 3), with direct apertures to the port interspace where maintenance hands-on operation will be carried out, it is essential to assess the shutdown dose rates (SDDR) in these maintenance areas. In this work, the D1S-UNED3.1.4 Monte-Carlo transport code, based on the implementation of the direct-one-step methodology in MCNP5 v1.60, was used to estimate the dose rate level 12 days (106 s) after shutdown in the port interspace. The results show that the CTS system does not contribute significantly to the SDDR in the area where hands-on maintenance is foreseen with contribution to dose rates less than 1 µSv/h. This is consistent with previous estimates, although with the most recent model of the CTS design there is a slight increase of the SDDR values. This deviation can be attributed to design changes and improved shielding modelling and/or most importantly, to statistical fluctuations of the D1S simulations. From a neutronics point of view, the increase in the SDDR falls within the range of the statistical fluctuations, and the design is still compliant with the radiation safety ALARA principle aiming at minimizing radiation doses, and there is no requirement for further design optimizations.

Thomson and collisional regimes of in-phase coherent microwave scattering off gaseous microplasmas

The total number of electrons in a classical microplasma can be non-intrusively measured through elastic in-phase coherent microwave scattering (CMS). Here, we establish a theoretical basis for the CMS diagnostic technique with an emphasis on Thomson and collisional scattering in short, thin unmagnetized plasma media. Experimental validation of the diagnostic is subsequently performed via linearly polarized, variable frequency (10.5–12 GHz) microwave scattering off laser induced 1–760 Torr air-based microplasmas (287.5 nm O2 resonant photoionization by ~ 5 ns, < 3 mJ pulses) with diverse ionization and collisional features. Namely, conducted studies include a verification of short-dipole-like radiation behavior, plasma volume imaging via ICCD photography, and measurements of relative phases, total scattering cross-sections, and total number of electrons $$N_{e}$$ N e in the generated plasma filaments following absolute calibration using a dielectric scattering sample. Findings of the paper suggest an ideality of CMS in the Thomson “free-electron” regime—where a detailed knowledge of plasma and collisional properties (which are often difficult to accurately characterize due to the potential influence of inhomogeneities, local temperatures and densities, present species, and so on) is unnecessary to extract $$N_{e}$$ N e from the scattered signal. The Thomson scattering regime of microwaves is further experimentally verified via measurements of the relative phase between the incident electric field and electron displacement.

Thomson Scattering in the Lower Corona in the Presence of Sunspots

Polarized scattered light from low (few tens of megameter altitudes) coronal transients has been recently reported in Solar Dynamics Observatory/Helioseismic and Magnetic Image (HMI) observations. In a classic paper, Minnaert (1930) provided an analytic theory of polarization via electron scattering in the corona. His work assumed axisymmetric input from the photosphere with a single-parameter limb-darkening function. This diagnostic has recently been used to estimate the free-electron number and mass of HMI transients near the solar limb, but it applies equally well to any coronal material, at any height. Here we extend his work numerically to incorporate sunspots, which can strongly effect the polarization properties of the scattered light in the low corona. Sunspot effects are explored first for axisymmetric model cases, and then applied to the full description of two sunspot groups as observed by HMI. We find that (1) as previously reported by Minnaert, limb darkening has a strong influence, usually increasing the level of linear polarization tangential to the limb; (2) unsurprisingly, the effects of the sunspot generally increase at the lower scatterer altitudes, and increase the larger the sunspot is and the closer to their center the scatterer subpoint is; (3) assuming the Stokes Q > 0 basis to be tangential to the limb, sunspots typically decrease the Stokes Q/I polarization and the perceived electron densities below the spotless case, sometimes dramatically; and (4) typically, a sizeable non-zero Stokes U/I polarization component will appear when a sunspot’s influence becomes non-negligible. However, that is not true in rare cases of extreme symmetry (e.g., scattering mass at the center of an axisymmetric sunspot). The tools developed here are generally applicable to an arbitrary image input.