Abstract
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 100s. 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 2400K) than the maximum value recommended by Langmuir [Phys. Rev. 22, 357-398 (1923)] (about 1900K). Our test in a superconducting magnet revealed that for a long-pulse operation of 30min or more in a 3.1T 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.
Highlights
Hot-cathode pressure gauges are used to measure the neutral pressure in strong magnetic fields of nuclear fusion devices
We present the results of a model of the heating circuit that allowed us to derive the cathode temperature from the heating current
To estimate the cathode temperature of the ASDEX pressure gauges (APGs) in Wendelstein 7-X, we developed a model of the heating circuit
Summary
Hot-cathode pressure gauges are used to measure the neutral pressure in strong magnetic fields of nuclear fusion devices. They are known as ASDEX pressure gauges (APGs). APGs were used in the Large Helical Device (LHD) and in the stellarators Wendelstein 7-AS8 and Wendelstein 7-X.9 In all of these experiments, the same nonplanar cathode geometry was used (see Fig. 1) with a wire diameter of 0.6 mm, which was the standard geometry of thoriated tungsten cathodes for a long time. Among them is the stellarator Wendelstein 7-X It has not yet reached the designed 30 min discharge lengths, and the neutral pressure is not yet measured over such a long time, but already during shorter operation times up to 500 s, some cathodes in the APGs were deformed. We report on experimental observations of the behavior of the tungsten cathodes in a superconducting magnet and in Wendelstein 7-X in order to better understand the stability problems and to assess the consequences for long-pulse operation (30 min). VI, we will discuss alternative cathode designs that have the potential to improve the situation for long-pulse operation
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