Dynamics of density fluctuations and impact of flow on multi-scale physics and turbulence saturation in magnetized plasmas – DYNAFLUC

In fusion plasmas, performance is directly linked to the quality of magnetic confinement, which remains a key issue for the design of future reactors. Radial energy transport is caused by both collisional processes and the development of small scale turbulence. In high-performance machines such as tokamaks operating at low collisionality and optimised stellarators, turbulent transport dominates. Performance can be improved by forming a strong shear in the plasma velocity that locally reduces turbulent transport. This is notably what happens through a complex mechanism of flow self-organization, during the transition from a low to high-confinement regime. This points to the importance of the interactions between flows and turbulence as a key mechanism, while the establishment of the transport barrier and the dynamics of the transition are still not fully understood. A better understanding of the mechanisms of turbulence saturation, flow generation and their influence on the saturated state would be a major step forward, both in predicting the performance of future machines and in optimising them. The DYNAFLUC project addresses this topic by combining experimental and theoretical studies, based on detailed measurements of density fluctuations velocity on WEST tokamak and the W7-X stellarator. The interest in a combined study of these two types of magnetic confinement devices arises from the fact that although the two devices have many differences, the underlying nature of physics of plasmas should be the same, especially involving turbulence and flow interaction. As a result, they produce similar plasmas but with different sources of mean flows and different damping mechanisms. From this perspective, it is possible to study the same natural system in two completely different configurations. Within a magnetically confined plasma, a large-scale mean flow is established, associated with the ‘stationary’ velocity of the plasma via collisional mechanisms, in addition to the zonal flows generated by turbulence, which, in turn, they regulate. The respective weight of these two contributions varies as a function of the damping of the flows, which itself varies as a function of the magnetic configuration.
In this project, the idea is to use the dynamics of density fluctuations in the plasma frame as a function of their spatial sizes to identify the saturation mechanisms at work within the plasma.
For this purpose, the project has two main objectives: validate and explore measurements of the density fluctuations velocity in WEST and W7X; and link the behaviour of these experimental velocities dependencies to the multi-scale interactions operating in the plasma. Precise measurements of density fluctuations will be obtained by combining Doppler backscattering and poloidal correlation reflectometry, which can detect density fluctuations in different ranges of complementary spatial and temporal scales. These challenging measurements will be carried out on the two machines and validated by a dedicated study based on fullwave simulations using the reference code in the field. All the experimental results, obtained in different turbulence and flow regimes, will be interpreted by comparing them with the results from reduced models and direct simulations.
The DYNAFLUC project offers an excellent opportunity to combine a unique range of expertise in the field of turbulence and flow characterisation.