Step 8: Thermal Limitations
A critical value which must be known is the anode’s heat storage capacity. The tungsten/copper anode has a limited thermal conductance, thereby limiting its ability to dissipate the great heat generated by the focused electron beam. To cope with this Coolidge tubes often are run at a duty cycle, limited by both the operating power, and the anode’s heat storage capacity. In a typical Coolidge tube this heat capacity is usually 7kJ.
Fortunately, manufacturers of both x-ray machines and x-ray tubes are required by federal law [CFR Title 21] to provide anode heating and cooling curves for their devices [image 1]. It's evident that operating this tube at a power of 225W would limit the maximum exposure time to a bit less than 1 minute, with a 5 minute cool-down period. Of course x-ray exposures are never actually 1 minute long; usually they are only a few seconds at most.
Provided the tube is not abused, 225W would not be an unreasonable power to operate it at.