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The photograph shows a well used anode that has also suffered an exposure when the disk was not rotating. The repeated heating and cooling of the focal track has roughened the surface, and the stationary exposure has produced localized melting of the tungsten. The intense heating of this small area has also caused expansion that has resulted in a small crack developing. If the anode were now to be rotated, this crack would propagate, leading to complete fracture of the disk.
The roughening of the focal track will cause a drop in output, and a change in the spectrum of the radiation output.
The rate at which roughening builds up can be reduced by incorporating a small amount of Rhenium into the tungsten.