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 confinement magnetic field. When these neutral particles are ionized by collision with the plasma particles, they are kept in the plasma by the confining magnetic field, and can transfer most of their energy by further collisions with the plasma. By tangential injection in the torus, neutral beams provide also momentum to the plasma and current drive, one essential feature for long pulses of burning plasmas. 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. In the 90s there has been impressive progress in negative ion sources and accelerators with the construction of multi-megawatt negative ion based NBI systems at LHD (H0, 180 keV) and JT-60U (D0, 500 keV). The NBI designed for ITER is a substantial challenge (D0, 1MeV, 40A) and a prototype is being constructed to optimize its performance in view of the ITER future operations. Other ways to heat plasma for nuclear fusion include RF heating, electron cyclotron resonance heating (ECRH), and ion cyclotron resonance heating (ICRH).
This is typically done by:
- Making a plasma. This can be done by microwaving a low pressure gas.
- Electrostatic ion acceleration. This is done dropping the positively charged ions towards negative plates. As the ions fall, the electric field does work on them, heating them to fusion temperatures.
- Reneutralizing the hot plasma by adding in the opposite charge. This gives the fast moving beam no charge.
- Injecting the fast moving hot neutral beam in the machine.
It is critical to inject neutral material into plasma, because if it is charged, it can start harmful plasma instabilities. Most fusion devices inject isotopes of hydrogen, such as pure deuterium or a mix of deuterium and tritium. This material becomes part of the fusion plasma. It also transfers its energy into the existing plasma within the machine. This hot stream of material should raise the overall temperature. Although the beam has no electrostatic charge when it enters, as it passes through the plasma, the atoms are ionized. This happens because the beam bounces off ions already in the plasma.
Neutral Beam Injectors installed in fusion experiments
At present, all main fusion experiments use NBIs. Traditional positive ion based injectors (P-NBI) are installed for instance in the JET, or in ASDEX-U. To allow power deposition in the center of the burning plasma in larger devices, a higher neutral beam energy is required. High energy (>100keV) systems require the use of negative ion technology (N-NBI).
|Precursor ion beam||D−||H−||D−|
|Max acceleration voltage (kV)||400||190||1000|
|max power per installed beam (MW)||5.8||6.4||16.7|
|Pulse duration (s)||30 (2MW, 360kV)||128 (at 0.2MW)||3600 (at 16.7MW)|
Coupling with fusion plasma
Because the magnetic field inside the torus is circular, these fast ions are confined to the background plasma. The confined fast ions mentioned above are slowed down by the background plasma, in a similar way to how air resistance slows down a baseball. The energy transfer from the fast ions to the plasma increases the overall plasma temperature.
It is very important that the fast ions are confined within the plasma long enough for them to deposit their energy. Magnetic fluctuations are a big problem for plasma confinement in this type of device (see plasma stability) by scrambling what were initially well-ordered magnetic fields. If the fast ions are susceptible to this type of behavior they can escape very quickly, however some evidence suggests they are not susceptible.
- LR Grisham, P Agostinetti, G Barrera, P Blatchford, D Boilson, J Chareyre, et al., Recent improvements to the ITER neutral beam system design, Fusion Engineering and Design 87 (11), 1805-1815
- V. Toigo, D. Boilson, T. Bonicelli, R. Piovan, M. Hanada, et al. 2015 Nucl. Fusion 55:8 083025
- "Neutral beam powers into the record books, 09/07/2012".