1. An x-ray tube assembly comprising:a housing that encloses an inner volume;
a movable divider within the inner volume, the movable divider dividing the inner volume into a first volume and a second volume, wherein the inner volume is cylindrical about a central axis and the movable divider is a sliding piston that slides along the direction of the central axis;
an x-ray tube within the first volume;
the first volume between the housing and the x-ray tube filled with an insulating fluid; and
the second volume containing a compressible gas.