The suite of measurement techniques used to determine the temperature, density, composition, and behaviour of fusion plasmas — from lasers and microwaves to neutron detectors and spectroscopy.
Plasma diagnostics are measurement systems that provide information about the state of a fusion plasma without significantly disturbing it. Because fusion plasmas are too hot and tenuous to insert physical probes (except at the very edge), most diagnostics rely on electromagnetic radiation emitted by or scattered from the plasma, or on particles escaping from it.[1]
Thomson scattering: Pulsed laser measures electron temperature and density (the gold standard). Interferometry: Microwave or infrared beam measures line-integrated electron density. Electron cyclotron emission (ECE): Detects microwave radiation from electrons to map temperature profiles. Charge exchange recombination spectroscopy (CXRS): Measures ion temperature, rotation, and impurity density. Neutron diagnostics: Count fusion neutrons to determine fusion power and fuel composition. Bolometry: Measures total radiated power to track impurity behaviour.[2]
In a burning D-T plasma, diagnostics must survive 14 MeV neutron fluences that degrade optical fibres, darken windows, damage electronics, and transmute sensor materials. Developing radiation-hard diagnostics is one of the major engineering challenges for ITER and DEMO.[3]