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Subject: magnetic-fields ×

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    Tracing dynamics of laser-induced fields on ultra-thin foils using complementary imaging with streak deflectometry
    (College Park, MD : American Physical Society, 2016) Abicht, Florian; Braenzel, Julia; Priebe, Gerd; Koschitzki, Christian; Andreev, Alexander; Nickles, Peter; Sander, Wolfgang; Schnürer, Matthias
    We present a detailed study of the electric and magnetic fields, which are created on plasma vacuum interfaces as a result of highly intense laser-matter interactions. For the field generation ultrathin polymer foils (30–50 nm) were irradiated with high intensity femtosecond (1019–1020  W/cm2) and picosecond (∼1017  W/cm2) laser pulses with ultrahigh contrast (1010–1011). To determine the temporal evolution and the spatial distribution of these fields the proton streak deflectometry method has been developed further and applied in two different imaging configurations. It enabled us to gather complementary information about the investigated field structure, in particular about the influence of different field components (parallel and normal to the target surface) and the impact of a moving ion front. The applied ultrahigh laser contrast significantly increased the reproducibility of the experiment and improved the accuracy of the imaging method. In order to explain the experimental observations, which were obtained by applying ultrashort laser pulses, two different analytical models have been studied in detail. Their ability to reproduce the streak deflectometry measurements was tested on the basis of three-dimensional particle simulations. A modification and combination of the two models allowed for an extensive and accurate reproduction of the experimental results in both imaging configurations. The controlled change of the laser pulse duration from 50 femtoseconds to 2.7 picoseconds led to a transition of the dominating force acting on the probing proton beam at the rear side of the polymer foil. In the picosecond case the (⇀vx⇀B)-term of the Lorentz force dominated over the counteracting ⇀E-field and was responsible for the direction of the net force. The applied proton deflectometry method allowed for an unambiguous determination of the magnetic field polarity at the rear side of the ultrathin foil.
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    Revisiting the Space Weather Environment of Proxima Centauri b
    (London : Institute of Physics Publ., 2022) Garraffo, Cecilia; Alvarado-Gómez, Julián D.; Cohen, Ofer; Drake, Jeremy J.
    Close-in planets orbiting around low-mass stars are exposed to intense energetic photon and particle radiation and harsh space weather. We have modeled such conditions for Proxima Centauri b, a rocky planet orbiting in the habitable zone of our closest neighboring star, finding a stellar wind pressure 3 orders of magnitude higher than the solar wind pressure on Earth. At that time, no Zeeman-Doppler observations of the surface magnetic field distribution of Proxima Cen were available and a proxy from a star with a similar Rossby number to Proxima was used to drive the MHD model. Recently, the first Zeeman-Doppler imaging (ZDI) observation of Proxima Cen became available. We have modeled Proxima b’s space weather using this map and compared it with the results from the proxy magnetogram. We also computed models for a high-resolution synthetic magnetogram for Proxima b generated by a state-of-the-art dynamo model. The resulting space weather conditions for these three scenarios are similar with only small differences found between the models based on the ZDI observed magnetogram and the proxy. We conclude that our proxy magnetogram prescription based on the Rossby number is valid, and provides a simple way to estimate stellar magnetic flux distributions when no direct observations are available. Comparisons with models based on the synthetic magnetogram show that the exact magnetogram details are not important for predicting global space weather conditions of planets, reinforcing earlier conclusions that the large-scale (low-order) field dominates, and that the small-scale field does not have much influence on the ambient stellar wind.