Strong higher-order resonant contributions to x-ray line polarization in hot plasmas

We studied angular distributions of x rays emitted in resonant recombination of highly charged iron and krypton ions, resolving dielectronic, trielectronic, and quadruelectronic channels. A tunable electron beam drove these processes, inducing x rays registered by two detectors mounted along and perpendicular to the beam axis. The measured emission asymmetries comprehensively benchmarked full-order atomic calculations. We conclude that accurate polarization diagnostics of hot plasmas can only be obtained under the premise of inclusion of higher-order processes that were neglected in earlier work.

Figure 1

Experimental setup: Ions are produced and trapped within a monoenergetic electron beam. Its energy was tuned into the recombination resonances, and the subsequently emitted x rays were observed by two germanium detectors at $0^{\circ }$ and ${90}^{\circ }$ with respect to the beam axis. Upper two-dimensional (2D) plot: Intensity of x rays along beam axis as a function of electron and x-ray energy. Lower 2D plot: Difference in intensity between the two $0^{\circ }$ and ${90}^{\circ }$ detectors. Solid line: Projection of this difference. Some individual resonances are marked exemplarily for the different resonant channels: DR, TR, and QR.

Figure 2

X-ray intensities measured as a function of collision energy for iron (lower left panel) and krypton ions (lower right panel). The solid lines and the solid areas correspond respectively to the x rays observed side on and head on to the beam axis. DR, TR, and QR resonances are indicated by the gray atomic symbols of the initial ion on the upper panels and by the configurations of the intermediate states. The extracted emission asymmetries $R$ (solid circles) for the individual resonances and the corresponding fac predictions (open circles) are shown on the upper panels. Solid vertical lines indicate the resonances used for the intensity normalization.

Figure 3

Charge state distribution in an optically thin iron plasma (upper panel) and polarization of iron $K\alpha$ x-ray line $P\left(T\right)$ (middle panel) calculated with fac as functions of temperature. The contributions of hitherto neglected higher-order transitions and the Breit interaction to the total polarization are shown in the middle and lower panels.