Filamentation
Filamentation
Striations or string-like structures,[27] also known as birkeland currents, are seen in many plasmas, like the plasma ball, the aurora,[28] lightning,[29] electric arcs, solar flares,[30] and supernova remnants.[31] They are sometimes associated with larger current densities, and the interaction with the magnetic field can form a magnetic rope structure.[32] High power microwave breakdown at atmospheric pressure also leads to the formation of filamentary structures.[33] (See also Plasma pinch)
Filamentation also refers to the self-focusing of a high power laser pulse. At high powers, the nonlinear part of the index of refraction becomes important and causes a higher index of refraction in the center of the laser beam, where the laser is brighter than at the edges, causing a feedback that focuses the laser even more. The tighter focused laser has a higher peak brightness (irradiance) that forms a plasma. The plasma has an index of refraction lower than one, and causes a defocusing of the laser beam. The interplay of the focusing index of refraction, and the defocusing plasma makes the formation of a long filament of plasma that can be micrometers to kilometers in length.[34] One interesting aspect of the filamentation generated plasma is the relatively low ion density due to defocusing effects of the ionized electrons.
