Abstract: A large cathode structure having a diameter on the order of 50 cm for producing uniform plasmas in a magnetic field. The cathode is oxide-coated and indirectly heated by a plurality of heater wires disposed in insulating tubes arranged parallel to each other. The cathode is pressed against the insulating tubes thereby to maintain it substantially flat in spite of expansion and contraction due to the heating and cooling effects. Optionally, the insulating tubes carrying the heater wires may be backed up by a second set of insulating tubes disposed at right angles to the first set. An annular ring is disposed about the cathode and is provided with tungsten springs for forcing the cathode against the heater tubes. This ring may additionally serve as a heat shield and maintain the temperature more uniform about the periphery of the cathode. For large cathodes, the structure may be water cooled by the provision of two copper tubes through which the water flows.