5. Measuring the plasma

Measuring the key plasma properties is one of the most challenging aspects of fusion research. Knowledge of the important plasma parameters (temperature, density, radiation losses etc) is very important in increasing understanding of plasma behaviour and designing, with confidence, future devices. However, as the plasma is contained in a vacuum vessel and its properties are extreme (extremely low density and extremely high temperature), conventional methods of measurement are not appropriate. Thus, plasma diagnostics are normally very innovative and often measure a physical process from which information on a particular parameter can be deduced.

Measurement techniques can be categorised as active or passive. In active plasma diagnostics, the plasma is probed (via laser beams, microwaves, probes etc) - to see how the plasma responds. For instance, in inteferometers, the passage of a microwave beam through the plasma will be slowed by the presence of the plasma (compared to the passage through vacuum). This measures the refractive index of the plasma from which the density of plasma ions/electrons can be interpreted. With all active diagnostics, it must be ensured that the probing mechanism does not significantly affect the behaviour of the plasma.

With passive plasma diagnostics, radiation and particles leaving the plasma are measured - and this knowledge is used to deduce how the plasma behaves under certain conditions. For instance, during D-T operation on JET, neutron detectors measure the flux of neutrons emitted form the plasma. All wavelengths of radiated waves (visible, UV waves, X-rays etc) are also measured - often from many locations in the plasma. Then a detailed knowledge of the process which created the waves can enable a key plasma parameter to be deduced.

For more details see Focus on JET Diagnostics.