On the representation of dense plasma focus as a circuit element

TL;DR

This paper critically examines how dense plasma focus devices are modeled as circuit elements, highlighting issues with traditional inductance representations and proposing a more accurate approach based on first principles and dynamo effects.

Contribution

It introduces a first principles method to represent dense plasma focus as a circuit element, addressing limitations of the traditional time-varying inductance model.

Findings

Traditional inductance models face conceptual difficulties.

Anomalous impedance is necessary to reconcile models.

Dynamo effects influence the effective inductance.

Abstract

The dense plasma focus is a plasma discharge powered by a capacitor bank. Standard diagnostics include measurement of the time derivative of the current through and the voltage across its connections with the capacitor bank. Interpretation of this diagnostic data often involves some assumptions regarding the representation of the dense plasma focus as a time varying inductance. One of the characteristic features of the current derivative waveform is a relatively sharp dip and an associated sharp voltage spike. This has often been interpreted as a result of a rapid rise in the time varying inductance of the plasma. Sometimes, an anomalous plasma impedance is invoked. This Letter discusses instances where such interpretation creates conceptual difficulties. A first principles approach to the representation of the dense plasma focus as a circuit element reveals some fundamental problems with the traditional representation of plasma focus as a time varying inductance. The anomalous impedance is shown to be necessary to account for the difference in the motional impedance implied by a time-varying inductance in the circuit element representation and a first principles description based on Poynting's Theorem. Dynamo effects that convert post-stagnation local motion of plasma into 3-dimensional magnetic fields are shown to contribute to the effective inductance of the plasma focus and resolve the observed conceptual difficulties