The evolution of the plasmoidal structure in the pinched column in plasma focus discharge

Abstract

In this paper, a description is provided of the evolution of the dense spherical-like structuresplasmoids-formed in the pinched column of the dense plasma focus at the current of 1 MA at the final phase of implosion of the deuterium plasma sheath and at the phase of evolution of instabilities both at the time of HXR and neutron production. At the stratification of the plasma column, the plasma injected to the dense structures from the axially neighboring regions forms small turbulences which increase first the toroidal structures, and finally generates a nonchaotic current plasmoidal structure with central maximal density. This spontaneous evolution supports the hypothesis of the spheromak-like model of the plasmoid and its sub-millimeter analogy, high-energy spot. These spots, also called nodules formed in the filamentary structure of the current can be a source of the energy capable of accelerating the fast charged particles.

Evolution of a Pinch Column During the Acceleration of Fast Electrons and Deuterons in a Plasma-Focus Discharge

“…The plasmoid was formed at the axial position of a smaller plasma lobule of the former toroid. During the formation of the plasmoid one could observe some internal coalescence of plasma turbulences [28]. The internal density gradients in the ordered structures (with the noticeable symmetry) could be caused by the internal closed currents.…”

“…The first plasmoid moved toward the top of the dense plasma column with a velocity equal to about 10 5 m/s, as a consequence of the inertia of confined plasma at its formation [see Figs. 2(b) and (c)] [28]. In the later phase, the stagnated plasma column was subjected to the development of the strong MHD instability.…”

“…A longer production of the fusion-produced neutrons corresponded to the decay of the observed plasmoids, during which their magnetic energy was released, as one can see in Figs. 3(b), 4(b), and 4(c) [28], [30]. It should be noted that in the recorded interferometric fringes, there was no experimental information about the acceleration of fast charged particles in the interrupted pinch region (filled up with low-density plasma).…”

“…Plasma of the parameters described above was a convenient medium for the self-organization and self-generation of a magnetic field, because of relatively high Reynolds magnetic numbers (ratio of frozen and dissipative characteristics [4]) which reached 10 3 . A scenario of the evolution of the ordered structures was described in [28] and [32]. The process was started by some concentration of a spontaneously generated azimuthal current component into the toroidal structure.…”

Update on the Scientific Status of the Plasma Focus

“…The interferometric system described in Section 3.2.1 enabled visualization of internal structure of the plasma from radial implosion phase until plasma decay in a sequence of 15 frames recorded in a single shot. Results of such investigations are described in a series of publications [103,[105][106][107]126,[391][392][393][394][395][396][397], including an overview paper [103]. Measurements of neutron, soft X-ray (SXR) and hard X-ray (HXR) emissions provided context for the interferometric observations.…”