Neutral Beam Injection
A widespread technique for additional plasma heating is based on the injection of powerful beams of neutral atoms into ohmically pre-heated plasma. The beam atoms carry a large uni-directional kinetic (motional) energy. In the plasma, beam atoms lose electrons due to collisions, i.e. they get ionised (electrically charged) and as a consequence 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 group velocity of beam atoms is transferred into an increased mean velocity of the chaotic motion of all plasma particles. The action is similar to the opening break in the game of pool, when a fast motion of one ball can cause the seemingly chaotic motion of all balls. However, the world of plasma particles is inconceivably small, and many billions of particles are in play. We sense an increase in their chaotic motion as an increase in temperature. In other words, a neutral beam heats the plasma – and that is what we desire!
In fusion experiments, the neutral beams are usually formed by atoms of hydrogen isotopes (hydrogen, deuterium or even tritium at JET). Notice that we always speak about a ‘Neutral beam’ and its ‘atoms’. Indeed, the beam needs to consist of neutral atoms (as opposed to electrically charged ions) otherwise it could not penetrate the strong magnetic field that confines fully ionised plasmas. The energy of the beam (corresponding to the velocity of its atoms) must be sufficient to reach the plasma centre – if the beam atoms were too slow, they would get ionised immediately at the plasma edge. At the same time, the beam is supposed to have power sufficient to deliver significant amounts of fast atoms into the plasma, otherwise the heating effect would not be noticeable. At JET, the beam energy is 80 or 140 keV, corresponding in the case of a deuterium beam to 2800 or 3600 km/s which is approximately five times faster than the mean velocity of the ions in a JET deuterium plasma. The total power of beam heating at JET is as much as 23 MW (million Watts). With this power, the number of beam atoms per second corresponds approximately to 10% of the total number of JET plasma ions.
It is not at all straightforward to generate powerful neutral beams of very fast atoms. 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. The accelerated ions get neutralised in charge-exchange interactions with a gas cloud, however, some leave the cloud still in a charged state. These residual fast ions must be deflected by a dedicated electromagnet to a cooled ion dump that can withstand heavy ion bombardment. Last but not least, powerful vacuum pumping must assure that practically no slow atoms from the neutralising gas cloud can diffuse as far as to the plasma chamber, so that the fast neutral atoms have free access to burst into the plasma.