Advances on X-rays

THE shortness and brightness of X-ray pulses can help researchers explore phenomena over very short time and length scales. The shorter the pulse, the better its ability to capture events occurring on ultrashort timescales, such as changes in atomic configuration. The brighter the X-ray, the stronger the signal, which allows a researcher to conduct multiple experiments in a single shot This is beneficial when probing dense plasmas and other phenomena that naturally emit X-rays or visible light.

A team of researchers from the Lawrence Livermore Laboratory (LLL), University of California at Los Angeles, and the SLAC National Accelerator Laboratory has produced X-ray pulses with these characteristics and on a large tabletop-sized system. Pulses from Livermore’s 200-terawatt Callisto laser are focussed onto various gases, generating relativistic electrons that, in turn, produce high-energy betatron X-rays in the femtosecond regime.

The laser uses chirped-pulse amplification to produce a 60-femtosecond-long pulse of ultrahigh intensity light. The laser’s 13 cm diameter beam is focussed within a spot diameter of 12 micrometres under vacuum onto a cell filled with gas, typically helium.

As the leading edge of the pulse encounters the edge of the gas cloud, the pulse strips electrons from the gas molecules, creating a plasma of positively charged ions and free electrons. The pulse creates a plasma cloud as it continues to travel through the gas. The light pressure of the pulse’s peak pushes the free electrons in the cloud away from the laser pulse similar to how the prow of a boat pushes water away from the bow. The ions are too heavy to be moved by the light pressure. Thus, as the pulse passes by, it leaves a positively charged “bubble” populated only by ions in its wake.

Just as some of the water displaced by a boat will circle around and be trapped by its wake, some of the electrons shoved aside by the light pulse will circle back and be drawn into the ion-filled bubble. The “wakefield” electrons accelerate through the positively charged bubble, oscillating rapidly in a sinusoidal path and emitting X-rays which are synchronous with the initial laser pulse on the femtosecond timescale.