X-ray Tube - Rotating Anode Tube

Rotating Anode Tube

Imagine thinning the tube. The anode can then be rotated by electromagnetic induction from a series of stator windings outside the evacuated tube.

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. 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.

The anode must be constructed of high temperature materials. The focal spot temperature can reach 2,500 °C (4,530 °F) during an exposure, and the anode assembly can reach 1,000 °C (1,830 °F) following a series of large exposures. Typical materials are a tungsten-rhenium target on a molybdenum core, backed with graphite. The rhenium makes the tungsten more ductile and resistant to wear from the impact of the electron beams. The molybdenum conducts heat from the target. The graphite provides thermal storage for the anode, and minimizes the rotating mass of the anode.

Increasing demand for high-performance Computed tomography(CT) scanning and angiography systems has driven development of very high performance medical X-ray tubes. Contemporary CT tubes have power ratings of up to 100 kW and anode heat capacity of 6 MJ, yet retain an effective focal spot area of less than 1 mm2.