Papers by Daniel López-Bruna
arXiv (Cornell University), Jan 9, 2023
In this paper, we analyze the results of neutral-beam current drive (NBCD) experiments, performed... more In this paper, we analyze the results of neutral-beam current drive (NBCD) experiments, performed in the TJ-II stellarator, with the aim of validating the theoretical predictions. Both parallel and anti-parallel injection with respect to the magnetic field were explored using co (NBI1) and counter (NBI2) beams at different injected beam power and plasma densities. The fast-ion current driven by both beams was simulated with the Monte Carlo code ASCOT and the electron response to the fast-ion current was calculated analytically using a model valid for an arbitrary magnetic configuration and a low collisionality plasma. The model reproduces with rather good agreement the toroidal current measured in NBI2 plasmas while the current driven by NBI1 is less than half the predicted one. Possible reasons for this discrepancy are discussed.
Se describe, analiza y prueba un modelo general de formacion de barreras de transporte en disposi... more Se describe, analiza y prueba un modelo general de formacion de barreras de transporte en dispositivos de fusion por confinamiento magnetico. El modelo explica la generalidad del fenomeno de las bifurcaciones a regimenes de confinamiento mejorado como las observadas en numerosos dispositivos de ese tipo. El fenomeno asemeja una transicion de fase termodinamica. El modelo reproduce las facetas mas generales del proceso y predice soluciones del tipo ciclo limite compatibles con las observaciones experimentales. Sus ingredientes basicos son la definicion de un campo escalar relacionado con la envolvente del nivel de fluctuaciones (e.g.de la densidad), que evoluciona en base a sus mecasnismos de crecimiento, saturacion y supresion. Este campo es responsable del transporte anomalo. La evolucion del transporte anomalo cierra un lazo de retroalimentacion entre los perfiles termodinamicos del plasma capaz de dar lugar a la formacion de barreras de transporte. La generalidad del modelo permi...
Heat and particle transport in fusion plasmas is generally due to turbulent processes associated ... more Heat and particle transport in fusion plasmas is generally due to turbulent processes associated with small-scale instabilities driven by the inhomogeneity in plasma profiles. The magnitude of turbulent transport is probably the dominant parameter affecting the global confinement properties and hence the understanding of turbulence is a fundamental issue for the success of magnetic nuclear fusion. At the same time, the development of inverse cascade of turbulent energy spectra and condensation of the spectrum can form meso-scale structures that can control turbulent radial diffusion. Among these structures zonal flows (ZFs) and geodesic acoustic modes (GAMs) are known to play a role in regulating plasma turbulence and consequently anomalous transport [1, 2 and references therein]. In addition, ZFs-like structures can also be generated by fast particles and MHD instabilities [3]. Also, it
Experiments in the TJ-II heliac show a correlation between the position of magnetic rational surf... more Experiments in the TJ-II heliac show a correlation between the position of magnetic rational surfaces and a modification of the electron temperature profile [1], measured using Electron Cyclotron Emission (ECE) in low density Electron Cyclotron Resonance heated discharges. On plasma discharges heated using Neutral Beam Injection, ECE cannot be used due to the high density; however, using the heliac’s flexibility, different rational surfaces can be swept along the minor radius by varying the helical current and it was found that the transport was reduced at the position of the rational surfaces. Bolometry studies have also shown a correlation between transport barriers appearing on rational surfaces and MHD activity [2]. This may also lead to transitions of the L-H type. These observations suggest that the plasma confinement can be improved by strategically placing rational surfaces, which in in turn give rise to a transport barrier. Since magnetic islands are likely to form at ratio...
Transport analysis is of great importance for interpreting and predicting the plasma behavior in ... more Transport analysis is of great importance for interpreting and predicting the plasma behavior in fusion plasmas. The main outcome is transport related magnitudes, like confinement times and transport coefficients. However, the difficulty of applying such analysis arises from the need to correctly obtain the profiles of the particle and heat sources, which are often strongly dependent on the input parameters, such as gas puffing, recycling and power efficiency coefficients. This work aims to study NBI discharges, using ASTRA (Automated System for TRansport Analysis) and other peripheral codes to obtain the particle confinement time, τp, for different plasma scenarios. In particular, the focus is given to both, finding the sources, and investigating the dependence of the particle confinement time on the electron density, to check the expectation that higher densities accompany higher confinement times [1]. For this purpose a set of discharges with lithiumand boron-coated walls and wit...
i scan of ECH plasmas based on Thomson Scattering data [2]. Differences between the ¯ i in vacuum... more i scan of ECH plasmas based on Thomson Scattering data [2]. Differences between the ¯ i in vacuum and in the presence of the plasma can come from internal currents. A way to overcome this uncertainty is studying discharges with induced currents much larger than the bootstrap one. In this way, the changes in electron temperature (Te) gradients can be confidently related to the moving position of the magnetic resonances.
This paper presents an overview of experimental results and progress made in investigating partic... more This paper presents an overview of experimental results and progress made in investigating particle control using Li-coating, transport and L-H transitions in TJ-II. The Li coating changes drastically the plasmawall interaction, decreasing the recycling, and enlarges substantially the operational range of the device avoiding the appearance of radiative collapse, which permits to study confinement properties at much higher densities. Moreover, L-H mode transition has been only achieved after Li-coating in TJ-II. The effect of rationals on heat transport is studied showing a decrease of heat diffusivity close to their position, and it is also seen that rationals located in the edge make L-H transition easier. TJ-II findings provide a new guideline for understanding the trigger mechanism of the L-H transition pointing out the importance of low frequency fluctuating sheared ExB flows. The properties of fast ion confinement are also described as well as the effects of impurities on radia...
SE RESUELVE LA ECUACION CINETICA DE PLASMAS EN LA APROXIMACION DE CENTRO GUIA (5-D) CON EL OBJETO... more SE RESUELVE LA ECUACION CINETICA DE PLASMAS EN LA APROXIMACION DE CENTRO GUIA (5-D) CON EL OBJETO DE SIMULAR PLASMAS ATRAPADOS EN DISPOSITIVOS DE CONFINAMIENTO MAGNETICO PARA LA FUSION TERMONUCLEAR. EL TRABAJO DESCRIBE TANTO EL MARCO TEORICO DEL MODELO COMO LOS METODOS DE DISCRETIZACION EN LAS TRES DIMENSIONES ESPACIALES (SDM), DOS EN VELOCIDAD (ESPECTRAL) Y UNA TEMPORAL (METODOS DIRECTOS E INDIRECTOS DE AVANCE, ESTACIONARIOS). LA MODELIZACION TEORICA Y LA DISCRETIZACION SE HAN REUNIDO EN UN CODIGO QUE PERMITE SIMULAR LOS PLASMAS EN LAS CONDICIONES MENCIONADAS. SE HAN LLEVADO A CABO DOS TIPOS DE SIMULACIONES: - COMPARACIONES PARA TOKAMAK AXISIMETRICO FRENTE A LA TEORIA NEOCLASICA. - PRIMEROS CALCULOS PARA DEMOSTRAR EL OBJETIVO PRINCIPAL DEL CODIGO: SU CAPACIDAD PARA RESOLVER GEOMETRIAS MAGNETICAS TAN COMPLEJAS COMO LA DEL HELIAC TJ-II, EL DISPOSITIVO EXPERIMENTAL DEL LABORATORIO NACIONAL DE FUSION (CIEMAT, MADRID). POR ULTIMO, SE DETALLA EL SOFTWARE IMPLEMENTADO PARA GENERAR MALLAS ...
The influence of plasma density and edge gradients in the development of perpendicular sheared fl... more The influence of plasma density and edge gradients in the development of perpendicular sheared flows and radial electric fields, has been investigated in the plasma edge region of the TJ-II stellarator. Experimental results show that the development of the “naturally occurring” velocity shear layer requires a minimum plasma density (or gradient). Near this critical density, the level of edge turbulent transport and the turbulent kinetic energy significantly increase in the plasma edge; once sheared flows are fully developed the level of fluctuations and turbulent transport slightly decreases whereas edge gradients and plasma density increase. Furthermore, the resulting shearing rate of the spontaneous sheared flows turns out to be close to the one needed to trigger a transition to an improved confinement regimen (H-mode like regimes), suggesting that spontaneous sheared flows and fluctuations keep themselves near marginal stability. These findings provide the first experimental evid...
The relaxation of the constraint of periodicity imposed by the external confining magnetic field ... more The relaxation of the constraint of periodicity imposed by the external confining magnetic field coils in a nominally 4-field period Helias Advanced Stellarator configuration produces weak periodicity-breaking deformations of the plasma. The corrugations are driven by the interaction of the pressure gradient with the magnetic field line curvature and correspond to saturated ideal magnetohydrodynamic interchanges with a mode structure dominated by nonresonant m = 1, n = ±1 Fourier components. Very similar low order mode number oscillations are observed in the 4-field period TJ-II Heliac stellarator. The conditions of quasi-isodynamicity of the Helias reactor system investigated are not significantly altered by the periodicity-breaking distortions.
Nuclear Fusion
The relaxation of the constraint of periodicity imposed by the external confining magnetic field ... more The relaxation of the constraint of periodicity imposed by the external confining magnetic field coils in a nominally 4-field period Helias Advanced Stellarator configuration produces weak periodicity-breaking deformations of the plasma. The corrugations are driven by the interaction of the pressure gradient with the magnetic field line curvature and correspond to saturated ideal magnetohydrodynamic interchanges with a mode structure dominated by nonresonant m = 1, n = ±1 Fourier components. Similar low order mode number oscillations are observed in the 4-field period TJ-II Heliac stellarator [Alejaldre et al., Fusion Technol. 17 131 (1990)]. The conditions of quasi-isodynamicity of the Helias reactor system investigated are not significantly altered by the periodicity-breaking distortions.
Nuclear Fusion
In toroidal magnetic confinement fusion research, one-dimensional (1D) transport models rely on o... more In toroidal magnetic confinement fusion research, one-dimensional (1D) transport models rely on one radial coordinate that labels nested toroidal flux surfaces. The presence of magnetic islands in the magnetic geometry does not impede making 1D transport calculations if the island regions are excluded and then, if necessary, treated separately. In this work we show a simple way to modify the flux-surface coordinate and corresponding metric coefficients when an island region is excluded. Comparison with the metrics obtained from Poincaré plots are shown, as well as applications to two types of plasma: Heliac (TJ-II, CIEMAT, Spain), where the geometrical effects alone cannot explain the experimental results when islands move throughout minor radius; and Heliotron (LHD, NIFS, Japan), where we estimate the effect of possible heat losses in flux-gradient relations.
Nuclear Fusion
This paper reports the main recent results of the RFX-mod fusion science activity. The RFX-mod de... more This paper reports the main recent results of the RFX-mod fusion science activity. The RFX-mod device is characterized by a unique flexibility in terms of accessible magnetic configurations. Axisymmetric and helically shaped Reversed-field pinch equilibria have been studied, along with tokamak plasmas in a wide range of q(a) regimes (spanning from 4 down to 1.2 values). The full range of magnetic configurations in between the two, the so-called ultra low-q ones, has been explored, with the aim of studying specific physical issues common to all equilibria, such as, for example, the density limit phenomenon. The powerful RFX-mod feedback control system has been exploited for MHD control, which allowed to extend the range of experimental parameters, as well as to induce specific magnetic perturbations for the study of 3D effects. In particular, transport, edge and isotope effect in 3D equilibria have been investigated, along with runaway mitigations through induced magnetic perturbations. The first transitions to an improved confinement scenario in circular and D-shaped tokamak plasmas have been obtained thanks to an active modification of the edge electric field through a polarized electrode. The experiments are supported by intense modelling with 3D MHD, gyrokinetic, guiding center and transport codes. Proposed modifications to the RFX-mod device, which will enable further contributions to the solution of key issues in the roadmap to ITER and DEMO, are also briefly presented.
Nuclear Fusion
In this contribution we propose a new method to analyze the modulation electron cyclotron resonan... more In this contribution we propose a new method to analyze the modulation electron cyclotron resonance heating (MECH) experiment, aiming to examine the classical local transport model. The method is applied to the MECH experiments performed in various helical and tokamak devices, i.e., the Large Helical Device (LHD), the TJ-II, the Korea Superconducting Tokamak Advanced Research (KSTAR), and the Doublet III-D (DIII-D) with different plasma conditions. The thermal diffusivity and the convective velocity are obtained not only at the fundamental MECH frequency but also at its higher harmonics, as well as from both outward and inward propagating pulses, providing different transport coefficients at a radial region. Results clearly show universality of the violation of the classical local model.
Nuclear Fusion
New observations in the Large Helical Device (LHD) show that the magnetic islands externally impo... more New observations in the Large Helical Device (LHD) show that the magnetic islands externally imposed by m/n = 1/1 resonant magnetic perturbation (RMP) can be maintained in an intermediate state with a finite phase shift away from the value present in vacuum. Given the previous experimental observation that the saturated magnetic islands show either growth or healing, the intermediate states are realized in the "healing region" in the beta and collisionality space, which implies that a parameter, other than beta and collisionality should exist in order to determine the island state. Theories based on the competition between electromagnetic torques and poloidal flow-induced viscous torques provide a prediction for the intermediate state. These two kinds of torques might be balanced to realize the steadily maintained intermediate state whereas the islands are placed in growth or healing state in the case in which the balance is broken. The experimental observation shows that there is a possibility for the magnetic island phase to deviate from its designed position. If the parameters are controlled properly, it is possible to control the phase of the magnetic island, which may permit continued utilization of the island divertor concept.
Nuclear Fusion
The effects of 3D geometry are explored in TJ-II from two relevant points of view: neoclassical t... more 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 Alfvén Eigenmodes properties are also explored, showing the possibility of controlling Alfvén 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.
Plasma Physics and Controlled Fusion, 2016
We study radial particle transport in stellarator plasmas using cryogenic pellet injection. By me... more We study radial particle transport in stellarator plasmas using cryogenic pellet injection. By means of perturbative experiments, we estimate the experimental particle flux and compare it with neoclassical simulations. Experimental evidence is obtained of the fact that core depletion in helical devices can be slowed-down even by pellets that do not reach the core region. This phenomenon is well captured by neoclassical predictions with DKES and FORTEC-3D.
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Papers by Daniel López-Bruna