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Managing the Heat Produced by X-ray Tubes

The most frequent mode of failure of X-ray tubes is the failure to adequately dissipate the heat generated during normal operation.

Greater than 99% of the kinetic energy imparted on the electron beam is lost in the form of heat at the anode target. Thus, a 50W X-ray tube will produce roughly 49.8W of energy in the form of heat just through the conversion process. Add to this the thermal energy produced by the helical tungsten filament and one can readily see that heat dissipation is a major factor.

Inadequate cooling of an X-ray tube can cause it to fail in two ways.

The first is sublimation of the anode target material. In converting the anode target material directly from a solid to a gas (sublimation), the resulting vapor rapidly degrades the internal high vacuum necessary for proper operation on the X-ray tube. The loss of high vacuum results in a failure of the X-ray tube to withstand the high voltage gap between the cathode electron source (helical tungsten filament) and the target anode. The X-ray tube begins to short circuit, or arc, which in turn liberates more gas that further degrades the internal vacuum, resulting in an X-ray tube that no longer functions.

The second failure mode caused by improper heat dissipation is the liberation of damaging ions. If the X-ray tube anode is allowed to surpass the vapor pressure point of the target material, ions will be liberated. These ions are attracted back toward the helical tungsten filament and begin to erode the filament through an ion scrubbing process. This can cause the filament to break, creating an open circuit.

Prevention of both of these failure modes is made possible by ensuring that the X-ray tube is not allowed to overheat. This means careful monitoring of the cooling system with fault protection in the event of a cooling system failure.

Many of our packaged X-ray tubes offer an integrated thermal switch that helps prevent permanent damage to the X-ray tube.

Please confirm that your cooling system can maintain the temperature range that is recommended on our products.

Author: Oxford Instruments X-ray Technologies

Category: Application Note

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