A preliminary life test of a low-power DC arcjet anode was conducted using a 500-W-class laboratory-model arcjet. After a continuous operation of 90-h, we compared the degradation of two types of anodes: one is a conventional pure tungsten anode, which is made of multiple fine grains of several tens of micrometer in size; another is a tungsten anode made of only four coarse grains. Both materials showed nearly the same amount of degradation from the viewpoint of so-called a constrictor closure phenomenon, in which the radius of the constrictor decreases during the continuous operation. However, micrograph analyses showed different degradation features for the two anodes: for the conventional tungsten anode, grains were severely embrittled in the recrystalization processes after the thruster operation accompanied by many cracks or even drops out of grains; in contrast, the anode made of four coarse grains showed a few cracks only along the grain boundaries, and its thrust performance was unchanged before and after the 90-h test. Hence there is a possibility to suppress the grain embrittlement by completely removing these grain boundaries from the tungsten anode by making the anode from one huge grain.

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Kenichi Okamoto

Papers

A Study of Degradation Mechanism of a Low-Power DC Arcjet Anode