Observation of poloidal magnetic flux emission from a low-pressure spark: validation of the hypothesis of constrained plasma dynamics?
This paper revisits an earlier discussion of the hypothesis of constrained dynamics [IEEE Trans. Plasma Sci. 41, 2013 pp. 437-446] with better experiments and insights. This hypothesis pertains to the observation that the standard single fluid equation of motion of a current-carrying plasma has terms governed by diverse physical phenomena which are constrained to evolve at widely mismatched characteristic space and time scales. It should then be possible to construct counterexamples where the equation cannot be satisfied at all times. The hypothesis posits that the usually neglected electron inertia terms come into play in such examples and create "anomalous" electron currents whose signatures can be detected outside the plasma. One of these signatures is emission of poloidal magnetic flux not attributable to a helical deformation of the current channel. Following the cue of the above-mentioned paper, a series of low-pressure spark experiments have been performed. Clockwise and counterclockwise diamagnetic loops, an electrostatically shielded magnetic probe oriented to intercept the azimuthal magnetic flux and an electric displacement probe are deployed outside the spark channel. Observation of several expected signatures provides a tentative validation of this hypothesis. This suggests that summary neglect of electron inertia in continuum models of plasma needs to be replaced by a redevelopment of these models based on perturbation theory with an electron-mass-related small parameter. Major insights could then arise into unsolved problems such as the origin of axial magnetic field in astrophysical jets. Near-solid-density magneto-inertial confinement fusion may be a potential practical application.