Neutral-beam injection
Neutral-beam injection (NBI) is one method used to heat plasma inside a fusion device consisting in a beam of high-energy neutral particles that can enter the magnetic confinement field.
Neutral-beam injection is a flexible and reliable technique, which has been the main heating system on a large variety of fusion devices.
To date, all NBI systems were based on positive precursor ion beams.
Although the beam has no electrostatic charge when it enters, as it passes through the plasma, the atoms are ionized.
Traditional positive-ion-based injectors (P-NBI) are installed for instance in JET[3] and in AUG. To allow power deposition in the center of the burning plasma in larger devices, a higher neutral-beam energy is required.
High-energy (>100 keV) systems require the use of negative ion technology (N-NBI).
Because the magnetic field inside the torus is circular, these fast ions are confined to the background plasma.
It is very important that the fast ions are confined within the plasma long enough for them to deposit their energy.
If the fast ions are susceptible to this type of behavior, they can escape very quickly.
[citation needed] The interaction of fast neutrals with the plasma consist of The adsorption length
For a fusion-relevant plasma, the required fast neutral energy gets in the range of 1 MeV.
[5] Caesium, deposited at the source walls, is an efficient electron donor; atoms and positive ions scattered at caesiated surface have a relatively high probability of being scattered as negatively charged ions.
[6] For a precursor negative-ion beam at fusion-relevant energies, the key collisional processes are:[7] Underline indicates the fast particles, while subscripts i, j of the cross-section
The fractions of negatively charged, positively charged, and neutral particles exiting the neutraliser gas cells depend on the integrated gas density or target thickness
In the case of D− beams, the maximum neutralisation yield occurs at a target thickness
Typically, the background gas density shall be minimised all along the beam path (i.e. within the accelerating electrodes, along the duct connecting to the fusion plasma) to minimise losses except in the neutraliser cell.
Therefore, the required target thickness for neutralisation is obtained by injecting gas in a cell with two open ends.
A peaked density profile is realised along the cell, when injection occurs at mid-length.
is adopted, but this solution is unlikely in future devices due to the limited volume inside the bioshield protecting from energetic neutron flux (for instance, in the case of JT-60U the N-NBI neutraliser cell is about 15 m long, while in the ITER HNB its length is limited to 3 m).
