Princeton University Special Astrophysics Talk
Unraveling key transport mechanisms for the sustainment of the high confinement mode in tokamaks
In magnetically confined fusion devices, enhanced particle and energy transport induced by magnetohydrodynamic (MHD) fluctuations can deteriorate the plasma confinement and endanger the integrity of the device. One of the most prominent MHD fluctuations in a tokamak plasma is the edge localized mode (ELM) which expels a jet of hot plasma, similar to solar flares on the edge of the Sun. ELMs appear during a mode of tokamak operation in which energy is retained more effectively and pressure builds up at the plasma edge (pedestal region). This mode of operation is called high confinement mode (H-mode) and is the operational regime foreseen for ITER. For future fusion devices, the achievement of a highperformance plasma core coupled to a boundary solution without ELMs remains one of our grand challenges towards fusion energy. Thus, a thorough understanding of the edge stability, ELM-induced transport and ELM control is ubiquitous to advance in our goal.
The small spatial width of the pedestal (outermost 5% of the confined plasma) and the fast temporal changes associated to ELMs (duration of about 1 ms, corresponding to 1-2% of the confinement time for the ASDEX Upgrade tokamak, AUG) require high-resolution measurements to enable the analysis of the pedestal transport. Recent advances in the diagnostic capabilities at AUG enabled the measurement of a number of quantities that are believed to be key for the understanding of a tokamak H-mode and edge stability; the edge Er profile, ion heat transport and high-field-side to low-field-side flow and impurity density asymmetries. The measurements showed that the Er well is sustained by the gradients of the main ion species and that the radial location of the maximum Er shear coincides with that of the maximum ion pressure gradient. They both lie in the inner part of the Er well, thus indicating that the negative shear region is the important region for turbulence reduction observed in H-mode. The extension of the rotation measurements to the high-field side of AUG revealed an asymmetry in the flow structure at the plasma edge. An excess of impurity density at the high-field-side following the condition of divergence-free flows on a flux surface explains the impurity flow asymmetry. Comparison to theory shows that the poloidal centrifugal force plays a key role in the formation of these asymmetries.
Unprecedented measurements of the ion temperature with a temporal resolution of several tens of μs allowed us to study in detail the edge ion heat transport during edge localized mode cycles. The data shows that the ion heat diffusivity is close to the neoclassical level, at all times except for the ELM crash itself. Comparison to the electrons revealed that the ion and electron energy transport recover on different timescales, with the electrons recovering on a slower timescale. The dominant mechanism for the additional energy transport in the electron channel that could cause the delay in the electron temperature gradient recovery is attributed to the depletion of energy caused by the ELM. The local sources and sinks for the electron channel in the steep gradient region are much smaller compared to the energy flux arriving from the pedestal top, indicating that the core plasma may dictate the local dynamics of the electron temperature gradient recovery during the ELM cycle.
Identifying the dominant transport mechanisms allows us to get a better understanding of the ELM and is key to develop ELM-free high confinement regimes for future fusion devices.