A possible approach to overcome the saturation of the neutron yield in a Plasma Focus and to achieve breakeven

Abstract

Saturation of the neutron yield with increasing energy of the condenser bank in a Plasma Focus led to the shutdown of PF research focussed on controlled nuclear fusion in the past. We review available models of saturation and develop further the model of Lee S., Applied Phys. Lett. 95, 151503, 2009. This model relies on the well-known and generally accepted model of Lee S., J. Fusion Energy 2014, 33, 319 of Plasma Focus discharges and describes saturation in terms of the dynamic resistance, i.e. the rate of change of PF inductance due to the motion of the plasma sheath during rundown. A model of this sheath discussed in Di Vita A., J. Plasma Physics, 1993, 50, 1 shows that its spontaneous filamentation rules the dynamic resistance, spoiling the power supply from the condenser bank to the plasma at the values of condenser bank energy above 0.5 MJ values which are relevant to a fusion reactor. Together, these two models lead to the conclusion that suppression of such filamentation prevents saturation, multiplies the PF drive parameter by a factor 3 at least and allows breakeven in a 224 kV, 10 MJ Plasma Focus working with DT. We can suppress filamentation by superimposing a radial magnetic field to the interelectrode region of the Plasma Focus where rundown occurs. A conservative estimate shows that a 1.4 T radial magnetic field is enough to suppress many known filamentation instabilities. Suitably located magnets can generate this field. Their layout resembles the layout of the sources of radial magnetic field in the cylindrical geometry of a Hall thruster for space propulsion. For high temperature superconducting magnets, the required current density is too small to trigger quenching.