  Striking pictures showing filamentary structures in plasma focus have
intrigued researchers from the early days of plasma focus research. A
definitive understanding of their occurrence, origin, structure and role in
plasma focus physics is still not in sight as summarized in a recent
comprehensive review. This is because they are often not observed in a
"standard mode" of plasma focus operation with pure deuterium, particularly in
large installations, but are found in smaller experiments or those with gaseous
admixtures. This has led to the suspicion that filaments are not a native
feature of the plasma focus phenomenon. Recent success in observation of
filaments in PF-1000 in a pure deuterium operation by a novel modification of
the interferometer system that allows simultaneous interferometry and schlieren
photography changes this situation. This Letter looks at the implications of
this development in the larger context of plasma focus physics.
Conceptualization of filamentation as a native feature of the traveling current
distribution behind an ionizing strong shock wave is shown to be a feasible
paradigm that can be formulated as a computable model for filamentation in the
plasma focus.
