Structured Plasma Boosts X-Ray Flux

High-intensity sources of femtosecond x-ray pulses are a must-have tool for studying ultrafast processes. Unfortunately, only a handful of kilometer-long free-electron lasers can produce these pulses, making them inaccessible to most researchers. Now, Michaela Kozlova at the Czech Academy of Sciences, Prague, and colleagues demonstrate improvements to an alternative, tabletop x-ray emitter—a laser-based scheme known as a wakefield accelerator—that could make such capabilities more broadly available.

In a wakefield accelerator, a femtosecond laser pulse travels through a plasma, pushing electrons away from its path and forming intense plasma waves in its wake. Electrons “surfing” on these waves are accelerated to relativistic speeds. They also wiggle around the laser-beam axis, emitting x rays whose frequency and intensity depend on the electrons’ speed and on the frequency and amplitude of their oscillations.

Previously, high-energy x-ray sources employing this technique required impractical lasers that output hundreds of terawatts. Kozlova and colleagues achieved similar results with a much smaller terawatt-class laser by tailoring the density of the laser-generated plasma. First, a gradual density increase in the direction of laser propagation increased the electrons’ speed and oscillation frequency. Second, a sharp density variation along a direction angled diagonally with respect to the beam axis increased the electrons’ oscillation amplitude. The two effects boosted the total x-ray flux tenfold compared to a homogeneous plasma, with a 100-fold improvement at hard x-ray wavelengths. The modified device produced x-ray pulses with a broad spectrum of frequencies that are suitable for performing ultrafast x-ray absorption spectroscopy experiments. These experiments enable the observation of changes in atomic and electronic structures occurring on femtosecond timescales.

Particle-in-cell simulations showing the laser propagation (thick curve) and the particle trajectories (thin curves) in (z,y) plane. The orbits are colored according to the laser peak field and the particle energy, respectively. Gray levels represent plasma density. (a) Up-ramp gradient. (b) Up-ramp gradient and transverse density gradient.