Abstract: A high vacua diffusion pump for use in a vacuum system including a chamber to be evacuated and a positive displacement pump. The diffusion pump includes a pump body structure having an open upper end and a closed lower end. A boiler and jet assembly is yieldably supported within the pump body and produces jet streams that impel the residual air molecules through the exit port. A ceramic heat break having a labyrinth passage therethrough positively separates the temperature of the condensing wall surface from the boiler temperature. A cooling system permits selective rapid cooling of the boiler and is openable to permit rapid re-heating of the boiler. In one embodiment, the pump includes a room temperature condensing surface, and in another embodiment a non-room temperature condensing surface is provided.