How fast ions mitigate turbulence and enhance confinement in tokamak fusion plasmas

Along with high temperature and density, magnetic fusion requires good confinement and a degree of transport control for thermal plasmas. Meanwhile, fast ions are generated by the external heating used to raise plasma temperature and by the fusion reactions. As a result, the fusion plasmas are effectively rendered into systems with two coexisting populations of main interest — namely, the fast ions and the thermal plasma. Interestingly, several recent experiments indicate that the fast-ion population can improve the confinement of the thermal plasmas by mitigating turbulence. In this Review, we describe the physical mechanisms that underpin the improved confinement and discuss recent experimental results in terms of these mechanisms. Experiments have shown that fast ions generated in fusion plasmas improve confinement by mitigating plasma turbulence. This Review explores possible physical mechanisms to explain these experimental observations.