Self-Organized Electromagnetic Field Structures in laser-Produced Counter-Streaming Plasmas
Nowadays, the internet is decentralised and unorganised. Users interact dynamically almost randomly by clicking, sharing and creating webpages. However, it is not hard for one to imagine a global structure emerging from this disorderliness: E.g. people are organised in a spider-web-liked structure on Facebook.
The internet is an example of self-organisation in which macroscopic structures are formed out of chaos. This phenomenon might be counterintuitive as random systems should naturally become more and more randomised. After all, a messy room can only get messier over time unless some mechanisms intervene, such as a person cleaning up the room.
While the self-organisation mechanism is well-studied in many regimes of physics such as crystallisation and fluid dynamics, those in the area of astrophysics are generally not. For example, supernova remnants, solar wind shocks and astrophysical bodies like the Herbig-Haro object are results of global structures (e.g. collision-less shockwave) emerging from turbulent plasma that are not well understood.
Although these phenomena have long been studied using satellites, their mechanism and even the necessary pre-conditions are still unknown. Researches have now tried to recreate stable and macroscopic structures in plasma in laboratories, hoping that greater parameter control could generate more insightful models. For the first time, physicists could make accurate measurements of plasma self-organisation processes that take place deep in the cosmos.
2 Counter-streaming plasma were created by focusing high energy beams on target disks. The intensity of the beams was a quadrillion time more than that of the sun on earth’s surface. The counter-stream meant high relative velocity, which rendered inter-beam ion exchange rare and facilitated the formation of collisionless shock — an important macro-feature of interest. The interactions were recorded using proton imaging.
After two nanoseconds — the time needed for light to travel 60 cm — turbulent fields developed. Shortly afterward, sharp caustics structures started to form. Caustic structures were similar to the pattern of light formed when sunlight shines through rippled water. Then, the structure arranged itself into two horizontal regions, leaving an empty centre.
Remarkably, these structures spanned distances much longer than any other plasma feature. They were also so stable that they persisted during the entire experiment window. Given these time and length, these features were definitely “global” structures that emerge from the chaotic plasma beam.
A paper published a year later hypothesised a few explanations. Some models relied on electrostatic phenomenon while others explained using the formation of toroidal (doughnut-shaped) magnetic fields. The latter successfully accounted for the general features but relied on too many assumptions.
Further investigations are currently underway with improved data collection and tomographic techniques. Indeed, merely analysing proton imaging data is inconclusive. Methods that examine density gradients and magnetic field patterns have to be deployed. Future experiments can also be conducted at the National Ignition Facility, which allows researchers to produce these plasma structures at a much larger scale.
Ultimately, models of plasma self-organisation developed in the laboratory can help astrophysicists to understand the emergence of cosmic structures better. In the long term, the implications go beyond astronomy and facilitate a better understanding of self-organisation processes from microscopic to cosmological scales.
