Construction of the ITER diagnostic neutral beam system in India.

ITER’s goal is not only to create fusion plasma but also to measure and understand it in unprecedented detail. Around 50 diagnostic systems will monitor more than 100 plasma and machine parameters, including temperature, density, shape, position, impurity content and fusion power. Many of these measurements will also provide real-time input to the plasma control system. These systems will provide the principal information about conditions inside the vacuum vessel, which will be inaccessible to personnel during operation and cannot be observed directly.

Vacuum Vessel Condition

A separate set of viewing and instrumentation systems will monitor the condition of the vacuum vessel and its plasma-facing components. In-vessel lighting and viewing systems will inspect blanket and divertor surfaces, while temperature, pressure and other sensors will monitor thermal loads, structural behaviour and vacuum integrity.

Plasma Shape and Position

Langmuir probes (green) installed between divertor target elements.

Accurate knowledge of plasma shape and position is essential for plasma control. Magnetic diagnostics, including flux loops and magnetic probes installed around the vacuum vessel, will measure the magnetic equilibrium, plasma current, stored energy, boundary shape and position as they evolve during a pulse. Microwave diagnostics will provide additional information on plasma position and conditions in the main plasma and divertor region. These measurements will feed the plasma control system, which can adjust the poloidal field coils and in-vessel coils to maintain the required plasma position and shape.

Radiation Losses

A substantial fraction of the plasma power must be radiated before reaching the divertor in order to reduce local heat loads on plasma-facing components. Bolometric diagnostics measure the total radiated power and its spatial distribution throughout the main plasma and divertor region. These measurements help the control system maintain an appropriate balance between radiative cooling and plasma performance.

A basic bolometer uses a thin metal absorber that heats when it receives radiation. The resulting temperature-dependent change in electrical resistance provides a measure of the absorbed power. Arrays of thin-film bolometers with multiple lines of sight will measure radiation over a broad spectral range and allow tomographic reconstruction of the spatial distribution of radiated power.

If radiative losses are too low, controlled injection of impurity species can be used to increase radiation and reduce the heat reaching the divertor. If radiation becomes excessive, impurity injection can be reduced or other plasma-control parameters adjusted to restore the required operating conditions.

Plasma Parameters

Spectroscopic instruments covering wavelengths from the visible to the X-ray range will measure impurity species and concentrations, particle fluxes, ion temperature, helium density, fuelling ratio, plasma rotation and other parameters. Thomson scattering will provide electron-temperature and electron-density profiles, while interferometry will measure line-integrated electron density.

Langmuir probes will provide local measurements of electron temperature and density in the divertor plasma. Because direct exposure to the hot core plasma would rapidly destroy solid probes, they are installed in protected locations between divertor target elements, where plasma temperatures and heat loads are much lower. Around 400 Langmuir probes are planned for ITER.

Neutron Flux

During deuterium–tritium operation, neutron diagnostics will provide one of the principal measurements of fusion power. ITER will use neutron cameras, spectrometers, flux monitors and micro-fission chambers to measure neutron emission and its spatial and energy distributions. A micro-fission chamber is a small gas-filled detector containing a thin layer of fissile material, such as uranium.

When a neutron induces fission in a fissile nucleus, the energetic fission fragments ionise the surrounding gas. An electric field between the detector electrodes collects the resulting charge, producing a measurable signal that can be calibrated to determine the neutron flux.

Requirements

ITER will use many additional diagnostic techniques, all of which must operate reliably in an exceptionally demanding nuclear environment. Components installed close to the plasma must withstand intense neutron and gamma radiation, thermal loads, electromagnetic forces and long pulse durations. Many diagnostic components will be difficult to access after assembly, so reliability, remote maintenance capability and in-vessel calibration are important design requirements.

ITER diagnostics must combine rapid response, high accuracy and long-term reliability under conditions far more severe than in present-day tokamaks. Neutral-particle flux, neutron flux and neutron fluence will be significantly higher, while high-performance plasma pulses will typically last hundreds of seconds. Diagnostic systems must therefore maintain stable performance throughout long pulses and over the lifetime of the machine.