Ultrabroadband ultraviolet to x-ray radiation from microscale magnetic field arrays

High-brilliance x-ray sources from synchrotron radiation or free-electron lasers are widely applied across disciplines, including scientific research and industry. To reduce the cost and footprint of synchrotron or free-electron light sources, a microundulator utilizing microscale-ordered electromagnetic fields offers an innovative approach. Here, using 3D particle-in-cell (PIC) simulations, we demonstrate that the amplified quasiperiodic magnetic field arrays generated in the interactions of counterstreaming flows can give rise to a novel undulatorlike light source. The magnetic field arrays, which are amplified to ${10}^{4}\phantom{\rule{0.25em}{0ex}}\mathrm{T}$ by the Weibel instability, cause transverse oscillation of the electrons to form a helical undulator or wiggler with periods in the micrometer scale. We show that the growth rate, intensity, and structure of the magnetic field arrays are tunable by varying the density and energy of the injected and background electrons in the plasma. This flexibility of modification enables the production of compact and ultrabroadband radiation sources ranging from ultraviolet to x-ray with photon energy in hundreds of electron volts and a peak brilliance above $5\ifmmode\times\else\texttimes\fi{}{10}^{22}\phantom{\rule{0.25em}{0ex}}\mathrm{photons}\phantom{\rule{0.25em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}\phantom{\rule{0.25em}{0ex}}\mathrm{m}{\mathrm{m}}^{\ensuremath{-}2}\phantom{\rule{0.25em}{0ex}}\mathrm{mra}{\mathrm{d}}^{\ensuremath{-}2}$ per $0.1\mathrm{%}$ bandwidth.