Neutral Beam Injection

Neutral Beam Injection

Neutral beam injection (NBI) is the most powerful fusion plasma heating and current drive method. Only the neutral beam can penetrate the intensive magnetic field of the tokamak. The neutral beam can heat the ions and drive the plasma current. An ion beam is formed in an ion source, energized in an accelerator, and converted to a neutral beam in a neutralizer.

Negative Ion Source

A neutralization efficiency of a positive ion beam decreases rapidly with increasing beam energy. While the neutralization efficiency of K-STAR NBI, whose beam energy is 100 keV, is about 60%, it goes down to less than 10% if a 300 keV positive ion beam. However, the neutralization efficiency of the negative ion beam is sustained at over 50% even if its energy is higher than 1000 keV. Therefore, to improve the operational efficiency of the NBI for the fusion power plant, the negative ion source must be developed. In addition, a weakness of the conventional negative ion source induced by using cesium has to be solved. FLASH develops a novel negative ion source based on a volume production mechanism of the negative ions. We research the plasma physics and engineering improvement, of the negative ion source. 

Neutralizer

An electron of the negative ion is detached and the ion beam is converted to the neutral beam in the neutralizer. In a conventional gas neutralizer, the negative ion is neutralized via charge exchange between the ion and neutral gas particles. Thus, the neutralization efficiency is directly related to pressure and path length inside the neutralizer. To enhance the neutralization efficiency, the pressure should increase, meanwhile, it requests a massive vacuum system to sustain the tokamak vacuum pressure. Thus, FLASH researches a photo-neutralizer based on laser technology.

We develop the next-generation NBI that adopts innovative technology such as cesium-free negative ion source and photo-detachment.