Dust / Pellet Injector
Frozen-hydrogen ice cubes shot at 10× the speed of sound — fuel and emergency brake, all in one barrel.
Delivers solid material into a fusion plasma in three roles: (1) fuelling, by shooting frozen D-T pellets so the plasma core gets fresh hydrogen isotopes; (2) edge-localised-mode pacing, by injecting smaller deuterium pellets at high frequency to trigger many small ELMs instead of one large one; (3) disruption mitigation, by firing shattered ice pellets (SPI) of neon, argon, or hydrogen to radiate plasma energy uniformly when a disruption is detected, protecting the first wall.
Pellet formation: hydrogen isotopes are condensed inside a cryogenic extruder at ~10 K to form a solid ribbon of D-T or D₂ ice; a punch chops the ribbon into cylindrical pellets typically 2–10 mm in diameter. Acceleration is done in one of three ways. (a) Pneumatic gas guns use 4–10 MPa of helium to launch pellets at 1–3 km/s through a 1–5 m guide tube. (b) Centrifugal injectors spin pellets up in a rotor and release them tangentially, achieving 0.5–1 km/s with mechanical robustness. (c) Two-stage light-gas guns, electrothermal guns, and railguns have all been studied as routes to >10 km/s, the velocity needed for deep core fueling in larger tokamaks like ITER. To date, reliable repetitive injection at ≥10 km/s has not been achieved — it is a target, not a demonstrated capability. For shattered-pellet injection (SPI), large 10–25 mm cryogenic pellets of neon-deuterium or pure neon are shattered against an angled tube wall just before entering the vacuum vessel, producing a cloud of small fragments and gas that radiates the plasma stored energy in milliseconds. SPI is the leading disruption-mitigation system for ITER and JET.
In plain terms
A tokamak's plasma is like an extremely hot soup — and you need to keep adding ingredients without splashing it. So you freeze the new ingredients into bullets and fire them straight at the hot core. They vaporise instantly, supplying fresh fuel deep inside without ever touching the walls. Other times, when something is going wrong, you fire shattered ice cubes from another barrel to dump the energy safely before the plasma damages the machine.
Also called: pellet injector, dust injector, shattered pellet injector, SPI.
Why it matters
- Only proven method to fuel a burning-plasma tokamak core (gas puffing alone deposits at the edge).
- Disruption-mitigation injectors are part of the safety case that makes ITER's design defensible against runaway-electron damage.
- Edge-localised-mode pacing using small pellets has been demonstrated to suppress damaging large ELMs at JET and DIII-D.
- Same technology generalises to lithium-dust injection for first-wall conditioning and divertor protection.
Where it lives
- ITER pellet-injection system (in design; SPI for disruption mitigation)
- JET shattered-pellet injector (deployed)
- DIII-D pellet injector (operational)
- ASDEX Upgrade pellet injector
Key numbers
- Pellet diameter: 2–25 mm
- Pellet velocity (pneumatic): 1–3 km/s
- Centrifugal injector velocity: 0.5–1 km/s
- Target high-speed velocity: ~10 km/s (Not yet routinely achieved)
- Pellet temperature: ~10 K (Cryogenic D-T ice)
Surprising facts
- Deuterium ice melts at 18.65 K and is roughly six times denser than the same molar amount of D₂ gas at STP — a single 5 mm pellet contains about 10²⁰ deuterium atoms.
- Shattered-pellet injection on ITER will fire 25-mm-diameter neon-deuterium ice cubes at >300 m/s through a tube that shatters them against an angled wall — three barrels firing within milliseconds of detected disruption.
- Tokamak fueling at ITER's design density ultimately requires pellet velocities of ~10 km/s (about 30 times the speed of sound) to penetrate the magnetic field and deposit fuel at mid-radius — a velocity not yet routinely achieved.
- Edge-localised-mode pacing uses small pellets at 10–50 Hz to trigger frequent harmless ELMs in place of the rare violent ones that would otherwise dump megajoules into the divertor.
Open questions
- Pellet ablation asymmetry: Pellets injected from the high-field side ablate differently than those from the low-field side, with deeper deposition than gas-dynamic models predict. The mechanism (ExB drift of the ablation cloud) is qualitatively understood but not quantitatively closed — first-principles ablation codes still disagree by tens of percent with experiment.
- Disruption-time delay scatter: On JET and DIII-D, the time between SPI trigger and thermal quench varies shot-to-shot in ways not fully reducible to pellet trajectory or fragment-size distribution. Until this scatter is mechanistically understood, ITER's safety margin must be set conservatively.
A short history
First cryogenic pellet injection into a tokamak was performed at ORNL on ISX-B in 1980 (Milora and Foster). Centrifugal injectors followed in the 1990s for ASDEX Upgrade. Shattered-pellet injection was developed at General Atomics in the 2010s for DIII-D and selected as the ITER baseline disruption-mitigation method. JET's SPI system has been used in real DT campaigns since 2021. As of 2026, reliable, repetitive 10 km/s pellet injection — the goal for deep core fueling in burning plasmas — remains a research target rather than a routine capability.
Sources
- Milora et al., Nucl. Fusion (1995) — review of pellet fueling (https://iopscience.iop.org/article/10.1088/0029-5515/35/6/I02)
- Combs et al., Fusion Eng. Des. (2017) — shattered-pellet injection (https://www.sciencedirect.com/science/article/pii/S0920379617306191)
- ITER Disruption Mitigation System overview (https://www.iter.org/mach/disruption-mitigation)
- JET DT campaign — disruption mitigation results (2024) (https://www.euro-fusion.org/news/)