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One of our grand challenges towards fusion energy is the achievement of a high-performance plasma core coupled to a boundary solution. The high confinement mode (H-mode) provides such a high-performance fusion core due to the build-up of an edge transport barrier leading to a pedestal. However, it usually features type-I edge localized modes (ELMs) which pose a threat for long-duration plasma operation in future fusion devices as they induce large energy fluences onto the plasma facing components and typically are projected to damage the first wall. For future fusion devices, the integration of a stationary no-ELM regime with a power exhaust solution is indispensable. Several no-ELM and small-ELM regimes have extended their operational space in the past years, with the ultimate goal of providing an alternative core–edge solution to ITER and EU-DEMO. Prominent no-ELM or small-ELM alternatives include the I-mode, QH-mode, EDA H-mode, quasi-continuous exhaust (QCE) and ‘grassy’ ELM regimes, X-point radiator scenarios and negative triangularity L-mode. The state-of-the-art, including access conditions and main signatures, of these alternative regimes is reviewed. Many of these regimes partly match the operational space of ITER and EU-DEMO, however, knowledge gaps remain. Besides compatibility with divertor detachment and a radiative mantle, these include extrapolations to high Q operations, low core collisionality, high Greenwald fractions, impurity transport, amongst others. The knowledge gaps and possible strategies to close these gaps to show their applicability to ITER and EU-DEMO are discussed.

Periodic edge localized modes (ELMs) are the non-linear consequences of pressure-gradient-driven ballooning modes and current-driven peeling modes becoming unstable in the pedestal region of high confinement fusion plasmas. In future tokamaks like ITER, large ELMs are foreseen to severely affect the lifetime of wall components as they transiently deposit large amounts of heat onto a narrow region at the divertor targets. Several strategies exist for avoidance, suppression, or mitigation of these instabilities, such as the naturally ELM-free quiescent H-mode (QH-mode). In the present article, an ASDEX Upgrade (AUG) equilibrium that features a QH-mode is investigated through non-linear extended magneto-hydrodynamic simulations covering the dynamics over tens of milliseconds. The equilibrium is close to the ideal peeling limit and non-linearly develops saturated modes at the edge of the plasma. A dominant toroidal mode number of n = 1 is found, for which the characteristic features of the edge harmonic oscillation are recovered. The saturated modes contribute to heat and particle transport preventing pedestal build-up to the ELM triggering threshold. The non-linear dynamics of the mode, in particular its interaction with the evolution of the edge safety factor, are studied, and suggest a possible new saturation mechanism for the QH-mode. The simulations show good qualitative and quantitative agreement with experiments in AUG. In particular, the processes leading to the termination of QH-mode above a density threshold are studied, which results in the transition into an ELM regime. In the vicinity of this threshold, limit cycle oscillations are observed.

Nuclear fusion, the energy source of the stars, is expected to be a clean and abundant source of energy on Earth in the near future. However, fusion reactors have to face different challenges due to the high temperatures required to reach ignition. One of the major concerns is the integrity of the first wall of the machine. If enough heat is transported from the fuel, which is in plasma state, to the wall of the reactor, the integrity of the machine could be at stake. The injection of impurities in the plasma has shown to reduce the heat loads in the reactor walls via radiation. Hence, understanding the physics of these impurities and their distribution along the plasma edge is a requirement for future fusion devices. Furthermore, some impurity species have been shown to be beneficial for the plasma confinement and thus, for fusion performance. The work presented in this thesis is based on the Charge eXchange Recombination Spectroscopy (CXRS) technique. This technique exploits the light emitted after the charge transfer between injected neutrals and ionized impurities of the plasma. The emitted light gives information on the temperature, rotation and density of the observed species. Normally, the CXRS measurements are taken at the Low Field Side (LFS). However, this work is focussed on the study of the impurity properties at two different poloidal locations. For this purpose, the High Field Side (HFS) gas puff based CXRS system of the ASDEX Upgrade tokamak has been upgraded. This diagnostic injects thermal neutrals into the plasma to produce the charge exchange reactions. A new piezo driven gas valve provides higher signal to noise ratio and a better characterization of the background light. The upgraded optical heads have 16 lines of sight (LOS) each, covering around 7 cm of the HFS edge region. While impurity temperature and rotation can be obtained directly from the emitted light, the evaluation of the impurity density requires information on the injected neutral

We present a non-parametric inference of impurity transport coefficients by using charge exchange recombination spectroscopy measurements of Ne X, Ne VIII, O VIII, and C VI lines. Due to their close atomic numbers, neon, oxygen and carbon impurity ions are assumed to have the same diffusion coefficient D and convection velocity v. Unlike conventional techniques that modulate or perturb the impurity contents, we employ a quasi-stationary plasma with static impurity profiles. Since the ratio of v to D only describes the equilibrated profile of the sum of all impurity charge states, steady-state measurements can still decouple D and v if different charge states are simultaneously observed. We have formulated a non-parametric analysis framework based on the Bayesian probability theory and conducted transport coefficient measurements for a Type III ELMy H-mode plasma at ASDEX Upgrade. The charge exchange reactions with the background neutrals, which are known to affect the impurity charge state balance, are taken into account by introducing additional free parameters. While D at the pedestal is close to the neoclassical level ( < 1 m s-2), a large diffusion coefficient and a strong outward convection are inferred right inside the pedestal top.

DIII-D physics research addresses critical challenges for the operation of ITER and the next generation of fusion energy devices. This is done through a focus on innovations to provide solutions for high performance long pulse operation, coupled with fundamental plasma physics understanding and model validation, to drive scenario development by integrating high performance core and boundary plasmas. Substantial increases in off-axis current drive efficiency from an innovative top launch system for EC power, and in pressure broadening for Alfven eigenmode control from a co-/counter-Ip steerable off-axis neutral beam, all improve the prospects for optimization of future long pulse/steady state high performance tokamak operation. Fundamental studies into the modes that drive the evolution of the pedestal pressure profile and electron vs ion heat flux validate predictive models of pedestal recovery after ELMs. Understanding the physics mechanisms of ELM control and density pumpout by 3D magnetic perturbation fields leads to confident predictions for ITER and future devices. Validated modeling of high-Z shattered pellet injection for disruption mitigation, runaway electron dissipation, and techniques for disruption prediction and avoidance including machine learning, give confidence in handling disruptivity for future devices. For the non-nuclear phase of ITER, two actuators are identified to lower the L–H threshold power in hydrogen plasmas. With this physics understanding and suite of capabilities, a high poloidal beta optimized-core scenario with an internal transport barrier that projects nearly to Q = 10 in ITER at ∼8 MA was coupled to a detached divertor, and a near super H-mode optimized-pedestal scenario with co-Ip beam injection was coupled to a radiative divertor. The hybrid core scenario was achieved directly, without the need for anomalous current diffusion, using off-axis current drive actuators. Also, a controller to assess proximity to stability limits and

The SMall Aspect Ratio Tokamak is a spherical tokamak that is in building process in the University of Seville. Various magnetic diagnostic systems based on inductive sensors have been designed for their installation in this tokamak. Among them there is a set of Rogowski coils that are employed to measure the plasma current and the Eddy currents in the walls of the vacuum chamber. Moreover there is also an array of Mirnov coils used for the plasma equilibrium reconstruction and another set for the detection of unstable modes of up to 200kHz of frequency. These diagnostics are described in detail and so are the considerations taken into account for their design along with a proposal for their calibration., El SMall Aspect Ratio Tokamak es un tokamak esférico que se encuentra en proceso de construcción en la Universidad de Sevilla. Varios sistemas de diagnóstico magnético basados en sensores inductivos han sido diseñados para su instalación en este tokamak. Entre ellos se incluye un conjunto de bobinas de Rogowski empleadas para medir la corriente del plasma y las corrientes de Eddy en las paredes de la cámara de vacío. También se incluye un conjunto de bobinas de Mirnov utilizadas para la reconstrucción del equilibrio del plasma y otro conjunto para la detección de modos inestables de hasta 200kHz de frecuencia. Estos diagnósticos son descritos en detalle y también lo son las consideraciones tenidas en cuenta para su diseño así como una propuesta para su calibración.

A new method was developed to model the neutral population produced by the gas puff based charge exchange recombination spectroscopy systems at ASDEX Upgrade (AUG). With this method, the edge impurity density on the high field side (HFS) and low field side (LFS) can be obtained without the need to apply a neutral beam injection system. The neutral penetration needed for the calculation of the impurity density is obtained with a new gas puff module implemented in the FIDASIM code. The LFS impurity density profile evaluated with the new gas puff module matches the impurity density calculated with standard beam-based charge exchange diagnostics. Impurity temperature, rotation and density profiles at the HFS and LFS of an AUG H-mode discharge are presented. Edge impurity toroidal and poloidal flows show asymmetric structures. The impurity density asymmetries obtained with the new gas puff module are consistent with the observed flow structure.

El método GP-CXRS (del inglés "Gas Pu -Charge Exchange Recombination Spectros- copy") es una técnica que permite conocer la densidad, velocidad de rotación y temperatura de las impurezas del plasma. Este método se basa en el uso de un gas de neutrales que al interactuar con los iones del plasma los excita, mediante intercambio de carga, y se emiten fotones que pueden ser capturados mediante distintos dispositivos ópticos. En este trabajo hemos desarrollado tres métodos que permiten caracterizar los espectros obtenidos mediante este diagnóstico; el primero de ellos consiste en ajustar el espectro incluyendo las líneas moleculares de los neutrales, en nuestro caso moléculas de deuterio, el segundo de ellos trata de eliminar estas líneas para posteriormente ajustar el espectro y el último de ellos considera únicamente la presencia del ion emisor del espectro. Estos tres métodos se han aplicado al espectro de emisión del nitrógeno totalmente ionizado (NVII)a y se han obtenido los per les de intensidad, velocidad de rotación y temperatura de este ion. Los resultados muestran que el método que mejor consigue ajustar el espectro es el que realiza el ajuste considerando las líneas moleculares; sin embargo, resultados muy similares se obtienen en los diferentes per les independientemente del método utilizado. Las principales diferencias se encuentran en regiones externas a la separatrix, última super cie cerrada con ujo magnético constante, por lo que para un diagnóstico centrado en regiones internas a la separatrix se concluye que el método que sólo considera al ion emisor es el más adecuado debido a su simpleza. Por otra parte, si se considera un diagnóstico centrado en regiones externas a la separatrix, se recomiendan los métodos que consideran las líneas moleculares, teniendo el primero de ellos el respaldo de que ajusta mejor los espectros.

The dynamic behaviour of the ion and electron energy, particle and momentum transport measured during type-I edge localized mode (ELM) cycles at ASDEX Upgrade is presented. Fast measurements of the ion and electron temperature profiles revelead that the ion and electron energy transport recover on different timescales, with the electrons recovering on a slower timescale (Cavedon et al 2017 Plasma Phys. Control. Fusion 59 105007). The dominant mechanism for the additional energy transport in the electron channel that could cause the delay in the electron temperature gradient (VTe) recovery is attributed to the depletion of energy caused by the ELM. The local sources and sinks for the electron channel in the steep gradient region are much smaller compared to the energy flux arriving from the pedestal top, indicating that the core plasma may dictate the local dynamics of the VTe recovery during the ELM cycle. A model for the edge momentum transport based on toroidal torque balance that takes into account the existence of poloidal impurity asymmetries has been developed. The analysis of the profile evolution during the ELM cycle shows that the model captures the dynamics of the rotation both before the ELM crash and during the recovery phase.

Within the EUROfusion MST1 Work Package, a series of experiments has been conducted on AUG and TCV devices to disentangle the role of plasma fueling and plasma shape for the onset of small ELM regimes. On both devices, small ELM regimes with high confinement are achieved if and only if two conditions are fulfilled at the same time. Firstly, the plasma density at the separatrix must be large enough (ne,sep/nG ∼ 0.3), leading to a pressure profile flattening at the separatrix, which stabilizes type-I ELMs. Secondly, the magnetic configuration has to be close to a Double Null (DN), leading to a reduction of the magnetic shear in the extreme vicinity of the separatrix. As a consequence, its stabilizing effect on ballooning modes is weakened.

The upgrade of the high field side (HFS) edge charge exchange recombination spectroscopy (CXRS) system of ASDEX Upgrade is presented. This diagnostic provides temperature, rotation and radiance measurements of impurity species by taking advantage of the gas puff based CXRS technique (GP-CXRS). The system is formed by a fast piezoelectric valve, that injects thermal neutrals into the plasma, and two optical heads. The localized gas injection together with properly aligned lines of sights (LOS) lead to a high spatial resolution of 5–19 mm. Fast gas puff modulation allows a precise subtraction of the passive part of the signal. The existing poloidal optical head has been replaced with a new one to increase the radial resolution. The number of lines of sight (LOS) of the poloidal optical head has been increased from 8 to 16 covering around 7 cm of the plasma edge at the HFS. The same radial range is also viewed by a toroidal optical head. The neutral deposition, needed to calculate the impurity density profile, has been modelled using the FIDASIM code. A realistic gas puff geometry has been implemented in the code. The first measurements of impurity temperature, rotation and radiance utilizing the upgraded diagnostic are presented.

The high confinement mode (H-mode) is a very important regime for future fusion devices. In this regime, the global confinement is increased and a pedestal structure is developed in the profiles. However, a complete understanding of how it is formed is still missing. In this regime, the toroidal impurity velocity profile exhibits a local minimum close to the separatrix and, under certain conditions, the impurities at the plasma edge can rotate in the opposite direction compared to the plasma core. A pedestal database was compiled with data from ASDEX Upgrade to try to progress in understanding the pedestal physics. This thesis is focussed on the study of the ion temperature and toroidal impurity velocity profiles, obtained with the charge exchange recombination spectroscopy system, at low and high collisionality. A correlation between the characteristics of the pedestal with the minimum in the toroidal impurity velocity was studied. It has been observed that the minimum in the toroidal impurity velocity reaches negative values in low collisionality discharges, while it is positive in high collisionality discharges. Moreover, the position of the minimum in the toroidal impurity velocity is correlated with the position of the ion temperature and density pedestal tops in high collisionality discharges, while it is only correlated with the position of the ion temperature pedestal top in low collisionality discharges