Climbing the Staircase to Fusion

The Science

Most of fusion science assumes that tokamaks (machines used to study fusion) need to suppress turbulence at the edge of fusion plasmas. It seemed like this control was necessary to keep fusion reactors hot enough. However, recent experiments in the DIII-D tokamak show the opposite. They demonstrate that more turbulence at the edge of the plasma can result in it being hotter. This surprising result arises when the turbulence is sandwiched in between two insulating layers of plasma. The turbulence makes the plasma near the edge cooler, but the plasma core in the middle of the tokamak hotter. The graph showing how the temperature changes along the edge of the plasma then takes on the shape of a staircase, where the edge becomes cooler and the core becomes hotter.

The Impact

In a device like ITER – a massive fusion experiment under construction in France – enhanced turbulence may appear in the edge of its plasma. Enhanced turbulence is thought to degrade fusion performance. However, this work demonstrates that enhanced turbulence in the edge of ITER may actually improve ITER’s fusion performance. The study suggests that even if ITER doesn’t fully suppress turbulence along the edge of the plasma, the core temperature and fusion power output may actually increase. That may happen if the turbulent region is completely surrounded by an insulating layer free of turbulence.  

Summary

Turbulence, a commonly observed feature in natural systems, is one of the obstacles towards achieving fusion in tokamak reactors. Fusion reactors such as ITER will require the suppression of turbulence in the plasma edge to prevent excessive heat loss. The plasma layer near the edge where turbulence is suppressed is called an edge transport barrier and this barrier acts much like the thermal insulation surrounding hot water pipes that prevent excessive heat loss. Recent theoretical studies have suggested that the formation of an edge transport (or insulating) layer may be more difficult in large-scale fusion devices such as ITER. This is because gradients in the plasma flow break apart the turbulence and larger scale fusion plasmas are expected to have weaker flows. Experiments in the DIII-D tokamak show that wide edge transport barriers can be formed with enhanced turbulence contained inside the transport barrier. The region of enhanced turbulence in DIII-D coincides with the region of weak electric field gradients, as predicted from theoretical considerations based on the amount of electric field shear required to suppress long wavelength turbulence. The effect of the enhanced turbulence is to locally flatten the edge temperature profile. However, so long as the edge transport barrier surrounds the region of enhanced turbulence, thermal insulation actually improves. This result has implications for large-scale fusion reactors such as ITER. Even if edge turbulence is not fully suppressed in a fusion reactor, thermal insulation can be maintained and even improved if the turbulence is contained within the transport barrier.

Funding

This research was supported by the Department of Energy (DOE), Office of Science, Office of Fusion Energy Sciences. It used the DIII-D National Fusion Facility and the National Energy Research Scientific Computing Center (NERSC), which are both a DOE Office of Science user facilities.

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