Periodic Reporting for period 1 - FICOP (Optimization of fast-ion confinement against edge instabilities for future fusion reactors)
Reporting period: 2022-09-01 to 2024-12-31
The quality-of-life of modern societies has reached unprecedented levels, and this is usually linked to the large (electrical) energy consumption per capita. The main energy sources that have made this possible in the last century are fossil fuels and nuclear fission. However, the sustainability of these resources is now in question and there is an increasing concern in the society about the impact of these on the environment and its consequences. In this context, nuclear fusion emerges as a promising solution to the global energy demands.
Nowadays, the most advanced concept for making nuclear fusion possible for energy production is the tokamak. A tokamak is a device based on magnetic confinement fusion. The fuel (in state of plasma) is heated up to temperatures of the order of millions degrees Celsius, in order to make the fusion reactions possible. The plasma, this is, an ionized quasi-neutral gas, is kept confined and away from the vessel walls by means of complex magnetic fields. In a tokamak fast (energetic) ions play a key role: they are responsible for the heating and current drive of the plasma. If not well confined, the loss of fast-ions is detrimental for the tokamak efficiency and poses a risk to the machine integrity. For this reason, it is of paramount importance to understand the behaviour of fast-ions and their interaction with plasma instabilities.
The goal of this work is the optimization of fast-ion confinement in tokamaks. In particular, the study of the interaction between fast-ions and instabilities that appear in the edge of the plasma, the region which is closer to the walls. These instabilities are characteristic of the different confinement regimes in which a tokamak can be operated. Thus, understanding the behaviour of fast-ions in the presence of these, may help to guide the operational point of future fusion reactors.
Nowadays, the most advanced concept for making nuclear fusion possible for energy production is the tokamak. A tokamak is a device based on magnetic confinement fusion. The fuel (in state of plasma) is heated up to temperatures of the order of millions degrees Celsius, in order to make the fusion reactions possible. The plasma, this is, an ionized quasi-neutral gas, is kept confined and away from the vessel walls by means of complex magnetic fields. In a tokamak fast (energetic) ions play a key role: they are responsible for the heating and current drive of the plasma. If not well confined, the loss of fast-ions is detrimental for the tokamak efficiency and poses a risk to the machine integrity. For this reason, it is of paramount importance to understand the behaviour of fast-ions and their interaction with plasma instabilities.
The goal of this work is the optimization of fast-ion confinement in tokamaks. In particular, the study of the interaction between fast-ions and instabilities that appear in the edge of the plasma, the region which is closer to the walls. These instabilities are characteristic of the different confinement regimes in which a tokamak can be operated. Thus, understanding the behaviour of fast-ions in the presence of these, may help to guide the operational point of future fusion reactors.
The work carried out in this project has been supported by two main pillars. On one hand, the development of novel diagnostic tools and data analysis techniques for the experimental characterization of plasma parameters, in particular, the fast-ion distribution function. On the other hand, the use of these tools to investigate the interaction between fast-ions and different plasma edge instabilities. In particular, we have focused on the effect of edge localized modes (ELMs), externally applied magnetic perturbations (MPs), and other instabilities characteristic of alternative confinement regimes such as filaments in the quasi-coherent exhaust regime (QCE). Additionally, the conditions upon which ion runaway can occur in tokamak plasmas have been explored.
During the course of this project the first measurements of two novel plasma diagnostics have been obtained in the ASDEX Upgrade tokamak: the imaging heavy ion beam probe (iHIBP), for the characterization of plasma edge parameters such as density, magnetic field, and electrostatic potential; and the imaging neutral particle analyzer (INPA), for the characterization of the confined fast-ion population in velocity-space. Additionally, novel tomographic inversion techniques (needed for the correct interpretation of these measurements), have been developed. These include iterative and anisotropic regularization methods, as well as reconstruction methods based on the use of neural networks.
The interaction between fast-ions and ELMs has been further studied by means of orbit following simulations. In these, the ELM has been modelled by means of advanced hybrid kinetic-MHD simulations. Going beyond the standard ELMy H-mode regime (considered as the baseline for some of the future fusion reactors), the interaction between fast-ions and edge instabilities in other operating regimes has been experimentally explored. For this, a multi-machine effort has been carried out, collecting fast-ion loss measurements in the ASDEX Upgrade (Germany), TCV (Switzerland) and MAST-Upgrade (United Kingdom) tokamaks. A variety of operating regimes has been covered, including: low and high collisionality H-modes, MP ELM mitigated and suppressed H-modes, I-mode, L-mode or QCE. The characteristics of fast-ion losses have been systematically analyzed in a threefold scheme: velocity-space domain, time domain, and frequency domain. Additionally, the characteristics of positive and negative fast-ion losses have been explored in the EAST tokamak.
During the course of this project the first measurements of two novel plasma diagnostics have been obtained in the ASDEX Upgrade tokamak: the imaging heavy ion beam probe (iHIBP), for the characterization of plasma edge parameters such as density, magnetic field, and electrostatic potential; and the imaging neutral particle analyzer (INPA), for the characterization of the confined fast-ion population in velocity-space. Additionally, novel tomographic inversion techniques (needed for the correct interpretation of these measurements), have been developed. These include iterative and anisotropic regularization methods, as well as reconstruction methods based on the use of neural networks.
The interaction between fast-ions and ELMs has been further studied by means of orbit following simulations. In these, the ELM has been modelled by means of advanced hybrid kinetic-MHD simulations. Going beyond the standard ELMy H-mode regime (considered as the baseline for some of the future fusion reactors), the interaction between fast-ions and edge instabilities in other operating regimes has been experimentally explored. For this, a multi-machine effort has been carried out, collecting fast-ion loss measurements in the ASDEX Upgrade (Germany), TCV (Switzerland) and MAST-Upgrade (United Kingdom) tokamaks. A variety of operating regimes has been covered, including: low and high collisionality H-modes, MP ELM mitigated and suppressed H-modes, I-mode, L-mode or QCE. The characteristics of fast-ion losses have been systematically analyzed in a threefold scheme: velocity-space domain, time domain, and frequency domain. Additionally, the characteristics of positive and negative fast-ion losses have been explored in the EAST tokamak.
The new tomographic techniques developed in this project have been applied to real measurements of scintillator based fast-ion loss detectors (for the study of ELM-induced fast-ion loss and acceleration) and INPA diagnostics (for the study of fast-ion transport induced by different magnetohydrodynamic instabilities). The modelling work on the fast-ion and ELMs has shown an interplay between these two mediated by a resonant interaction. This is consistent with the experimentally measured transport and acceleration of fast-ions in the presence of ELMs. Furthermore, it has been noticed that the fast-ions may have a potential impact on the stability of the ELM.
The study of fast-ion transport induced by externally applied MPs in the ASDEX Upgrade tokamak has shown the potential of this actuator as an active controller of Alfven Eigenmodes (instabilities known to have a large impact on the confinement of fast-ions). At the same time, the experiments in MAST-Upgrade have corroborated previous measurements at ASDEX Upgrade, demonstrating that the fast-ion transport induced by MPs depends on a number of control parameters: the MP spectrum, its intensity, its toroidal orientation and the velocity-space of the fast-ions. While analysis is still on-going and further research is needed, preliminary results may suggest a possible role of the coupling between the MPs and the machine intrinsic error fields.
The comparison between fast-ion losses in different confinement regimes has shown differences in the probability density function of the fast-ion losses, possibly linked to the characteristic intermittency of edge parameters amongst the different regimes. Additionally, for the first time, negative fast-ion losses have been measured in the EAST tokamak, originating in the scrape-off layer of the tokamak through different reaction channels. These measurements suggest that further investigation is needed for an accurate prediction of fast-ion heat loads to the first wall. This may be of great importance for future reactors, where a high scrape-off layer density is expected.
The study of fast-ion transport induced by externally applied MPs in the ASDEX Upgrade tokamak has shown the potential of this actuator as an active controller of Alfven Eigenmodes (instabilities known to have a large impact on the confinement of fast-ions). At the same time, the experiments in MAST-Upgrade have corroborated previous measurements at ASDEX Upgrade, demonstrating that the fast-ion transport induced by MPs depends on a number of control parameters: the MP spectrum, its intensity, its toroidal orientation and the velocity-space of the fast-ions. While analysis is still on-going and further research is needed, preliminary results may suggest a possible role of the coupling between the MPs and the machine intrinsic error fields.
The comparison between fast-ion losses in different confinement regimes has shown differences in the probability density function of the fast-ion losses, possibly linked to the characteristic intermittency of edge parameters amongst the different regimes. Additionally, for the first time, negative fast-ion losses have been measured in the EAST tokamak, originating in the scrape-off layer of the tokamak through different reaction channels. These measurements suggest that further investigation is needed for an accurate prediction of fast-ion heat loads to the first wall. This may be of great importance for future reactors, where a high scrape-off layer density is expected.