Just like a car’s exhaust system, you have to get the burnt fuel out of the combustion chamber after you fuse it.
This is the third article in The Fusion Report series on critical supporting technologies for commercial fusion energy. In today’s article, we’ll look at a class of technologies called divertor and heat exhaust systems. But first, let’s look at which of these systems are required for what types of fusion machines.
Magnetic Fusion Energy (MFE) Machines – Tokamaks, Stellarators
The classical definition of divertors are deliberately sacrificial exhaust regions of magnetic fusion energy (MFE) machines: magnetic field lines guide helium “ash,” fuel ions, and heat out of the plasma core onto specially cooled targets. In massive tokamaks, such as the International Thermonuclear Experimental Reactor (ITER), the usual arrangement is an axisymmetric single-null or double-null X-point divertor, typically positioned at the bottom (and, for double-null operation, also the top) of the torus. The X-point separates confined field lines from open lines that intersect divertor targets, allowing particles and heat to be removed while helping shield the main chamber wall. For example, ITER’s divertor uses tungsten armor because of its exceptionally high melting point, and is designed to extract fusion heat and ash while limiting plasma contamination.
In commercial reactor-scale tokamaks, the central problem is that a compact conventional divertor can concentrate enormous heat fluxes onto a small wetted area. That has motivated “advanced” tokamak layouts: snowflake divertors use a near-second-order magnetic null to broaden and split the exhaust pattern, while long-leg concepts such as Super-X route exhaust along a longer path to a larger-radius target, increasing flux expansion and giving the plasma more opportunity to radiate and cool before reaching material surfaces. These approaches have been tested or developed in devices including the Swiss TCV, the Princeton Plasma Physics Laboratory (PPPL) National Spherical Torus Experiment (NSTX), the General Atomics DIII-D facility, China’s Experimental Advanced Superconducting Tokamak (EAST), and the UK Atomic Energy Authority (UK-AEA) MAST Upgrade.
Stellarators require a substantially different divertor philosophy because their externally generated, three-dimensional helical magnetic fields lack the tokamak’s simple rotational symmetry and generally do not rely on a large plasma current. Rather than using a circular, axisymmetric X-point and ring-like targets, stellarators commonly employ island or helical divertors that exploit naturally occurring edge magnetic islands and direct open field lines to multiple shaped target modules around the vessel. Wendelstein 7-X is the best-known example: its modular divertor system must conform to a strongly three-dimensional field-line topology, making the geometry, target shaping, pumping placement, and heat-load distribution more complex than in a tokamak. The payoff is potentially steady-state operation and the ability to tailor the edge magnetic geometry through coil and boundary design; the challenge is ensuring that the island structure distributes heat broadly enough and does not create localized hot spots. Other fusion concepts may depart further: spherical tokamaks face severe space constraints and often pursue compact, high-flux-expansion or long-leg divertors, while some low-temperature, open-field or inertial fusion concepts do not use a tokamak- or stellarator-style divertor at all. Thus, the key distinction is not merely material choice, but the machine’s magnetic topology: relatively symmetric X-point exhaust in tokamaks versus intrinsically 3-D island/helical exhaust in stellarators.
Magnetic Mirrors
Magnetic mirrors are much closer to having a divertor analogue, because their magnetic field lines are inherently open. A mirror confines many charged particles by increasing the magnetic-field strength at each end, reflecting particles whose pitch angle is sufficiently large. Particles inside the loss cone instead stream out along the machine axis through the ends—known as end loss. In a simple mirror, this is a primary confinement limitation, not a small peripheral exhaust like in a tokamak. Consequently, mirror systems require engineered end regions to receive this escaping particle and power flow. Depending on the design, those regions may include:
- An end tank or expander, where field lines expand and the heat/particle flux can spread over a larger area.
- A cooled end plate, collector, or plasma-facing target to tolerate residual particle and thermal loads.
- Neutral pumping and fueling hardware to control the end-region plasma and remove particles.
- A direct-energy converter (DEC), which uses electrostatic fields or grids to decelerate escaping charged particles and recover part of their kinetic energy directly as electricity rather than merely turning it into heat.
That last option is distinctive: in a magnetic mirror, a stream of charged particles exiting through an end loss cone can be a usable output channel. Direct conversion has long been investigated for mirror end losses, and current mirror developers are pursuing it as a way to improve plant efficiency. It cannot capture neutron energy, which still has to be recovered in a surrounding blanket, but it may recover some energy carried by escaping ions and electrons.
Inertial Fusion Energy (IFE) Machines
For IFE machines, a tiny target is compressed and ignited in the center of a chamber by lasers, ion beams, or another driver. The burn lasts only briefly and produces an outward, nearly radial pulse of X-rays, charged target debris, and 14.1-MeV neutrons; there is no magnetically confined edge plasma that must be led along open field lines to a divertor. Accordingly, IFE plants need a fusion chamber and first-wall protection system, but not a divertor in a classical sense. The engineering task is to survive repetitive micro-explosions (potentially several per second), while preserving clear beam paths, handling debris and tritium, recovering heat, and limiting damage to structural materials. There can still be specialized debris collectors, gas-handling equipment, vacuum pumping, liquid cleanup, and heat-extraction systems in an IFE plant, but calling those a “divertor” would be misleading.
Conclusion: Divertors and Heat Exhaust Systems Are MFE Issues
Different fusion energy systems provide different challenges, and divertors and heat exhaust systems are primarily an issue for magnetic fusion energy (MFE) approaches, as well as to a lesser extent for magnetic mirrors. The primary reason that the divertor is an issue for MFE systems is that they have to be able to exhaust burned products at the same time they are still continuously burning new hydrogen into helium, while not breaking magnetic field confinement. IFE systems can simply pump out the helium gas and any other byproducts, or capture them in a liquid first wall blanket. Magnetic mirrors provide different challenges than MFE or IFE machines, with the potentially added benefit of direct energy conversion (DEC), especially for aneutronic fusion fuels where the exhaust is primarily charged particles.
