Fusion plasmas loose energy at their edge and must therefore continously be heated to make up for these losses. Tokamaks offer three main methods to heat a plasma:
The plasma current which is induced in order to provide the poloidal magnetic field, produces heat just like a wire warms up when an electric current flows through it. At JET, approximately 1 megawatt (MW) of power is supplied by the Ohmic heating, which creates plasma currents of up to five million amperes. Ohmic heating is limited by two facts: Firstly the plasma current is induced via transformer action, which employs an increasing magnetic field. It is therefore pulsed and does not allow continous plasma operation. Secondly the electric resistance, which produces the heat, decreases with plasma temperature.
Neutral Beam Heating
A widespread technique for additional plasma heating is based on the injection of powerful beams of neutral atoms into ohmically pre-heated plasma. The total power of neutral beam heating at JET is as much as 35 MW.
The beam consists of neutral atoms which are able to penetrate the strong magnetic field that confines the plasma. In the plasma, the beam atoms lose electrons due to collisions, they become electrically charged and are captured by the magnetic field of the tokamak. These (new) ions are much faster then average plasma particles. In a series of subsequent ion-ion, ion-electron and electron-electron collisions, the beam atoms is transfer their velocity to all plasma particles.
The neutral beams are usually formed by atoms of hydrogen isotopes (hydrogen, deuterium or even tritium at JET). The only way to form the neutral beam is to produce large amounts of ions first, then to accelerate the ions in a strong high-voltage electric field and finally to neutralise the accelerated beam via charge-exchange interactions with a gas cloud.
As the plasma ions and electrons rotate around the magnetic field lines, electromagnetic waves of the right frequency can resonate or damp their wave power into the plasma particles.The rotational frequencies of the plasma particles depend on their charge and mass and on the magnetic field strength. Thus one can target certain particles by injecting waves of the respective frequency.
Ion cyclotron resonant heating (ICRH) is routinely applied on JET. The available resonant frequencies are in the range of 23-57 MHz (megahertz). 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 (LHCD) employs frequencies lying between ion cyclotron and electron cyclotron ones. While LHCD has an inefficient heating effect, it can drive electric current because it increases the mean velocity of the electrons and thus creates a net electric current. can increase the velocity of the plasma electrons. The JET LHCD system works at a frequency of 3.7 GHz and can supply 12 MW of additional power.