Neutral-beam injection
Neutral-beam injection (NBI) is one method used to heat
Mechanism
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 with 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
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 JET[3] and 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 (>100 keV) systems require the use of negative ion technology (N-NBI).
Magnetic confinement device | P-NBI | N-NBI | ECRH | ICRH
|
LH | Type | First operation |
---|---|---|---|---|---|---|---|
JET | 34 | — | — | 10 | 7 | Tokamak | 1983 |
JT-60U | 40 | 3 | 4 | 7 | 8 | Tokamak | 1985 |
TFTR
|
40 | — | — | 11 | — | Tokamak | 1982 |
EAST | 8 | — | 0.5 | 3 | 4 | Tokamak | 2006 |
DIII-D
|
20 | — | 5 | 4 | — | Tokamak | 1986 |
ASDEX-U | 20 | — | 6 | 8 | — | Tokamak | 1991 |
JT60-SA* | 24 | 10 | 7 | — | — | Tokamak | 2020 |
ITER *
|
— | 33 | 20 | 20 | — | Tokamak | 2026 |
LHD[4] | 9 (H+) 20 (D+) |
15 (H−) 6 (D−) |
? | ? | ? | Stellarator | 1998 |
Wendelstein 7-X | 8 | — | 10 | ? | — | Stellarator | 2015 |
- Legend
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 that they are not susceptible.[citation needed]
The interaction of fast neutrals with the plasma consist of
- ionisation by collision with plasma electrons and ions,
- drift of newly created fast ions in the magnetic field,
- collisions of fast ions with plasma ions and electrons by Coulomb collisions (slow-down and scattering, thermalisation) or charge exchange collisions with background neutrals.
Design of neutral beam systems
Beam energy
The adsorption length for neutral beam ionization in a plasma is roughly
with in m, particle density n in 1019 m−3, atomic mass M in amu, particle energy E in keV. Depending on the plasma minor diameter and density, a minimum particle energy can be defined for the neutral beam, in order to deposit a sufficient power on the plasma core rather than to the plasma edge. For a fusion-relevant plasma, the required fast neutral energy gets in the range of 1 MeV. With increasing energy, it is increasingly difficult to obtain fast hydrogen atoms starting from precursor beams composed of positive ions. For that reason, recent and future heating neutral beams will be based on negative-ion beams. In the interaction with background gas, it is much easier to detach the extra electron from a negative ion (H− has a binding energy of 0.75 eV and a very large cross-section for electron detachment in this energy range) rather than to attach one electron to a positive ion.
Charge state of the precursor ion beam
A neutral beam is obtained by neutralisation of a precursor ion beam, commonly accelerated in large
Existing and future negative-ion-based neutral beam systems (N-NBI) are listed in the following table:
JT-60U | LHD | ITER **
| |
---|---|---|---|
Precursor ion beam | D− | H− / 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) |
Ion beam neutralisation
Neutralisation of the precursor ion beam is commonly performed by passing the beam through a gas cell.[6] For a precursor negative-ion beam at fusion-relevant energies, the key collisional processes are:[7]
- D− + D2 → D0 + e + D2 (singe-electron detachment, with −10=1.13×10−20 m2 at 1 MeV)
- D− + D2 → D+ + e + D2 (double-electron detachment, with −11=7.22×10−22 m2 at 1 MeV)
- D0 + D2 → D+ + e + D2 (reionization, with 01=3.79×10−21 m2 at 1 MeV)
- D+ + D2 → D0 + D2+ (charge exchange, 10 negligible at 1 MeV)
Underline indicates the fast particles, while subscripts i, j of the cross-section ij indicate the charge state of fast particle before and after collision.
Cross-sections at 1 MeV are such that, once created, a fast positive ion cannot be converted into a fast neutral, and this is the cause of the limited achievable efficiency of gas neutralisers.
The fractions of negatively charged, positively charged, and neutral particles exiting the neutraliser gas cells depend on the integrated gas density or target thickness with the gas density along the beam path . In the case of D− beams, the maximum neutralisation yield occurs at a target thickness m−2.
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. For a given gas throughput [Pa·m3/s], the maximum gas pressure at the centre of the cell depends on the gas conductance [m3/s]:
and in molecular-flow regime can be calculated as
with the geometric parameters , , indicated in figure, gas molecule mass, and gas temperature.
Very high gas throughput is commonly adopted, and neutral-beam systems have custom vacuum pumps among the largest ever built, with pumping speeds in the range of million liters per second.[8] If there are no space constraints, a large gas cell 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
See also
References
- ^ L. R. 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.
- S2CID 124477971.
- ^ "Neutral beam powers into the record books, 09/07/2012". Archived from the original on 2017-03-24.
- .
- S2CID 33934446.
- hdl:11577/3227451.
- ^ IAEA Aladdin database.
- .