Design of the NSTX heating and cooling system

NSTX requires that its internal plasma facing components (PFCs) reach 350degreesC during "bakeout" conditioning of the vacuum vessel. This paper describes a helium system designed to meet this requirement as well as provide cooling during plasma operations. The NSTX vacuum vessel's PFCs were designed to be heated or cooled by flowing a fluid medium through tubing attached to the PFC's copper backing plates. The heating / cooling system must move enough fluid at a sufficient rate with a high enough heat capacity through these restrictive paths. After the evaluation of several approaches including the use of heat transfer oils and steam, a compressed helium system was determined to be the optimal choice. The helium system utilizes a blower operating inside of a pressure vessel. This arrangement allows the base pressure to be raised to 20 atmospheres. With the system pressure elevated, the helium blower need only provide the motive force for overcoming 28 psi of friction losses and is not encumbered with compressing the gas. At 20 atmospheres the density of the helium is high enough to provide the heat capacity necessary to meet the NSTX requirements of 66 kW for heating and 82 KW for cooling. The paper will detail the unique design problems associated with a high pressure high temperature helium system as well as review the overall design, and modes of operation.