One of the challenges of fusion tokamaks is how to keep the core of a plasma hot enough that fusion can occur while preventing the tokamak walls from melting from that heat. This problem is even more difficult if instabilities at the plasma edge release energy in short bursts instead of a steady flow. Large tokamaks must operate without edge instabilities in order to avoid potential damage. Recent experiments on the DIII-D tokamak have demonstrated that such operation is possible using techniques that can be used in a fusion power plant. The key is enhancing energy flow in the plasma edge due to turbulent fluctuations so that energy bleeds smoothly out of the plasma. This prevents excessive pressure that can lead to sudden bursts of energy. This continuous energy release scenario produces a peak power density on the walls of the tokamak that is as much as 100 times smaller than that of the “bursty” energy release scenario.
To generate power efficiently, future fusion power plants will need to produce large amounts of power relative to their sizes. To do so, they will need to operate for long periods with high heat flow to the surfaces of the power plant that surround the plasma. Design calculations show that extra bursts of heat from impulsive edge instabilities would severely damage some of these surfaces. To prevent this damage, fusion power plants must operate without these instabilities. One operating mode that provides the confinement necessary to produce the hot plasma core but avoids the edge instabilities is called the wide pedestal quiescent H-mode (QH-mode). Experiments at the DIII-D National Fusion Facility show that plant operators can achieve this mode with the control techniques anticipated in future fusion devices.
Tokamak plasmas in ITER and other future reactors need the effective energy confinement of the so-called H-mode to maintain high temperatures in the core. H-mode has a self-generated insulating layer in the plasma edge. The energy confinement provided by this layer is good enough that the outflowing energy piles up until an instability known as the Edge Localized Mode (ELM) occurs. The challenge is to retain the layer’s excellent energy confinement while preventing the ELMs. The QH-mode operates naturally with sufficient additional turbulent edge energy transport to eliminate the ELMs while retaining excellent energy confinement in the plasma core.
The elegant feature of QH-mode is that the plasma generates this edge turbulence modification by itself. When first discovered, QH-mode was created using rapid plasma rotation driven by torque injected using the neutral hydrogen heating beams. However, the effective torque in much larger future machines will be too small to produce enough rotation. Recent experiments on DIII-D created and sustained a variant of QH-mode (wide pedestal QH-mode) using no net neutral beam torque. As a bonus, the wide pedestal feature improves edge stability, allowing operation at higher edge plasma pressure by widening the pedestal, even though the local temperature and density gradients decrease.
Keith H. Burrell
This material is based on work supported by the Department of Energy Office of Science, Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility.
Burrell, K.H. et al., Creation and sustainment of wide pedestal quiescent H-mode with zero net neutral beam torque, Nuclear Fusion 60, 086005 (2020). [DOI: 10.1088/1741-4326/ab940d]