An ion source with improved temperature control is disclosed. A portion of the ion source is nestled within a recessed cavity in a heat sink, where the portion of the ion source and the recessed cavity are each shaped so that expansion of the ion source causes high pressure thermal contact with the heat sink. For example, the ion source may have a tapered cylindrical end, which fits within a recessed cavity in the heat sink. Thermal expansion of the ion source causes the tapered cylindrical end to press against the recessed cavity in the heat sink. By proper selection of the temperature of the heat sink, the temperature and flow of coolant fluid through the heat sink, and the size of the gap between the heat sink and the ion source, the temperature of the ion source can be controlled.