On the stability of thoriated tungsten cathodes in strong magnetic fields

Thoriated tungsten cathodes, first studied by Langmuir [Phys. Rev. 22, 357-398 (1923)], are used in many applications as efficient electron emitters. However, neutral pressure gauges with thoriated tungsten cathodes (or ASDEX pressure gauges) are not reliable when operated in the strong magnetic field of fusion devices of several Tesla. We have identified the reason for the bad performance in the Wendelstein 7-X stellarator during the operation of several 100 s. Not only were slow, creeping mechanical deformations of the cathodes observed, but also fast events, such as sudden short circuits. The temperature of the cathode is often much higher (about 2400 K) than the maximum value recommended by Langmuir [Phys. Rev. 22, 357-398 (1923)] (about 1900 K). Our test in a superconducting magnet revealed that for a long-pulse operation of 30 min or more in a 3.1 T field, there is an additional effect. We observed that the cathodes required a very high heating current after 6 h of operation. As a consequence, the possible temperature range of the thoriated tungsten cathodes became very small near to an experimentally determined failure limit. In fusion devices with long-pulse operation or in reactors, new cathode types must be used. We give a brief overview of alternative designs that are currently under development.