Nonlinear Burn Condition and Kinetic Profile Control in Tokamak Fusion Reactors

One of the most promising devices for realizing power production through nuclear fusion is the tokamak. In order to maximize performance, it is preferable that tokamaks achieve operating scenarios characterized by good plasma confinement, improved magnetohydrodynamic stability, and a largely non-inductively driven plasma current. Such scenarios could enable steady-state reactor operation with high fusion gain, the ratio of fusion power produced to the external heating power needed to sustain reactions. There are many experimental tokamaks around the world, each exploring different facets of plasma physics and fusion technology. These experiments have reached the point where the power released from fusion is nearly equal to the power input required to heat the plasma. The next experimental step is ITER, which aims to reach a fusion gain exceeding ten for short pulses, and to sustain a gain of five for longer pulses (around 1000 s). In order for ITER to be a success, several challenging control engineering problems must be addressed.Among these challenges is to precisely regulate the plasma density and temperature, or burn condition. Due to the nonlinear and coupled dynamics of the system, modulation of the burn condition (either during ramp-up/shut-down or in response to changing power demands) without a well designed control scheme could result in undesirable transient performance. Feedback control will also be necessary for responding to unexpected changes in plasma confinement, impurity content, or other parameters, which could significantly alter the burn condition during operation. Furthermore, although stable operating points exist for most confinement scalings, certain conditions can lead to thermal instabilities. Such instabilities can either lead to quenching or a thermal excursion in which the system moves to a higher temperature equilibrium point. In any of these situations, disruptive plasma instabilities could be triggered, stopping operation and potentially causing damage to the confinement vessel.In this work, the problem of burn condition control is addressed through the design of a nonlinear control law guaranteeing stability of desired equilibria. Multiple actuation methods, including auxiliary heating, isotopic fueling, and impurity injection, are used to ensure the burn condition is regulated even when actuators saturate. An adaptive control scheme is used to handle model uncertainty, and an online optimization scheme is proposed to ensure that the plasma is driven to an operating point that minimizes an arbitrary cost function. Due to the possible limited availability of diagnostic systems in ITER and future reactors, an output feedback control scheme is also proposed that combines the nonlinear controller with an observer that estimates the states of the burning plasma system based on available measurements. Finally, the control scheme is tested using the integrated modeling code METIS.The control of spatial profiles of parameters, including current, density, and temperature, is also an important challenge in fusion research, due to their effect on MHD stability, non-inductive current drive, and fusion power. In this work, the problem of kinetic profile control in burning plasmas is addressed through a nonlinear boundary feedback control law designed using a technique called backstepping. A novel implementation of the backstepping technique is used that enables the use of both boundary and interior actuation. The backstepping technique is then applied to the problem of current profile control in both low-confinement and high-confinement mode discharges in the DIII-D tokamak based on a first-principles-driven model of the current profile evolution. Both designs are demonstrated in simulations and experimental tests

Development of a concept and basis for the DEMO diagnostic and control system

An initial concept for the plasma diagnostic and control (D&C) system has been developed as part of European studies towards the development of a demonstration tokamak fusion reactor (DEMO). The main objective is to develop a feasible, integrated concept design of the DEMO D&C system that can provide reliable plasma control and high performance (electricity output) over extended periods of operation. While the fusion power is maximized when operating near to the operational limits of the tokamak, the reliability of operation typically improves when choosing parameters significantly distant from these limits. In addition to these conflicting requirements, the D&C development has to cope with strong adverse effects acting on all in vessel components on DEMO (harsh neutron environment, particle fluxes, temperatures, electromagnetic forces, etc.). Moreover, space allocation and plasma access are constrained by the needs for first wall integrity and optimization of tritium breeding. Taking into account these boundary conditions, the main DEMO plasma control issues have been formulated, and a list of diagnostic systems and channels needed for plasma control has been developed, which were selected for their robustness and the required coverage of control issues. For a validation and refinement of this concept, simulation tools are being refined and applied for equilibrium, kinetic and mode control studies

Data-driven model for divertor plasma detachment prediction

We present a fast and accurate data-driven surrogate model for divertor plasma detachment prediction leveraging the latent feature space concept in machine learning research. Our approach involves constructing and training two neural networks. An autoencoder that finds a proper latent space representation (LSR) of plasma state by compressing the multi-modal diagnostic measurements, and a forward model using multi-layer perception (MLP) that projects a set of plasma control parameters to its corresponding LSR. By combining the forward model and the decoder network from autoencoder, this new data-driven surrogate model is able to predict a consistent set of diagnostic measurements based on a few plasma control parameters. In order to ensure that the crucial detachment physics is correctly captured, highly efficient 1D UEDGE model is used to generate training and validation data in this study. Benchmark between the data-driven surrogate model and UEDGE simulations shows that our surrogate model is capable to provide accurate detachment prediction (usually within a few percent relative error margin) but with at least four orders of magnitude speed-up, indicating that performance-wise, it has the potential to facilitate integrated tokamak design and plasma control. Comparing to the widely used two-point model and/or two-point model formatting, the new data-driven model features additional detachment front prediction and can be easily extended to incorporate richer physics. This study demonstrates that the complicated divertor and scrape-off-layer plasma state has a low-dimensional representation in latent space. Understanding plasma dynamics in latent space and utilizing this knowledge could open a new path for plasma control in magnetic fusion energy research.Comment: 24 pages, 15 figure

Physics through the 1990s: Plasmas and fluids

The volume contains recommendations for programs in, and government support of, plasma and fluid physics. Four broad areas are covered: the physics of fluids, general plasma physics, fusion, and space and astrophysical plasmas. In the first section, the accomplishments of fluid physics and a detailed review of its sub-fields, such as combustion, non-Newtonian fluids, turbulence, aerodynamics, and geophysical fluid dynamics, are described. The general plasma physics section deals with the wide scope of the theoretical concepts involved in plasma research, and with the machines; intense beam systems, collective and laser-driven accelerators, and the associated diagnostics. The section on the fusion plasma research program examines confinement and heating systems, such as Tokamaks, magnetic mirrors, and inertial-confinement systems, and several others. Finally, theory and experiment in space and astrophysical plasma research is detailed, ranging from the laboratory to the solar system and beyond. A glossary is included

Fast transport simulations with higher-fidelity surrogate models for ITER

A fast and accurate turbulence transport model based on quasilinear gyrokinetics is developed. The model consists of a set of neural networks trained on a bespoke quasilinear GENE dataset, with a saturation rule calibrated to dedicated nonlinear simulations. The resultant neural network is approximately eight orders of magnitude faster than the original GENE quasilinear calculations. ITER predictions with the new model project a fusion gain in line with ITER targets. While the dataset is currently limited to the ITER baseline regime, this approach illustrates a pathway to develop reduced-order turbulence models both faster and more accurate than the current state-of-the-art

Fast-Ion Transport and Acceleration Induced by Edge Localized Modes in MAST Upgrade and ASDEX Upgrade

La fusión nuclear se esboza como una fuente de energía sostenible, de bajas emisiones y que aportará potencia de base al mix eléctrico. Los tokamaks son los dispositivos más avanzados para la obtención de energía mediante fusión nuclear y operan en condiciones cercanas a las de las futuras centrales nucleares. Hoy en día, los tokamaks existentes tienen como objetivo resolver los desafíos científicos y técnicos que plantean las futuras plantas de fusión nuclear. En los tokamaks, los iones rápidos, -- aquellos iones cuya energía es superior a la del resto del volumen de plasma --, se emplean para aumentar la temperatura del plasma hasta energías donde la fusión se hace patente. Los iones rápidos pueden perder el confinamiento debido a interacciones con perturbaciones electromagnéticas de diversa naturaleza, siendo esto un riesgo para el rendimiento del plasma y para la integridad del reactor. En el modo de operación de alto confinamiento, conocido por H-mode, aparecen de forma repetitiva inestabilidades magnetohidrodinámicas (MHD) explosivas localizadas en el borde, conocidas como Edge Localized Modes (ELMs). Los ELMs liberan una gran cantidad de energía y momento hacia las paredes del plasma, cuyo efecto se estima intolerable en futuros tokamaks. Además, investigaciones recientes en el tokamak ASDEX Upgrade han observado un aumento en las pérdidas de iones rápidos a energías por encima de su energía de inyección, que aparecen relacionadas con la actividad de los ELMs. Estas observaciones sugieren que existe una interacción entre los ELMs y los iones rápidos, que resulta en la aceleración y pérdida de estos últimos. Este trabajo tiene como objetivo de estudio las pérdidas de iones rápidos inducidos por ELMs. Para conseguir este objetivo, la tesis ha abarcado el desarrollo de herramientas numéricas y el diseño de diagnósticos experimentales. En el aspecto numérico, se han utilizado códigos de seguimiento de órbitas como ASCOT5 y se han implementado perturbaciones electromagnéticas 3D que evolucionan en el tiempo. Esto ha permitido investigar los principales mecanismos de transporte y aceleración planteados para esclarecer la pérdida de iones rápidos inducida por ELMs. En el ámbito experimental, durante esta tesis se ha diseñado el primer detector de pérdidas de iones rápidos (FILD) en el tokamak esférico MAST-U. El diagnóstico está montado sobre un mecanismo de rotación y traslación que permite adaptar la sonda a diferentes orientaciones [0o, 90o] y posiciones radiales [1.40 m, 1.60 m]. Los primeros datos experimentales del FILD de MAST-U se esperaban obtener a lo largo de esta tesis. Sin embargo, distintos retrasos en el comienzo de la campaña experimental de MAST-U han impedido poner el diagnóstico en funcionamiento. Por este motivo, los planes experimentales y numéricos han tenido que seguir distintos planteamientos. Los experimentos, que se han realizado en ASDEX Upgrade, han tenido como objetivo expandir el alcance de las observaciones de pérdida de iones rápidos inducidas por ELMs, llevando a cabo nuevos rastreos para detectar los parámetros que más afectan en la interacción entre los iones rápidos y los ELMs. Con respecto al modelado, se busca realizar un modelado que permita reproducir las principales observaciones experimentales en ASDEX Upgrade y revele los mecanismos básicos de transporte y aceleración de iones rápidos durante los ELMs. Además, el modelado en MAST-U ha permitido preveer la señal de FILD, que se ha podido comparar con la señal en ASDEX Upgrade