When a laser beam hits a gas, it tears electrons from atoms and accelerates them to nearly the speed of light. As these electrons fly through the plasma, they emit X-rays, like a glowing trail from a fast particle. In the new work, physicists made a single laser pulse create not one but two such trails in succession.
To do this, they made the gas target non-uniform: in one place, the gas density changes sharply, like a step. The first electron bunch is born when the laser ionizes the gas, and the second when the plasma wave hits this step. Each bunch gives its own X-ray flash, and they follow each other with a tiny delay.
Such a pair of pulses is a tool for studying ultrafast processes: how electrons jump between orbits, how atoms shift positions in a molecule. Previously, paired X-ray flashes could only be produced at giant accelerators the size of a football field. Now the same trick has been achieved on a setup that fits in a laboratory.
The scientists measured the electron spectrum and the angular distribution of the X-ray radiation. It turned out that the two pulses originate from different points in the plasma channel and differ slightly in direction. Computer simulations confirmed that each pulse is associated with its own electron bunch. In a single shot from a 40-terawatt laser, about ten billion X-ray photons are born — enough to examine fast atomic motions.
The duration of each X-ray pulse is a few femtoseconds. That's like stretching one second into thirty million years and then taking a single instant from it.