---
title: "How a tokamak holds heat and measures where it goes"
description: "Twisted magnetic fields guide charged particles. Scattered laser light and calibrated heat-flow measurements reveal how much thermal energy remains."
canonical_url: "https://sandboxphysics.com/stories/instruments/tokamak-iter/"
language: "en"
page_type: "story"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/stories/instruments/tokamak-iter/"
last_modified: "2026-11-13"
---

[How physics instruments measure](https://sandboxphysics.com/stories/instruments/)

# How a tokamak holds heat and measures where it goes

A hot gas would quickly cool against the walls of its container. A tokamak uses magnetic fields to guide its charged particles around a ring and reduce their contact with those walls. To understand the heating, an experiment must also measure how much energy remains and where the rest goes. A laser sent through the gas helps make that measurement.

Sandbox Physics · 2026-11-13

## 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.

$$
\frac{dW}{dt}=P_{\mathrm{deposited}}-P_{\mathrm{radiated}}-P_{\mathrm{transport}}
$$

W is the total random thermal energy of electrons and ions, and t is time. The left side is its change per second. The right side subtracts radiated and non-radiative transported power from deposited heating power. Inferring W from electron diagnostics retains the stated ion-temperature assumption.

Sources: [3] [4] [5]

## [Compare magnetic surfaces, electron spectra and heat flows](https://sandboxphysics.com/labs/tokamak-iter/)

Compare heating and free cooling, then move the laser off axis and inspect electron heat alongside energy readings for the same interval.

A prescribed axisymmetric cold field with a passive low-pressure thermal perturbation and finite diagnostic geometry. Electron heat is measured; ion-inclusive heat requires the supplied equal-temperature closure.

## Sources and further reading

1. [ITER · What is a tokamak?](https://www.iter.org/machine/what-tokamak)

   Checked 13 November 2026: toroidal chamber, charged plasma and magnetic confinement; ITER tests at reactor scale and is not equipped to generate electricity. Component roles only; no temperature targets or performance figures are used.

2. [ITER · Magnets](https://www.iter.org/machine/magnets)

   Checked 13 November 2026: toroidal confinement, plasma shaping and central-solenoid induction. Facility coil dimensions, field strengths and stored energies are not teaching-model inputs.

3. [ITER · External heating systems](https://www.iter.org/machine/supporting-systems/external-heating-systems)

   Checked 13 November 2026: current heating, collisional energy deposition from neutral beams and wave heating. The article distinguishes supply, transmission and deposited heat without repeating facility power figures or operating plans.

4. [ITER · Divertor](https://www.iter.org/machine/divertor)

   Checked 13 November 2026: particles reach targets, deposit kinetic energy and transfer heat to active water cooling. Facility heat-flux values and target counts are not used.

5. [ITER · Diagnostics](https://www.iter.org/machine/supporting-systems/diagnostics)

   Checked 13 November 2026: distinct measurements of magnetic state, electron temperature/density profiles, radiated-power distribution and material surfaces. Bolometric sightlines motivate the spatial information needed to estimate total radiation.

6. [Petra Bílková · Thomson Scattering, 13th ITER International School (2024)](https://www.iter.org/sites/default/files/media/2025-01/l6_bilkova_thomson_scattering.pdf)

   Checked 13 November 2026, PDF pages 15–23: finite scattering geometry, velocity-dependent shifts, spectral width/temperature, absolute yield/density and high-temperature relativistic spectra. The page-21 numerical example conflicts with its formula and is not used. No slide or image is reproduced.

[How a vernier reads between millimetres](https://sandboxphysics.com/stories/instruments/length-metrology/) [Air helps hold up the object on a balance](https://sandboxphysics.com/stories/instruments/precision-balance/) [How Cavendish Measured the Attraction Between Spheres](https://sandboxphysics.com/stories/instruments/cavendish-balance/) [Which pressure does a liquid column measure?](https://sandboxphysics.com/stories/instruments/manometer/) [The cup absorbs heat too](https://sandboxphysics.com/stories/instruments/calorimeter/) [Rotational rheometers need both torque and speed](https://sandboxphysics.com/stories/instruments/rotational-rheometer/) [Why the air beyond a pipe changes its resonance](https://sandboxphysics.com/stories/instruments/resonance-tube/) [How a seismometer reads the ground through motion inside its case](https://sandboxphysics.com/stories/instruments/seismometer/) [Why does a meter change the 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atomic spacing](https://sandboxphysics.com/stories/instruments/powder-xray-diffractometer/) [How Small-Angle X-ray Scattering Reads a Particle Halo](https://sandboxphysics.com/stories/instruments/small-angle-xray-scattering/) [Why X-ray fluorescence depends on sample thickness](https://sandboxphysics.com/stories/instruments/xray-fluorescence-spectrometer/) [Why an AFM makes a bump look wider](https://sandboxphysics.com/stories/instruments/atomic-force-microscope/) [How a small indentation separates hardness from elasticity](https://sandboxphysics.com/stories/instruments/nanoindenter/) [How Differential Scanning Calorimetry Reads a Melting Peak](https://sandboxphysics.com/stories/instruments/differential-scanning-calorimeter/) [How thermogravimetry separates mass loss from the gas around it](https://sandboxphysics.com/stories/instruments/thermogravimetric-analyzer/) [How a diamond anvil cell reads pressure in ruby light](https://sandboxphysics.com/stories/instruments/diamond-anvil-cell/) [How diamond fluorescence reveals a magnetic-field direction](https://sandboxphysics.com/stories/instruments/nv-center-magnetometer/) [Reading a magnetic field in the rhythm of atoms](https://sandboxphysics.com/stories/instruments/optically-pumped-magnetometer/) [Keeping a laser in time with atoms](https://sandboxphysics.com/stories/instruments/optical-lattice-clock/) [Why a Kibble balance needs two experiments](https://sandboxphysics.com/stories/instruments/kibble-balance/) [How a dilution refrigerator keeps carrying heat away](https://sandboxphysics.com/stories/instruments/dilution-refrigerator/) [Why a bigger vacuum pump may barely help the sample](https://sandboxphysics.com/stories/instruments/ultra-high-vacuum-system/) [How microwaves read a superconducting qubit](https://sandboxphysics.com/stories/instruments/superconducting-qubit-readout/) [How current leaves a voltage across a thin sheet](https://sandboxphysics.com/stories/instruments/four-probe-hall/) [How an ion flight becomes a mass spectrum](https://sandboxphysics.com/stories/instruments/time-of-flight-mass-spectrometer/) [How four straight rods select ion mass](https://sandboxphysics.com/stories/instruments/quadrupole-mass-filter/) [How a rapidly reversing field can hold ions](https://sandboxphysics.com/stories/instruments/paul-ion-trap-mass-spectrometer/) [How a cyclotron keeps a proton on the accelerating beat](https://sandboxphysics.com/stories/instruments/cyclotron-synchrocyclotron/) [Why the drift tubes in an RF linac get longer](https://sandboxphysics.com/stories/instruments/radio-frequency-linac/) [The directions hidden inside a small beam spot](https://sandboxphysics.com/stories/instruments/beam-diagnostics/) [How defocusing magnets help a beam stay in a ring](https://sandboxphysics.com/stories/instruments/synchrotron-storage-ring/) [Why a collider cannot keep every collision](https://sandboxphysics.com/stories/instruments/collider-trigger-daq/) [Getting synchrotron light onto a sample](https://sandboxphysics.com/stories/instruments/synchrotron-beamline/) [How a free-electron laser gathers electrons into a brighter beam](https://sandboxphysics.com/stories/instruments/xray-free-electron-laser/) [Following energy exchange with neutron flight times](https://sandboxphysics.com/stories/instruments/pulsed-neutron-time-of-flight/) [Why stellarator coils bend in three dimensions](https://sandboxphysics.com/stories/instruments/w7x-stellarator/)

## Keep reading

How physics instruments measure · Reading path

### How physics instruments measure

1. [Article **How a vernier reads between millimetres**](https://sandboxphysics.com/stories/instruments/length-metrology/)
2. [Article **Air helps hold up the object on a balance**](https://sandboxphysics.com/stories/instruments/precision-balance/)
3. [Article **How Cavendish Measured the Attraction Between Spheres**](https://sandboxphysics.com/stories/instruments/cavendish-balance/)
4. [Article **Which pressure does a liquid column measure?**](https://sandboxphysics.com/stories/instruments/manometer/)
5. [Article **The cup absorbs heat too**](https://sandboxphysics.com/stories/instruments/calorimeter/)
6. [Article **Rotational rheometers need both torque and speed**](https://sandboxphysics.com/stories/instruments/rotational-rheometer/)
7. [Article **Why the air beyond a pipe changes its resonance**](https://sandboxphysics.com/stories/instruments/resonance-tube/)
8. [Article **How a seismometer reads the ground through motion inside its case**](https://sandboxphysics.com/stories/instruments/seismometer/)
9. [Article **Why does a meter change the circuit?**](https://sandboxphysics.com/stories/instruments/electrical-meters/)
10. [Article **How a null detector reveals a resistance**](https://sandboxphysics.com/stories/instruments/resistance-bridges/)
11. [Article **Why a capacitor can behave like an inductor**](https://sandboxphysics.com/stories/instruments/lcr-impedance/)
12. [Article **Where does the slow wave on an oscilloscope come from?**](https://sandboxphysics.com/stories/instruments/digital-oscilloscope/)
13. [Article **How a lock-in amplifier finds a signal buried in noise**](https://sandboxphysics.com/stories/instruments/lock-in-amplifier/)
14. [Article **Where a small peak in a spectrum can come from**](https://sandboxphysics.com/stories/instruments/spectrum-analyzer/)
15. [Article **How a network analyzer separates the cable from the device**](https://sandboxphysics.com/stories/instruments/vector-network-analyzer/)
16. [Article **When a slower quartz crystal can measure added mass**](https://sandboxphysics.com/stories/instruments/quartz-microbalance/)
17. [Article **How an echelle spectrograph gives overlapping colors a second direction**](https://sandboxphysics.com/stories/instruments/echelle-spectrograph/)
18. [Article **How total brightness reveals many infrared colors**](https://sandboxphysics.com/stories/instruments/ftir-spectrometer/)
19. [Article **Measuring a Solution’s Concentration with Light**](https://sandboxphysics.com/stories/instruments/uv-vis-spectrophotometer/)
20. [Article **How an ellipsometer reads thin-film thickness from reflected light**](https://sandboxphysics.com/stories/instruments/spectroscopic-ellipsometer/)
21. [Article **How a polarimeter reveals mixed polarization**](https://sandboxphysics.com/stories/instruments/mueller-polarimeter/)
22. [Article **How individual photons reveal a fluorescence lifetime**](https://sandboxphysics.com/stories/instruments/tcspc-fluorescence-lifetime/)
23. [Article **Brillouin Microscopy Listens to Sound with Light**](https://sandboxphysics.com/stories/instruments/brillouin-microscope/)
24. [Article **How a white sphere compares a lamp’s total output**](https://sandboxphysics.com/stories/instruments/integrating-sphere-radiometer/)
25. [Article **How powder rings reveal atomic spacing**](https://sandboxphysics.com/stories/instruments/powder-xray-diffractometer/)
26. [Article **How Small-Angle X-ray Scattering Reads a Particle Halo**](https://sandboxphysics.com/stories/instruments/small-angle-xray-scattering/)
27. [Article **Why X-ray fluorescence depends on sample thickness**](https://sandboxphysics.com/stories/instruments/xray-fluorescence-spectrometer/)
28. [Article **Why an AFM makes a bump look wider**](https://sandboxphysics.com/stories/instruments/atomic-force-microscope/)
29. [Article **How a small indentation separates hardness from elasticity**](https://sandboxphysics.com/stories/instruments/nanoindenter/)
30. [Article **How Differential Scanning Calorimetry Reads a Melting Peak**](https://sandboxphysics.com/stories/instruments/differential-scanning-calorimeter/)
31. [Article **How thermogravimetry separates mass loss from the gas around it**](https://sandboxphysics.com/stories/instruments/thermogravimetric-analyzer/)
32. [Article **How a diamond anvil cell reads pressure in ruby light**](https://sandboxphysics.com/stories/instruments/diamond-anvil-cell/)
33. [Article **How diamond fluorescence reveals a magnetic-field direction**](https://sandboxphysics.com/stories/instruments/nv-center-magnetometer/)
34. [Article **Reading a magnetic field in the rhythm of atoms**](https://sandboxphysics.com/stories/instruments/optically-pumped-magnetometer/)
35. [Article **Keeping a laser in time with atoms**](https://sandboxphysics.com/stories/instruments/optical-lattice-clock/)
36. [Article **Why a Kibble balance needs two experiments**](https://sandboxphysics.com/stories/instruments/kibble-balance/)
37. [Article **How a dilution refrigerator keeps carrying heat away**](https://sandboxphysics.com/stories/instruments/dilution-refrigerator/)
38. [Article **Why a bigger vacuum pump may barely help the sample**](https://sandboxphysics.com/stories/instruments/ultra-high-vacuum-system/)
39. [Article **How microwaves read a superconducting qubit**](https://sandboxphysics.com/stories/instruments/superconducting-qubit-readout/)
40. [Article **How current leaves a voltage across a thin sheet**](https://sandboxphysics.com/stories/instruments/four-probe-hall/)
41. [Article **How an ion flight becomes a mass spectrum**](https://sandboxphysics.com/stories/instruments/time-of-flight-mass-spectrometer/)
42. [Article **How four straight rods select ion mass**](https://sandboxphysics.com/stories/instruments/quadrupole-mass-filter/)
43. [Article **How a rapidly reversing field can hold ions**](https://sandboxphysics.com/stories/instruments/paul-ion-trap-mass-spectrometer/)
44. [Article **How a cyclotron keeps a proton on the accelerating beat**](https://sandboxphysics.com/stories/instruments/cyclotron-synchrocyclotron/)
45. [Article **Why the drift tubes in an RF linac get longer**](https://sandboxphysics.com/stories/instruments/radio-frequency-linac/)
46. [Article **The directions hidden inside a small beam spot**](https://sandboxphysics.com/stories/instruments/beam-diagnostics/)
47. [Article **How defocusing magnets help a beam stay in a ring**](https://sandboxphysics.com/stories/instruments/synchrotron-storage-ring/)
48. [Article **Why a collider cannot keep every collision**](https://sandboxphysics.com/stories/instruments/collider-trigger-daq/)
49. [Article **Getting synchrotron light onto a sample**](https://sandboxphysics.com/stories/instruments/synchrotron-beamline/)
50. [Article **How a free-electron laser gathers electrons into a brighter beam**](https://sandboxphysics.com/stories/instruments/xray-free-electron-laser/)
51. [Article **Following energy exchange with neutron flight times**](https://sandboxphysics.com/stories/instruments/pulsed-neutron-time-of-flight/)
52. [You are here **How a tokamak holds heat and measures where it goes**](https://sandboxphysics.com/stories/instruments/tokamak-iter/)
53. [Article **Why stellarator coils bend in three dimensions**](https://sandboxphysics.com/stories/instruments/w7x-stellarator/)