Abstract: A power supply for the cathode of an electron beam gun disposed in a vacuum chamber comprises a transformer for frequencies above about 20 kHz and having a primary winding and a secondary winding connected to the cathode. The transformer is mounted so as to integrate same into the vacuum chamber to dispose the primary winding in communication with the atmosphere and the secondary winding in communication with the vacuum chamber. A partition is disposed between the primary winding and the secondary winding to seal the primary winding from the vacuum and the seondary winding from the atmosphere. A control circuit receptive of a direct current supply has the output thereof connected to the primary winding of the transformer and has pulse width controlled transistors for the chopping of the direct current at a frequency above about 20 kHz.