Gyrotrons used as microwave sources for the electron cyclotron resonance heating system.

A successful deuterium–tritium fusion reaction requires a plasma temperature of about 150 million K, which is extremely difficult to achieve.

Joule Heating

At the beginning of a plasma pulse, an induced electric current heats the plasma through its electrical resistance, a process known as ohmic or Joule heating. As the plasma temperature rises, however, its electrical resistance decreases sharply, so ohmic heating becomes progressively less effective above roughly 10–15 million K.

Heating Systems

ITER will use three external heating and current-drive systems: neutral beam injection, ion cyclotron resonance heating and electron cyclotron resonance heating. Together, they will raise the plasma to fusion temperatures and, where required, help drive and shape the plasma current for long-pulse operation.

Neutral Beam Injection

Neutral beam injector (right) connected to the vacuum vessel (left).

Neutral beam injection uses high-energy neutral particles that penetrate deep into the plasma and transfer their kinetic energy through collisions with plasma particles. ITER will be equipped with two heating neutral beam injectors as well as a smaller diagnostic neutral beam; space is reserved for a third heating injector if required.

Each heating neutral beam injector is a very large particle-accelerator system, approximately 25 metres long and about 5 metres high and wide. It accelerates deuterium ions to an energy of 1 MeV and delivers up to 16.5 MW of neutral-beam power to the plasma. Before entering the plasma, the accelerated ions must be neutralised because charged particles would otherwise be deflected by the tokamak’s magnetic field and would not penetrate deeply into the plasma.

Because of ITER’s large plasma volume, the injected deuterium atoms must travel at speeds of roughly 10,000 km/s to penetrate sufficiently far into the plasma. At such high energies, positive ions are difficult to neutralise efficiently, so ITER uses negative deuterium ions as the starting point. These ions are accelerated and then passed through a neutraliser before entering the tokamak. Although negative ions are easier to neutralise efficiently at high energy, they are more difficult to produce and handle than positive ions, which makes the ITER neutral beam system technically demanding.

Ion Cyclotron Resonance Heating

Ion cyclotron resonance heating antenna.

Ion cyclotron resonance heating uses radio-frequency waves in the range of about 40–55 MHz, chosen to match ion cyclotron resonances in the plasma. Absorbed wave energy increases the kinetic energy of the ions. Two large antennas, each about 3.5 metres high with a cross-section of approximately 2 × 2 metres and weighing 45–50 tonnes, will be installed in equatorial ports to deliver up to 20 MW of heating power to the plasma. ITER is initially expected to operate with 10 MW of ICRH power, increasing to the full 20 MW in later phases.

Electron Cyclotron Resonance Heating

Electron cyclotron resonance heating transfers energy directly to plasma electrons using 170 GHz microwaves. The heated electrons then transfer energy to ions through collisions. In the current ITER baseline, ECRH will provide about 40 MW during Start of Research Operation and up to approximately 67 MW in later phases. Because the power can be deposited very precisely, ECRH can also assist plasma initiation, current drive and the suppression of certain magnetohydrodynamic instabilities.

The high-frequency electromagnetic waves used by ITER’s electron and ion cyclotron heating systems will be generated by equipment housed in the Radiofrequency Building.

Burning Plasma

One of ITER’s principal goals is to achieve a burning plasma, in which energetic helium nuclei produced by fusion provide the dominant source of plasma heating. ITER is designed to reach a fusion gain of Q ≥ 10, producing about 500 MW of fusion power from 50 MW of external plasma-heating power. In this regime, alpha-particle self-heating will dominate, while external heating can be reduced and used mainly for control.