Papers by Jens Juul Rasmussen
Physics of Plasmas, May 1, 2020
Field-aligned filaments, the so-called blobs, born at the edge of the magnetically confined regio... more Field-aligned filaments, the so-called blobs, born at the edge of the magnetically confined region of tokamaks propagate radially outward into the scrape-off layer (SOL) region that allows for a substantial population of neutral particles compared to the region of confinement. The electrons and ions constituting the blob undergo both elastic and inelastic collisions with the neutral particles, and the latter leads to sources and sinks of the blob density, momentum, and heat. The influence of the inelastic collisions with neutrals on the evolution of seeded blobs is investigated numerically by the nHESEL drift-fluid model through a series of discrete scans in interactions, active source terms, and blob plasma parameters. In light of the results, the potential influence of local inelastic collisions on the SOL density shoulder formation is discussed. It is found that density sources increase the blob compactness, which delays the blob dispersion and decreases the dispersion rate. Density sources or momentum sinks also influence the blob dynamics by increasing the vorticity layer around the perturbation, whereas the pressure sources/sinks only affect the blob dynamics marginally. The change to the vorticity structure leads, in most cases, to a decrease in the radial velocity of the blob center of mass, although, at high source rates, a radial acceleration of the blob center of mass is also observed. Density sources may, thus, contribute to shoulder formation not only by increasing the density locally but also by changing the filament dynamics.
Physics of Plasmas, Mar 1, 2020
Transport in the edge and scrape-off layer mediated by turbulent fluctuations is often studied us... more Transport in the edge and scrape-off layer mediated by turbulent fluctuations is often studied using drift fluid models. In this work, we expand previous work on a two-fluid single ion species drift model to a multi-ion-species model that incorporates collisional interactions between the individual species while conserving energy. The model is simplified into a set of equations that are computationally realizable. This is used to study the dependency of seeded blob propagation on different mixes of deuterium and tritium isotopes in the background and blob, respectively. We find that the background mix is initially the dominant driver that determines propagation, but the blob mix becomes the dominating factor for continued evolution. It is found that the maximum velocity of the blob scales stronger with the initial blob mix than the background mix.
Plasma Physics and Controlled Fusion, Jul 11, 2018
The scrape-off layer power fall-off length, λ q , for parameters relevant to ASDEX Upgrade (AUG) ... more The scrape-off layer power fall-off length, λ q , for parameters relevant to ASDEX Upgrade (AUG) L-mode discharges is examined by means of numerical simulations. The simulation data is acquired using synthetic probe data from turbulence simulations, which gives a high temporal resolution on the full density and pressure fields, required for an accurate evaluation of λ q due to the strongly intermittent signals in the scrape-off layer (SOL). Electron conduction is found to dominate the parallel heat flux close to the separatrix, while ion convection dominates in the far SOL. Good agreement is found with the experimental scaling for AUG Lmode [Sieglin et al., Plasma Phys. Contr. Fusion 2016;58(5):055015], where λ q is found to scale almost linearly with the safety factor, q, and to be weakly dependent on the power across the last-closed flux surface (LCFS), P . However, P depends on a wide range of parameters. In this paper we trace this dependence and the resulting fit of λ q reveals a scaling proportional to the inverse square root of the electron temperature at the LCFS, T -1/2 e,LCF S , and a linear dependence on q.
Nuclear Fusion, Jul 3, 2018
The effects of enhanced electron and ion pressure perturbations mediated in filamentary structure... more The effects of enhanced electron and ion pressure perturbations mediated in filamentary structures (blobs) on the densities of neutral atoms and molecules are investigated through a self-consistent dynamical fluid model for plasma and neutral fields. The electron and ion densities and pressures, and the generalized vorticity, are simulated by a 2D drift-fluid model in an edge and scrape-off layer slab domain of a toroidally magnetically confined plasma. The plasma dynamics are coupled with a diffusion model for densities of neutral atoms and molecules. The combined model allows for determining the response of the density of neutrals with various temperatures to blobs. It is found that blobs locally deplete densities of molecules and atoms that do not originate from dissociation of molecules, whereas the density of atoms created by dissociation may increase during blob events. The neutral species, their temperature, and origin should thus be taken into consideration when estimating the effect of blobs on neutral density perturbations when calculating emission rates, e.g., for gas puff imaging.
Plasma Physics and Controlled Fusion, Jan 21, 2016
Three-dimensional and two-dimensional seeded blob simulations are performed with five different f... more Three-dimensional and two-dimensional seeded blob simulations are performed with five different fluid models, all based on the drift-reduced Braginskii equations, and the numerical results are compared among themselves and validated against experimental measurements provided by the TORPEX device [A. Fasoli et al., Phys. Plasmas 13, 055902 (2006)]. The five models are implemented in four simulation codes, typically used to simulate the plasma dynamics in the tokamak SOL, namely BOUT++ [B.
Plasma Physics and Controlled Fusion, Mar 22, 2022
In this paper we present an analysis of the conservation of currents in a full-F electromagnetic ... more In this paper we present an analysis of the conservation of currents in a full-F electromagnetic gyro-kinetic model in the long-wavelength limit. This equation corresponds to what is usually named "vorticity equation", which is not strictly correct as it cannot be formulated as the curl of a velocity equation. In the paper we will therefore use the term "current conservation equation" instead. Our results are relevant to reduced plasma descriptions like gyro-kinetic, drift-kinetic, gyro-fluid and drift-fluid models for tokamaks and stellarators. The equation describes the change of the polarization charge density (often called "vorticity") in terms of the polarization stress due to the E × B flow, external sources and three currents: the parallel current, the curvature current and a current related to the magnetic field fluctuations. We compare this equation with previous driftand gyro-fluid equations and find general agreement except in the vorticity source terms where previous drift-fluid models fail to capture the heating and density sources. We discuss the role of the currents in the dynamics of diamagnetic and E × B flow shear. The possible connection between these currents with phenomena observed in experiments that influence the radial electric field in the edge of tokamak plasmas, like resonant magnetic perturbations, different magnetic field configurations and shapes, is presented.
Plasma Physics and Controlled Fusion, Apr 8, 2021
Fluid models used to study the edge plasma region need to be benchmarked against similar conditio... more Fluid models used to study the edge plasma region need to be benchmarked against similar conditions given that models can strongly differ in complexity and therefore the results they produce. Via this validation study undertaken through the framework of EUROfusion Enabling Research, four state-ofthe art models -GBS, Hermes/BOUT++, HESEL and TOKAM3X -are compared to experimental plasma turbulence measurements on the ISTTOK tokamak. Statistical comparisons of simulation and experiment data show that fluid models used here can replicate most of the experiment in terms of I sat and V f loat fluctuations. Furthermore, it is shown that without including more complex information (like core turbulence information and domain geometry details and magnetic topological aspects) in fluid models, the results recovered can fall short from the experimental results. Via the simulations using these codes, it is demonstrated that fluid models continue to be a good cost-effective tool in recovering many global aspects of edge plasma behaviour.
Physical Review Letters, May 8, 2014
An electrostatic coherent mode near the electron diamagnetic frequency (20-90 kHz) is observed in... more An electrostatic coherent mode near the electron diamagnetic frequency (20-90 kHz) is observed in the steep-gradient pedestal region of long pulse H-mode plasmas in the Experimental Advanced Superconducting Tokamak, using a newly developed dual gas-puff-imaging system and diamond-coated reciprocating probes. The mode propagates in the electron diamagnetic direction in the plasma frame with poloidal wavelength of ∼8 cm. The mode drives a significant outflow of particles and heat as measured directly with the probes, thus greatly facilitating long pulse H-mode sustainment. This mode shows the nature of dissipative trapped electron mode, as evidenced by gyrokinetic turbulence simulations.
Nuclear Fusion, May 5, 2022
The scrape-off layer power width (λq) is an important parameter for characterizing the divertor h... more The scrape-off layer power width (λq) is an important parameter for characterizing the divertor heat loads. Many experimental, theoretical, and numerical studies on λq 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 experimental λq scalings for L-mode plasmas and experiments with a typical EAST H-mode discharge. The simulated λq is found to agree quite well with the Eich scaling [Eich et al. 2013 Nucl. Fusion 53 093031] for the EAST H-mode discharge and the comparison of the probability distribution function of the parallel particle flux with the measurements by reciprocating probes is also consistent. The code is utilized to simulate the ITER 15 MA baseline scenario [Kim et al. 2018 Nucl. Fusion 58 056013]. The ITER simulation reveals that the radial particle/heat transports are dominated by blobby transports, and predicts λq,ITER = 9.6 mm, which is much larger than the prediction by the Eich scaling (λq,ITER ≈ 1 mm). Based on the EAST modified cases, an estimated HESEL H-mode scaling, 𝜆 𝑞 = 0.51𝑅 𝑐 1.1 𝐵 𝑡 -0.3 𝑞 95 1.1 is proposed. This scaling predicts λq,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 this paper can reproduce the Eich scaling for current devices, but predict a much larger λq for ITER. According to the simulation results, the strong positive scaling dependence of λq 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 E × B turbulent heat transports when the machine size is increased.
Physics of Plasmas, Feb 1, 2010
Drift-Alfvén vortex filaments associated with electromagnetic turbulence were recently identified... more Drift-Alfvén vortex filaments associated with electromagnetic turbulence were recently identified in reversed field pinch devices. Similar propagating filamentary structures were observed in the Earth magnetosheath, magnetospheric cusp and Saturn's magnetosheath by spacecrafts. The characteristics of these structures closely resemble those of the so-called mesoscale coherent structures, prevailing in fusion plasmas, known as "blobs" and "edge localized mode filaments" in the boundary region, and propagating avalanchelike events in the core region. In this paper the fundamental dynamics of drift-Alfvén vortex filaments in a nonuniformly and strongly magnetized plasma are revisited. We systemize the Lagrangian-invariant-based method. Six Lagrangian invariants are employed to describe structure motion and the resultant convective transport, namely, magnetic flux, background magnetic energy, specific entropy, total energy, magnetic momentum, and angular momentum. The perpendicular vortex motions and the kinetic shear Alfvén waves are coupled through the parallel current and Ampere's law, leading to field line bending. On the timescale of interchange motion Ќ , a thermal expansion force in the direction of curvature radius of the magnetic field overcomes the resultant force of magnetic tension and push plasma filament to accelerate in the direction of curvature radius resulting from plasma inertial response, reacted to satisfy quasineutrality. During this process the internal energy stored in the background pressure gradient is converted into the kinetic energy of convective motion and the magnetic energy of field line bending through reversible pressure-volume work as a result of the plasma compressibility in an inhomogeneous magnetic field. On the timescale of parallel acoustic response ʈ ӷ Ќ , part of the filament's energy is transferred into the kinetic energy of parallel flow. On the dissipation timescale d ӷ Ќ , the kinetic energy and magnetic energy are eventually dissipated, which is accompanied by entropy production, and in this process the structure loses its coherence, but it has already traveled a distance in the radial direction. In this way the propagating filamentary structures induce intermittent convective transports of particles, heat, and momentum across the magnetic field. It is suggested that the phenomena of profile consistency, or resilience, and the underlying anomalous pinch effects of particles, heat, and momentum in the fusion plasmas can be interpreted in terms of the ballistic motion of these solitary electromagnetic filamentary structures.
44th EPS Conference on Plasma Physics, 2017
Radial transport of poloidal momentum in the scrape-off layer (SOL) of ASDEX Upgrade was investig... more Radial transport of poloidal momentum in the scrape-off layer (SOL) of ASDEX Upgrade was investigated. A reciprocating probe was used with a probe head containing six Langmuir probes with which the poloidal and radial electric field components and the density were determined. Separating all quantities into stationary and fluctuating parts, four components of the momentum transport were evaluated separately. We see a distinct change in behaviour between L-and H-mode discharges. Particularly the direction of momentum flux at the probe location is opposite between L-and H-mode.
The transport in the edge region and through the scrape-off-layer (SOL) at the outboard mid-plane... more The transport in the edge region and through the scrape-off-layer (SOL) at the outboard mid-plane of magnetically confined toroidal plasmas is known to be strongly intermittent and dominated by filamentary structures of enhanced plasma pressure – plasma blobs. These structures are generated in the edge region and propagate through the SOL with velocities up to a few percent of the ion acoustic speed. They play a significant role in setting up the plasma profiles in the SOL and for the power deposition profiles. We have investigated the transport dynamics, the ensuing density and pressure profiles in the SOL, and estimated power depositions on plasma facing components by applying the HESEL code. HESEL is an energy conserving four-field Braginskii model governing the dynamical evolution of generalized vorticity, density, electron, and ion pressures [1]. The parallel dynamics in the SOL is parametrized and different divertor conditions attached or detached are mimicked by applying shea...
Measurements of current carrying filaments associated with edge localized modes (ELMs) have recen... more Measurements of current carrying filaments associated with edge localized modes (ELMs) have recently been made in the Experimental Advanced Superconducting tokamak (EAST) by direct probing the edge plasma using magnetic probes. Prior to type-III ELMs, magnetic precursor develop with decreasing frequency until the phase delay between radial and poloidal magnetic components reach π/2,which seems to behave as the trigger condition for the type-III ELM crash. In addition, there is no remarkable current structure to be observed releasing from the pedestal during type-III ELMs. On the contrary, the pronounced magnetic perturbation associated with current filaments has been detected commonly in type-I like ELMs without magnetic precursor. In the scrape off-layer (SOL), type-I like ELMs manifest themselves as filamentary, field aligned structures, rotating toroidally/poloidally and moving radially on EAST. Further more, a strong SOL current is found to arise just prior to the transport phas...
Fusion Engineering and Design, 2020
Nuclear Fusion, 2017
International Atomic Energy Agency a In the future we will refer to the author list of the paper ... more International Atomic Energy Agency a In the future we will refer to the author list of the paper as the EUROfusion MST1 Team.
Turbulent transport in the scrape-off layer of TCV has been investigated by probe measurements an... more Turbulent transport in the scrape-off layer of TCV has been investigated by probe measurements and direct comparison with two-dimensional fluid simulations of interchange motions at the outer midplane. The experiments demonstrate that the fluctuation statistics in the region where plasma profiles are broad are invariant with respect to changes in the line-averaged core plasma density. Good agreement is found between measurements and an interchange turbulence simulation at high density, indicating that the turbulent transport is dominated by the radial motion of field-aligned plasma filaments. Moreover, the plasma flux onto the main chamber wall at the outer midplane scales linearly with the local particle density, implying that the particle flux at the wall radius can be parameterized in terms of an effective convection velocity. Strong ballooning of the edge turbulence would also result in toroidal field direction independent transport-driven parallel flows in the scrape-off layer. The magnitude of this flow estimated from pressure fluctuation statistics in the simulations is found to compare favorably with the measured flow offset obtained when averaging data obtained from flow profiles observed in matched forward and reversed field discharges.
Journal of Fluid Mechanics, 1994
The long-time evolution of monopolar and dipolar vortices influenced by the largescale gradient o... more The long-time evolution of monopolar and dipolar vortices influenced by the largescale gradient of the ambient potential vorticity (the β-effect) is studied by direct numerical solutions of the equivalent barotropic quasi-geostrophic equation. Translation and reorganization of vortical structures are shown to depend strongly on their intensity. Transport of trapped fluid by vortical structures is illustrated by calculating particle trajectories and by considering closed isolines of potential vorticity and the streamfunction in a co-moving reference frame.The initial behaviour of strong monopoles is found to be well described by a recent approximate theory for the evolution of azimuthal mode one, even for times longer than the linear Rossby wave period. In the long-time limit, strong monopoles transport particles mainly westward, although the meridional displacement is several times larger than the initial vortex size. The appearance of an annulus with opposite radial gradient of the...
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Papers by Jens Juul Rasmussen