Neutral Beam Injector (NBI)
A particle gun that turns ions into ghosts so they can slip past a magnetic forcefield.
Neutral beam injectors are one of the main external heating systems for fusion reactors. A plasma cannot reach fusion temperature (150 million °C) on its own from the start; it needs external energy input. NBIs fire a beam of very fast, uncharged hydrogen or deuterium atoms directly into the plasma. Once inside, the atoms are ionized by the plasma itself and the magnetic field traps them, where they slow down by colliding with other particles and depositing their kinetic energy as heat.
Building a neutral beam requires several steps. First, a plasma source generates negative deuterium ions (D⁻, atoms with an extra electron). An electric grid accelerates these ions to high energy — ITER's system uses 1 megavolt (1 MV) of accelerating voltage, giving each deuterium ion 1 MeV of kinetic energy. The ions then pass through a gas neutralization cell, where about 55% of them gain back their missing electron and become fast neutral atoms. The remaining charged ions are deflected away by a dump magnet. The surviving neutrals shoot into the tokamak's plasma. ITER's two main injectors will each deliver 16.5 MW of heating power, for a combined 33 MW from neutral beams alone.
In plain terms
Imagine a magic cannon that fires cannonballs at a castle protected by a force field that stops everything with an electric charge. The trick? Just before the cannonball hits the force field, you strip away its charge so it turns invisible to the field, passes right through, and smashes into the inside — depositing all its energy right where you want it.
Also called: NBI, neutral beam heating, negative ion beam injector.
Why it matters
- Neutral beams are the most powerful auxiliary heating method for large tokamaks; ITER relies on them for 33 of its required 50 MW of external heating
- By injecting beams tangentially, NBIs also spin the plasma (drive plasma rotation), which helps suppress instabilities
- Neutral beams deliver energy directly into the plasma core, not just the edge — this is essential for heating the dense center where fusion reactions must occur
- ITER's 1 MV NBI system represents a factor-of-three increase over any previous neutral beam voltage, requiring entirely new power supply and accelerator technology
Where it lives
- ITER Heating Neutral Beam — two injectors, 16.5 MW each, 1 MeV deuterium, up to 3,600-second pulses (under construction, Padua test facility MITICA)
- JT-60SA (Japan) — negative-ion NBI up to 500 kV, targeting 10 MW; first plasma 2023
- Large Helical Device (LHD, Japan) — NBI up to 190 kV, 6.4 MW per beam
Key numbers
- Acceleration voltage (ITER): 1 MV (1,000,000 V) (3× higher than any previous NBI)
- Beam energy per particle: 1 MeV (Deuterium atoms; needed to reach ITER plasma core)
- Power per injector (ITER): 16.5 MW (Total from two injectors: 33 MW)
- Maximum pulse length (ITER): 3,600 s (One hour continuous operation)
- Neutralization efficiency at 1 MeV: ~55% (Fraction of D⁻ ions successfully neutralized)
Surprising facts
- ITER's neutral beam injectors will operate at 1,000,000 volts — nearly eight times the voltage of a high-voltage power transmission line — for pulses lasting up to one hour
- Only about 55% of the accelerated ions are successfully neutralized in the gas cell at 1 MeV; the rest are dumped, making neutral beam efficiency an active engineering challenge
- The full ITER neutral beam power supply infrastructure covers 11,700 m² of buildings — larger than two American football fields — just to feed two beam injectors
- At 1 MeV, a deuterium atom travels at roughly 10% of the speed of light (about 30,000 km/s), giving it enough energy to penetrate deep into the plasma core of a large reactor
Open questions
- At 1 MeV, the neutralization efficiency of D⁻ beams drops to only about 55%: At 1 MeV, the neutralization efficiency of D⁻ beams drops to only about 55% — far lower than at lower energies — because the neutralization cross-section decreases rapidly with energy; this fundamental quantum-mechanical limitation means that roughly half of the accelerated beam power is wasted in the ion dump, a persistent engineering challenge for future reactors
- Negative deuterium ions (D⁻) are extremely fragile: Negative deuterium ions (D⁻) are extremely fragile: the extra electron is bound with only 0.75 eV of energy (compared to ~13.6 eV for a normal hydrogen electron), meaning even a tiny amount of background gas in the beam line can strip it and re-ionize the beam before it reaches the plasma
A short history
Neutral beam heating was developed in the 1970s at Lawrence Berkeley National Laboratory and Oak Ridge National Laboratory, originally using positive ion beams. Positive ions cannot be efficiently neutralized above about 100 keV, so as tokamaks grew larger and required higher-energy beams, researchers developed negative-ion-based NBIs. Japan's JT-60U demonstrated the first negative-ion NBI exceeding 100 keV in the early 2000s. ITER's requirement for 1 MeV beams led to the construction of the MITICA (Megavolt ITER Injector and Concept Advancement) test facility in Padua, Italy, where ITER's full-scale prototype NBI is being qualified. As of 2025, MITICA successfully completed high-voltage vacuum tests exceeding the maximum operating voltage.
Sources
- ITER Organization — Neutral Beam Test Facility (https://www.iter.org/project/iter-neutral-beam-test-facility)
- ITER Organization — Neutral Beam Power Supply (https://www.iter.org/project/construction-archives/neutral-beam-power-supply)
- Wikipedia — Neutral-beam injection (mechanism overview) (https://en.wikipedia.org/wiki/Neutral-beam_injection)
- Fusion for Energy — First deliveries for ITER's Neutral Beams (https://fusionforenergy.europa.eu/news/first-deliveries-to-light-iters-neutral-beams/)