FIELD: X-ray instruments. SUBSTANCE: incandescence cathode is located in focusing electrode which is insulated from cathode and which cross section is shaped as step- wise groove in which root cathode is at least partially buried. Width H of external part of step-wise groove is greater than its depth G. Preferable H/G ratio is in range of 1.5-2.5. This activates mechanism which limits area of cathode emitting surface by three- dimensional charge. Cathode emitting surface emits thermal- emission electrons from cathode unit in direction of anode under voltage applied between cathode and anode. Said emission depends on bias voltage which is applied to focusing electrode. EFFECT: optimal use of load capacity of tube. 2 cl, 2 dwg
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