In a plasma gun in which a plasma power source coupled between an anode and a cathode partly disposed within a central plasma chamber within the anode combines with the introduction of inert gas in the region of the cathode to form a plasma arc and a resulting plasma stream flowing from the anode, an improved cooling system is provided for the anode. Cooling fluid in the form of water from a booster pump is introduced generally tangentially into an annular cooling chamber formed between a cylindrical outer surface of the anode and a housing in which the anode is mounted. A helical groove formed by a set of threads in the outer cylindrical surface of the anode provides a spiral flow path for the cooling water within the annular cooling chamber such that the cooling water advances axially along the length of the anode as it flows around the outside of the anode. This enhance the heat exchanging ability of the cooling water and thereby the cooling of the anode, particularly in the case of small plasma guns whose miniaturized cooling passages and other scaled-down dimensions make anode cooling difficult. The cooling water eventually exits the annular chamber for return to the booster pump via an outlet passage disposed generally in line with the central axis of the anode to encourage multiple revolutions of the cooling water within the annular chamber before exiting.