Plasma diagnostic aims to determine plasma temperature and density, to measure plasma particle and radiation losses, to find out the magnetic topology and to observe plasma flows and fluctuations. The task is not straightforward. One cannot simply put a sensitive element inside the hot plasma – not only would it sublimate, but more importantly, the experiment would be lost as the plasma would cool down and become impure.
Instead, scientists use two strategies:
Firstly, they observe the plasma from the outside, applying as many different methods as possible and exploiting a great variety of physical phenomena, ranging from atomic effects and nuclear reactions to radiation propagation and electromagnetism.
Secondly, they send a tiny harmless probe into the plasma, like a beam of atoms, laser light or a microwave frequency, and observe its behaviour.
This section introduces some of the methods employed at JET.
JET diagnostic – the best from all over Europe
JET has the most complete set of diagnostics – about 100 different methods – for reactor grade plasmas in the world. It benefits from being Europe’s joint fusion experiment: Many EUROfusion Research Units continue to develop new diagnostics for JET or upgrade the present ones. JET also serves as a unique test bed for the development of diagnostics for the future fusion reactor machine, ITER.
At JET, signals from all diagnostic systems are digitised and stored in a central database. Every JET pulse produces almost 8 GBytes of raw diagnostics data. Most of the data need further processing – this is done automatically where possible by dedicated computer codes, but in many cases human intervention and/or data validation is required. All data are accessible to all scientists on the JET site and, moreover, any scientist from any EUROfusion Research Unit has remote access to the data from their home institute.