To create a plasma for fusion requires heating the fuels to between 100 and 200 million degrees Celsius. In the case of JET, this entails putting tens of millions of watts of power – equivalent to about 10,000 fan heaters – into a mere couple of grams of fuel.
- Ohmic Heating or Current Drive
The coils around the central pole of the torus act as the primary coil for a huge transformer. The plasma itself is the secondary loop, and so has a large current induced in it. This plasma current produces heat, just as a wire warms up when an electric current flows through it. Tokamaks largely rely on plasma current not only for heating, but also for the poloidal magnetic field. At JET, approximately 1 megawatt of power is supplied by the Ohmic heating, which creates plasma currents of up to five million amperes.
- Neutral Beam Heating
High energy particle beams injected into the plasma transfer their energy as they collide with the plasma ions. A particle beam is generated by accelerating ions with high voltage. Since the charged particles cannot penetrate the magnetic field around the plasma, they are turned into neutral atoms just before injection. At JET, about 35 megawatts of energy (equivalent to 200 Porsche 944 turbos) can be provided by this system.
- Radio-Frequency Heating
As plasma particles move they spiral along the magnetic field lines with a regular frequency, typically in the radio-frequency (RF) region of the electromagnetic spectrum. Hence, if one can inject radio waves into the plasma at exactly the right frequency, they will resonate with this rotation, and transfer their energy to the plasma particles. The plasma particles have different resonance frequencies, depending on their mass and charge and the magnetic field strength at their location. Therefore the heating can be applied selectively to a defined group of particles in a defined location in the plasma, by injecting radiation at just the right frequency. This is known as Ion-Cyclotron Resonance Heating (ICRH). JET has a number of ICRH antennae in the vessel wall which, combined can supply up to 20 megawatts of power.
- Lower Hybrid Current Drive
A second RF heating method is called Lower Hybrid Current Drive (LHCD). This involves sending microwaves around the vessel at just the right frequencies that “push” electrons travelling in one particular speed and direction. This effect is used to add to the plasma current. In JET, the LHCD system generates 10 megawatts, creating plasma currents of up to three million amperes.