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1. Low-temperature synthesis of crystalline vanadium oxide films using oxygen plasmas

   https://doi.org/10.1116/6.0002383  

   Mohammad, Adnan; Joshi, Krishna D; Rana, Dhan; Ilhom, Saidjafarzoda; Wells, Barrett; Willis, Brian; Sinkovic, Boris; Okyay, A K; Biyikli, Necmi (May 2023, Journal of Vacuum Science & Technology A)

   Vanadium oxide (VOx) compounds feature various polymorphs, including V2O5 and VO2, with attractive temperature-tunable optical and electrical properties. However, to achieve the desired material property, high-temperature post-deposition annealing of as-grown VOx films is mostly needed, limiting its use for low-temperature compatible substrates and processes. Herein, we report on the low-temperature hollow-cathode plasma-enhanced atomic layer deposition (ALD) of crystalline vanadium oxide thin films using tetrakis(ethylmethylamido)vanadium and oxygen plasma as a precursor and coreactant, respectively. To extract the impact of the type of plasma source, VOx samples were also synthesized in an inductively coupled plasma-enhanced ALD reactor. Moreover, we have incorporated in situ Ar-plasma and ex situ thermal annealing to investigate the tunability of VOx structural properties. Our findings confirm that both plasma-ALD techniques were able to synthesize as-grown polycrystalline V2O5 films at 150 °C. Postdeposition thermal annealing converted the as-grown V2O5 films into different crystalline VOx states: V2O3, V4O9, and VO2. The last one, VO2 is particularly interesting as a phase-change material, and the metal-insulator transition around 70 °C has been confirmed using temperature-dependent x-ray diffraction and resistivity measurements. 

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2. Benchmarking of hydrodynamic plasma waveguides for multi-GeV laser-driven electron acceleration

   https://doi.org/10.1103/PhysRevAccelBeams.27.081302  

   Miao, B; Rockafellow, E; Shrock, J E; Hancock, S W; Gordon, D; Milchberg, H M (August 2024, Physical Review Accelerators and Beams)

   Hydrodynamic plasma waveguides initiated by optical field ionization have recently become a key component of multi-GeV laser wakefield accelerators. Here, we present the most complete and accurate experimental and simulation-based characterization to date, applicable to current multi-GeV experiments and future 100 GeV-scale laser plasma accelerators. Crucial to the simulations is the correct modeling of intense Bessel beam interaction with meter-scale gas targets, the results of which are used as initial conditions for hydrodynamic simulations. The simulations are in good agreement with our experiments measuring evolving plasma and neutral hydrogen density profiles using two-color short pulse interferometry, enabling realistic determination of the guided mode structure for application to laser-driven plasma accelerator design. Published by the American Physical Society2024 

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3. Target sensitivity study of density transition-injected electrons in laser wakefield accelerators

   https://doi.org/10.1103/PhysRevAccelBeams.27.111301  

   Cobo, C C; Arran, C; Bourgeois, N; Calvin, L; Carderelli, J; Cavanagh, N; Colgan, C; Dann, S_J D; Fitzgarrald, R; Gerstmayr, E; et al (November 2024, Physical Review Accelerators and Beams)

   While plasma-based accelerators have the potential to positively impact a broad range of research topics, a route to application will only be possible through improved understanding of their stability. We present experimental results of a laser wakefield accelerator in the nonlinear regime in a helium gas jet target with a density transition produced by a razor blade in the flow. Modifications to the target setup are correlated with variations in the plasma density profile diagnosed via interferometry and the shot-to-shot variations of the density profile for nominally equal conditions are characterized. Through an in-depth sensitivity study using particle-in-cell simulations, the effects of changes in the plasma density profile on the accelerated electron beams are investigated. The results suggest that blade motion is more detrimental to stability than gas pressure fluctuations, and that early focusing of the laser may reduce the deleterious effects of such density fluctuations. Published by the American Physical Society2024 

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4. A model-based, Bayesian approach to the CF4/Ar etch of SiO2

   https://doi.org/10.1117/12.2297482  

   Chopra, Meghali J.; Zhu, Xilan; Zhang, Zizhuo Z.; Helpert, Sofia; Verma, Rahul; Bonnecaze, Roger (March 2018, Proc. SPIE 10588, Design-Process-Technology Co-optimization for Manufacturability XII)

   The design and optimization of highly nonlinear and complex processes like plasma etching is challenging and timeconsuming. Significant effort has been devoted to creating plasma profile simulators to facilitate the development of etch recipes. Nevertheless, these simulators are often difficult to use in practice due to the large number of unknown parameters in the plasma discharge and surface kinetics of the etch material, the dependency of the etch rate on the evolving front profile, and the disparate length scales of the system. Here, we expand on the development of a previously published, data informed, Bayesian approach embodied in the platform RODEo (Recipe Optimization for Deposition and Etching). RODEo is used to predict etch rates and etch profiles over a range of powers, pressures, gas flow rates, and gas mixing ratios of an CF4/Ar gas chemistry. Three examples are shown: (1) etch rate predictions of an unknown material “X” using simulated experiments for a CF4/Ar chemistry, (2) etch rate predictions of SiO2 in a Plasma-Therm 790 RIE reactor for a CF4/Ar chemistry, and (3) profile prediction using level set methods. 

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5. X-Rays from RS Ophiuchi’s 2021 Eruption: Shocks In and Out of Ionization Equilibrium

   https://doi.org/10.3847/1538-4357/ad1041  

   Islam, Nazma; Mukai, Koji; Sokoloski, J L (January 2024, The Astrophysical Journal)

   Abstract The recurrent nova RS Ophiuchi (RS Oph) underwent its most recent eruption on 2021 August 8 and became the first nova to produce both detectable GeV and TeV emission. We used extensive X-ray monitoring with the Neutron Star Interior Composition Explorer Mission (NICER) to model the X-ray spectrum and probe the shock conditions throughout the 2021 eruption. The rapidly evolving NICER spectra consisted of both line and continuum emission that could not be accounted for using a single-temperature collisional equilibrium plasma model with an absorber that fully covered the source. We successfully modeled the NICER spectrum as a nonequilibrium ionization collisional plasma with partial covering absorption. The temperature of the nonequilibrium plasma shows a peak on day 5 with akTof approximately 24 keV. The increase in temperature during the first five days could have been due to increasing contribution to the X-ray emission from material behind fast polar shocks or a decrease is the amount of energy being drained from the shocks into particle acceleration during that period. The absorption showed a change from fully covering the source to having a covering fraction of roughly 0.4, suggesting a geometrical evolution of the shock region within the complex global distribution of the circumstellar material. These findings show evidence of the ejecta interacting with some dense equatorial shell initially, and with less dense material in the bipolar regions at later times during the eruption. 

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