Transport barriers at the plasma edge are key elements of high confinement regimes in fusion devices. In typical configurations, such barriers are not stable but exhibit quasi-periodic relaxation ...oscillations. In this work, one-dimensional and zero-dimensional models for such oscillations are presented that give insight into the underlying mechanisms which are found to be intrinsically nonlinear. The models are systematically derived from three-dimensional turbulence simulations based on a fluid description of the plasma. In these simulations, a transport barrier is generated by an imposed E × B shear flow. This barrier exhibits quasiperiodic relaxation oscillations even if the E × B shear flow is frozen. The models presented here are therefore different from those based on turbulent shear flow generation. They allow to isolate and illustrate a different mechanism for barrier oscillations. Furthermore, these models reproduce regimes characterized by a decrease of the oscillation frequency with the E × B flow shear.
A major goal in fusion research is understanding the anomalous thermal transport of energy and particles in a tokamak. In particular, a crucial issue is the origin of its empirical dependence on the ...various dimensionless parameters controlling the energy confinement. This paper addresses the impact of ion-ion collisions and of self-generated zonal flows (largescale E × B sheared poloidal flows) on turbulent ion thermal transport, in the core of a tokamak plasma. A three-dimensional fluid model is used, which describes flux-driven electrostatic plasma turbulence, generated by Ion Temperature Gradient (ITG) instabilities. The model includes curvature effects, parallel Landau damping and a collisional damping of the poloidal flows. Simulation results show a stabilization of the turbulence and a rise of the energy confinement time when the collisionality is lowered, that is when the zonal flows are weakly damped by ion-ion collisions. The mechanism responsible for the turbulence stabilization at low collisionality is identified as the non-linear upshift of the effective threshold for the ITG turbulence onset. This upshift is governed by an increase of the zonal flow shear.
The spontaneous pedestal formation above a power threshold at the edge of magnetically confined plasma is modelled for the first time in flux driven three-dimensional fluid simulations of ...electromagnetic turbulence with the code EMEDGE3D. The role of the collisional friction between trapped and passing particles is shown to be the key ingredient for shearing the radial electric field, hence stabilizing the turbulence, rather than the Maxwell and Reynolds stresses. The isotope effect, observed in many tokamaks worldwide, is recovered in EMEDGE3D simulations: the power threshold for pedestal formation is lower for Tritium than for Deuterium. The turbulence auto-correlation time is found to increase with the ion mass easing the radial electric shear stabilization, hence the pedestal formation.
To understand how massive galaxies at high-\(z\) co-evolve with enormous reservoirs of halo gas, it is essential to study the coldest phase of the circum-galactic medium (CGM), which directly relates ...to stellar growth. The SUPERCOLD-CGM survey is the first statistical survey of cold molecular gas on CGM scales. We present ALMA+ACA observations of CO(4-3) and continuum emission from 10 Enormous Ly\(\alpha\) Nebula (ELANe) around ultraluminous type-I QSOs at \(z\sim2\). We detect CO(4-3) in 100\(\%\) of our targets, with 60\(\%\) showing extended CO on scales of 15\(-\)100 kpc. Q1228+3128 reveals the most extended CO(4-3) reservoir of \(\sim\)100 kpc and is the only radio-loud target in our sample. The CO reservoir is located along the radio axis, which could indicate a link between the inner radio-jet and cold halo gas. For the other five radio-quiet ELANe, four of them show extended CO(4-3) predominantly in the direction of their companions. These extended CO(4-3) reservoirs identify enrichment of the CGM, and may potentially contribute to widespread star formation. However, there is no evidence from CO(4-3) for diffuse molecular gas spread across the full extent of the Ly\(\alpha\) nebulae. One target in our sample (Q0107) shows significant evidence for a massive CO disk associated with the QSO. Moreover, 70\(\%\) of our QSO fields contain at least one CO companion, two of which reveal extended CO emission outside the ELANe. Our results provide insight into roles of both the cold CGM and companions in driving the early evolution of massive galaxies.
Knowledge of the Earth's atmospheric optical turbulence is critical for astronomical instrumentation. Not only does it enable performance verification and optimisation of existing systems but it is ...required for the design of future instruments. As a minimum this includes integrated astro-atmospheric parameters such as seeing, coherence time and isoplanatic angle, but for more sophisticated systems such as wide field adaptive optics enabled instrumentation the vertical structure of the turbulence is also required. Stereo-SCIDAR is a technique specifically designed to characterise the Earth's atmospheric turbulence with high altitude resolution and high sensitivity. Together with ESO, Durham University has commissioned a Stereo-SCIDAR instrument at Cerro Paranal, Chile, the site of the Very Large Telescope (VLT), and only 20~km from the site of the future Extremely Large Telescope (ELT). Here we provide results from the first 18 months of operation at ESO Paranal including instrument comparisons and atmospheric statistics. Based on a sample of 83 nights spread over 22 months covering all seasons, we find the median seeing to be 0.64" with 50% of the turbulence confined to an altitude below 2 km and 40% below 600 m. The median coherence time and isoplanatic angle are found as 4.18 ms and 1.75" respectively. A substantial campaign of inter-instrument comparison was also undertaken to assure the validity of the data. The Stereo-SCIDAR profiles (optical turbulence strength and velocity as a function of altitude) have been compared with the Surface-Layer SLODAR, MASS-DIMM and the ECMWF weather forecast model. The correlation coefficients are between 0.61 (isoplanatic angle) and 0.84 (seeing).
A novel and generic understanding of spontaneous generation of transport barriers and zonation regimes in turbulent self-organization is presented. It associates the barrier onset to the development ...of a spectral gap between large scale flows and turbulence modes leading to a zonation regime. A robust barrier builds-up at the interface of such a region and a neighboring one with reduced zonal flow generation. This more complex and generic transition paradigm could fit the numerous and sometimes conflicting observations as in fusion plasma experiments. Barrier relaxation by bursts of turbulence regenerate the zonal flows that are eroded by viscous (collisional-like) damping. The duration of the quiescent phase between the quasi-periodic relaxation events is governed by this damping process, hence the barrier collision frequency for fusion plasmas.
The Projection on Proper elements (PoPe) is a novel method of code control dedicated to 1) checking the correct implementation of models, 2) determining the convergence of numerical methods and 3) ...characterizing the residual errors of any given solution at very low cost. The basic idea is to establish a bijection between a simulation and a set of equations that generate it. Recovering equations is direct and relies on a statistical measure of the weight of the various operators. This method can be used in any dimensions and any regime, including chaotic ones. This method also provides a procedure to design reduced models and quantify the ratio costs to benefits. PoPe is applied to a kinetic and a fluid code of plasma turbulence.
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