Dense Plasma Focus: A question in search of answers, a technology in search of applications

Abstract

Diagnostic information accumulated over four decades of research suggests a directionality of toroidal motion for energetic ions responsible for fusion neutron production in the Dense Plasma Focus (DPF) and existence of an axial component of magnetic field even under conditions of azimuthal symmetry. This is at variance with the traditional view of Dense Plasma Focus as a purely irrotational compressive flow. The difficulty in understanding the experimental situation from a theoretical standpoint arises from polarity of the observed solenoidal state: three independent experiments confirm existence of a fixed polarity of the axial magnetic field or related azimuthal current. Since the equations governing plasma dynamics do not have a built-in direction, the fixed polarity must be related with initial conditions: the plasma dynamics must interact with an external physical vector in order to generate a solenoidal state of fixed polarity. Only four such external physical vectors can be identified: the earth's magnetic field, earth's angular momentum, direction of current flow and the direction of the plasma accelerator. How interaction of plasma dynamics with these fields can generate observed solenoidal state is a question still in search of answers; this paper outlines one possible answer. The importance of this question goes beyond scientific curiosity into technological uses of the energetic ions and the high-power-density plasma environment. However, commercial utilization of such technologies faces reliability concerns, which can be met only by first-principles integrated design of globallyoptimized industrial-quality DPF hardware. Issues involved in the emergence of the Dense Plasma Focus as a technology platform for commercial applications in the not-too-distant future are discussed.

Update on the Scientific Status of the Plasma Focus

“…Experimental reports on axial magnetic field at Frascati [172], Stuttgart [42,173] and PF-1000 [212] mention that the polarity of the axial magnetic field does not change from shot to shot. Since the equations of plasma dynamics do not contain a preferred direction, the existence of a fixed polarity of axial magnetic field must be explained in terms of initial conditions, or equivalently, in terms of interaction of the plasma with an external physical vector field [463]. Only four such external vector fields are identifiable: Earth's magnetic field, Earth's angular momentum, direction of current density and direction of the plasma accelerator (from breech towards muzzle of the coaxial gun) [463].…”

“…Since the equations of plasma dynamics do not contain a preferred direction, the existence of a fixed polarity of axial magnetic field must be explained in terms of initial conditions, or equivalently, in terms of interaction of the plasma with an external physical vector field [463]. Only four such external vector fields are identifiable: Earth's magnetic field, Earth's angular momentum, direction of current density and direction of the plasma accelerator (from breech towards muzzle of the coaxial gun) [463]. It is difficult to imagine how the last three vector fields interact with plasma dynamics in order to create an axial magnetic field with a fixed polarity, leaving the geomagnetic field as the most likely reason behind the observation of the fixed polarity of the axial magnetic field.…”

Axial magnetic field and toroidally streaming fast ions in the dense plasma focus are natural consequences of conservation laws in the curved axisymmetric geometry of the current sheath

“…However, what sort of fast ion population would give rise to observed properties of the neutron emission remains a subject of active research. A recent summary of the status [7] mentions the pioneering observations [33][34][35][36] The combination of reports of ions moving along the axis [33][34][35][36], moving radially [40], moving in loops in the plane comprising device axis and a horizontal line [41] , opposite to the axis [37][38][39] and around the axis [13,42] (over axially limited region) can be comprehensively described as representing toroidal motion [43]. The signature of such toroidal motion is predominantly seen in the width (and not the average energy which remains ~2.45 MeV for neutrons) of the space-averaged side-on neutron spectrum, and in the space space-resolved sideon neutron or fusion proton spectra.…”

“…All the geometric source terms are proportional to coefficients , , A F, (see Appendix) which are related to the curvature of the local coordinate system; the timedependent source term is related to the scaled current profile determined from the resistive GV model [23]. An indication of their influence can be seen from equations 25 and 27: even a small ambient field like the earth's magnetic field [7,49] can lead to growth of magnetic field components other than the azimuthal magnetic field driving the plasma.…”

“…Azimuthal symmetry allows counter-streaming azimuthal flow of electrons and ions without being limited by any opposing forces such as those resulting from charge separation, and without violating overall conservation of charge or of azimuthal momentum. Hall Effect plays a role [26] in this process by providing a mechanism for choosing between + θ and − θ directions with the help of a non-zero but small ambient magnetic field [7]. The transition zone can therefore be expected to contain strong azimuthal sheet currents limited only by conservation laws, generating components of magnetic field other than the azimuthal in the post-shock region, accompanied with azimuthal flow.…”