  Experimental data compiled over five decades of dense plasma focus research
is consistent with the snowplow model of sheath propagation, based on the
hypothetical balance between magnetic pressure driving the plasma into neutral
gas ahead and wind pressure resisting its motion. The resulting sheath
velocity, or the numerically proportional drive parameter, is known to be
approximately constant for devices optimized for neutron production over 8
decades of capacitor bank energy. This paper shows that the validity of the
snowplow hypothesis, with some correction, as well as the non-dependence of
sheath velocity on device parameters, have their roots in local conservation
laws for mass, momentum and energy coupled with the ionization stability
condition. Both upper and lower bounds on sheath velocity are shown to be
related to material constants of the working gas and independent of the device
geometry and capacitor bank impedance.
