Applied and fundamental aspects of fusion science
TL;DR: In this paper, the authors discuss selected developments in diagnostics and present-day research topics in high-temperature plasma physics, and present day research topics for high temperature plasma physics.
Abstract: Fusion research is driven by the applied goal of energy production from fusion reactions. There is, however, a wealth of fundamental physics to be discovered and studied along the way. This Commentary discusses selected developments in diagnostics and present-day research topics in high-temperature plasma physics.
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TL;DR: In this paper, the authors review the basic physics underlying magnetic fusion: past achievements, present efforts and the prospects for future production of electrical energy, and discuss questions related to the safety, waste management and decommissioning of a future fusion power plant.
Abstract: Our modern society requires environmentally friendly solutions for energy production. Energy can be released not only from the fission of heavy nuclei but also from the fusion of light nuclei. Nuclear fusion is an important option for a clean and safe solution for our long-term energy needs. The extremely high temperatures required for the fusion reaction are routinely realized in several magnetic-fusion machines. Since the early 1990s, up to 16 MW of fusion power has been released in pulses of a few seconds, corresponding to a power multiplication close to break-even. Our understanding of the very complex behaviour of a magnetized plasma at temperatures between 150 and 200 million °C surrounded by cold walls has also advanced substantially. This steady progress has resulted in the construction of ITER, a fusion device with a planned fusion power output of 500 MW in pulses of 400 s. ITER should provide answers to remaining important questions on the integration of physics and technology, through a full-size demonstration of a tenfold power multiplication, and on nuclear safety aspects. Here we review the basic physics underlying magnetic fusion: past achievements, present efforts and the prospects for future production of electrical energy. We also discuss questions related to the safety, waste management and decommissioning of a future fusion power plant. One way of realizing controlled nuclear fusion reactions for the production of energy involves confining a hot plasma in a magnetic field. Here, the physics of magnetic-confinement fusion is reviewed, focusing on the tokamak and stellarator concepts.
149 citations
TL;DR: In this article, the authors analyzed the geodesic acoustic modes (GAMs) on the T-10 tokamak since the last IAEA FEC 2014 and found that the frequency at the edge of the GAM peak with frequency 17 kHz in potential fluctuations, and noticeable peak of quasi-coherent density and potential fluctuations with frequency 40-100 kHz and HFHM ~30 kHz.
Abstract: New findings in study of geodesic acoustic modes (GAMs) on the T-10 tokamak since the last IAEA FEC 2014 are described. The broadband fluctuations of potential and electron density n e in Ohmic and ECRH regimes are analyzed with Heavy Ion Beam Probing along with fluctuations of poloidal magnetic field B pol. At the edge, at ρ > 0.8, the dominated GAM peak with frequency 17 kHz in potential fluctuations, and noticeable peak of quasi-coherent density and potential fluctuations with frequency 40–100 kHz and HFHM ~30 kHz are observed. During ECRH of high density ~ 4 × 1019 m−3 plasmas, the level of broadband fluctuations decreases, but the energy confinement degrades and the GAM amplitude on rises. The bi-spectral analysis of and n e fluctuations demonstrates the existence of statistically significant auto- and cross-bicoherence at the GAM frequency that points out to three-wave interaction between GAM and broadband electrostatic turbulence, while the cross-bicoherence for , n e and B pol indicates three-wave interaction between GAM and broadband electromagnetic turbulence. These three-wave interactions may be explained by quadratic character of nonlinear GAM generation, e.g. owing to Reynolds stress.
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TL;DR: In toroidal systems with geodesic curvature, an electrostatic acoustic mode occurs with plasma motion in the magnetic surfaces, perpendicular to the field as discussed by the authors, and this mode should dominate ordinary sound waves associated with motion along the field.
Abstract: In toroidal systems with geodesic curvature an electrostatic acoustic mode occurs with plasma motion in the magnetic surfaces, perpendicular to the field. In typical stellarators this mode should dominate ordinary sound waves associated with motion along the field.
624 citations
TL;DR: Alfven wave instability in toroidally confined plasmas is studied in this paper, where the authors identify three types of Alfven wave instabilities: frequency crossings of counterpropagating waves, extremum of the continuous spectrum, and reversed shear Alfven eigenmode.
Abstract: Superthermal energetic particles (EP) often drive shear Alfven waves unstable in magnetically confined plasmas. These instabilities constitute a fascinating nonlinear system where fluid and kinetic nonlinearities can appear on an equal footing. In addition to basic science, Alfven instabilities are of practical importance, as the expulsion of energetic particles can damage the walls of a confinement device. Because of rapid dispersion, shear Alfven waves that are part of the continuous spectrum are rarely destabilized. However, because the index of refraction is periodic in toroidally confined plasmas, gaps appear in the continuous spectrum. At spatial locations where the radial group velocity vanishes, weakly damped discrete modes appear in these gaps. These eigenmodes are of two types. One type is associated with frequency crossings of counterpropagating waves; the toroidal Alfven eigenmode is a prominent example. The second type is associated with an extremum of the continuous spectrum; the reversed shear Alfven eigenmode is an example of this type. In addition to these normal modes of the background plasma, when the energetic particle pressure is very large, energetic particle modes that adopt the frequency of the energetic particle population occur. Alfven instabilities of all three types occur in every toroidal magnetic confinement device with an intense energetic particle population. The energetic particles are most conveniently described by their constants of motion. Resonances occur between the orbital frequencies of the energetic particles and the wave phase velocity. If the wave resonance with the energetic particle population occurs where the gradient with respect to a constant of motion is inverted, the particles transfer energy to the wave, promoting instability. In a tokamak, the spatial gradient drive associated with inversion of the toroidal canonical angular momentum Pζ is most important. Once a mode is driven unstable, a wide variety of nonlinear dynamics is observed, ranging from steady modes that gradually saturate, to bursting behavior reminiscent of relaxation oscillations, to rapid frequency chirping. An analogy to the classic one-dimensional problem of electrostatic plasma waves explains much of this phenomenology. EP transport can be convective, as when the wave scatters the particle across a topological boundary into a loss cone, or diffusive, which occurs when islands overlap in the orbital phase space. Despite a solid qualitative understanding of possible transport mechanisms, quantitative calculations using measured mode amplitudes currently underestimate the observed fast-ion transport. Experimentally, detailed identification of nonlinear mechanisms is in its infancy. Beyond validation of theoretical models, the future of the field lies in the development of control tools. These may exploit EP instabilities for beneficial purposes, such as favorably modifying the current profile, or use modest amounts of power to govern the nonlinear dynamics in order to avoid catastrophic bursts.
431 citations
TL;DR: Fusion materials research started in the early 1970s following the observation of the degradation of irradiated materials used in the first commercial fission reactors as mentioned in this paper, and has been the subject of decades of worldwide research efforts underpinning the present maturity of the fusion materials research program.
Abstract: Fusion materials research started in the early 1970s following the observation of the degradation of irradiated materials used in the first commercial fission reactors. The technological challenges of fusion energy are intimately linked with the availability of suitable materials capable of reliably withstanding the extremely severe operational conditions of fusion reactors. Although fission and fusion materials exhibit common features, fusion materials research is broader. The harder mono-energetic spectrum associated with the deuterium–tritium fusion neutrons (14.1 MeV compared to <2 MeV on average for fission neutrons) releases significant amounts of hydrogen and helium as transmutation products that might lead to a (at present undetermined) degradation of structural materials after a few years of operation. Overcoming the historical lack of a fusion-relevant neutron source for materials testing is an essential pending step in fusion roadmaps. Structural materials development, together with research on functional materials capable of sustaining unprecedented power densities during plasma operation in a fusion reactor, have been the subject of decades of worldwide research efforts underpinning the present maturity of the fusion materials research programme. For achieving proper safety and efficiency of future fusion power plants, low-activation materials able to withstand the extreme fusion conditions are needed. Here, the irradiation physics at play and fusion materials research is reviewed.
326 citations
TL;DR: In this paper, the Thomson scattering on T3 has been used to obtain temperatures of 100 eV up to 1 keV and densities in the range 1 −3 × 1013 cm−3.
Abstract: Electron temperatures of 100 eV up to 1 keV and densities in the range 1–3 × 1013 cm−3 have been measured by Thomson scattering on Tokamak T3. These results agree with those obtained by other techniques where direct comparison has been possible.
207 citations
TL;DR: Geodesic acoustic modes (GAMs) were investigated on the T-10 tokamak using heavy ion beam probe, correlation reflectometry and multipin Langmuir probe diagnostics as mentioned in this paper.
Abstract: Geodesic acoustic modes (GAMs) were investigated on the T-10 tokamak using heavy ion beam probe, correlation reflectometry and multipin Langmuir probe diagnostics. Regimes with Ohmic heating and with on- and off-axis ECRH were studied. It was shown that GAMs are mainly the potential oscillations. Typically, the power spectrum of the oscillations has the form of a solitary quasi-monochromatic peak with the contrast range 3–5. They are the manifestation of the torsional plasma oscillations with poloidal wavenumber m = 0, called zonal flows. The frequency of GAMs changes in the region of observation and decreases towards the plasma edge. After ECRH switch-on, the frequency increases, correlating with growth in the electron temperature Te. The frequency of the GAMs depends on the local Te as , which is consistent with a theoretical scaling for GAM, where cs is the sound speed within a factor of unity. The GAMs on T-10 are found to have density limit, some magnetic components and an intermittent character. They tend to be more excited near low-q magnetic surfaces.
187 citations