arXiv subjects

A. H. Nielsen

Publications and source records attributed to A. H. Nielsen.

7 recordsLinked to original sources

arXiv2022

Simulations of scrape-off layer power width for EAST H-mode plasma and ITER 15 MA baseline scenario by 2D electrostatic turbulence code

The scrape-off layer power width (λ_q) is an important parameter for characterizing the divertor heat loads. Many experimental, theoretical, and numerical studies have been performed in recent years. In this paper, a 2D electrostatic turbulence code, BOUT-HESEL, has been upgraded to simulate H-mode plasmas for the first time. The code is validated against the previous implementation and the experiments. The simulated λ_q is found to agree quite well with the Eich scaling for the EAST H-mode discharge. The code is utilized to simulate the ITER 15MA baseline scenario. The ITER simulation reveals that the radial particle/heat transports are dominated by blobby transports, and predicts λ_{q,ITER} = 9.6 mm, which is much larger than the prediction by the Eich scaling. Based on the EAST modified cases, an estimated HESEL H-mode scaling, λ_q=0.51R_c^1.1B_t^(-0.3)q_95^1.1 is proposed. This scaling predicts λ_{q,ITER} = 9.3 mm, which agrees surprisingly well with that for the ITER case. A further investigation combined with the basic parameters in the database of the Eich scaling shows that the missing positive scaling dependence on the machine size (Rc) in the Eich scaling appears to be shaded by the negative scaling dependence on the toroidal magnetic field (Bt) for current devices. This is however not the case for ITER, explaining why simulations in recent studies and in this paper can reproduce the Eich scaling for current devices, but predict a much larger λ_q for ITER. According to the simulation results, the strong positive scaling dependence of λ_q on Rc is due to a combination of slowing down the parallel heat transports by increasing the parallel connection length and the enhancement of the radial ExB turbulent heat transports when the machine size is increased.

physics.plasm-ph↗

arXiv2021

CloudCast: A Satellite-Based Dataset and Baseline for Forecasting Clouds

Forecasting the formation and development of clouds is a central element of modern weather forecasting systems. Incorrect clouds forecasts can lead to major uncertainty in the overall accuracy of weather forecasts due to their intrinsic role in the Earth's climate system. Few studies have tackled this challenging problem from a machine learning point-of-view due to a shortage of high-resolution datasets with many historical observations globally. In this paper, we present a novel satellite-based dataset called ``CloudCast''. It consists of 70,080 images with 10 different cloud types for multiple layers of the atmosphere annotated on a pixel level. The spatial resolution of the dataset is 928 x 1530 pixels (3x3 km per pixel) with 15-min intervals between frames for the period 2017-01-01 to 2018-12-31. All frames are centered and projected over Europe. To supplement the dataset, we conduct an evaluation study with current state-of-the-art video prediction methods such as convolutional long short-term memory networks, generative adversarial networks, and optical flow-based extrapolation methods. As the evaluation of video prediction is difficult in practice, we aim for a thorough evaluation in the spatial and temporal domain. Our benchmark models show promising results but with ample room for improvement. This is the first publicly available global-scale dataset with high-resolution cloud types on a high temporal granularity to the authors' best knowledge.

cs.CV↗

arXiv2018

Radial structure of vorticity in the plasma boundary of ISTTOK tokamak

The first experimental measurements of vorticity and vorticity flux in a fusion device were performed in tokamak ISTTOK. This is an important achievement since vorticity plays a key role in the transport of energy and particles in plasmas and fluids. The measurements were performed with a specifically designed array of Langmuir probes in the plasma edge of the small tokamak ISTTOK. The experimental results presented in this paper, allowing for the first time a direct comparison with theoretical models, show that the vorticity flux feeds into the shear flow in the tokamak plasma edge region. The Probability Distribution Function of the vorticity exhibit fat tails with a q-Gaussian shape typical of a non-equilibrium process. Self-similarity in the probability distribution function of several parameters, including vorticity and vorticity flux, is observed indicating that there is no morphological change in the coherent structures in the plasma boundary region and that the fluctuations in the Reynolds stress, vorticity and vorticity flux follow a universal shape.

physics.plasm-ph↗

arXiv2017

ExB mean flows in finite ion temperature plasmas

The impact of ion pressure dynamics on E x B mean flows is investigated. Using a simplified, two-dimensional, drift ordered fluid model in the thin-layer approximation, three stresses in addition to the Reynolds stress are shown to modify the E x B mean flow. These additional terms in the stress tensor all require ion pressure fluctuations. Quasi-linear analysis show that these additional stresses are as important as the Reynolds stress, and hence must be taken into account in analysis of transport barriers in which sheared E x B mean flows are key ingredients.

physics.plasm-ph↗

arXiv2013

Experimental and numerical characterisation of the turbulence in the Scrape-Off Layer of MAST

Numerical simulations of interchange turbulence in the Scrape-Off Layer are performed in a regime relevant for a specific L-mode MAST (Mega Ampere Spherical Tokamak) discharge. Such a discharge was diagnosed with a reciprocating arm equipped with a Gundestrup probe. A detailed comparison of the average and statistical properties of the simulated and experimental ion saturation current is performed. Good agreement is found in the time averaged radial profile, in the probability distribution functions (PDFs) and in qualitative features of the signals such as the shape, duration and separation of burst events. These results confirm the validity of the simple interchange model used and help to identify where it can be improved. Finally, the simulated data are used to assess the importance of the temperature fluctuations on plasma potential and radial velocity measurements acquired with Langmuir probes. It is shown that the correlation between the actual plasma quantities and the signal of the synthetic diagnostics is poor, suggesting that accurate measurements of the temperature fluctuations are needed in order to obtain reliable estimates of the perpendicular fluxes.

physics.plasm-ph↗

arXiv2004

Shear Flow Generation and Energetics in Electromagnetic Turbulence

Zonal flows are recognised to play a crucial role for magnetised plasma confinement. The genesis of these flows out of turbulent fluctuations is therefore of significant interest. We investigate the relative importance of zonal flow generation mechanisms via the Reynolds stress, Maxwell stress, and geodesic acoustic mode (GAM) transfer in drift-Alfvén turbulence. By means of numerical computations we quantify the energy transfer into zonal flows owing to each of these effects. The importance of the three driving ingredients in electrostatic and electromagnetic turbulence for conditions relevant to the edge of fusion devices is revealed for a broad range of parameters. The Reynolds stress is found to provide a flow drive, while the electromagnetic Maxwell stress is in the cases considered a sink for the flow energy. In the limit of high plasma beta, where electromagnetic effects and Alfvén dynamics are important, the Maxwell stress is found to cancel the Reynolds stress to a high degree. The geodesic oscillations, related to equilibrium pressure profile modifications due to poloidally asymmetric transport, can act as both sinks as drive terms, depending on the parameter regime. For high beta cases the GAMs are the main drive of the flow. This is also reflected in the frequency dependence of the flow, showing a distinct peak at the GAM frequency in that regime.

physics.plasm-ph↗

arXiv2004

Sharp vorticity gradients in two-dimensional hydrodynamic turbulence

The appearance of sharp vorticity gradients in two-dimensional hydrodynamic turbulence and their influence on the turbulent spectra is considered. We have developed the analog of the vortex line representation as a transformation to the curvilinear system of coordinates moving together with the di-vorticity lines. Compressibility of this mapping can be considered as the main reason for the formation of the vorticity discontinuities at high Reynolds numbers. For two-dimensional turbulence in the case of strong anisotropy the vorticity discontinuities can generate spectra with the fall-off at large $k$ proportional to $k^{-3}$ resembling the Kraichnan spectrum for the enstrophy cascade. For turbulence with weak anisotropy the $k$ dependence of the spectrum due to discontinuities coincides with that of the Saffman spectrum: $k^{-4}$. We have compared the analytical predictions with direct numerical solutions of the two-dimensional Euler equation for decaying turbulence. We observe that the di-vorticity is reaching very high values and is distributed locally in space along piecewise straight lines. Thus, indicating strong anisotropy and accordingly we found a spectrum close to the $k^{-3}$-spectrum.

physics.flu-dyn↗