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  • Publication
    Online experiments and modelling with a detailed reaction scheme of single particle biomass pyrolysis
    (Elsevier, 2017-09-01) Anca Couce, Andres; Sommersacher, Peter; Scharler, Robert
    Detailed reaction schemes and experimental data for the online release of pyrolysis volatiles are required to gain a more fundamental understanding of biomass pyrolysis, which would in turn allow the process to be controlled in a more precise way and the development of more targeted applications. A detailed online characterisation of pyrolysis products has been conducted in single particle experiments with spruce pellets at different temperatures, obtaining a good closure of the elemental mass balances. The yields and online release of CO, CO2, H2O, CH4, other light hydrocarbons and total organic condensable species, as well as char yield and composition, can be predicted with a reasonable accuracy with the application of a single particle model, coupled with a detailed pyrolysis scheme, and a simple one-step scheme for tar cracking. In order to achieve it, improvements have been conducted in the pyrolysis scheme, mainly concerning the release of light hydrocarbons and char yield and composition. Deviations are still present in the different groups in which organic condensable species can be classified.
  • Publication
    Enhanced confinement induced by pellet injection in the stellarator TJ-II
    (2023-07-01) Garcia Cortes, Maria Isabel; Mccarthy, K. J.; Estrada, T.; Tribaldos Macía, Víctor; Medina-Roque, D.; Van Milligen, B.; Ascasibar, E.; Carrasco, R.; Chmyga, A. A.; Garcia, R.; Hernandez Sanchez, J.; Hidalgo, C.; Kozachek, A. S.; Medina, F.; Ochando, M.A.; De Pablos, J.L.; Panadero, N.; Pastor, I.; European Commission; Ministerio de Economía y Competitividad (España)
    Enhanced confinement is observed in neutral beam injector (NBI)-heated hydrogen discharges made in the stellarator TJ-II after the injection of a single cryogenic fuel pellet into the plasma core. In addition to the expected increase in electron density, ne, in the core after pellet injection (PI), the plasma diamagnetic energy content is seen to rise, with respect to similar discharges without PI, by up to 40%. Furthermore, the energy confinement time, sE diag, as determined using a diamagnetic loop, is enhanced when compared to predictions obtained using the International Stellarator Scaling law [H. Yamada et al., Nucl. Fusion 45, 1684 (2005)] and the triple product, ne _ Ti _ sE diag, exhibits a clear bifurcation point toward an improved confinement branch as compared to the branch product predicted by this scaling law. In general, once such a pellet-induced enhanced confinement (PiEC) phase has been established, it is characterized by steepened radial density gradients, by more negative plasma potential in the core, more negative radial electric fields, Er, across a broad plasma region, as well as by reductions in density and plasma potential fluctuations in the density gradient region. In addition, experimental observations show increased peaking of core radiation losses, this pointing to edge/core plasma decoupling. In parallel, neoclassical simulations of reference and PiEC plasmas predict increased particle and energy confinement times during a PiEC phase together with a more negative Er profile. Qualitative rather than quantitative agreement with experimental parameters is found, indicating that turbulence seems to play a significant role here. In summary, single cryogenic pellet injection facilitates the achievement of an enhanced operational regime that was previously not observed in NBIheated discharges of the TJ-II.
  • Publication
    Study of Alfvén eigenmode stability in Quasi-Poloidal Stellarator (QPS) plasma using a Landau closure model
    (IOP Publishing, 2023-05-01) Ortiz Luengo, Juan; Varela Rodríguez, Jacobo; Spong, Donald; García Gonzalo, Luis; Ghai, Yashika; Comunidad de Madrid
    The aim of this study is to analyze the linear stability of Alfvén eigenmodes (AE) in the QPS device heated by a tangential neutral beam injector (NBI). The analysis is performed using the gyro-fluid code FAR3d, that solves the reduced MHD equations for the thermal plasma coupled with moments of the kinetic equation for the energetic particles (EP). The AE stability is calculated in several operational regimes of the tangential NBI: EP β between 0.001 and 0.1, EP energy between 12 and 180 keV and different radial locations of the beam. The analysis is performed for vacuum and finite β equilibria as well as QPS configurations with two and three periods. The EP β threshold in the vacuum case is 0.001 and the AE frequency is lower as the energy of the EP population decreases. Toroidal Alfvén eigenmodes with f = 80–120 kHz and elliptical AE between f = 120–350 kHz are triggered between the middle-outer plasma region (r/a > 0.5). The AE stability improves in the simulations with finite β equilibria and three period configurations with respect to the vacuum case with two periods because the continuum gaps are slender, leading to a higher threshold of the EP β, above 0.03 for the AEs triggered by the helical mode families. Helical effects are not strong enough to destabilize Helical Alfvén eigenmodes, the AEs with the largest growth rates are triggered by the n=1 and n=2 toroidal families.
  • Publication
    3D effects on transport and plasma control in the TJ-II stellarator
    (IOP Science, 2017-10) Castejón, F.; Bustos Molina, Andrés de; García Gonzalo, Luis; Liu, Bing; Martínez, M.; European Commission; Ministerio de Economía y Competitividad (España)
    The effects of 3D geometry are explored in TJ-II from two relevant points of view: neoclassical transport and modification of stability and dispersion relation of waves. Particle fuelling and impurity transport are studied considering the 3D transport properties, paying attention to both neoclassical transport and other possible mechanisms. The effects of the 3D magnetic topology on stability, confinement and Alfven Eigenmodes properties are also explored, showing the possibility of controlling Alfven modes by modifying the configuration; the onset of modes similar to geodesic acoustic modes are driven by fast electrons or fast ions; and the weak effect of magnetic well on confinement. Finally, we show innovative power exhaust scenarios using liquid metals.
  • Publication
    Analysis of the ECH effect on EPM/AE stability in Heliotron J plasma using a Landau closure model
    (IOP Science, 2023-02) Varela Rodríguez, Jacobo; Nagasaki, K.; Kobayashi, S.; Nagaoka, K.; Adulsiriswad, P.; Cappa, A.; Yamamoto, S.; Watanabe, K. Y.; Spong, D. A.; García Gonzalo, Luis; Ghai, Y.; Ortiz Luengo, Juan; Comunidad de Madrid
    The aim of the present study is to analyze the effect of the electron cyclotron heating (ECH) on the linear stability of Alfvén eigenmodes (AEs) and energetic particle modes (EPMs) triggered by energetic ions in Heliotron J plasma. The analysis is performed using the FAR3d code that solves a reduced MHD model to describe the thermal plasma coupled with a gyrofluid model for the energetic particle (EP) species. The simulations reproduce the AE/EPM stability trends observed in the experiments as the electron temperature (Te) increases, modifying the thermal plasma β, EP β and EP slowing-down time. Particularly, the $n/m = 1/2$ EPM and $2/4$ Global AE are stabilized in the low-bumpiness (LB) configuration due to an enhancement of the continuum, finite Larmor radius and e-i Landau damping effects as the thermal β increases. On the other hand, a larger ECH injection power cannot stabilize the AE/EPM in medium-bumpiness and high-bumpiness (HB) configurations because the damping effects are weaker compared to the LB case, unable to balance the further destabilization induced by an enhanced EP resonance as the EP slowing-down time and EP β increases with Te.
  • Publication
    The potential impact of climate change on the efficiency and reliability of solar, hydro, and wind energy sources
    (MDPI, 2022-08) Bhatt, Uma S.; Carreras, Benjamin A.; Reynolds Barredo, José Miguel; Newman, David E.; Collet, Pere; Gomila, Damia; Ministerio de Ciencia e Innovación (España)
    Climate change impacts the electric power system by affecting both the load and generation. It is paramount to understand this impact in the context of renewable energy as their market share has increased and will continue to grow. This study investigates the impact of climate change on the supply of renewable energy through applying novel metrics of intermittency, power production and storage required by the renewable energy plants as a function of historical climate data variability. Here we focus on and compare two disparate locations, Palma de Mallorca in the Balearic Islands and Cordova, Alaska. The main results of this analysis of wind, solar radiation and precipitation over the 1950–2020 period show that climate change impacts both the total supply available and its variability. Importantly, this impact is found to vary significantly with location. This analysis demonstrates the feasibility of a process to evaluate the local optimal mix of renewables, the changing needs for energy storage as well as the ability to evaluate the impact on grid reliability regarding both penetration of the increasing renewable resources and changes in the variability of the resource. This framework can be used to quantify the impact on both transmission grids and microgrids and can guide possible mitigation paths.
  • Publication
    Analysis of the blackout risk reduction when segmenting large power systems using lines with controllable power flow
    (Elsevier, 2023-06) Gomila, D.; Carreras, Benjamín A.; Reynolds Barredo, José Miguel; Colet, P.; Gomis Bellmunt, O.; European Commission; Ministerio de Economía y Competitividad (España); Ministerio de Ciencia e Innovación (España)
    Large electrical transmission networks are susceptible to undergo very large blackouts due to cascading failures, with a very large associated economical cost. In this work we propose segmenting large power grids using controllable lines, such as high-voltage direct-current lines, to reduce the risk of blackouts. The method consists in modifying the power flowing through the lines interconnecting different zones during cascading failures in order to minimize the load shed. As a result, the segmented grids have a substantially lower risk of blackouts than the original network, with reductions up to 60% in some cases. The control method is shown to be specially efficient in reducing blackouts affecting more than one zone.
  • Publication
    Assessing blackout risk with high penetration of variable renewable energies
    (IEEE, 2021-09-20) Carreras, Benjamín A.; Colet, Pere; Reynolds Barredo, José Miguel; Gomila, Damià; Ministerio de Economía y Competitividad (España); Ministerio de Ciencia e Innovación (España)
    We propose a method to analyze the risk of blackouts with high penetration of variable renewable energy sources (VRESs). We consider a model for the long-term evolution of the power grid including propagation of cascading failures, day-to-day fluctuations of renewable generation and moderate use of storage. We analyze grid resilience and stress as VRESs are progressively incorporated. We also evaluate the VRES performance as the average fraction of daily demand covered by renewables.We find that in general, VRES intrinsic variability increases the grid stress and the blackout risk. However, if VRESs are implemented in a distributed way, the spatial spreading of the generation may have a positive effect on grid resilience. As a case study, we analyze the replacement of conventional power plants by solar photovoltaic generation combined with storage in the power grid of the Balearic Islands. We also consider the use of source redundancy and briefly discuss the potential of wind energy.
  • Publication
    MHD study of extreme space weather conditions for exoplanets with earth-like magnetospheres: On habitability conditions and radio-emission
    (Wiley, 2022-11) Varela Rodríguez, Jacobo; Brun, A. S.; Zarka, P.; Strugarek, A.; Pantellini, F.; Reville, V.; Comunidad de Madrid; Universidad Carlos III de Madrid
    The present study aims at characterizing the habitability conditions of exoplanets with an Earth-like magnetosphere inside the habitable zone of M and F stars, caused by the direct deposition of the stellar wind on the exoplanet surface. Also, the radio emission generated by exoplanets with a Earth-like magnetosphere is calculated for different space weather conditions. The study is based on a set of MHD simulations performed by the code PLUTO. Exoplanets hosted by M stars at 0.2 au are protected from the stellar wind during regular and coronal mass ejection (CME)-like space weather conditions if the star rotation period is slower than 3 days. Exoplanets hosted by a F stars at 2.5 au are protected during regular space weather conditions, but a stronger magnetic field compared to the Earth is mandatory if the exoplanet is close to the inner edge of the star habitable zone (2.5 au) during CMEs. The range of radio emission values calculated in the simulations are consistent with the scaling proposed by Zarka (2018, https://doi.org/10.1007/978-3-319-55333-7_22) during regular and common CME-like space weather conditions. If the radio telescopes measure a relative low radio emission signal with small variability from an exoplanet, that may indicate favorable exoplanet habitability conditions. The radio emission power calculated for exoplanets with an Earth-like magnetosphere is in the range of 3 × 107 to 2 × 1010 W for SW dynamic pressures between 1.5 and 100 nPa and interplanetary magnetic field intensities between 50 and 250 nT, and is below the sensitivity threshold of present radio telescopes at parsec distances.
  • Publication
    Rational surfaces, flows and radial structure in the TJ-II stellarator
    (IOP Science, 2023-01) Van Milligen, B.Ph.; Voldiner, I.; Carreras Verdaguer, Benjamin A.; Ochando, Maria A.; García Gonzalo, Luis; European Commission; Ministerio de Ciencia, Innovación y Universidades (España)
    In this work, we report on the results obtained by measuring several turbulent quantities well inside the plasma edge by means of a Langmuir probe during dynamical rotational transform scans in the TJ-II stellarator, while applying a radial electric field to the edge plasma using a biasing probe. By calculating the intermittence parameter from floating potential measurements, we are able to identify a major low order rational surface and hence relate the probe measurements to the local value of the rotational transform. Based on the former, we are able to show that the poloidal plasma velocity (and hence radial electric field) has a significant radial structure that is clearly related to the rotational transform profile and in particular the lowest order rational surfaces in the range studied. The poloidal velocity is also affected by the edge biasing. The particle flux was also found to exhibit a radial pattern, as did the flow shear suppression term wExB1, but the relation of the former to the low-order rational surfaces was less clear. We surmise that this lack of direct correspondence is due to an unknown term in the turbulence evolution equation: the instability growth rate, y. We make use of a reduced Magnetohydrodynamic turbulence model to interpret the results. Overall, a picture is obtained in which the plasma self-organizes towards a state with a clear radial pattern of the radial electric field, in line with expectations from some numerical studies describing the spontaneous formation of an E x B staircase, consisting of alternating layers with fast and slow radial transport. In this state, the radial profiles of various quantities (density, temperature, pressure) will not be smooth.
  • Publication
    The interplay of network structure and dispatch solutions in power grid cascading failures
    (AIP Publishing, 2016-11) Reynolds Barredo, José Miguel; Newman, David; Carreras Verdaguer, Benjamin A.; Dobson, Ian; Ministerio de Economía y Competitividad (España)
    For a given minimum cost of the electricity dispatch, multiple equivalent dispatch solutions may exist. We explore the sensitivity of networks to these dispatch solutions and their impact on the vulnerability of the network to cascading failure blackouts. It is shown that, depending on the heterogeneity of the network structure, the blackout statistics can be sensitive to the dispatch solution chosen, with the clustering coefficient of the network being a key ingredient. We also investigate mechanisms or configurations that decrease discrepancies that can occur between the different dispatch solutions.
  • Publication
    Powering stellar magnetism: energy transfers in cyclic dynamos of sun-like stars
    (IOP Science, 2022-02-10) Brun, Allan Sacha; Strugarek, Antoine; Noraz, Quentin; Perri, Barbara; Varela Rodríguez, Jacobo; Augustson, Kyle; Charbonneau, Paul; Toomre, Juri
    We use the anelastic spherical harmonic code to model the convective dynamo of solar-type stars. Based on a series of 15 3D MHD simulations spanning four bins in rotation and mass, we show what mechanisms are at work in these stellar dynamos with and without magnetic cycles and how global stellar parameters affect the outcome. We also derive scaling laws for the differential rotation and magnetic field based on these simulations. We find a weaker trend between differential rotation and stellar rotation rate, (ΔΩ∞(IΩI/Ω⊛) 0.46) in the MHD solutions than in their HD counterpart ((IΩI/Ω⊛) 0.66), yielding a better agreement with the observational trends based on power laws. We find that for a fluid Rossby number between 0.15 ≲ Rof ≲ 0.65, the solutions possess long magnetic cycle, if Rof ≲ 0.42 a short cycle and if Rof ≲ 1 (antisolar-like differential rotation), a statistically steady state. We show that short-cycle dynamos follow the classical Parker–Yoshimura rule whereas the long-cycle period ones do not. We also find efficient energy transfer between reservoirs, leading to the conversion of several percent of the starʼs luminosity into magnetic energy that could provide enough free energy to sustain intense eruptive behavior at the star’s surface. We further demonstrate that the Rossby number dependency of the large-scale surface magnetic field in the simulation ( BL,surf ~ Rof -1.26) agrees better with observations ( Bv~ Ros -1.4 ± 0.1) and differs from dynamo scaling based on the global magnetic energy (Bbulk ~Rof -0.5).
  • Publication
    Transport dynamics of self-consistent, near-marginal drift-wave turbulence. I. Investigation of the ability of external flows to tune the non-diffusive dynamics
    (American Institute of Physics, 2017-07) Ogata, D.; Newman, D. E.; Sánchez Fernández, Luis Raúl; Ministerio de Economía y Competitividad (España)
    The reduction of turbulent transport across sheared flow regions has been known for a long time in magnetically confined toroidal plasmas. However, details of the dynamics are still unclear, in particular, in what refers to the changes caused by the flow on the nature of radial transport itself. In Paper II, we have shown in a simplified model of drift wave turbulence that, when the background profile is allowed to evolve self-consistently with fluctuations, a variety of transport regimes ranging from superdiffusive to subdiffusive open up depending on the properties of the underlying turbulence [D. Ogata et al., Phys. Plasmas 24, 052307 (2017)]. In this paper, we show that externally applied sheared flows can, under the proper conditions, cause the transport dynamics to be diffusive or subdiffusive.
  • Publication
    Width and rugosity of the topological plasma flow structures and their relation to the radial flights of particle tracers
    (IOP, 2015-11-01) García Gonzalo, Luis; Llerena Rodríguez, Irene; Carreras Verdaguer, Benjamin A.; Dirección General de Investigación Científica y Técnica (España)
    An analysis of the distributions of the width and rugosity of topological plasma flow structures is presented for some resistive pressure-gradient-driven turbulence results. The distributions of the radial excursions of particle tracers during trappings are compared with those of the width and rugosity of the flow structures.
  • Publication
    Three-dimensional linear peeling-ballooning theory in magnetic fusion devices
    (American Institute of Physics (AIP), 2014-04-23) Weyens, Toon; Sánchez Fernández, Luis Raúl; García Gonzalo, Luis; Loarte, A.; Huijsmans, G.; Comunidad de Madrid; Dirección General de Investigación Científica y Técnica (España)
    Ideal magnetohydrodynamics theory is extended to fully 3D magnetic configurations to investigate the linear stability of intermediate to high n peeling-ballooning modes, with n the toroidal mode number. These are thought to be important for the behavior of edge localized modes and for the limit of the size of the pedestal that governs the high confinement H-mode. The end point of the derivation is a set of coupled second order ordinary differential equations with appropriate boundary conditions that minimize the perturbed energy and that can be solved to find the growth rate of the perturbations. This theory allows of the evaluation of 3D effects on edge plasma stability in tokamaks such as those associated with the toroidal ripple due to the finite number of toroidal field coils, the application of external 3D fields for elm control, local modification of the magnetic field in the vicinity of ferromagnetic components such as the test blanket modules in ITER, etc.
  • Publication
    Fourier signature of filamentary vorticity structures in two-dimensional turbulence
    (IOP Publishing, 2016-08) Reynolds Barredo, José Miguel; Newman, D.E.; Terry, P. W.; Sánchez Fernández, Luis Raúl; Ministerio de Economía y Competitividad (España)
    It is shown that coherent regions of isotropic two-dimensional (2D) turbulence can be clearly identified in the phase part of the Fourier spectrum. Certain spectral phase events are particularly prominent, and are much stronger in the range of wave numbers corresponding to the dissipation range. It is shown that these events are associated with spatially localized filamentary structures in the 2D vorticity field that historically have been related to the intermittency of dissipation. The identified phase signature provides a particularly transparent diagnostic of the temporal evolution of the coherent coupling of disparate scales in anisostropic intermittent dissipative events. These results open the possibility of using the phase of the Fourier transform as a new turbulence diagnostic that identifies and quantitatively characterizes details pertaining to dissipative events.
  • Publication
    Bootstrap current control studies in the Wendelstein 7-X stellarator using the free-plasma-boundary version of the SIESTA MHD equilibrium code
    (IOP Publishing, 2018-02) Peraza Rodríguez, Hugo Alberto; Reynolds Barredo, José Miguel; Sánchez Fernández, Luis Raúl; Tribaldos Macía, Víctor; Geiger, John F.; Ministerio de Economía y Competitividad (España); Universidad Carlos III de Madrid
    The recently developed free-plasma-boundary version of the SIESTA MHD equilibrium code (Hirshman et al 2011 Phys. Plasmas 18 062504; Peraza-Rodriguez et al 2017 Phys. Plasmas 24 082516) is used for the first time to study scenarios with considerable bootstrap currents for the Wendelstein 7-X (W7-X) stellarator. Bootstrap currents in the range of tens of kAs can lead to the formation of unwanted magnetic island chains or stochastic regions within the plasma and alter the boundary rotational transform due to the small shear in W7-X. The latter issue is of relevance since the island divertor operation of W7-X relies on a proper positioning of magnetic island chains at the plasma edge to control the particle and energy exhaust towards the divertor plates. Two scenarios are examined with the new free-plasma-boundary capabilities of SIESTA: a freely evolving bootstrap current one that illustrates the difficulties arising from the dislocation of the boundary islands, and a second one in which off-axis electron cyclotron current drive (ECCD) is applied to compensate the effects of the bootstrap current and keep the island divertor configuration intact. SIESTA finds that off-axis ECCD is indeed able to keep the location and phase of the edge magnetic island chain unchanged, but it may also lead to an undesired stochastization of parts of the confined plasma if the EC deposition radial profile becomes too narrow.
  • Publication
    Optimized implementation of power dispatch in the OPA model and its implications for dispatch sensitivity for the WECC power network
    (ELSEVIER BV, 2020-05) Reynolds Barredo, José Miguel; Newman, D.E.; Carreras Verdaguer, Benjamin Andres; Ministerio de Economía y Competitividad (España); Universidad Carlos III de Madrid
    The social and economic costs of large blackouts in power transmission networks make it critical to properly understand their dynamics. The OPA model was developed with this objective in mind and has previously been applied to power grids of small and medium size, some of them properly modeling realistic cases such as the simplified WECC network, covering the Western region of the US. The bulk of the OPA model's computational cost comes from the repeated solution of a linear programming problem using the Simplex method which is difficult to parallelize. In this paper we introduce important improvements to the modeling part of the linear problem, accelerating the previous implementation by a factor of up to 200, depending on the network. These improvements make it possible, from a practical point of view, to simulate the largest, most detailed, WECC network consisting of 19,402 nodes, reducing the wall-clock time of the simulation from two years to only 10 days. The first simulations show an interesting result: the detailed 19,402 nodes network displays a reduced sensitivity of the dynamics to the dispatch, when compared to the previously used simplified WECC models containing only 1553 and 2504 nodes.
  • Publication
    Extension of the SIESTA MHD equilibrium code to free-plasma-boundary problems
    (AIP Publishing, 2017-08) Peraza Rodríguez, Hugo Alberto; Reynolds Barredo, José Miguel; Sánchez Fernández, Luis Raúl; Geiger, Joachim; Tribaldos Macía, Víctor; Hirshman, S. P.; Cianciosa, M.; Ministerio de Economía y Competitividad (España); Universidad Carlos III de Madrid
    is a recently developed MHD equilibrium code designed to perform fast and accurate calculations of ideal MHD equilibria for three-dimensional magnetic configurations. Since SIESTA does not assume closed magnetic surfaces, the solution can exhibit magnetic islands and stochastic regions. In its original implementation SIESTA addressed only fixed-boundary problems. That is, the shape of the plasma edge, assumed to be a magnetic surface, was kept fixed as the solution iteratively converges to equilibrium. This condition somewhat restricts the possible applications of SIESTA. In this paper, we discuss an extension that will enable SIESTA to address free-plasma-boundary problems, opening up the possibility of investigating problems in which the plasma boundary is perturbed either externally or internally. As an illustration, SIESTA is applied to a configuration of the W7-X stellarator.
  • Publication
    A positioning algorithm for SPH ghost particles in smoothly curved geometries
    (ELSEVIER BV, 2019-06) Vela Vela, Luis Ernesto; Reynolds Barredo, José Miguel; Sánchez Fernández, Luis Raúl; Ministerio de Economía y Competitividad (España)
    An algorithm to place ghost particles across the domain boundary in the context of Smoothed Particle Hydrodynamics (SPH) is derived from basic principles, and constructed for several simple, three-dimensional, geometries. The performance of the algorithm is compared against the more commonly used ‘‘mirrored with respect to the local tangent plane" approach and shown to converge to it whenever the distance of the particles to the reflecting boundary is much smaller than a local measure of the surface’s curvature. The algorithm is demonstrated, tested and compared against the usual approach via simulations of a compressible flow around a cylinder, and the numerical cost of implementing it is addressed. We conclude that use of ghost particles to enforce boundary conditions is not only viable in the presence of smoothly curved boundaries, but more robust than the usual method for low-resolution scenarios.