Power limits for dynamical pinch discharges

Discharge initiation and sheath formation are serious problems of high current (several MA) dynamical pinches because of the enormous power input. The fast 300 kV plasma focus SPEED 2 allowed studying the discharge behaviour in a wide range of power input (10-100 GW) previously not accessible. The main results are: (i) high power input causes fast sheath formation (<100 ns); (ii) there is an upper sheath energy density limit (<or approximately=100 J cm^-2) that decreases with increasing insulator radius; (iii) this power and energy density limit is enhanced by surface conditioning either using high energy (>100 keV) photons with which the insulator surface is irradiated during discharge initiation and sheath formation or using insulators with coppered surface.