Alignment dynamics in a laser-produced plasma

We observe the time evolution of ground-state ion alignment in a laser-produced plasma. Krypton ions produced in a strong, linearly polarized optical laser field $({10}^{14}–{10}^{15}\mathrm{W}∕{\mathrm{cm}}^2)$ are aligned along the field polarization axis. Using microfocused, tunable x rays from Argonne’s Advanced Photon Source, we measure orbital alignment as a function of time. For plasma densities of the order of ${10}^{14}{\mathrm{cm}}^{-3}$, the alignment decays within a few nanoseconds. A quantitative model explains the decay in terms of electron-ion collisions in the plasma. By applying an external magnetic field, we are able to suppress the disalignment and induce coherent spin precession of the Kr ions, thus providing an in situ monitor of magnetic fields in a plasma.

Figure 1

(Color online) Experimental setup with magnetic field, laser, and x-ray orientations. X rays and laser copropagate into the plane of the page. For details see text.

Figure 2

${\mathrm{Kr}}^{+}$ orbital alignment decay for various densities. ${\mathrm{Kr}}^{+}$ $1s\to 4p$ resonance fluorescence versus laser/x-ray time delay for ${\widehat{ϵ}}_L$ and ${\widehat{ϵ}}_X$ parallel $\left(I_{\Vert }\right)$ and perpendicular $\left(I_{\perp }\right)$. Target density $n_0=1.3\times {10}^{14}{\mathrm{cm}}^{-3}$. Laser intensity is $1.0\left(0.2\right)\times {10}^{15}\mathrm{W}∕{\mathrm{cm}}^2$. Solid curves were generated by theory.

Figure 3

${\mathrm{Kr}}^{+}$ $1s\to 4p$ resonance fluorescence as a function of laser/x-ray delay for low $(<30\mathrm{G})$ and high $\left(410\mathrm{G}\right)$ magnetic fields. Target density, $n=2.3\times {10}^{15}∕{\mathrm{cm}}^3$.

Figure 4

(Color online) Top: Differential x-ray absorption $(I_{\Vert }-I_{\perp })$ as a function of time delay for $\mathbf{B}\perp {\widehat{ϵ}}_L$. Bottom: Fourier transform of $(I_{\Vert }-I_{\perp })$. Target density, $n=2.3\times {10}^{15}∕{\mathrm{cm}}^3$.