Plasma is a hot gas in which electrons are torn from atoms. Instabilities and turbulence constantly arise in it: small perturbations grow, and energy flows between structures of different sizes. These processes determine how plasma moves and how energy is distributed. Understanding them is important for thermonuclear fusion, astrophysics and the physics of laser plasma.
The difficulty is that structures in plasma change from experiment to experiment even with small differences in conditions. This is why measurements are needed that cover many scales at once in a single "shot." This is especially true of high-energy-density plasma: there instabilities develop in an extremely short time, and the earliest stages must be captured in time.
An international group led by Associate Professor Masato Ota of the National Institute for Fusion Science (at the time of the work, a graduate student at Osaka University) and Associate Professor Yoichi Sakawa of the Institute of Laser Engineering at Osaka University solved this problem. The researchers combined a new method of small-angle X-ray scattering without a central stop (it is called a beamstop) with an X-ray free-electron laser and a target made of microscopic rods. The results were published in the journal Communications Physics.
In conventional small-angle scattering, the center of the beam is covered with a special stop so as not to damage the camera. Because of this, the signal from ultra-small angles is lost. The new scheme uses a set of metal foils of different thicknesses instead of a stop: they attenuate the bright central beam and expand the range of intensities that a single camera can record by more than a hundred times. Thanks to this, both weak and strong signals can be captured simultaneously.
The experiments were conducted at the SACLA X-ray laser (SPring-8 Angstrom Compact free electron LAser) in Japan's Hyogo Prefecture. A powerful femtosecond laser instantly turned a target of silicon rods into plasma, and an X-ray pulse recorded how the scattering pattern changed. Thus the scientists for the first time in the world measured structures in the range from 10 to 500 nanometers in a single pulse and a single camera, with a time resolution of less than a picosecond (a picosecond is one trillionth of a second).
By comparing the data for a solid target and plasma, the researchers calculated the plasma expansion rate and the electron temperature. Together with numerical modeling, they showed that if the initial plasma on the surface becomes just a few nanometers larger, the fraction of absorbed laser energy falls by about a factor of four. This is not a causal conclusion from a single observation, but the result of comparing measurements with a model.
The authors note that the method will help study laser ion acceleration and the interaction of plasma with the walls of thermonuclear reactors in more detail. In addition, it may be useful outside plasma physics - for example, for living cells, which are difficult to measure repeatedly because their state changes over time.