PURPOSE: To make inverse magnetron discharge persist in without being affected by the direction of a magnetic field by shaping the potential of discharge space into spherical symmetry.

CONSTITUTION: The anode 1 is spherical and is fixed to the supporting rod 1A. The one end of the supporting rod 1A is fixed to the end plate 14 which avoids parasitic discharge in the regions except for the desired one and the anode 1 is directly grounded through the supporting 1A and the end plate 14. The cathode 23 is formed by clamping two metallic round rods hollowed out in a semi-sphere except for the penetration sections 231 and 232 through which the supporting rod 1A penetrates, with the screw 233. The two rods which form the cathode 23 fix the supporting frame 7 made of electrical insulator between them. Negative high voltage is applied to the cathode 23 through the current measuring device 5 and the DC high voltage power supply 4.