A Theoretical Comparison of the Performance of Stationary-Target Crystallographic X-Ray Tubes Operating on Alternating and Direct Voltages

Abstract

IT has been suggested that if a normally self-rectifying crystallographic X-ray tube is operated on D.C. instead of A.C., it should be possible approximately to double the power input to a tube and therefore similarly to increase the X-ray output. Unfortunately, this does not take into account the fact that the maximum target loading, determined by consideration of the heat dissipation, is normally obtained in a tube using A.C., so that at first sight it would seem to be impossible to increase the power input by operating the tube on D.C. However, as Terrill and Ulrey¹ point out, the temperature of the focal spot will fluctuate with each half-cycle in the case of A.C. ; if the maximum temperature obtained is equal to the maximum permissible temperature for the target, which is also the steady temperature in the D.C. case, it follows that the average temperature will be lower for A.C. than for D.C. Thus the power input with A.C. must be made smaller than with D.C. in order that the maximum temperature shall not exceed the safe value. Terrill and Ulrey give no actual figures for this difference, but DeGraaf and Oosterkamp² state that with a four-valve circuit the maximum load is roughly 15 per cent higher than for A.C., and that for a well-smoothed 'Greinacher' voltage it is about 20 per cent higher.