Boiling helium heat transfer characteristics in narrow cooling channel

For design of a stable superconducting magnet with pool cooling method, the size of the cooling channel is one of the important factors to determine its overall current density. We measured the steady state boiling helium heat transfer characteristics in a 600 mm long vertical cooling channel. The result shows that heat transfer characteristics in film boiling region do not become worse so far as the channel is not extremely narrow and rather becomes better adding some amount of bubbles in the channel, whereas that in nucleate boiling region becomes worse monotoneously and rapidly as the thickness of the channel decreases. From these results, it proves to be much better to choose the superconductor whose critical temperature is high to get the high current density stable magnets and also proves to be able to design stable pool-boiling magnets as large as toroidal coils for Tokamak machines of the next generation.