APPLIED PHYSICS LETTERS 95, 151503 (2009) published online 15 October 2009
Abstract: Plasma focus research in the direction of fusion energy faces the limitation of observed neutron saturation; the scaling deteriorating as storage energy E0 increases toward 1 MJ. Numerical experiments confirm this deterioration of scaling. This paper points out that the cause is the dynamic resistance of the axial phase that is constant for all plasma foci. This dynamic resistance dominates the circuit as capacitor bank surge impedance becomes insignificant at large E0, causing current, hence neutron “saturation.” With the cause thus identified, the paper also suggests a cure; to operate plasma focus at high voltages and with current-step technology.
Summary by Lerner:
Professor Lee provides a theoretical study of why earlier DPF experiments showed an apparent maximum or saturation of neutron production and how this could be overcome. Using his own group’s one-dimensional simulations of the run-down and pinch of DPFs, he shows that if capacitor banks are just built larger, with more capacitance, with longer and longer pulses, DPF electrodes will also get longer. This in turn increases the inductance, a measure of how much magnetic energy is generated by the currents between the electrodes. With more and more of the bank’s energy going to supply this magnetic energy, it becomes impossible to increase peak current beyond around 4 MA, even with huge capacitor banks of 45 MJ or more. Lee proposes the solution to this dilemma is to use capacitors operating at higher voltages. This can increase peak current without increasing pulse length.