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[en] Full text: In the framework of the EUROfusion work package “Heating and current drive”, a conceptual design of the neutral beam injector (NBI) for DEMO, has been developed by Consorzio RFX in collaboration with other European institutes. High efficiency is a fundamental requirement for DEMO, this has been taken into great consideration for the DEMO NBI, as a fundamental part of the maximization of RAMI performance. To increase the efficiency of the system, innovative solutions have been introduced for the neutralizer and the vacuum pumping systems. In particular, the design of a neutralizer based on the “closed recirculating cavity with nonlinear gating” (RING) photoneutralizer concept, using the second harmonic of a laser trapped in cavity through which the beam passes, has been implemented in the DEMO NBI conceptual design. The DEMO NBI has been designed to be also compatible with a gas neutralizer. Non-evaporable getter (NEG) pumps are foreseen to provide the required vacuum pumping inside the vessel. Compared to cryopumps, NEG pumps present numerous advantages: they are more resistant to neutron radiation and they do not need any continuous energy supply system for the operation. In order to increase the reliability and availability of the beam source, the DEMO NBI features a beam source composed of 20 subsources (two adjacent columns of 10 subsources each), following a modular design concept. Each subsource features its radio frequency driver. Such a modular solution is capable to provide a better alignment among the corresponding apertures of the accelerator grids, because the modules have a significantly smaller size than the whole accelerator, hence the horizontal and vertical deformations are also reduced compared with a nonmodular solution. To increase the maintainability of the system, the DEMO NBI has been designed in such a way that all the main components can be substituted without removing other components. For example, the beam source can be removed from the lateral opening of the beam source vessel, the neutralizer and the residual ion dump from dedicated upper flanges, the duct from the equatorial port close to the NBI port. Several analyzes have been carried out to investigate and optimize this conceptual design, namely optics, electrostatics, magnetics, neutronics and thermo-mechanics assessments. (author)
[en] Full text: Plasma exhaust is a critical aspect of DEMO-class devices, so there needs to be confidence that it will work. This paper considers the methodology to establish confidence in potential solutions, drawing on approaches inside and outside fusion, including evolving high power computing tools — these approaches could also help find improved solutions, possibly where all the plasma and materials ingredients operate in known regimes, reducing uncertainty. Crucially, the elements need to be integrated into an overall solution that can meet the demanding performance requirements and constraints of a fusion plant yet also accommodate significant uncertainties in plasma, materials and component behaviour. A prior full scale test of a DEMO exhaust solution is not feasible, almost by definition. The reference approach is to take the best available design, with various uncertainties and unknowns, and use margins and risk mitigation tools to address these. We explore a complementary approach based on models for the final step to give more confidence in the performance and uncertainty range of the design. The two approaches could be combined. For the plasma aspects, qualification will be eased if solutions have resilience to uncertainties and variations, ideally with natural “springiness”, or damping of transients. These can be tested with integrated models containing all relevant mechanisms and interactions, suitably validated. Materials and components have comparable modelling and integration challenges, in particular predicting the effects of combined loads (e.g., neutron, thermal, mechanical). A possible strategy is to combine measured and predicted materials properties and failure mechanisms (such as crack propagation, deformation) into a hierarchical multiscale model from atom-scale up. Such a modelling workflow would be well suited to high levels of parallelization and would improve over use of average material properties. In-silico qualification of such a large and complex system is very challenging, but has large potential benefits in cost, time, flexibility and optimization. Fortunately essentially all science issues are being addressed in the community (e.g., in EUROfusion). The computational demands are excessive today, but the rapid development of both computing power and numerical techniques is likely to transform the situation in the next 10–20 years. (author)
[en] This paper intends to overview the status of fusion activities and to present emerging issues related to the management of resources and knowledge in fusion projects; they can be better addressed by looking at appropriate methodologies and tools in the thematic areas of knowledge management. After a short introduction outlining the present transition phase of the worldwide fusion activity, I will present a preliminary analysis of emerging requirements and challenges, which create the foundations for knowledge management practices for the Fusion Sector. Differences between the fusion and the fission sector will also be discussed, and appropriate practices for some selected challenges will be proposed and analysed. (author)
[en] In 2018 75 projects of new fusion machines and fission reactors were reported. Concerning thermonuclear fusion, there are 2 kinds of projects, those that revisit concepts that were left aside by national laboratories and those that intend to complete present works on ITER by proposing innovating technologies. The discovery in 2015 of a new alloy allowing the construction of superconducting coils operating at higher temperatures and generating more intense magnetic fields has opened the way to more compact machines than ITER. Commonwealth Fusion Systems (CFS), Lockheed Martin and Tokamak Fusion are enterprises working on such machines. To illustrate the second type of projects, there is the Canadian start-up General Fusion that is studying a concept that relies on the use of an array of pistons driving a pressure wave to compress liquid metal rotating around a 3 million degrees plasma. The compression of the liquid metal heats the plasma triggering the fusion of hydrogen nuclei. Most of the projects are financed by the private sector. (A.C.)
[en] Full text: During the preconceptual design phase of fusion devices such as the European demonstration fusion power plant (DEMO), systems codes provide a fast evaluation of optimal design points and highlight high impact areas. However, determining or evaluating a design point at such an early stage comes with uncertainties in many of the design parameters. These uncertainties are both associated with the physics as well as the engineering basis of the European DEMO design. This work applies an uncertainty quantification analysis to the 2017 pulsed European DEMO design using the PROCESS systems code. It assumes that DEMO will be built as suggested by the baseline and explores what implications the currently known physics and engineering uncertainties have on the expected performance parameters (net electric output and pulse length), while optimizing the fusion gain Q or the pulse length. It furthermore compares the analysis of the conservative DEMO baseline design to the more advanced Flexi-DEMO option. A more detailed single parameter analysis is clearly identifying high impact parameters. This is confirming previous investigations as well as revealing new areas that warrant deeper investigation. (author)
[en] Tungsten (W) is considered as a promising candidate for plasma-facing materials for future nuclear fusion devices, and selecting optimal alloying constituents is a critical issue to improve radiation resistance of the W alloys as well as to improve their mechanical properties. We conducted in the current study a series of first-principles calculations for investigating solvent-solute mixed dumbbells in W crystals. The results suggested that titanium (Ti), vanadium (V), and chromium (Cr) are favorable as solutes for W alloys from irradiation-effect perspectives because these elements are expected to promote vacancy-interstitial recombination without causing radiation-induced precipitation that reduces ductility of irradiated materials.
[en] In a variety of magnetized plasma geometries, it has long been known that highly charged impurities tend to accumulate in regions of higher density. This “collisional pinch” is modified in the presence of additional forces, such as those might be found in systems with gravity, fast rotation, or non-negligible space charge. In the case of a rotating, cylindrical plasma, there is a regime in which the radially outermost ion species is intermediate in both mass and charge. As a result, this could have implications for fusion devices and plasma mass filters.
[en] Full text: Megawatt (MW) gyrotrons with a wide frequency range from 14 to 300 GHz are being developed in a collaborative ECH study for advanced fusion devices and a DEMO. 1) Detailed designs of a 14 GHz 1 MW gyrotron has been started for actual fabrication. For a 14 GHz RF beam with high divergence, a calculated transmission efficiency of 85% to the corrugated waveguide coupling position was initially obtained by minimizing the RF transmission path. 2) In the experimental tests of a new 28/35 GHz dual-frequency gyrotron, the cooling characteristics of an optimal-structure double-disk sapphire window was evaluated. We confirmed that operating at 0:4 MW with a continuous wave (CW) at 28 GHz is possible, which is two times the output power reported in previous studies. 3) A 77/51 GHz dual frequency gyrotron with an output of over 1 MW is presented. 4) In an experiment with a 300 GHz gyrotron, the influence of the reflected wave from the window was reduced by tilting the output window, and mode competition in the cavity was suppressed. An output power of 0.62 MW with a pulse width of 1 ms, which is the new record in this frequency, was obtained. (author)
[en] Full text: Extensive measurements are carried out on microturbulence because of their possible role in causing anomalous particle and energy transport in fusion devices. Outcome from past investigations suggest that the electron temperature gradient (ETG) driven turbulence is considered presently as a major source of anomalous plasma transport in fusion devices, as transport by ion scale turbulence is largely understood. Direct measurement of ETG is extremely difficult in fusion devices because of its extremely small scale length (∼ μm). In this background, efforts were made in large volume plasma device (LVPD), to produce plasma suitable for carrying out investigations on ETG turbulence (∼mm). Introduction of electron energy filter (EEF) divides LVPD plasma into three distinct regions of source, EEF and target plasmas. In the core region of target plasma (x ≤ 45 cm), unambiguous, identification of ETG turbulence is successfully demonstrated. Simultaneous measurement of fluctuations in electron temperature (10%-30%), plasma density (5%-10%) and potential (0.5%-2.5%) are carried out. Particle and energy transport are estimated from <ñe T-tildeθ> and
e Latin Capital Letter E With Tildeθ correlations. It was observed that electrostatic particle transport agrees well with theoretical estimates while, electromagnetic particle flux satisfies the relationship Γes ∼ 10-5 x Γem. Strong negative correlation is observed between fluctuations of density and temperature with potential fluctuations, showing correlation coefficients, Cne,φ-~ ∼ -0.8 and CT-tildee,φ-~ ∼ -0.7 respectively. This paper will present results on work carried out for energy transport due to ETG turbulence. Details on adopted diagnostic methods, for accurate measurement of temperature fluctuations will also be presented. A comparison will be made of experimentally derived energy transport with theoretically estimated values. (author)
[en] Structural materials present in and around any fusion device will face stringent conditions due to the high-energy and high-flux neutrons emitted from the fusion plasma. These neutrons can cause induced radioactivity, gas production, energetic knock out atoms, atomic displacement and decay heat in these materials. This would have a significant life-limiting impact on the materials and would also cause biological hazards and radioactive waste. Hence designing low activation materials for fusion devices is warranted. This paper presents a novel tool for quantification of radiological responses in terms of the elements present in the initial material composition. Such a framework would help in the identification and optimization of the fraction of most dangerous elements/isotopes from the material composition. In practical scenarios, the material encounters a large spatial and temporal variation of neutron fluxes. This problem has been effectively treated in the present work using a multi-parameter optimization scheme. The scheme optimizes the material composition (elemental or isotopic) based on various nuclear responses and there spatial and temporal variations within the user-defined constraints. This scheme is further included in the multi-point activation code ACTYS-1-GO. The tool provides a comprehensive picture of the material response during neutron irradiation and after shutdown, enabling the assessment of structural integrity of components in a fusion device. As an aide to the material optimization process, this paper also introduces a visual representation of the evaluated information like quantification of radiological responses produced by the parent elements/isotopes in a material. This has been implemented through a series of spectrum independent and spectrum-dependent diagrams called the radiation response diagrams. These diagrams show the variation of contributing parents toward the radiological responses as a function of cooling time. Such a graph could be very useful as a first approximation for material design. (paper)