Week 7: Defrosting with a difference
Week 7 of the shutdown has been busy again. At the time of writing there are more than thirty people in the JET torus hall, and this is typical of the rest of the week. The machine and its infrastructure has seen many changes since commissioning in 1983.
The vessel is well hidden behind diagnostics, cables and transmission lines. That is why preparations are continuing to gain access to the torus (see week 6). One of the two remote handling ‘booms’ has now been carried in and positioned ready for operation. As the photograph show, this enclosure has been designed to be as large as can fit through the doors and the lifting operation requires great care.
In week 4 it was also mentioned that some of the large water cooled components of the neutral beam heating system were lifted out of the torus hall. These contain complex and convoluted cooling water channels and it is not possible to drain all of the cooling water out of them effectively. During the last few weeks they have been being dried using a specially designed vacuum system with a ‘cold trap’. The water gradually evaporates when a partial vacuum is created. This water vapour is captured by the cold trap in the form of ice just as you might see in your freezer at home. The cold traps have to be defrosted once or twice per day by warming them to room temperature.