THERMAL analysis of high power x-ray target: scaling effects

High resolution x-ray imaging systems require small focal spots ranging from 1 μm to 1 mm. In NDE applications, the demand for small spot sizes for high spatial resolution conflicts with the need for increased x-ray flux for faster scan times. In this paper, a finite element model is developed to compute the temperature of a stationary x-ray target exposed to micrometer-sized high power (10’s to 100’s of watts) electron beams. Such extremely high power densities at the focal spot are the limiting factor in both performance and life of many x-ray imaging system. This model is used to demonstrate the effect of focal spot size – diameter, on the heat dissipation capability. As the spot size reduces, a higher power density may be sustained by the target. This effect is explained by increased lateral heat conduction. The peak temperature of a small focal spot also becomes more sensitive to the current density distribution of the incident electron beam. The relationship of the peak power and electron beam profile, volumetric power deposition into the x-ray target and focal spot aspect ratio are discussed. Some experimental data demonstrating such scaling effects is included. General design rules for higher-flux capable targets leveraging these scaling effects are also proposed.