Diagnostics

The challenge of characterising extreme conditions of nuclear fusion plasmas both spatially and temporally has inspired JET to produce an impressive array of diagnostic techniques. Drawing from fields as diverse as neutronics, spectroscopy, lasers and microwaves, JET is a leader in the art of measurement”

Andrea Murari, Task Force Leader – Diagnostics

Methods of plasma observation

The aim of fusion research is the achievement of the conditions required for plasma to start producing energy from the fusion of hydrogen atomic nuclei. It is an extremely challenging task, as plasma heated to hundreds of millions of degrees needs to be confined well enough at sufficient density. The simultaneous task of diagnosing such a plasma, of measuring its characteristics, is not straightforward either! 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.

What can be done then? Firstly, one can simply try and 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. Quite a few tricky computing methods (including tomography, better known in its medical applications) provide information about plasma’s internal properties purely from external measurements. Secondly, one can send a tiny harmless probe into the plasma, like a beam of atoms, laser light or a microwave frequency, and observe its behaviour in the hot plasma. In both cases, a good understanding of the physics underlying the measurements is essential to get sensible results.