The biggest tokamak is back online. They talk about it here, here, here and here (OK most articles only are excerpts of the original news release by CCFE . During the last months, JET underwent a longer maintenance (22 months) than usual to proceed with two major upgrades of the machine: a change of first wall materials and an increase of the auxiliary power heating.
The change of material, with the introduction of beryllium and tungsten in replacement of carbon, is of primary importance for ITER and for future reactors: it has a higher melting point and brings thus a better resistance of the machine to high power loads, especially in the divertor, which is the part of the chamber which receives the highest fluxes (both of power and particles). The problem with tungsten is that its density in the plasma should be kept very low otherwise it acts as an impurity which radiates a lot of power and provokes a degradation of confinement. Therefore, it is necessary to maintain the erosion and “sputtering” of tungsten within acceptable limits. It is one of the main difficulties encountered on the ASDEX Upgrade Tokamak which is entirely covered with this material.
The first plasma discharge (featured here) was concluding with, apparently, a low level of impurity (as measured by spectroscopy).
The Neutral Beam Injection system, which is used to heat the plasma, was also improved with an increase of total power from 23MW to 34MW. The system has not been tested yet in the new configuration.
The first tests take place only in pure Deuterium (with a low rate of nuclear reactions) to avoid high level of radiated powers and limit the flux of fast neutrons, which activate the walls of the machine.
Now the real scientific work is about to begin and I am looking forward to seeing the new results of this configuration with, hopefully, some good surprises.