Control of trapped electron mode turbulence with an electron cyclotron resonant source
The performance of a tokamak plasma largely depends on to the level of turbulent transport. Trapped electron modes are one of the main instabilities responsible for turbulence in tokamaks. On the other hand, electron cyclotron resonance heating is a generic heating system for tokamaks. Both physical processes rely on resonant interactions with electrons. Non-linear interaction between the resonant processes is theoretically possible. This thesis aims to evaluate the possibility of exploiting this non-linear interaction to stabilize the trapped electron modes instability within tokamak plasmas, using a heating source present on many tokamaks, including ITER. This control technique could improve the performance of certain tokamaks without any extra cost.
The thesis will be based on a theoretical understanding of the two processes studied, will require the use of the gyrokinetic code GYSELA to model the non-linear interactions between resonant processes, and will include an experimental aspect to validate the identified turbulence control mechanism.
Physics & control of dissipative divertor regime in WEST tokamak experiments
The success of the magnetic confinement fusion program relies on the control of the interaction between the hot confined plasma, where the fusion reactions take place, and the wall of the vacuum vessel in which this plasma is maintained. Currently, this interaction is managed by a hardware and magnetic configuration called the divertor, which aims to concentrate the lost plasma fluxes through a dedicated volume (the divertor volume) towards high flux components (surface components of the divertor). The control of dissipative phenomena in this divertor volume is a critical objective that shall allow maintaining high confinement performances in the core (hot plasma) while maintaining fluxes to the components below technological limits. The WEST tokamak, currently operated at CEA Cadarache, has as its main objective the control of this interaction, in close support with the ITER project. The thesis project aims to improve the physical understanding of the control experiments started on WEST, through advanced experimental analysis, to the optimization of a robust and generic control model that can be deployed on WEST to conduct scenarios representative of ITER conditions. The project will also be part of a very active international context on the subject, both in Europe (EUROfusion Activities), in Asia and in the United States, offering a wide spectrum of visibility and possibilities for collaborations and developments. The results will be published in peer-reviewed journals with possibly high impact factors, and may be presented at international conferences.
Transport in runaway electron companion plasmas: impact on mitigation and extrapolation to ITER
Disruptions are abrupt interruptions of plasma discharges in tokamaks. They are due to instabilities leading to the loss of thermal energy and magnetic energy of the plasma over periods of the order of a few tens of milliseconds. Disruptions can generate so-called relativistic runaway electron beams reaching energies up to several MeV and potentially carrying a large part of the initial current. It is crucial to control or stop them to ensure a reliable operation of future tokamaks such as ITER. The proposed thesis project focuses on the mitigation of runaway electrons by massive injection of deuterium or hydrogen into the beam. This scenario leads to a drastic decrease in the energy deposited on the wall by the runaway electrons, through two phenomena: a magnetohydrodynamic instability and the absence of regeneration of the runaway electrons in the final loss of the plasma current. These two conditions are obtained when the plasma created by the interaction between the runaway electron beam and the neutral gas remains cold enough to recombine. The recombination mechanism relies on energy transport processes by the neutrals and a decrease in the interaction between the runaway electrons and the background plasma. Limits to this scheme were found on current tokamaks; they must be understood in order to extrapolate to future machines. The first part of the thesis will focus on the characterization of the cold plasma: density profiles, deuterium/hydrogen or heavy impurity concentration, current profile. We will be particularly interested in the quantities related to transport phenomena in the plasma: heat conduction, particle diffusion or radiation transport. This experimental characterization will quickly call upon numerical modelling to confirm the role of the various transport mechanisms in keeping the conditions required for the dissipation of the beam without damage. An extrapolation towards ITER will then be considered via simulations.
Exploring the High-Frequency fast Electron-Driven Instabilities towards application to WEST
In current tokamaks, the electron distribution is heavily influenced by external heating systems, like Electron Cyclotron Resonance Heating (ECRH) or Lower Hybrid (LH) heating, which generate a large population of fast electrons. This is expected also in next-generation tokamaks, such as ITER, where a substantial part of input power is deposited on electrons. A significant population of fast electrons can destabilize high-frequency instabilities, including Alfvén Eigenmodes (AEs), as observed in various tokamaks. However, this phenomenon remains understudied, especially regarding the specific resonant electron population triggering these instabilities and the impact of electron-driven AEs on the multi-scale turbulence dynamics in the plasma complex environment.
The PhD project aims to explore the physics of high-frequency electron-driven AEs in realistic plasma conditions, applying insights to WEST experiments for in-depth characterization of these instabilities. The candidate will make use of advanced numerical codes, whose expertise is present at the IRFM laboratory, to analyze realistic plasma conditions with fast-electron-driven AE in previous experiments, to grasp the essential physics at play. Code development will also be necessary to capture key aspects of this physics. Once such a knowledge is established, predictive modeling for the WEST environment will guide experiments to observe these instabilities.
Based at CEA Cadarache, the student will collaborate with different teams, from the theory and modeling group to WEST experimental team, gaining diverse expertise in a stimulating environment. Collaborations with EUROfusion task forces will further provide an enriching international experience.
Magnetic fusion turbulence: where do reduced models fail, how to enrich them?
One of the key challenges facing the field of fusion plasma modeling is the nonlinear nature of the plasma response. This means that factors such as temperature and density gradients, flows, and velocity gradients all have an impact on the transport of heat, particles, and momentum in complex ways. Modeling such a system requires a range of approaches, from the highly detailed flux-driven gyrokinetics method to simpler quasilinear models within an integrated framework. These have proven effective in interpreting experimental data and predicting plasma behaviour. However, there are two significant challenges to this approach. Firstly, modeling the peripheral region of the plasma edge, at the transition between open and closed field lines, is challenging due to the confluence of significantly different underlying physics. Recent research indicates that current quasilinear transport models may have significant shortcomings in this region. Secondly, modeling the 'near marginality' regime is challenging due to the fact that it involves a state of dynamic equilibrium where the system's behaviour is self-regulated by slow, large-scale modes. Computing this state is challenging and requires a flux-driven gyrokinetic approach to move away from the typical assumption of time scale separation between turbulence and transport. Recent work from within our team indicates that current quasilinear transport models may also be facing significant shortcomings in this regime. It is crucial to understand this regime in depth as it is relevant for future machine operation. We are now in a position to address these two issues, as we have access to cutting-edge in-house tools relevant to both ends of the spectrum.
We plan to compare transport predictions in the edge and near marginality regimes from the advanced flux-driven gyrokinetic code GYSELA with those from the integrated framework using the reduced quasilinear QuaLiKiz model. The research will contribute to the development of robust reduced models for transport, crucial for the interpretation of current experimental data and for future burning plasma operation.