Impact Of The Plasma Geometry On The Divertor Power Exhaust In A Magnetic Fusion Reactor

A deep understanding of plasma transport at the edge of a magnetically confined fusion device is mandatory for a sustainable and controlled handling of the power exhaust. In the next-generation fusion device ITER, technological limits constrain the peak heat flux on the divertor. For a given exhaust power the peak heat flux is determined by the extent of the plasma footprint on the wall. Heat flux profiles at the divertor targets of X-point configurations can be parametrized by using two length scales for the transport of heat in SOL. In this work, we challenge the current interpretation of these two length scales by studying the impact of divertor geometry modifications on the heat exhaust. In particular, a significant broadening of the heat flux profiles at the outer divertor target is diagnosed while increasing the length of the outer divertor leg. Modelling efforts showed that diffusive simulations well reproduce the experimental heat flux profiles for short-legged plasmas. Conversely, the broadening of the heat flux for a long divertor leg is reproduced by a turbulent model, highlighting the importance of turbulent transport not only in the main SOL but also in the divertor. These results question the current interpretation of the heat flux width as a purely main SOL transport length scale. In fact, long divertor leg magnetic configurations highlighted the importance of asymmetric divertor transport. We therefore conclude that main SOL and divertor SOL transport cannot be arbitrarily disentangled and we underline the importance of the divertor magnetic geometry in enhancing asymmetric turbulent transport with the potential benefit of an unexpected power spreading.

A complete and up-to-date summary of power exhaust in fusion plasmas, for academic researchers and graduate students in plasma physics.

Plasma Science and Engineering transforms fundamental scientific research into powerful societal applications, from materials processing and healthcare to forecasting space weather. Plasma Science: Enabling Technology, Sustainability, Security and Exploration discusses the importance of plasma research, identifies important grand challenges for the next decade, and makes recommendations on funding and workforce. This publication will help federal agencies, policymakers, and academic leadership understand the importance of plasma research and make informed decisions about plasma science funding, workforce, and research directions.

Encyclopedia of Nuclear Energy provides a comprehensive and reliable overview of the many ways nuclear energy contributes to society. Comprised of four volumes, it includes topics such as generating clean electricity, improving medical diagnostics and cancer treatment, improving crop yields, improving food shelf-lives, and crucially, the deployment of nuclear energy as an alternative energy source, one that is proving to be essential in the management of global warming. Carefully structured into thematic sections, this encyclopedia brings together the vast and highly diversified literature related to nuclear energy into a single resource, with convenient to read, cross-referenced chapters. This book will serve as an invaluable resource for researchers in the fields of energy, engineering, material science, chemistry, and physics, from both industry and academia. Offers a contemporary review of current nuclear energy research and insights into the future direction of the field, hence negating the need for individual searches across various databases Written by academics and practitioners from different fields to ensure that the knowledge within is easily understood by, and applicable to, a large audience Meticulously organized, with articles split into sections on key topics and clearly cross-referenced to allow students, researchers and professionals to quickly and easily find relevant information

Large, localized plasma heat exhaust continues to be one of the critical problems for the development of tokamak fusion reactors. Excessive heat flux erodes and possibly melts plasma-facing materials, thereby dramatically shortening their lifetime and increasing the impurity contamination of the core plasma. A detailed assessment by the ITER team for their divertor has revealed substantial limitations on the operational space imposed by the divertor performance. For a fusion reactor, the problem becomes worse in that the divertor must accommodate 20% of the total fusion power (less any broadly radiated loss), while not allowing excess buildup of tritium in the walls nor excessive impurity production. This is an extremely challenging set of problems that must be solved for fusion to succeed as a power source; it deserves a substantial research investment. Material heat-flux constraints: Results from present-day tokamaks show that there are two major limitations of peak plasma heat exhaust. The first is the continuous flow of power to the divertor plates and nearby surfaces that, for present technology, is limited to 10-20 MW/m2. The second is the transient peak heat-flux that can be tolerated in a short time, [tau]{sub m}, before substantial ablation and melting of the surface occurs; such common large transient events are Edge Localized Mode (ELMs) and disruptions. The material limits imposed by these events give a peak energy/[tau]{sub m}12 parameter of H"40 MJ/m2s12 [1]. Both the continuous and transient limits can be approached by input powers in the largest present-day devices, and future devices are expected to substantially exceed the limits unless a solution can be found. Since the early 90's LLNL has developed the analytic and computational foundation for analyzing divertor plasmas, and also suggested and studied a number of solid and liquid material concepts for improving divertor/wall performance, with the most recent being the Snowflake divertor concept [2] and generating Resonant Magnetic Perturbations by the SOL currents [3]. However, the specific approaches discussed here are part of a wider class of innovative divertor ideas that have come from the community in the last several years, and we certainly advocate the need to consider a range of options. Indeed, the most effective solution to the heat-flux problem may well contain features of various ideas. For example, there are the X-divertor (Kotschenreuther et al. [4]) that expands the magnetic flux surface in the vicinity of the near-X-point divertor plate, and the super X-divertor (Valanju et al. [5]) that guides the near-separatrix SOL flux tubes to a larger major radius to increase the surface area available for power deposition. These approaches have the common feature of manipulation of the edge magnetic geometry. Another approach is the use of liquid divertor surfaces that can increase the heat-flux capability by flowing the heated material to a cooling region and eventually out of the machine, and/or by being able to withstand a higher peak heat flux [6]. All of these areas are only emerging concepts that require substantially more analysis and definitive experimental tests, and given the need for a large improvement in this area, we advocate a substantial program to systematically assess the approaches. Because of space limitation here, we present some details of one of the concepts, namely the Snowflake divertor configuration. The Snowflake (SF) divertor [2] exploits a tokamak geometry in which the poloidal magnetic field varies quadratically with distance from the X-point null, [Delta]r. The name stems from the characteristic hexagonal, snowflake-like, shape of the multi-branched separatrix for this exact second-order null. In contrast, the standard X-point configuration has a poloidal field varying linearly with?r. The different variations mean that a flux expansion is much larger in the vicinity of a null of a snowflake divertor, and one can try to exploit this fact for reducing the divertor heat load. A unique feature here is also that the shear in the magnetic field near the X-point is substantially larger for the SF configuration, which may favorably affect microinstabilities and ELMs. Practical realization appears straightforward; the SF can be obtained using existing poloidal field coils in various present-day devices, and in general can be produced with coils located well outside the vacuum vessel. The SF configuration increases the flux expansion near the X-point that can be exploited by some increase in the plate wetted area, longer field-line length, and larger volume for impurity radiation. Initial UEDGE simulations comparing the SF with the standard divertor for the same conditions show a reduction in the peak heat flux for the SF of H".2-1.6 when comparing cases for the same angle of the total magnetic field to the divertor plate [7].

Fusion offers the prospect of virtually unlimited energy. The United States and many nations around the world have made enormous progress toward achieving fusion energy. With ITER scheduled to go online within a decade and demonstrate controlled fusion ten years later, now is the right time for the United States to develop plans to benefit from its investment in burning plasma research and take steps to develop fusion electricity for the nation's future energy needs. At the request of the Department of Energy, the National Academies of Sciences, Engineering, and Medicine organized a committee to develop a strategic plan for U.S. fusion research. The final report's two main recommendations are: (1) The United States should remain an ITER partner as the most cost-effective way to gain experience with a burning plasma at the scale of a power plant. (2) The United States should start a national program of accompanying research and technology leading to the construction of a compact pilot plant that produces electricity from fusion at the lowest possible capital cost.

This book treats nonlinear dynamics in both Hamiltonian and dissipative systems. The emphasis is on the mechanics for generating chaotic motion, methods of calculating the transitions from regular to chaotic motion, and the dynamical and statistical properties of the dynamics when it is chaotic. The new edition brings the subject matter in a rapidly expanding field up to date, and has greatly expanded the treatment of dissipative dynamics to include most important subjects.

Magnetic Fusion Technology describes the technologies that are required for successful development of nuclear fusion power plants using strong magnetic fields. These technologies include: • magnet systems, • plasma heating systems, • control systems, • energy conversion systems, • advanced materials development, • vacuum systems, • cryogenic systems, • plasma diagnostics, • safety systems, and • power plant design studies. Magnetic Fusion Technology will be useful to students and to specialists working in energy research.

This well-illustrated resource provides vital cross-section information for the atomic and molecular collision processes taking place in the boundary region of magnetically confined fusion plasmas and in other laboratory and astrophysical low-temperature plasmas. The expertly assessed information in this noteworthy volume includes the most recent experimental and theoretical results presented in a convenient format. Coverage includes the processes of electron-impact excitation and ionization of plasma edge atoms, electron-ion recombination, dissociative collision processes involving electrons and much more.

Fundamentals of Magnetic Thermonuclear Reactor Design is a comprehensive resource on fusion technology and energy systems written by renowned scientists and engineers from the Russian nuclear industry. It brings together a wealth of invaluable experience and knowledge on controlled thermonuclear fusion (CTF) facilities with magnetic plasma confinement – from the first semi-commercial tokamak T-3, to the multi-billion international experimental thermonuclear reactor ITER, now in construction in France. As the INTOR and ITER projects have made an immense contribution in the past few decades, this book focuses on its practical engineering aspects and the basics of technical physics and electrical engineering. Users will gain an understanding of the key ratios between plasma and technical parameters, design streamlining algorithms and engineering solutions. Written by a team of qualified experts who have been involved in the design of thermonuclear reactors for over 50 years Outlines the most important features of the ITER project in France which is building the largest tokamak, including the design, material selection, safety and economic considerations Includes data on how to design magnetic fusion reactors using CAD tools, along with relevant regulatory documents