Thermal Behavior of SST-1 Vacuum Vessel and Plasma Facing Components during Baking

Abstract

For plasma physics experiment, the baking of vacuum vessel (VV) as well as plasma facing components (PFC) of Steady-state Superconducting Tokamak (SST-1) is very essential. SST-1 vacuum vessel consists of ultra-high vacuum (UHV) compatible sixteen sectors in which U-shaped baking channels are embedded on inner surfaces of each of them. Similarly, PFCs are comprised of modular graphite diverters and movable graphite based limiters and stainless steel (SS 304L) tubes are brazed on the back plate of PFC for baking. Baking of SST-1 vacuum vessel and plasma facing components are carried out using nitrogen gas heating and supply system. SST-1 main vacuum vessel is baked at 150°C by circulating hot nitrogen gas at 250°C at 4.5bar gauge (g) pressure through these U-shaped channels. The plasma facing components (PFC) are baked at 250°C or more in the similar fashion by passing hot nitrogen gas through these SS brazed tubes. Thermal analysis shows that the temperature of 150°C at the vacuum vessel is achieved within ten hours if hot nitrogen gas is passed at the ramp rate of 50°C/h while thermal shields are maintained at 85K. It is also observed that the baking of either of them at a given temperature could be possible through radiation if one of them is maintained at desired temperature. The vacuum vessel at room temperature could be baked to 150°C due to radiation from PFC after 40hours when PFC alone is baked at 150°C. The mass flow rate required to bake SST-1 vacuum vessel at 150°C is 1.074kg/s while that for raising PFCs temperature to 150°C is 0.57kg/s. The mass flow rate required to bake PFCs at 250°C is 0.80kg/s.