focusing cup

fo·cus·ing cup

(fōkŭs-ing kŭp)
An oval depression surrounding the cathode in a diagnostic x-ray tube
Medical Dictionary for the Health Professions and Nursing © Farlex 2012
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2.) The cathode, which is on the left, is not visible due to the angle of the recessed focusing cup. Its glowing light (space charge effect), is reflected off the interior surface of the glass tube housing above the anode.