An x-ray tube (10) includes an evacuated envelope (14), a cathode assembly (20) located in the evacuated envelope and a disk shaped anode assembly (18) located in the evacuated envelope in operative relationship with the cathode assembly for generating x-rays (40). The anode assembly includes an axis of rotation (26) and a target substrate (28) facing the cathode assembly. A heat pipe (33) is located within the anode assembly (18). The heat pipe is comprised of an evacuated shell (60) and is vacuum sealed at a first end (70) of the shell and at a second end (72). A material (80, 82) within the shell is a working fluid for the heat pipe at x-ray tube operating conditions. A porous wick (62) is located within the shell and the wick has a length extending from the first end (70) of the shell to the second end (72) of the shell. A shield (64) is attached to the wick to reduce working fluid loss out of the wick during x-ray tube operation.