Divertor

A divertor is a device that guides the outermost magnetic field lines of a confined plasma onto a dedicated target, where the heat flux, impurities, and helium (the “ash” from fusion burning) are exhausted in a concentrated way.

Simple Definition (High School)

Even when a plasma at 100 million degrees is confined, the particles at its outer edge inevitably touch the wall. If the entire vessel had to absorb this, the wall would be damaged, and atoms eroded from the wall would mix into the plasma and cool it down. So the divertor deliberately pulls the outer field lines toward a single location and directs all the heat and contamination onto one dedicated, rugged target. Like a kitchen exhaust fan, it acts as a single outlet that collects the dirt and pushes it out from one place.

Precise Definition (Undergraduate and Above)

Using the current in the poloidal field coils, a divertor creates a magnetic null point called the X-point outside the plasma cross section. Bounded by this X-point, the outermost of the closed magnetic surfaces is called the separatrix. The field lines outside the separatrix do not close but instead connect to the divertor target plates, and this region is called the scrape-off layer.

When the edge plasma diffuses outside the separatrix, it is carried along the field lines to the divertor plates, where it deposits its heat and particles. As a result, the heat flux is strongly concentrated on the target, and in steady-state operation the design must withstand a heat load exceeding 10 MW per square meter.

The operating regime that reduces this load is called detachment (a non-contact plasma). Impurity gases and the like are intentionally injected near the divertor to disperse energy through radiation, lowering the plasma temperature before it reaches the target and greatly reducing the heat flux and particle flux to the plate.

Role in Fusion Research

If the helium produced by fusion reactions (the burning ash) cannot be exhausted, the plasma becomes diluted and the reactions stop. The divertor is the key to continuously and stably exhausting this ash and heat, while at the same time suppressing the impurities that erode from the wall material and mix into the plasma. For ITER and future power reactors, how to reconcile this concentration of heat flux with detachment control is one of the central challenges that determine feasibility.

Learn More

• Divertor
• Plasma-Facing Materials

Related Terms

• Plasma - The state being exhausted
• Tokamak - A representative device equipped with a divertor
• Blanket - Another major structure that receives heat and neutrons
• Disruption - Another phenomenon where heat load becomes a problem