86 research outputs found
DREAM: a fluid-kinetic framework for tokamak disruption runaway electron simulations
Avoidance of the harmful effects of runaway electrons (REs) in
plasma-terminating disruptions is pivotal in the design of safety systems for
magnetic fusion devices. Here, we describe a computationally efficient
numerical tool, that allows for self-consistent simulations of plasma cooling
and associated RE dynamics during disruptions. It solves flux-surface averaged
transport equations for the plasma density, temperature and poloidal flux,
using a bounce-averaged kinetic equation to self-consistently provide the
electron current, heat, density and RE evolution, as well as the electron
distribution function. As an example, we consider disruption scenarios with
material injection and compare the electron dynamics resolved with different
levels of complexity, from fully kinetic to fluid modes.Comment: 32 pages, 11 figure
Runaway dynamics in the DT phase of ITER operations in the presence of massive material injection
A runaway avalanche can result in a conversion of the initial plasma current
into a relativistic electron beam in high current tokamak disruptions. We
investigate the effect of massive material injection of deuterium-noble gas
mixtures on the coupled dynamics of runaway generation, resistive diffusion of
the electric field, and temperature evolution during disruptions in the DT
phase of ITER operations. We explore the dynamics over a wide range of injected
concentrations and find substantial runaway currents, unless the current quench
time is intolerably long. The reason is that the cooling associated with the
injected material leads to high induced electric fields that, in combination
with a significant recombination of hydrogen isotopes, leads to a large
avalanche generation. Balancing Ohmic heating and radiation losses provides
qualitative insights into the dynamics, however, an accurate modeling of the
temperature evolution based on energy balance appears crucial for quantitative
predictions.Comment: 24 pages, 8 figure
Kinetic modelling of runaway electron generation in argon-induced disruptions in ASDEX Upgrade
Massive material injection has been proposed as a way to mitigate the
formation of a beam of relativistic runaway electrons that may result from a
disruption in tokamak plasmas. In this paper we analyse runaway generation
observed in eleven ASDEX Upgrade discharges where disruption was triggered
using massive gas injection. We present numerical simulations in scenarios
characteristic of on-axis plasma conditions, constrained by experimental
observations, using a description of the runaway dynamics with self-consistent
electric field and temperature evolution in two-dimensional momentum space and
zero-dimensional real space. We describe the evolution of the electron
distribution function during the disruption, and show that the runaway seed
generation is dominated by hot-tail in all of the simulated discharges. We
reproduce the observed dependence of the current dissipation rate on the amount
of injected argon during the runaway plateau phase. Our simulations also
indicate that above a threshold amount of injected argon, the current density
after the current quench depends strongly on the argon densities. This trend is
not observed in the experiments, which suggests that effects not captured by 0D
kinetic modeling -- such as runaway seed transport -- are also important.Comment: 17 pages, 15 figures, published in Journal of Plasma Physics (Invited
Contributions from the 18th European Fusion Theory Conference
Validity of models for Dreicer generation of runaway electrons in dynamic scenarios
Runaway electron modelling efforts are motivated by the risk these energetic
particles pose to large fusion devices. The sophisticated kinetic models can
capture most features of the runaway electron generation but have high
computational costs which can be avoided by using computationally cheaper
reduced kinetic codes. In this paper, we compare the reduced kinetic and
kinetic models to determine when the former solvers, based on analytical
calculations assuming quasi-stationarity, can be used. The Dreicer generation
rate is calculated by two different solvers in parallel in a workflow developed
in the European Integrated Modelling framework, and this is complemented by
calculations of a third code that is not yet integrated into the framework.
Runaway Fluid, a reduced kinetic code, NORSE, a kinetic code using non-linear
collision operator, and DREAM, a linearized Fokker-Planck solver, are used to
investigate the effect of a dynamic change in the electric field for different
plasma scenarios spanning across the whole tokamak-relevant range. We find that
on time scales shorter than or comparable to the electron collision time at the
critical velocity for runaway electron generation kinetic effects not captured
by reduced kinetic models play an important role. This characteristic time
scale is easy to calculate and can reliably be used to determine whether there
is a need for kinetic modelling, or cheaper reduced kinetic codes are expected
to deliver sufficiently accurate results. This criterion can be automated, and
thus it can be of great benefit for the comprehensive self-consistent modelling
frameworks that are attempting to simulate complex events such as tokamak
start-up or disruptions
Runaway electron synchrotron radiation in a vertically translated plasma
Synchrotron radiation observed from runaway electrons (REs) in tokamaks
depends upon the position and size of the RE beam, the RE energy and pitch
distributions, as well as the location of the observer. We show that
experimental synchrotron images of a vertically moving runaway electron beam
sweeping past the detector in the TCV tokamak agree well with predictions from
the synthetic synchrotron diagnostic Soft. This experimental validation lends
confidence to the theory underlying the synthetic diagnostics which are used
for benchmarking theoretical models of and probing runaway dynamics. We present
a comparison of synchrotron measurements in TCV with predictions of kinetic
theory for runaway dynamics in uniform magnetic fields. We find that to explain
the detected synchrotron emission, significant non-collisional pitch angle
scattering as well as radial transport of REs would be needed. Such effects
could be caused by the presence of magnetic perturbations, which should be
further investigated in future TCV experiments.Comment: 7 pages, 4 figures. Accepted for publication in Nuclear Fusio
Spatiotemporal analysis of the runaway distribution function from synchrotron images in an ASDEX Upgrade disruption
Synchrotron radiation images from runaway electrons (REs) in an ASDEX Upgrade discharge disrupted by argon injection are analysed using the synchrotron diagnostic tool Soft and coupled fluid-kinetic simulations. We show that the evolution of the runaway distribution is well described by an initial hot-tail seed population, which is accelerated to energies between 25-50 MeV during the current quench, together with an avalanche runaway tail which has an exponentially decreasing energy spectrum. We find that, although the avalanche component carries the vast majority of the current, it is the high-energy seed remnant that dominates synchrotron emission. With insights from the fluid-kinetic simulations, an analytic model for the evolution of the runaway seed component is developed and used to reconstruct the radial density profile of the RE beam. The analysis shows that the observed change of the synchrotron pattern from circular to crescent shape is caused by a rapid redistribution of the radial profile of the runaway density
EUROfusion-theory and advanced simulation coordination (E-TASC): programme and the role of high performance computing
The paper is a written summary of an overview oral presentation given at the 1st Spanish Fusion HPC Workshop that took place on the 27th November 2020 as an online event. Given that over the next few years ITER will move to its operation phase and the European-DEMO design will be significantly advanced, the EUROfusion consortium has initiated a coordination effort in theory and advanced simulation to address some of the challenges of the fusion research in Horizon EUROPE (2021-2027), i.e. the next EU Framework Programme for Research and Technological Development. This initiative has been called E-TASC that stands for EUROfusion-Theory and Advanced Simulation Coordination. The general and guiding principles of E-TASC are summarized in the paper. In addition, an overview of the scientific results obtained in a pilot phase (2019-2020) of E-TASC are provided while highlighting the importance of the required progress in computational methods and HPC techniques. In the initial phase, five pilot theory and simulation tasks were initiated: 1. Towards a validated predictive capability of the L-H transition and pedestal physics; 2. Electron runaway in tokamak disruptions in the presence of massive material injection; 3. Fast code for the calculation of neoclassical toroidal viscosity in stellarators and tokamaks; 4. Development of a neutral gas kinetics modular code; 5. European edge and boundary code for reactor-relevant devices. In this paper we report on recent progress made by each of these projects.</p
EUROfusion-theory and advanced simulation coordination (E-TASC) : programme and the role of high performance computing
This paper is a written summary of an overview oral presentation given at the 1st Spanish Fusion High Performance Computer (HPC) Workshop that took place on the 27 November 2020 as an online event. Given that over the next few years ITER24 will move to its operation phase and the European-DEMO design will be significantly advanced, the EUROfusion consortium has initiated a coordination effort in theory and advanced simulation to address some of the challenges of the fusion research in Horizon EUROPE (2021-2027), i.e. the next EU Framework Programme for Research and Technological Development. This initiative has been called E-TASC, which stands for EUROfusion-Theory and Advanced Simulation Coordination. The general and guiding principles of E-TASC are summarized in this paper. In addition, an overview of the scientific results obtained in the pilot phase (2019-2020) of E-TASC are provided while highlighting the importance of the required progress in computational methods and HPC techniques. In the initial phase, five pilot theory and simulation tasks were initiated: towards a validated predictive capability of the low to high transition and pedestal physics; runaway electrons in tokamak disruptions in the presence of massive material injection; fast code for the calculation of neoclassical toroidal viscosity in stellarators and tokamaks; development of a neutral gas kinetics modular code; European edge and boundary code for reactor-relevant devices. In this paper, we report on recent progress made by each of these projects.Peer reviewe
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