Influence of the Background Gas Pressure on the Expansion of the Arc-Cathode Plasma

Experimental evidence is presented for the effect of the background gas on the expansion of the metallic plasma cloud produced at the cathode spot of an arc (50 A, 25-300 ¿s, 0-50 torr He) with or without an applied magnetic field (850 G). Streak and frame high-speed photography and electrostatic probes were used. It appears that the background gas is swept up by the expanding plasma slowing it down. A snow-plow model is derived which accounts satisfactorily for the results.