Neutral Beam Injection Heating (NBI)

Neutral Beam Injection launches accelerated neutral particles which heat the plasma electrons and ions via collisions.

In a first step hydrogen ions are accelerated to high energies via electric fields. As the ions cannot penetrate the magnetic field they must pass a neutralizer target where they capture electrons resulting in high energetic neutral hydrogen atoms which subsequently enter the magnetic field of the stellarator. Due to collisions with plasma particles the neutral atoms become ionized again and are now captured as fast particles in the confining magnetic field. Their kinetic energy is then transferred to the plasma particles via further collisions. Thus besides heating the plasma NBI also contributes to plasma fuelling.

For Wendelstein 7-X two injector boxes with four ion sources each are planned with a technology based on the successful ASDEX Upgrade type. These injectors are not yet capable for steady state operation as some components will heat up. Instead 10 seconds lasting pulses are planned every 5 minutes during the otherwise ECRH heated plasmas. An R&D program aiming at steady state NBI is planned.

Total NBI heating power will be 7 MW using hydrogen atoms with a particle energy of 55 keV. Using deuterium atoms in future campaigns - deuterium is hydrogen with an additional neutron in the nucleus - one reaches slightly higher particle energies, namely 60 keV and a total heating power of 10 MW.

In operation phase OP1.2b one injector box equippped with two hydrogen sources was available and provided about 3 MW to the plasma. It is planned to operate both injectors with two hydrogen ion sources each, starting with experiment phase OP2.1.