Divertor Cassette
The hardest-working brick on the planet — a tungsten-armoured drawer that catches a star's exhaust without cracking.
Forms the receiving surface for exhaust plasma in a tokamak with a 'divertor magnetic configuration': field-line geometry that channels particles leaving the confinement region down onto a small target area at the bottom (or top) of the vacuum vessel. The cassette absorbs heat fluxes of up to 10–20 MW/m² in steady state and ~50 MW/m² during transient events, neutralises the incoming ions to gas, pumps tritium-bearing exhaust, and is exchanged remotely every few years as it erodes.
Magnetic geometry: a poloidal-field coil set creates a magnetic 'X-point' below the main plasma. Field lines outside the last closed flux surface become 'open' and lead from the plasma edge down through the divertor channel to the target plates. Particles streaming along these field lines slam into the inner ('vertical') and outer target plates of the cassette. Those plates are armoured with tungsten — the only metal whose melting point (3,422 °C) and thermal conductivity allow it to survive 10–20 MW/m² steady heat flux. The armour sits on copper-chromium-zirconium (CuCrZr) heat sinks bonded to stainless-steel (or Inconel) cooling tubes that carry pressurised water at 100–155 °C, 4 MPa, removing megawatts of heat per cassette. ITER's divertor is built from 54 individual cassettes, each ~10 tonnes, that slide circumferentially into the vacuum vessel from a port on the lower side; remote-handling robots replace them every ~2 full-power years. During plasma operation the divertor also hosts gas puffing, neutral-pressure measurement, and cryopump suction lines, and is the primary location where neutralised D-T fuel and helium ash are pumped away.
In plain terms
A tokamak is like a doughnut-shaped fireplace where the fire never touches the walls. Almost — the flames have to come out somewhere, so engineers point them at one carefully chosen drawer at the bottom, made of the toughest metal we have. The drawer slides in and out so it can be replaced when the heat finally wears it down. It survives heat hotter than a rocket exhaust nozzle for years at a time.
Also called: divertor module, ITER divertor cassette, tungsten divertor.
Why it matters
- Only credible technology for handling steady-state heat exhaust in burning-plasma tokamaks.
- Tungsten armour is what enables ITER, JET ITER-Like-Wall, and EAST to operate with metallic plasma-facing components instead of carbon (which retains tritium).
- Drives generations of materials science — tungsten-copper bonding, helium-bubble damage, neutron-induced transmutation.
- Sets the most aggressive heat-flux engineering target ever attempted on Earth: a sustained 10–20 MW/m² metallic surface.
Where it lives
- ITER divertor (54 cassettes, in fabrication)
- JET ITER-Like-Wall divertor (operational)
- ASDEX Upgrade tungsten divertor
- EAST tungsten divertor
- WEST tokamak (CEA) — tungsten-armoured divertor for steady-state research
Key numbers
- Steady-state heat flux: 10–20 MW/m²
- Transient heat flux (ELMs): up to ~50 MW/m²
- Coolant: Pressurised water 4 MPa, 100–155 °C
- Cassettes per ITER divertor: 54 (~10 tonnes each)
- Tungsten melting point: 3,422 °C
Surprising facts
- ITER's divertor handles heat fluxes up to 10–20 MW/m² in steady state — comparable to the surface of the Sun, and higher than the heat flux on a rocket nozzle throat.
- Each ITER divertor cassette weighs roughly 10 tonnes and is replaced entirely by remote-handling robots — humans never enter the vacuum vessel after first plasma.
- Tungsten was chosen because it has the highest melting point of any pure metal (3,422 °C) and the lowest sputtering yield under hydrogen bombardment of any candidate armour material.
- ITER's divertor will see neutron fluences of ~0.3 dpa (displacements per atom) per full-power year — enough to change tungsten's mechanical properties measurably over its planned lifetime.
Open questions
- Tungsten 'fuzz' growth under helium bombardment: Tungsten exposed to helium plasma at 1,000–2,000 °C grows a nanostructured 'fuzz' of fibrils within hours. The fuzz is mechanically fragile and changes thermal conductivity. The microscopic mechanism is still debated; mitigation strategies (alloying, surface engineering) remain partial.
- Inter-ELM heat-flux narrowing: The scrape-off-layer width on the divertor target narrows in surprisingly tokamak-independent ways with increasing plasma current. This Eich scaling has held across many machines but has no first-principles derivation that fully explains its near-universality, and projections to ITER carry significant uncertainty.
A short history
The divertor magnetic configuration was proposed by Lyman Spitzer for early stellarator designs and developed in tokamaks during the 1970s–80s (notably ASDEX, JET). Carbon armour was used universally until the 2000s, when concerns about tritium retention drove the move to tungsten. JET's ITER-Like-Wall (carbon-replaced-by-tungsten) campaign in 2011–2014 demonstrated that tungsten divertors could support high-performance H-mode operation. ITER's tungsten-armoured cassette design was finalised in the early 2010s and is in fabrication for first-plasma operation in the 2030s. EAST, WEST, and ASDEX Upgrade have provided long-pulse operational experience with tungsten plasma-facing components.
Sources
- Pitts et al., Nucl. Mater. Energy (2019) — ITER divertor R&D (https://www.sciencedirect.com/science/article/pii/S2352179119300596)
- ITER divertor description (https://www.iter.org/mach/divertor)
- JET ITER-Like-Wall results, Matthews et al., Phys. Scr. (2014) (https://iopscience.iop.org/article/10.1088/0031-8949/2014/T159/014015)
- EUROfusion divertor and PFC research overview (https://www.euro-fusion.org/)