Principle of neutral beam injection

Neutral beam injection is installed on the majority of the world‘s nuclear fusion experiments as the most powerful among the heating systems. Its principle is the injection of fast hydrogen atoms with a power of several megawatts into the fusion plasma.

In collisions with the ions and electrons in the plasma the fast neutral hydrogen atoms become ionized and subsequently confined in the magnetic field. In consecutive collisions they gradually transfer their energy to the electrons and ions in the plasma and thereby heat the plasma. Furthermore, the injected beam can drive a current in the plasma that can partially substitute for the inductively driven current. IPP's neutral beam injection group builds and operates the neutral beam injection systems on ASDEX Upgrade and W7-X and investigates the physics of neutral beam current drive in ASDEX Upgrade.

Sketch of neutral beam heating

In order to produce the highly energetic, powerful beams an ion beam is extracted from a plasma source and accelerated. The fast ion beam is subsequently neutralized as the fast ions would be deflected by the fusion plasma’s confining magnetic field. This neutralization happens while the ions transit a gas target, a tube filled with neutral hydrogen gas at low pressure. The remaining non-neutralized ions in the beam are deflected onto a dedicated ion dump.

The typical particle energies of present-day systems are in the range between 50 keV and 130 keV. For comparison, the thermal energy in the plasma center is at maximum 15 keV. However, owing to its size ITER demands a significantly higher particle energy of 1 MeV for its neutral beam injection. At this high energy neutralization of positive hydrogen ions becomes very inefficient. Therefore, ion sources have to be used that produce negative hydrogen ions. The ITER reference ion source is being developed by IPP's neutral beam injection group.

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