Pushing the physics of transport barriers towards the wall: How do boundary conditions impact the confinement transitions in tokamaks

Mathieu Peret

Postdoctoral Researcher
Oak Ridge Associated Universities

Seminar Information

Seminar Series
Energy: Joint Mechanical & Aerospace Engineering Dept & Center for Energy Research

Seminar Date - Time
February 15, 2023, 11:00 am
-
12:15

Seminar Location
Hybrid: In Person & Zoom (connection in link below)

Engineering Building Unit 2 (EBU2)
Room 479

Seminar Recording Available: Please contact seminar coordinator, Jake Blair at (j1blair@eng.ucsd.edu)


Abstract

This work deals with the understanding of transport barrier establishment in the edge of magnetically confined fusion plasmas. To that end, two main axes were explored. First, an experimental characterization of the rotation profiles by Doppler Back-Scattering reflectometry (DBS) have been performed in the WEST tokamak. On the other hand, a theoretical development of transport models implying a spectral description of the turbulence and its interplay with sheared flows have been developed. In fact, tokamak plasmas can be decomposed in three regions of interest: a confined core region where the fusion reactions take place, a plasma-wall interaction region where the plasma intercepts the wall leading to power and particle exhaust and a transition region between the two firsts called edge region. The establishment of a transport barrier in this latter is attributed to the generation of a strongly sheared flow leading to a mitigation of the turbulence. Experimentally, the build-up of the barrier appeared very sensitive to edge plasma conditions such as the magnetic configuration, i.e. the existence and the position of an X-point (where the poloidal magnetic field is null) as well as edge density amplitude.

Aiming at understanding these features, a theoretical development has been derived to describe both transport and sheared flow/turbulence interplay. Applied to the plasma-wall interaction region, this model gives predictions for particle exhaust characteristic width. Interestingly, the model remains simple enough to include more complex geometric and collisional effects. These effects are investigated and discussed regarding the impacts of plasma shaping and edge density on these control parameters. Then,
this model has been verified against a broad set of 2D flux-driven simulations with control parameters in the range of the experimental ones. Furthermore, a comparison of the model predictions with experimental data recovers turbulent spectra measured in the TJ-K torsatron, and the background density profile decay lengths measured with Langmuir probes in Tore Supra. Finally, the model of flow generation by the turbulence recovers the experimental observations concerning the impact of the magnetic geometry and the plasma current.

The model described above is generic enough to add more complex physics in its control parameters. Indeed, the inclusion of the divertor physics and the neutrals-plasma interactions can be added into the plasma parallel dynamic description. This will be the purpose of the postdoctoral project that will be managed in the next years.

Speaker Bio

Dr. Peret holds the position of a postdoc at the Oak Ridge Associated Universities on assignment to the General Atomics Theory and Computational Science Division under the supervision of Dr. Gary Staebler. He obtained his PhD in 2022 in France at Ecole Polytechnique after managing the doctorate research at IRFM (CEA Cadarache) on the impact of the boundary conditions (i.e. the edge plasma shaping) on the confinement features in tokamak plasmas. During his postdoctoral program, he will be focused on the impact of the divertor physics on the scrape-off layer transport via the extension of the reduced model he built during his PhD, numerical simulations and experimental data measured in DIII-D. In particular, he will include the neutrals-plasma interactions in the model and the simulations in order to shed light on their impact on the dynamics parallel to the magnetic field lines and on the radial turbulent transport.