neutral beam heating

Method of heating a plasma in a magnetic containment device (e.g. tokamak) by injecting a beam of highly energetic neutral atoms (usually deuterium) into an plasma.

The energy of the neutral beams is harnessed in two stages. Firstly, the atom is converted into an ion by either charge exchange, ionization by plasma ions or ionization by plasma electrons. The newly created beam ion will then be trapped in the tokamk magnetic field. Secondly, this energetic ion will lose energy to the other plasma particles through Coulomb collisions. This will eventually result in the thermalization of the beam ions and heating of the plasma. At the Joint Europeran tokamak (JET), 20MW of neutral beam power can be injected.

The neutral beam is produced outside the tokamak. A bucket ion source is filled with neutral deuterium gas which is ionized using tungsten filaments. These ions are accelerated through an aperture system by an electric field up to the required kinetic energy. However, the magnetic field would prevent ions from penetrating the plasma. Therefore, they are injected into a neutralizer gas to convert them to neutral atoms again. Unfortunately, they also get reionized in a competing process. Before entering the plasma, a magnet deflects the unwanted ions allowing only the neutral beam to enter the tokamak.

At present, postive ions are created in a neutral beam heating system which are then sent through a neutralizer. The fraction of neutrals that is produced decreases with energy. However, if negative ions are used the fraction does not decrease so much. This promises a more powerful neutral beam. The technology for negative ion beams is still being researched. It is hoped that such a system could be employed in the next generation tokamak, ITER.