A vacuum bakeout system for the HSX stellarator upgrade

Reducing neutral influx from the vessel wall is of critical importance for the higher power and higher density discharges of the HSX upgrade. A bakeout system has been designed for the HSX vacuum vessel, which is expected to pump out adsorbed gases, especially water vapor, from the stainless-steel wall. A thermal simulation has been carried out in SolidWorks to determine heater layout, power requirements, and insulation choice such that the vessel is evenly heated up to 150C with wire wound heating strips. Each half period of the vessel is divided into three heating zones, with an additional zone on the closest boxport face. These zones will be separately controlled by a PLC system which will provide individual PID control to each zone to ensure uniform heating, which is crucial to the delicate weld joints of the vessel. The control loop will be closed by thermistors and will use pulse width modulation to maintain all zones at the same temperature. The PLC will also provide startup control sequencing to achieve uniform rate of temperature increase across all zones.