How a tokamak holds heat and measures where it goes

The field winds around the ring and its cross-section

When electrons separate from atoms, the gas contains mobile electrons and positive ions. This charged gas is a plasma. A magnetic field bends charged-particle motion: particles gyrate as they move along the field.

In a tokamak, a field around the large ring combines with a field around the tube’s cross-section. The resulting lines wind around the torus. In an ideal axisymmetric picture they lie on nested toroidal surfaces, called magnetic surfaces. External coils and plasma current form and control the configuration; the changing flux of a central solenoid induces plasma current.

ITER is an experimental facility for testing long pulses and required technologies at reactor scale. It is not equipped to generate electricity. Diagnostics measure the magnetic configuration and the evolving plasma.

Sources: [2][1]

Heating power must reach the particles

A static magnetic field bends motion. Heating transfers energy into that motion. Current can heat a plasma; fast neutral particles can enter and transfer energy through collisions; suitable electromagnetic waves can be absorbed by the particles.

Energy supplied to the heating equipment passes through conversion and transmission stages. Losses, incomplete coupling and reflected waves reduce the heat deposited in the plasma. Measuring that deposited energy provides the input needed to interpret a temperature rise.

Sources: [3]

Light and particle transport remove heat

Radiation carries plasma energy to surrounding material. Particle motion and transport also carry energy towards the edge. A divertor directs part of the outgoing particle flow onto designated targets, where particles deposit energy and active cooling removes the heat.

A bolometer absorbs radiation through an aperture and reads the resulting heating. It sees a limited set of positions and directions. Estimating total radiated power needs several views or a specified spatial-emission model. The aperture and its orientation therefore belong in the measurement.

Sources: [4][5]

Scattered light carries information about electron motion

Electrons scatter a tiny fraction of a laser pulse crossing the plasma. Separating the collected light by frequency gives a spectrum. Random electron motion broadens that spectrum. This measurement of free-electron scattering is called Thomson scattering.

With known geometry and a velocity-distribution model, the spectral shape constrains electron temperature. Absolute calibration of laser energy, collection area and detector response lets the scattering yield constrain electron density. The two quantities come from different information in the same spectrum.

Sources: [6]

A chord samples a spatial mixture

A finite laser beam and receiver collect light from a volume. If the centre is hotter than the edge, the spectrum mixes contributions from different temperatures. The instrument also spreads a narrow spectral line; a known line measures that contribution. Moving the chord and adding observation positions then help recover spatial profiles with this broadening included.

In each small volume, multiply the number of electrons by their mean random energy. Adding those contributions gives the electron heat. A central temperature needs a measured profile or a supplied profile assumption to become a volume estimate. Thomson scattering diagnoses electrons. If electron and ion numbers are equal, doubling electron heat gives total heat only under an additional assumption of equal electron and ion temperatures.

Sources: [5][6]

Compare stored heat and energy flows over the same interval

A thermal measurement records electron conditions together with deposited heating, radiation and energy reaching material. A common clock aligns these readings. Their integrated difference can then be compared with the change in stored heat between the beginning and end.

Wall calorimetry can receive both transported heat and absorbed radiation. If total radiation is counted separately, its wall contribution must be separated before adding the transport loss. Energy-meter gains, laser references and spatial measurements must cover the interval being analysed. A shared calibration error affects many readings together.

After heating stops, radiation and transport continue to remove energy. Electron spectra follow the cooling, and bolometric and material heat measurements distinguish its routes. Together they describe how a hot plasma loses its stored heat over time.

Sources: [3][4][5]