In 2022, KSTAR will receive a new tungsten divertor that will allow it to operate with much hotter plasmas longer. The divertor, a part of the Plasma Facing Components (PFC) within a tokamak, cleans the impurities caused by plasma operations. However, not only the divertor needs to be changed for hotter plasmas. A brand new cooling water system is also being prepared for the KSTAR.

Cool enough for 100 million ℃

Plasma temperatures inside the KSTAR tokamak can reach over 100 million ℃. Although the heat does not directly reach the PFCs thanks to superconducting magnets, which create strong magnetic fields and suspend the plasmas in a vacuum, they still have to endure indirect heat whose temperatures are around 1,000℃. This is where the cooling water does its job.

The coolants run inside the PFCs through flow paths, cooling down the heat generated from plasma operations. This cooling is indispensable for stable plasma operations as well as to protect the tokamak components from extensive heat.

The cooling water system consists of a storage tank, circulation pumps, heat exchangers, valves, an automatic control system, power supplies, etc. Also, the cooling water needs a purity maintenance system and UV sterilizers to keep it ultrapure.

A baking system and drying system are integrated in the system as well - the former gets rid of the impurities within the vacuum vessel when KSTAR starts operating and the latter completely dries out any water left inside the device after experiments. The upgrade will comprehensively include all these systems.

The cooler the system, the longer the plasma operations

KSTAR is attempting to achieve 300-second-long continuous operation with 100 million ℃ plasma by 2026. For longer ultra-high temperature plasma operations, tungsten was selected to be the next PFC material because it is one of the most effective materials for the inner walls of a fusion reactor. ITER also chose tungsten for its divertor.

Since much more intense heat will come from the new tungsten divertor, it was only natural to improve the cooling water system to match it. The current system was designed to circulate 207.33ℓ of cooling water through pipes per second with 16 bar pressure. However, when the operation runs for 300 seconds, the flow and pressure of coolants should be drastically different. The new system will circulate 520ℓ of coolant per second with 30 bar pressure. In addition, the capacity of the storage tank will be enlarged from the current 90 tons to accommodate 150 tons of cooling water. The pipes will also be replaced to endure increased pressure.

The upgrade for the new cooling water system began in 2020. As of April 2022, 38% of the tasks had been completed. The new components such as the water tank, pumps, heat exchangers, and the purity maintenance system are already in the KSTAR’s basement, ready to be installed in August.

(New components in KSTAR basement)