Managing exhaust heat poses a key engineering barrier for commercial-scale reactors. In a large tokamak, escaping plasma can deliver parallel heat fluxes exceeding 1 GW/m², which is 100x greater than the ~5-10 MW/m² today’s toughest materials can withstand. A conventional divertor would fail in seconds, as exhaust temperatures exceed 10,000 °C and charged particles hammer the wall.
The standard mitigation is “detachment”: injecting gas to radiate away energy and create a neutral cushion that cools the exhaust before it strikes the divertor. Done well, this reduces surface heat flux by more than an order of magnitude. But it’s a narrow window. Too little gas and the divertor melts; too much and the fuel plasma is quenched.
Fast transients (bursts from instabilities or fueling) destabilize the balance, cycling the plasma between attached and detached states. Conventional divertors lack the passive resilience to absorb these shocks, making power-plant-scale operation risky.
The Super-X Solution
The Super-X divertor, originally developed at the University of Texas, tackles the problem by reshaping the magnetic geometry of the exhaust. The “legs” of the divertor (magnetic field lines guiding plasma to the wall) are extended, and the strike point is shifted to a larger radius. This lengthens the exhaust path, expands the contact area, and provides more room for plasma to cool before hitting the target.
Neutral gas baffling in the divertor chamber traps cold atoms, reinforcing detachment and stabilizing the exhaust. Together, these features create what is essentially a longer, cooler, and more forgiving exhaust pipe.
Building on these design principles, an international team led by Kevin Verhaegh (TU Eindhoven) and Bob Kool (DIFFER) validated the Super-X concept on the UK’s MAST Upgrade tokamak, demonstrating its performance under dynamic, reactor-relevant conditions for the first time.
The key findings:
Independent control: Each divertor leg could be tuned without disturbing the opposite leg or the plasma core (critical for double-null reactor designs).
Passive damping of transients: The long-legged geometry buffered sudden power surges, preventing destructive heat spikes.
Robust detachment: Exhaust conditions remained stable even during rapid perturbations, giving operators a larger safety margin.
Even intermediate geometries, the “elongated” divertor tested alongside Super-X, delivered measurable benefits, suggesting designers can capture much of the advantage with modest adjustments rather than radical re-engineering.
Divertor configuration of the MAST Upgrade tokamak. The transport of energetic core particles towards the outer divertor targets is indicated by the red arrows.
Implications for Reactor Design
For reactor designers, this is a key risk-reduction milestone. Power exhaust has been a make-or-break issue for compact, high-power devices. Super-X proves that clever magnetic shaping can keep divertors safe without sacrificing burn performance.
This has implications across a range of projects, including:
UK STEP: As a spherical tokamak, STEP directly inherits lessons from MAST-U. A validated exhaust solution de-risks the design.
CFS ARC/SPARC: Compact high-field tokamaks generate extreme power density; Super-X-like geometries may be essential to handle heat flux.
EU DEMO: Even at larger scale, DEMO faces unsustainable exhaust loads. Incorporating advanced divertors widens its operating window.
Scaling from MAST-U to pilot power plants will require further refinement: optimizing geometry, integrating advanced materials, and validating performance at higher power and longer pulses. But the trajectory is clear, and designs like Super-X could lay the foundation for future power plants that can run continuously without melting down their own exhaust pipes.