Joule Heating

Plasma discharge in the ASDEX Upgrade fusion device.

Since the magnetic confinement in a tokamak relies partly on the current flowing through the plasma, the early stages of a plasma discharge are also heated by Joule, or ohmic, heating. The initially weakly ionised gas resists the induced current and heats up. As the plasma temperature rises to about 10–15 million kelvin, however, its electrical resistivity decreases sharply and ohmic heating becomes increasingly ineffective.

A temperature of 10–15 million kelvin is not high enough for efficient deuterium–tritium fusion, so additional external heating is required. At sufficiently high temperature, density and confinement, alpha particles produced by fusion can transfer enough energy back to the plasma to sustain its temperature. This self-heated state is known as ignition and would no longer require external plasma heating. For D–T fusion, plasma temperatures of about 150 million kelvin are typically required.

Three main methods are used: neutral beam injection, ion cyclotron resonance heating, and electron cyclotron resonance heating.

Neutral Beam Injection

Interior of the Alcator C-Mod tokamak at the MIT Plasma Science and Fusion Center. Two ion cyclotron range of frequencies (ICRF) antennas are visible to the left of the central column. A magnetic field-aligned ICRF antenna is visible to the right.

Neutral beam injection is based on injecting high-energy particles into the plasma, where they transfer their kinetic energy to plasma particles through collisions. A neutral beam injector is essentially a particle accelerator. Hydrogen or deuterium atoms are first ionised and the resulting ions are accelerated by an electric field to high energy.

Charged ions cannot penetrate deeply into the plasma because they would be deflected by the magnetic field. Before injection, they therefore pass through a neutralisation chamber, where they are converted into neutral atoms. These can travel across the magnetic field and penetrate deep into the plasma, where they are re-ionised and transfer their energy to the plasma particles.

Ion Cyclotron Heating

Ion cyclotron resonance heating uses radio-frequency waves tuned to the cyclotron motion of ions in the magnetic field. When ions absorb energy from these waves, their kinetic energy increases. The waves are launched into the plasma by large antennas. In ITER, the system will operate at frequencies of about 40–55 MHz.

Electron Cyclotron Heating

Electron cyclotron heating system for the ITER tokamak.

Electrons can also be heated directly by electron cyclotron resonance heating. Electromagnetic waves are tuned to the electron cyclotron resonance frequency, allowing electrons to absorb energy efficiently and then transfer part of it to ions through collisions. In ITER, the system operates at 170 GHz.

Both methods use electromagnetic waves to transfer energy to the plasma, although ECRH operates at microwave frequencies while ICRH uses much lower radio frequencies.

Lower Hybrid Current Drive

Radio-frequency waves can also be used to drive plasma current non-inductively and thereby extend the duration of tokamak discharges. One such technique is lower hybrid current drive (LHCD), in which lower-hybrid waves transfer momentum preferentially to electrons moving in one direction, producing a net plasma current. LHCD has been demonstrated on several tokamaks, although it is not part of the current ITER baseline.

Bootstrap Current

To make a tokamak more viable as a power plant, the external power required to sustain the plasma current should be minimised. In a toroidal plasma with suitable pressure gradients, part of the current can arise naturally through a process known as the bootstrap current. This self-generated current can reduce the amount of externally driven current required and is therefore important for long-pulse and steady-state tokamak concepts.