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Static magnetic undulators used by x-ray light sources are fundamentally too limited to achieve shorter undulator periods and dynamic control. To overcome these limitations, we report experimental demonstration of a novel short-period microwave undulator, essentially a Thomson scattering device, that has yielded tunable spontaneous emission and seeded coherent radiation. Its equivalent undulator period (λ u) is 13.9 mm while it has achieved an equivalent magnetic field of 0.65 T. For future-generation light sources, this device promises a shorter undulator period, a large aperture, and fast dynamic control.

We use a Monte Carlo model to simulate the interaction of a beam of relativistic (0.5–10 MeV) electrons with the upper atmosphere as they are injected downward from a notional high‐altitude (thermospheric/ionospheric) injection platform. The beam parameters, defined by realistic parameters of a compact linear accelerator, are used to create a distribution of thousands of electrons. Each electron is injected downward from 300 km altitude toward the dense atmosphere, where it undergoes elastic and inelastic collisions, leading to secondary ionization, optical emissions, and X‐rays via bremsstrahlung. In this report we describe the model initialization (i.e., development of the electron distribution), essential features of the Monte Carlo model, and secondary outputs, including optical emissions, X‐ray fluxes, secondary ionization, and backscattered energetic electron fluxes. Optical emissions are propagated to the ground through the lower atmosphere, including the effects of atmospheric absorption and scattering, to estimate the brightness of the emission column for a given beam current and energy. Similarly, X‐ray fluxes are propagated to hypothetical detectors on balloons and satellites. Secondary ionization is used to estimate the radar signal returns from various ground‐based radar facilities. Finally, simulated backscattered electron fluxes are measured at the injection location. The simulation results show that each of these diagnostics should be readily detectable by appropriate instruments.

We propose an approach to search for axion dark matter with a specially designed superconducting radio frequency cavity, targeting axions with masses m a 10 −6 eV. Our approach exploits axion-induced transitions between nearly degenerate resonant modes of frequency ∼ GHz. A scan over axion mass is achieved by varying the frequency splitting between the two modes. Compared to traditional approaches, this allows for parametrically enhanced signal power for axions lighter than a GHz. The projected sensitivity covers unexplored parameter space for QCD axion dark matter for 10 −8 eV m a 10 −6 eV and axion-like particle dark matter as light as m a ∼ 10 −14 eV.

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