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X-ray tube


An X-ray tube is a vacuum tube that converts electrical input power into X-rays. X-ray tubes evolved from experimental Crookes tubes with which X-rays were first discovered on November 8, 1895, by the German physicist Wilhelm Conrad Röntgen. The availability of this controllable source of X-rays created the field of radiography, the imaging of partly opaque objects with penetrating radiation. In contrast to other sources of ionizing radiation, X-rays are only produced as long as the X-ray tube is energized. X-ray tubes are also used in CT scanners, airport luggage scanners, X-ray crystallography, material and structure analysis, and for industrial inspection.

The anode is supported on vacuum bearings and can be rotated by electromagnetic induction from a series of stator windings outside the evacuated tube. Typically, eddy currents are induced in a rotatable metal cylinder, the rotor. The superimposition of external magnetic induction and induction from the eddy currents, which changes direction with time, creates a driving mechanical momentum in the rotor.

Because the entire anode assembly has to be contained within the evacuated tube, heat removal is a serious problem, further exacerbated by the higher power rating available. Due to the high heat generation, a matched thermal expansion of glass and the anode material is necessary. For this, high-borate borosilicate glass pioneered by Otto Schott is often used. Direct cooling by conduction or convection, as in the Coolidge tube, is difficult. In most tubes, the anode is suspended on ball bearings with silver powder lubrication which provide almost negligible cooling by conduction.

A recent development has been liquid gallium lubricated fluid dynamic bearings which can withstand very high temperatures without contaminating the tube vacuum. The large bearing contact surface and metal lubricant provide an effective method for conduction of heat from the anode.


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