Posted on: 7th January 2013
2013 promises to be a busy year at JET, building on a successful experimental campaign in 2011-2012.
The 2011-2012 campaign was based around the newly installed ITER-Like Wall, so the first thing was to investigate how a completely fresh metal wall would behave. ITER will start from a similar state and so it was good to find that this went quite easily – the cleaner, metallic environment made for good plasma breakdown in every pulse. Next the focus moved to full characterisation of the wall and re-establishing the ITER baseline and hybrid scenarios. During 2013 these regimes of operation will be explored further, with a view to optimising the confinement – critical experiments will be carried out to help ITER make its decision on the choice of day 1 divertor material.
Much was achieved in the 2011-12 campaigns. Most importantly, the amount of fuel retained in the wall was found to be an order of magnitude lower than with carbon – a pleasing result, as an undesirable level of fuel retention was the main reason for changing to metallic tiles.
However, the new metallic wall is more prone to melting than the previous carbon tiles, so it was crucial that the plasma’s heat was controlled to prevent damage to the beryllium and tungsten tiles. In particular the mechanism of disruptions – where the magnetic energy of the plasma escapes into the wall in a localised area – has proven to be quite different in the ITER-Like environment, because the plasma is cleaner and so less energy is lost through radiation. To tackle this new behaviour a disruption mitigation system was implemented, which puffs massive amounts of argon and deuterium gas into the vessel to quench the plasma’s energy if required. This system allowed JET to safely operate plasmas at currents up to 3.5 mega-amps.
Ironically, having worked hard to prevent tiles from being melted, one of the goals for 2013 is to deliberately melt a tile – a request from ITER. As JET has been refurbished to be as close to ITER as possible, it is in a position to test a worst-case scenario by deliberately melting a tungsten tile. How the liquified area behaves, and what happens to the plasma during and after this event will give the ITER designers vital information, on which they will base their choice of material for ITER’s divertor. While bulk melting of tungsten has been investigated in other tokamaks under conditions of steady heat load, JET is the only machine with enough energy stored in its plasma that it can explore melting due fast plasma events such as Edge Localised Modes (ELMs) or disruptions – the biggest concern for ITER.
Amongst all this hard work there will be a little bit of time out for celebrations on June 25th – the day that JET turns thirty!