Electrostatic flat-top solitons near double layers and triple root structures in multispecies plasmas : how realistic are they?

Electrostatic flat-top solitons are a new acoustic-type nonlinear mode and found to be a generic feature accompanying the occurrence of double layers and/or triple root structures, in multispecies plasmas admitting the latter. Their existence domains can be parameterized by the difference between their velocities and the double layer or triple root velocities, but these velocity differences turn out to be extremely small, of the order 10 5 or less. The onset of their flat top character in the electrostatic potential is clearly seen in the corresponding electric field or charge density profiles. However, even at the limit of the numerical accuracy for vanishing velocity differences, their profiles are still soliton-like, very unlike those of double layers or triple root structures. So although the Sagdeev potential varies continuously as the structure velocity approaches that of the double layer or triple root structure, the character of the nonlinear modes changes in a discontinuous manner. For sufficiently wide flat-top solitons, the electric field signature looks very much like two unipolar signals with opposite polarities, where unipolar electric fields typically characterize double layers or triple root structures. We are not aware of flat-top solitons having been reported to date, and their extremely limited existence range raises the question of whether they may be observable at all, unless helped by a fortunate stroke of serendipity. This topic requires suitable numerical simulations to ascertain their stability and interaction properties.

Publication year:2020

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