Frontiers in Fusion Research II: Introduction to Modern Tokamak Physics

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International research progress preparing for burning plasma study on ITER has also increased the state of readiness to undertake research beyond ITER leading toward the construction of follow-on devices that demonstrate fusion power production and the potential for economical fusion electricity. In Europe, Japan, South Korea, and China, research beyond ITER is directed to develop the interconnected science and technology needed to design and construct a device to demonstrate fusion power.

Just as ITER has provided a research focus for international fusion researchers to advance burning plasma science, a strategy for the accompanying research and technology programs needed to progress beyond ITER to a commercial fusion reactor guides national research and innovation programs, helps to engage industrial partners, and sets national priorities.

While temperatures in the core of the magnetically confined plasma can be up to hundreds of millions of degrees Kelvin, the plasma cools as it is transported radially from core to edge through plasma transport, radiation, and other processes. This can introduce impurities into the plasma, which can degrade plasma performance and fusion gain.

There has been considerable progress over the last decade in mitigating heat fluxes escaping the plasma by various means that increase the area over which the heat is deposited. While detachment in a conventional divertor alone is estimated to reduce heat loading in ITER to an acceptable level by radiating percent of the escaping heat flux, a next-step burning plasma may have heating and fusion powers greater than those expected in ITER. With a conventional divertor, up to 90 percent of the heat exhaust would have to be radiated away to avoid material surface damage; at these levels, core plasma performance could be severely affected.

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Innovative divertor designs use more complex magnetic topologies to spread the heat flux over a wider area on the target plates. Also related to the Super-X is the X-point target divertor concept. Liquid metals are renewable, and they return to equilibrium after perturbations. LMs can handle heat fluxes up to factors of several over the upper limits for solid walls. Among the key challenges for obtaining a steady-state burning plasma beyond ITER is the capability to drive the plasma current noninductively.

This is of fundamental importance in tokamaks since tokamak confinement improves with the poloidal magnetic field that is produced by the plasma current. EC-CD is particularly attractive since it can achieve the highest transmission power density, and the space necessary for power injection is smallest. The challenges facing EC-CD include the relatively high cost of sources, the possible need for new source development if high fields are used, and the lower current drive efficiency for off-axis current generation which may be necessary for high bootstrap fraction.

In contrast, LH-CD already has readily available sources at lower costs and the off-axis current drive efficiency is perhaps the highest of any of the RF options. On the other hand, LH-CD has important challenges because of the need for a close-to-the-plasma-edge launching structure. Establishment of a high-bootstrap-current-fraction, fully noninductive tokamak plasma at sufficiently high normalized-beta using reactor-relevant and efficient current drive systems is a critical research subject for developing steady-state scenarios and being able to operate in the reduced tritium breeding blanket space of a compact configuration.

Perhaps the greatest challenge faced in a burning plasma regime is the simultaneous solution to all of the above issues to achieve the overall goal of a fusion power system capable of uninterrupted operation.

Measuring progress on multiple design metrics is not trivial. See Figure 4. Burning plasmas must simultaneously achieve a high triple product while eventually being sustained for months of steady-state operation. Longer pulse studies using experiments with superconducting magnets have been performed, , , , but these research devices are not large enough to operate at the high pressures needed for fusion gain. The highly nonlinear interactions between different phenomena in fusion equivalent regimes, defined at reactor relevant integration parameters, have not yet been examined.

A critical integration aspect upon reaching fusion equivalent regimes is to reconcile the core and the edge. Because the plasma core and edge strongly interact, the core-edge-divertor interactions are altered by increasing neutral opacity as reactor-like densities are approached.

PHYSICS of the ENERGY SOURCES: Laser Driven Thermonuclear Fusion; M. N. Rosenbluth(1991)

As the fusion equivalent regime is approached, the divertor, scrape-off layer, and pedestal become increasingly opaque, and pedestal profiles become more strongly dependent on transport and pinch effects. Divertor and plasma confinement are linked, and integrated solutions require both regions to be in the relevant density regimes. Another major issue is integrating the design and operation of tokamaks with requirements and technology imposed by future reactor engineering constraints.

Progress toward this integration has been slow since plasma science and device performance are each a necessary first step. As devices approach the burning plasma regime it is appropriate to embrace design choices that are compatible with. In addition, the tritium breeding ratio and corresponding blanket design should be compatible with overall plasma performance.

The study of very long pulse, toroidal magnetic confinement is under way with major research facilities using superconducting magnets. Because tokamaks and stellarators have strong magnetic fields and toroidal geometries, the fusion science and technologies of tokamaks and stellarators are similar.

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The fundamental dynamics of plasma confinement are described with the same methods; scientists produce and diagnose confined plasma using the same technologies; and tokamaks and stellarators are challenged to achieve and sustain the same fusion equivalent conditions. Stellarators and tokamaks differ by the degree by which the magnetic field is three dimensional. As the degree of three-dimensional magnetic field increases significantly, the usual confinement properties that arise from symmetry are broken and particle confinement requires carefully designed three-dimensional magnetic fields to avoid rapid particle loss.

Careful design is also necessary for the three-dimensional magnetic fields used to control tokamak plasmas, and this further motivates ongoing research linking the science of strong three-dimensional magnetic fields optimized for stellarators and the weaker three-dimensional magnetic fields needed to optimize performance of tokamaks, like ITER. Understanding transport in stellarators Figure 2. Transport in early stellarators was dominated by collisional, or neoclassical, processes. Neoclassical effects cause the cross-field transport of otherwise trapped particles as well as large impurity fluxes, which can dilute the plasma and compromise performance.

However, advances in theory and in numerical tools have led to identifying stellarator configurations with various optimizations of the magnetic field that predict a reduction in neoclassical transport to levels comparable to those in tokamaks.

:عنوان Frontiers in Fusion Research II: Introduction to Modern Tokamak Physics

The experiments will serve to validate results of several gyrokinetic codes, including the first fully global and physically comprehensive turbulence codes, which have recently been developed. Some MHD and energetic particles instabilities are predicted to behave differently in stellarators than in tokamaks. Understanding these differences will help to better control a burning plasma and to better predict fusion performance.

For example, some stellarator configurations, like that used in W7-X, minimize the toroidal plasma current, and plasma current induced MHD instabilities can potentially be avoided. Other stellarator configurations, like the National Compact Stellarator Experiment, are predicted to reduce the toroidal plasma current by more than half an equivalently sized tokamak.

Because relatively few stellarators have been built and studied, stellarators provide new tests of our understanding of plasma stability limits. Although disruptions are absent in existing low current stellarators, because high-pressure plasma confined within a quasi-symmetric stellarator is predicted to generate significant bootstrap current, current-driven instabilities and plasma current disruptions may need active methods for controlling the plasma profiles and mitigating the effects of transients just as in tokamaks.

Stellarators also provide opportunities to investigate plasma heat flux issues and to better understand how to design an optimized divertor. Because the stellarator. There are several types of divertors that are being assessed on stellarators, including the helical divertor , and the island divertor. Nevertheless, these challenges are common to both stellarators and tokamaks and include understanding the plasma boundary, including the effect of edge magnetic field shear, active control requirements, and assessing compatibility of divertor solutions in a high-performance integrated core-edge coupled regime.

For many years, the U.

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  7. Both analytic theory and reduced models and high-fidelity physics simulations development comprise this impressive set of accomplishments. Theory and simulation offer important new opportunities for accelerating progress toward the objective of economical fusion energy development by incorporating recent advances in theoretical understanding, validated physics models, computing infrastructure, and diagnosis of experiments. Such a predictive modeling capability is required—for example, for the interpretive analysis of existing devices as well as for minimizing risks and qualifying operating scenarios for next-step burning plasma experiments.

    In all future burning plasma facilities, the optimization of fusion performance and control scenarios will require predictive WDM with a quantified, validated uncertainty, as it will not be feasible to determine operational limits by running trial discharges. The following research elements are critical for realization of a successful WDM capability:. While present models can help develop operational scenarios for ITER and increase confidence in achieving its goals, a validated WDM capability would provide the confidence to explore the extreme parameter regimes of fusion reactors, informing decisions and lowering the future risks of building full-scale fusion power plants, and thus accelerating the fusion program.

    In addition to projects in support of ITER and domestic Department of Energy DOE Office of Fusion Energy Sciences research activities, the fusion program can leverage technology developments in areas that have significant potential to advance fusion energy development. The four top-tier TECs identified by the panel were: advanced algorithms, high critical temperature superconductors, advanced materials, and novel technologies for tritium fuel cycle control.

    Advanced control systems are required to control a burning plasma during the normal evolution of the plasma and also during transient events that need to be either predicted and avoided or mitigated. Advancements in feedback algorithms and intelligent systems may significantly improve the reliability of fusion power and perhaps enable operation at optimized operating points whose achievement and sustainment are impossible without high performance feedback control.

    As explained in the TEC Report, the area of advanced algorithms includes the related fields of mathematical control, machine learning, artificial intelligence, integrated data analysis, and other algorithm-based research and development. The importance of advanced algorithms is illustrated in Figure 2. As understanding of burning plasma operation improves, advanced control algorithms will support and accelerate the pace of physics understanding, enable the experimental realization of theoretically predicted operating scenarios, and build mathematical synergies with advances in other high-performance computing capabilities that will enable improved physics understanding.

    Machine learning and mathematical control can also help to bridge gaps in knowledge when these exist, for example to enable effective control of fusion plasmas with imperfect understanding of the plasma state. Advances in higher field superconductors present a major opportunity to enhance the performance and feasibility of fusion reactor designs. High-field, high-temperature superconductors would enable a new generation of relatively compact fusion experiments and power plants, dramatically speeding the development path and improving the overall attractiveness of fusion energy.

    Strong magnetic fields are critical to the success of magnetic fusion as a source of energy. Achieving higher magnetic field strength extends the allowable plasma properties to higher plasma density, higher plasma current, and higher plasma pressure while retaining the same dimensionless scaling parameters found at lower magnetic field strength.

    This extended range of plasma parameters from high-field magnets allows more compact tokamak devices that may provide a lower cost path to future fusion reactors. The behavior and integrity of materials in a fusion system are of great importance to the long-term viability of fusion energy. The high energy neutrons from the D-T fusion reaction generate between 50 to times higher helium to dpa ratio in materials such as ferritic steels than does fission reactor irradiation.

    Burning plasma experiments will thus aid in the development of high-heat-flux components and will serve as testbeds in which to evaluate the performance of the components in a reactor-like fusion environment.

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    The heat loads on components in a burning plasma experiment will be comparable to those expected in a reactor and will require the application of state-. Linear plasma simulators allow for long-duration study of material evolution under fusion-relevant plasma flux, but they are not useful to test integrated plasma-material effects expected in fusion divertors. Qualification of materials to operate safely in a fusion environment is of critical importance for moving forward.

    Material effects have to be strongly connected to any consideration of availability, reliability, and maintainability of the fusion core. All of these have a large impact on the three overarching factors of plant economics, public perception, and ability to license. As reported by the U. These include advanced metals manufacturing: electron beam melting, laser-blown metal powder deposition, laser sintering, and metal laser melting.

    Figure 2. With these emerging techniques, important new opportunities are emerging to develop the resilient materials and components for a fusion energy system.


    A fusion breeding blanket—that is, a nuclear system that creates tritium via interaction of the fusion-produced MeV neutrons with lithium—is a key fusion nuclear technology needed for the development of fusion energy. Fusion reactors should operate with more tritium produced and recovered than is burned.