The mission of the Fusion Energy Sciences is to expand the fundamental understanding of matter at very high temperatures and densities and to build the scientific foundation needed to develop a fusion energy source. Many of the frontiers of fusion science exist at the extremes of the plasma state, a state of matter where gases are hot enough that electrons disassociate from atomic nuclei (ions), forming an ensemble of ions and electrons that can conduct electrical currents and be confined by electric and magnetic fields.
Although tremendous scientific progress has been made since the inception of fusion energy research in the United States and internationally, fusion's research frontiers remain replete with open problems of critical importance. Their solution requires understanding not only of plasma phenomena itself but also of its interaction with surrounding material structures. Novel advanced diagnostics are needed to inform scientific understanding and enhance control capabilities. Sophisticated algorithms are needed to probe and control plasma dynamics. Improved theoretical and computational models are required to elucidate the physics of laboratory and space plasmas. Commensurate with these challenges is the promise of fusion energy, which can simultaneously provide base-load power production without producing greenhouse gases, with no possibility of runaway nuclear reactions or meltdown, minimal weapons proliferation risk, and minimal generation of long-lived radioactive waste.
A number of technical workshops were held in 2015 to develop community input on key topics within the FES research portfolio. The community focused on broad research areas critical to moving the field into the burning plasma science era when there will be a strong focus on the creation and study of plasmas where the energy to sustain fusion reactions is generated by the plasma itself. These areas were Transients, Plasma-Materials Interactions, and Integrated Simulations for Magnetic Fusion Energy Sciences. A fourth workshop was held focusing on the broader scientific opportunities at the frontiers of plasma science. Applicants to the FES Postdoctoral Program are strongly encouraged to propose research in the key areas highlighted in the reports generated by these community activities.
A main goal of tokamak research is to use magnetic plasma confinement to develop the means of operating high-pressure fusion plasmas near stability and controllability boundaries while avoiding the occurrence of transient events that can degrade performance or terminate the plasma discharge. Various events can lead to the sudden release of thermal and magnetic stored energy to plasma facing components, potentially damaging device components. Two events of particular concern are edge localized modes (ELMs) and disruptions. ELMs may drive repetitive (approx. 1 Hz) pulses of up to 10 percent or more of the plasma stored energy to the walls in ITER while disruptions may completely terminate the discharge, releasing all of the plasma's thermal and magnetic energy into nearby structures and potentially producing a runaway electron population that may cause localized damage. Present estimates indicate that sustained fusion performance in ITER and later reactors will require large reductions in the magnitude and frequency of both ELMs and major disruptions. A great deal of progress has been made in avoiding and mitigating the impact of transients and the United States is a world leader in addressing the challenges, particularly in identifying innovative control solutions and in understanding the relevant physics mechanisms to support extrapolation of these solutions to reactor-scale tokamaks.
Research in this area covers a wide range of activities including but not limited to the following areas:
- Developing passively stable plasma operating scenarios
- Active optimization of non-axisymmetric fields
- Real-time plasma stability sensing and stability limit forecasting
- Faster-than-real-time transport and stability calculations
- Optimizing active control of global magnetohydrodynamic (MHD) modes
- Establishing the physics basis for ELM control techniques
- Establishing the physical mechanisms for runaway electron production and dissipation
- Developing techniques to mitigate the effects of disruptions and runaway electrons
- Rapid plasma discharge shutdown techniques, and
- Integration of the above techniques into a comprehensive transient prediction and Plasma Control System capable of achieving disruption-free plasma operation.
Since success in any and all of these areas, discussed in detail in the Transients report, is essential for the long-term viability of the tokamak approach to fusion energy, FES encourages applications geared toward making meaningful progress in these areas. In particular, proposals focused on avoidance and mitigation of runaway electrons generated during tokamak disruptions will be given highest priority.
The pronounced science challenges associated with plasma-materials interactions (PMI) in confined fusion devices are enormous. These challenges include physical phenomena that occur over a vast range of length and time scales, and involve processes from, and interactions between, plasma edge physics and material science. New PMI solutions are required for practical heat and particle exhaust in future plasma systems as these considerations limit the operating space and drive the overall size and cost of net-energy producing fusion systems. Over the past decade, a number of both domestic and international strategic planning activities have highlighted PMI as a major knowledge gap and recommend that it be a priority for fusion research towards ITER and conceptual prototype reactors. Therefore, PMI research offers opportunities to advance the science of both plasma and materials, and the potential for new discoveries arising from their interactions.
Research is sought that explores both solid and liquid plasma facing components for:
- Qualifying divertor solutions for power exhaust and particle control
- Assessing boundary plasma solutions for main chamber wall components
- Developing the scientific basis for material evolution at reactor-relevant conditions
- Understanding how PMI influences pedestal and core performance
- Advancing the understanding of the present limits on power and particle handling/tritium control
For more information, please see the 2012 Fusion Energy Sciences Advisory Committee (FESAC) report titled Opportunities for Fusion Materials Science and Technology Research Now and During the ITER Era and the recent community report on Plasma-Materials Interactions.
The high cost of building future experiments and prototype fusion facilities combined with the complexity of these systems and the advances expected in extreme-scale computing over the coming decade provide strong motivation for developing integrated simulation capabilities where all relevant physical processes are included in a self-consistent way. Such tools aim to predict the key physical processes in these devices thus minimizing risk to the device and contributing to its successful operation. Two key components of this effort most likely to benefit from such an integrated simulation approach involve (1) disruption prediction, avoidance, and mitigation ("disruption physics"), and (2) plasma pedestal and scrape-off layer physics ("plasma boundary physics"). Simulations in these areas are an integral part of research in the Transients and Plasma-Materials Interface areas. The long term goal is the development of an experimentally validated Whole-Device Modeling (WDM) capability which will enable a transformation in predictive power based on fundamental science and high-performance computing.
In the disruption physics area, integrated simulation of all stages and forms of tokamak disruptions, with and without mitigation, are needed, and will require improving multiscale, multi-physics algorithms, managing large amounts of computational data, and taking advantage of new extreme-scale computing opportunities. Linear and nonlinear computational models must be validated in order to establish confidence in the prediction and understanding of tokamak disruption physics with and without mitigation.
In plasma boundary physics, models require integration of multiple physical processes that cover a wide range of overlapping spatial and temporal scales, from the hot, confined pedestal zone with sharp gradients, to the cooler unconfined edge and divertor plasma, and finally to the first few microns of the wall itself. As demonstrated in the recent past, these simulations will continue to benefit from expertise in applied mathematics, computer science, and high-performance computing. Required capabilities include kinetic turbulence simulations, related three-dimensional fluid simulations with gyro-Landau kinetic extensions, large-scale molecular dynamics simulations of materials, and couplings of lower-dimensional models (for fast analyses) to simulate interactions between zones and particle species.
In addition to these two high-priority topical areas, proposals addressing other WDM simulation needs (such as core transport, energetic particle effects, radio-frequency heating, etc.) will also be considered.
Those interested in submitting proposals in this area are encouraged to review the Integrated Simulations report. Given the overlap with topics in the Transients and/or Plasma-Material Interactions, applicants may also want to review the reports from those areas.
The Frontiers of Plasma Science involve research in largely unexplored areas of plasma physics, with a combination of theory, computer modeling, and experimentation. The plasma science frontier is often, but not limited to, the extremes of the plasma state, ranging from the very small (several atom systems) to the extremely large (plasma structure spanning light years in length), from the very fast (attosecond processes) to the very slow (hours), from the diffuse (interstellar medium) to the extremely dense (diamond compressed to tens of gigabar pressures), and from the ultracold (tens of micro kelvin) to the extremely hot (stellar core). Advancing the science of these unexplored areas creates opportunities for new and unexpected discoveries with potential to translate into practical applications.
Research supported in this priority area must be directed toward addressing problems at the frontiers of plasma science, specifically in the following areas:
- Extreme States of Matter and Plasmas, including Warm Dense Matter
- Understanding the Physics of Coherent Structures (Plasma Electrical Self-Organization)
- Understanding the Energetics of the Plasma Universe (Magnetic Self-Organization)
- The Physics of Disruptive Plasma Technologies (Transformative Research)
- Plasma at the Interface of Chemistry and Biology (Plasma Chemistry and Plasma Medicine)
For detailed descriptions of the research frontiers in each of these areas, please see the Report of the Panel on Frontiers of Plasma Science (https://science.energy.gov/~/media/fes/pdf/program-news/Frontiers_of_Plasma_Science_Final_Report.pdf).