4. Heating the plasma

One of the main requirements for fusion is to heat the plasma particles to very high temperatures or energies. The following methods are typically used to heat the plasma - all of them are employed on JET.

Ohmic Heating and Current Drive

Currents up to 5 million amperes (5MA) are induced in the JET plasma - typically via the transformer or solenoid. As well as providing a natural pinching of the plasma column away from the walls, the current inherently heats the plasma - by energising plasma electrons and ions in a particular toroidal direction. A few MW of heating power is provided in this way.

Neutral Beam Heating

Beams of high energy, neutral deuterium or tritium atoms are injected into the plasma, transferring their energy to the plasma via collisions with the plasma ions. The neutral beams are produced in two distinct phases. Firstly, a beam of energetic ions is produced by applying an accelerating voltage of up to 140,000 Volts. However, a beam of charged ions will not be able to penetrate the confining magnetic field in the tokamak. Thus, the second stage ensures the accelerated beams are neutralised (i.e. the ions turned into neutral atoms) before injection into the plasma. In JET, up to 21MW of additional power is available from the NBI heating systems.

Radio-Frequency Heating

As the plasma ions and electrons are confined to rotating around the magnetic field lines in the tokamak, electromagnetic waves of a frequency matched to the ions or electrons are able to resonate - or damp its wave power into the plasma particles. As energy is transferred to the plasma at the precise location where the radio waves resonate with the ion/electron rotation, such wave heating schemes have the advantage of being localised at a particular location in the plasma.

In JET, a number of antennae in the vacuum vessel propagate waves in the frequency range of 25-55 MHz into the core of the plasma. These waves are tuned to resonate with particular ions in the plasma - thus heating them up. This method can inject up to 20MW of heating power.

Waves can also be used to drive current in the plasma - by providing a "push" to electrons travelling in one particular direction. In JET, 10 MW of these so-called Lower Hybrid microwaves (at 3.7GHz) accelerate the plasma electrons to generate a plasma current of up to 3MA.

Self Heating of Plasma

The Helium ions (or so-called alpha-particles) produced when Deuterium and Tritium fuse remain within the plasma's magnetic trap for a time - before they are pumped away through the divertor. The neutrons (being neutral) escape the magnetic field and their capture in a future fusion powerplant will be the source of fusion power to produce electricity.

When fusion power out just equals the power required to heat and sustain plasma then a Breakeven is achieved. However, only the fusion energy contained within the Helium ions heats the Deuterium and Tritium fuel ions (by collisions) to keep the fusion reaction going. When this self-heating mechanism is sufficient to maintain the plasma temperature required for fusion the reaction becomes self-sustaining (i.e. no external plasma heating is required). This condition is referred to as Ignition. In magnetic plasma confinement of the D-T fusion reaction the condition for ignition is approximately six times more demanding (in confinement time or in plasma density) than the condition for breakeven.

For more details on plasma heating see Focus On : JET Plasma Heating and Current Drive.