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Abstract

The angular distribution and polarization of x rays emitted due to resonant recombination into highly charged ions were studied experimentally using an electron beam ion trap. In the first experiment, the linear polarization of x rays produced by dielectronic recombination into highly charged krypton ions was measured using the Compton polarimetry technique. In the second experiment, the electron-ion collision energy was scanned over the K-shell dielectronic, trielectronic and quadruelectronic recombination resonances in highly charged iron and krypton ions. The angular distribution of x rays following resonant recombination was measured by observing x rays in the direction along and perpendicular to the electron beam propagation direction. Both the x-ray angular distribution and polarization reveal the alignment of the populated excited states. The experiments revealed a high sensitivity of the x-ray angular distribution and polarization to the relativistic effects such as the Breit interaction. Measurements show that most of the transitions lead to polarization, including hitherto neglected trielectronic and quadruelectronic recombination channels. Moreover, these channels dominate the ionization balance and the polarization of the prominent Kα x rays emitted by hot anisotropic plasmas. The experimental results benchmark relativistic atomic calculations and are expected to play an important role in accurate diagnostics of hot astrophysical plasmas of solar flares and active galactic nuclei, and laboratory fusion plasmas of tokamaks.