A new shape to tame fusion's hottest challenge

Fusion energy, produced when atomic nuclei fuse to release tremendous amounts of energy, holds promise as a clean, abundant, and sustainable energy source. The primary approach to achieving fusion energy involves devices called tokamaks, donut-shaped reactors using powerful magnetic fields to confine gases of charged particles, known as plasma, at temperatures around 100 million degrees Celsius. ITER, the international experimental reactor currently under construction, will be the world’s largest tokamak and is intended to demonstrate the feasibility of magnetic fusion energy.

One of fusion's major challenges is protecting reactor walls from interactions with superhot plasma, which can locally produce heat fluxes exceeding the intensity found at the Sun's surface. Researchers at EPFL have now experimentally demonstrated a new experimental setup that significantly enhances tokamak power handling and tames these extreme conditions

"We are investigating alternative magnetic shapes for the plasma exhaust channel - known as the divertor - to optimize the dissipation of plasma thermal energy into uniformly distributed radiation, thus reducing local heat loads on reactor walls," explained Kenneth Lee, PhD student at EPFL's Swiss Plasma Center (SPC) and lead author of the paper reporting these results in Physical Review Letters.

The X-point is a location in the tokamak where the magnetic field runs purely in the toroidal direction

The TCV tokamak experiment at EPFL, renowned for its versatility in creating diverse plasma configurations, serves as a crucial platform for testing these alternative divertor configurations, supporting the development of future fusion power plants such as the European DEMO.

The team discovered the spontaneous formation of a strongly radiating region named the "X-point target radiator", strategically positioned between the main plasma and the reactor wall. "The X-point is a location in the tokamak where the magnetic field runs purely in the toroidal direction," Lee explained. "The idea is that plasma particles travel longer distances and collide more frequently with neutral or ion species before hitting the reactor wall, thus ‘cooling down’ via radiation".

Thanks to TCV’s flexibility, creating a second X-point is routinely achievable

Conventionally, a single X-point is created by external coils, shaping the plasma boundary into a diverted configuration to exhaust the energy and particles. "We wanted to see whether a radiative zone would form if we placed a second X-point along the divertor leg, sufficiently far from the fusion core," Lee continued. "Thanks to TCV’s flexibility, creating a second X-point is routinely achievable."

The secondary X-point radiation - the X-point target radiator (XPTR) - was successfully observed on TCV. Using the advanced spectral imaging system MANTIS, researchers captured plasma line-emission images comparing conventional divertor shape with the new ‘X-point target’ configuration. The XPTR appears as a glowing ring positioned in the middle of the tokamak’s bottom chamber, created by the additional X-point. "Under identical conditions of the main plasma, we observed an 80% reduction in peak heat flux on the wall compared to the conventional shape," added Lee.

We observed exceptional stability across a broad range of plasma conditions

A similar X-point radiating regime has previously been demonstrated near the main plasma, but its effects on fusion performance and stability are still uncertain. The new XPTR demonstration appears to circumvent such concerns. "We observed exceptional stability across a broad range of plasma conditions, without negatively affecting main plasma performance or inducing disruptive instabilities. This greatly enhances our ability to control this plasma state for safe power handling," said Prof. Christian Theiler, head of the TCV Boundary Group.

These findings hold substantial implications for the global fusion community. "Some future reactors, like SPARC and ARC, plan to incorporate the X-point target divertor into their baseline designs, making our findings timely and crucial," Lee noted. "Implementing this new geometry in reactors poses engineering challenges, but the significant benefits we have identified could strongly motivate its adoption, thus determining how a future fusion reactor may look like.”