| 1. |
- Bombarda, F., et al.
(författare)
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Runaway electron beam control
- 2019
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 61:1
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Tidskriftsartikel (refereegranskat)
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| 2. |
-
- 2018
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 58:1
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Forskningsöversikt (refereegranskat)
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| 3. |
- Harrison, J.R., et al.
(författare)
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Overview of new MAST physics in anticipation of first results from MAST Upgrade
- 2019
-
Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:11
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Forskningsöversikt (refereegranskat)abstract
- The mega amp spherical tokamak (MAST) was a low aspect ratio device (R/a = 0.85/0.65 ∼ 1.3) with similar poloidal cross-section to other medium-size tokamaks. The physics programme concentrates on addressing key physics issues for the operation of ITER, design of DEMO and future spherical tokamaks by utilising high resolution diagnostic measurements closely coupled with theory and modelling to significantly advance our understanding. An empirical scaling of the energy confinement time that favours higher power, lower collisionality devices is consistent with gyrokinetic modelling of electron scale turbulence. Measurements of ion scale turbulence with beam emission spectroscopy and gyrokinetic modelling in up-down symmetric plasmas find that the symmetry of the turbulence is broken by flow shear. Near the non-linear stability threshold, flow shear tilts the density fluctuation correlation function and skews the fluctuation amplitude distribution. Results from fast particle physics studies include the observation that sawteeth are found to redistribute passing and trapped fast particles injected from neutral beam injectors in equal measure, suggesting that resonances between the m = 1 perturbation and the fast ion orbits may be playing a dominant role in the fast ion transport. Measured D-D fusion products from a neutron camera and a charged fusion product detector are 40% lower than predictions from TRANSP/NUBEAM, highlighting possible deficiencies in the guiding centre approximation. Modelling of fast ion losses in the presence of resonant magnetic perturbations (RMPs) can reproduce trends observed in experiments when the plasma response and charge-exchange losses are accounted for. Measurements with a neutral particle analyser during merging-compression start-up indicate the acceleration of ions and electrons. Transport at the plasma edge has been improved through reciprocating probe measurements that have characterised a geodesic acoustic mode at the edge of an ohmic L-mode plasma and particle-in-cell modelling has improved the interpretation of plasma potential estimates from ball-pen probes. The application of RMPs leads to a reduction in particle confinement in L-mode and H-mode and an increase in the core ionization source. The ejection of secondary filaments following type-I ELMs correlates with interactions with surfaces near the X-point. Simulations of the interaction between pairs of filaments in the scrape-off layer suggest this results in modest changes to their velocity, and in most cases can be treated as moving independently. A stochastic model of scrape-off layer profile formation based on the superposition of non-interacting filaments is in good agreement with measured time-average profiles. Transport in the divertor has been improved through fast camera imaging, indicating the presence of a quiescent region devoid of filament near the X-point, extending from the separatrix to ψ n ∼ 1.02. Simulations of turbulent transport in the divertor show that the angle between the divertor leg on the curvature vector strongly influences transport into the private flux region via the interchange mechanism. Coherence imaging measurements show counter-streaming flows of impurities due to gas puffing increasing the pressure on field lines where the gas is ionised. MAST Upgrade is based on the original MAST device, with substantially improved capabilities to operate with a Super-X divertor to test extended divertor leg concepts. SOLPS-ITER modelling predicts the detachment threshold will be reduced by more than a factor of 2, in terms of upstream density, in the Super-X compared with a conventional configuration and that the radiation front movement is passively stabilised before it reaches the X-point. 1D fluid modelling reveals the key role of momentum and power loss mechanisms in governing detachment onset and evolution. Analytic modelling indicates that long legs placed at large major radius, or equivalently low at the target compared with the X-point are more amenable to external control. With MAST Upgrade experiments expected in 2019, a thorough characterisation of the sources of the intrinsic error field has been carried out and a mitigation strategy developed.
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| 4. |
- Labit, B., et al.
(författare)
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Dependence on plasma shape and plasma fueling for small edge-localized mode regimes in TCV and ASDEX Upgrade
- 2019
-
Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:8
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Tidskriftsartikel (refereegranskat)abstract
- © 2019 Institute of Physics Publishing. All rights reserved. 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.
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| 5. |
- Brodrick, Jonathan, et al.
(författare)
-
Improving the accuracy of hohlraum simulations by calibrating the `SNB' multigroup diffusion model for nonlocal heat transport against a VFP code
- 2016
-
Ingår i: 58th Annual Meeting of the APS Division of Plasma Physics. ; 61:18
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Nonlocal heat transport, occurring when temperature gradients become steep on the scale of the electron mean free path (mfp), has proven critical in accurately predicting ignition-scale hohlraum energetics. A popular approach, and modern alternative to flux limiters, is the `SNB' model\footnote{Schurtz \emph{et al.} \textbf{Phys. Plasmas} 7, 4238 (2000)}. This is implemented in both the HYDRA code used for simulating National Ignition Facility experiments and the CHIC code developed at the CELIA laboratory. We have performed extensive comparisons of the SNB heat flow predictions with two VFP codes, IMPACT\footnote{Kingham \& Bell \textbf{J. Comp. Phys.} 194 (2004)} and KIPP\footnote{Chankin \emph{et al.} \textbf{Contrib. Plasma Phys.} 52, 500 (2012)} and found that calibrating the mfp to achieve agreement for a linear problem also improves nonlinear accuracy. Furthermore, we identify that using distinct electron-ion and electron-electron mfp's instead of a geometrically averaged one improves predictive capability when there are strong ionisation (Z) gradients.
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| 6. |
- Brodrick, J. P., et al.
(författare)
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Testing nonlocal models of electron thermal conduction for magnetic and inertial confinement fusion applications
- 2017
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 24:9
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Tidskriftsartikel (refereegranskat)abstract
- Three models for nonlocal electron thermal transport are here compared against Vlasov-Fokker-Planck (VFP) codes to assess their accuracy in situations relevant to both inertial fusion hohlraums and tokamak scrape-off layers. The models tested are (i) a moment-based approach using an eigenvector integral closure (EIC) originally developed by Ji, Held, and Sovinec [Phys. Plasmas 16, 022312 (2009)]; (ii) the non-Fourier Landau-fluid (NFLF) model of Dimits, Joseph, and Umansky [Phys. Plasmas 21, 055907 (2014)]; and (iii) Schurtz, Nicolaï, and Busquet's [Phys. Plasmas 7, 4238 (2000)] multigroup diffusion model (SNB). We find that while the EIC and NFLF models accurately predict the damping rate of a small-amplitude temperature perturbation (within 10% at moderate collisionalities), they overestimate the peak heat flow by as much as 35% and do not predict preheat in the more relevant case where there is a large temperature difference. The SNB model, however, agrees better with VFP results for the latter problem if care is taken with the definition of the mean free path. Additionally, we present for the first time a comparison of the SNB model against a VFP code for a hohlraum-relevant problem with inhomogeneous ionisation and show that the model overestimates the heat flow in the helium gas-fill by a factor of ?2 despite predicting the peak heat flux to within 16%.
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| 7. |
- Buller, Stefan, 1991, et al.
(författare)
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Neoclassical flows in deuterium-helium plasma density pedestals
- 2017
-
Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 59:5, s. 055019-
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Tidskriftsartikel (refereegranskat)abstract
- In tokamak transport barriers, the radial scale of profile variations can be comparable to a typical ion orbit width, which makes the coupling of the distribution function across flux surfaces important in the collisional dynamics. We use the radially global steady-state neoclassical delta f code PERFECT [Landreman et al 2014 Plasma Phys. Control. Fusion 56 045005] to calculate poloidal and toroidal flows, and radial fluxes, in the pedestal. In particular, we have studied the changes in these quantities as the plasma composition is changed from a deuterium bulk species with a helium impurity to a helium bulk with a deuterium impurity, under specific profile similarity assumptions. In the presence of sharp profile variations, the poloidally resolved radial fluxes are important for the total fluxes to be divergence-free, which leads to the appearance of poloidal return-flows. These flows exhibit a complex radial–poloidal structure that extends several orbit widths into the core and is sensitive to abrupt radial changes in the ion temperature gradient. We find that a sizable neoclassical toroidal angular momentum transport can arise in the radially global theory, in contrast to the local.
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| 8. |
- Buller, Stefan, 1991, et al.
(författare)
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Neoclassical study of the isotope effect in density pedestals
- 2017
-
Ingår i: 44th EPS Conference on Plasma Physics, EPS 2017. - 9781510849303 ; part 2, s. 841- 844
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Konferensbidrag (refereegranskat)abstract
- The isotope mass scaling of the energy confinement time in tokamak plasmas typically differs from gyro-Bohm estimates. This phenomenon – known as the isotope effect – remains an open issue in plasma physics, with important implications for the extrapolation from present day, mostly deuterium (D), fusion experiments to future deuterium-tritium (D-T) reactors. Differences in mass scaling in L-mode and various H-mode regimes suggest that the isotope effect may, in large part, originate from the pedestal. In the pedestal, sharp gradients render local diffusive estimates invalid, and global effects due to orbit-width scale profile variations have to be taken into account, potentially leading to mass scalings different from gyro-Bohm. We calculate cross-field fluxes from a radially-global linearized drift-kinetic equation using the PERFECT code, to study isotope composition effects in density pedestals. We define dimensionless parameters from the ratios of density length scale, pedestal width and orbit width, and study global effects in terms of these parameters for different pedestal profiles and bulk species. Quantifying global effects by the relative difference between peak heat-flux values in global and local simulations, we find that this quantity saturates at an isotope-dependent value, but the dimensionless parameters do not capture all the isotope dependencies. We also consider D-T and H-D mixtures, and compare the calculated heat fluxes to fluxes calculated from single species simulations with artificial "DT" and "HD" species.
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| 9. |
- Dudson, Benjamin, et al.
(författare)
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Verification of BOUT++ by the method of manufactured solutions
- 2016
-
Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 23:6
-
Tidskriftsartikel (refereegranskat)abstract
- BOUT++ is a software package designed for solving plasma fluid models. It has been used to simulate a wide range of plasma phenomena ranging from linear stability analysis to 3D plasma turbulence and is capable of simulating a wide range of drift-reduced plasma fluid and gyro-fluid models. A verification exercise has been performed as part of a EUROfusion Enabling Research project, to rigorously test the correctness of the algorithms implemented in BOUT++, by testing order-of-accuracy convergence rates using the Method of Manufactured Solutions (MMS). We present tests of individual components including time-integration and advection schemes, non-orthogonal toroidal field-aligned coordinate systems and the shifted metric procedure which is used to handle highly sheared grids. The flux coordinate independent approach to differencing along magnetic field-lines has been implemented in BOUT++ and is here verified using the MMS in a sheared slab configuration. Finally, we show tests of three complete models: 2-field Hasegawa-Wakatani in 2D slab, 3-field reduced magnetohydrodynamics (MHD) in 3D field-aligned toroidal coordinates, and 5-field reduced MHD in slab geometry.
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| 10. |
- Easy, L, et al.
(författare)
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Investigation of the effect of resistivity on scrape off layer filaments using three-dimensional simulations
- 2016
-
Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 23:1, s. 012512-
-
Tidskriftsartikel (refereegranskat)abstract
- The propagation of filaments in the Scrape Off Layer (SOL) of tokamaks largely determines the plasma profiles in the region. In a conduction limited SOL, parallel temperature gradients are expected, such that the resistance to parallel currents is greater at the target than further upstream. Since the perpendicular motion of an isolated filament is largely determined by balance of currents that flow through it, this may be expected to affect filament transport. 3D simulations have thus been used to study the influence of enhanced parallel resistivity on the dynamics of filaments. Filaments with the smallest perpendicular length scales, which were inertially limited at low resistivity (meaning that polarization rather than parallel currents determines their radial velocities), were unaffected by resistivity. For larger filaments, faster velocities were produced at higher resistivities due to two mechanisms. First parallel currents were reduced and polarization currents were enhanced, meaning that the inertial regime extended to larger filaments, and second, a potential difference formed along the parallel direction so that higher potentials were produced in the region of the filament for the same amount of current to flow into the sheath. These results indicate that broader SOL profiles could be produced at higher resistivities.
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| 11. |
- Militello, Fulvio, et al.
(författare)
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Characterisation of the L-mode scrape off layer in MAST: decay lengths
- 2016
-
Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 56:1, s. 016006-
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Tidskriftsartikel (refereegranskat)abstract
- This work presents a detailed characterisation of the MAST Scrape Off Layer in L-mode. Scans in line averaged density, plasma current and toroidal magnetic field were performed. A comprehensive and integrated study of the SOL was allowed by the use of a wide range of diagnostics. In agreement with previous results, an increase of the line averaged density induced a broadening of the midplane density profile. This increase was not correlated with divertor detachment, as confirmed by the systematic increase of the target ion flux and decrease of the ${{D}_{\gamma}}/{{D}_{\alpha}}$ emission. Also, no clear correlation is found with the density of the neutral particles at the wall. At comparable density levels, discharges with higher current did not show broadening. Outer target ion saturation current and heat flux decay lengths were measured and compared with midplane data. For the saturation current, the upstream projections of the target values, based on diffusive models, did not match the midplane measurements, neither in amplitude nor in trend, while agreement was found for the heat fluxes, suggesting a different perpendicular transport mechanism for the two channels. Furthermore, the value of the target heat flux decay length was quite insensitive to changes in the thermodynamic conditions, in agreement with recent scaling laws. In all the cases studied, sawtooth oscillations were present but they simply rescaled self-similarly the target profiles. The separatrix conditions changed significantly during a sawtooth cycle, but the target heat flux decay length and divertor spreading factor remained nearly constant, indicating that these quantities are rather insensitive to the upstream thermodynamic state of the SOL.
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| 12. |
- Militello, F., et al.
(författare)
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On the relation between non-exponential scrape off layer profiles and the dynamics of filaments
- 2016
-
Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 58:12
-
Tidskriftsartikel (refereegranskat)abstract
- A theoretical framework is developed to clarify the relation between the profiles of density and temperature in the scrape off layer (SOL) with the fluctuations (filaments) that generate them. The framework is based on the dynamics of independent filaments and on their statistical behaviour and can be used to rigorously understand the mechanisms that lead to the non-exponential nature of the radial SOL profiles as well as the increase of the relative fluctuation amplitude in the far SOL. Several models for the dynamics of the filaments, which can be applied to the framework, are derived and discussed for the purpose of identifying how different assumptions lead to the emergence of features in the profiles. It is found that multiple alternative models can explain the observations, thus motivating more stringent and focused experimental analysis. In particular, radially accelerating filaments, less efficient parallel exhaust and also a statistical distribution of the velocity of the filaments can all contribute to induce flatter profiles in the far SOL. A quite general result is the resiliency of the non-exponential nature of the profiles. At the same time, several of the models discussed can also capture the increase of the relative fluctuation amplitude observed in the far SOL. It is also shown that several scenarios are compatible with the broadening of the SOL, which could be caused by charge exchange interactions with neutral particles or by a significant radial acceleration of the filaments.
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| 13. |
- Militello, F., et al.
(författare)
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Scrape off layer profiles interpreted with filament dynamics
- 2016
-
Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 56:10, s. 104004-
-
Tidskriftsartikel (refereegranskat)abstract
- A theoretical framework is developed to link the density profiles in the scrape off layer (SOL) with the fluctuations (filaments) that generate them. The framework is based on the dynamics of independent filaments and their statistical behaviour and can be used to rigorously understand the mechanisms that lead to flattening and broadening of the SOL profiles as well as the radial increase of the relative fluctuation amplitude.
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| 14. |
- Omotani, John, 1985, et al.
(författare)
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Edge momentum transport by neutrals
- 2016
-
Ingår i: Proceedings of 26th IAEA Fusion Energy Conference, Kyoto, Japan. ; , s. TH/P3-18
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Due to their high cross-field mobility, neutrals can contribute to momentum transport evenat the low relative densities found inside the separatrix and they can generate intrinsicrotation. We use a charge-exchange dominated solution to the neutral kinetic equation,coupled to neoclassical ions, to evaluate the momentum transport due to neutrals. Numer-ical solutions to the drift-kinetic equation allow us to model the intermediate collisionalitytypical of the tokamak edge. In the edge there are several processes likely to contribute tomomentum transport in addition to neutrals. Therefore, we present here an interpretativeframework that can evaluate the momentum transport through neutrals based on radialplasma profiles. We demonstrate its application with representative artificial profiles andan analytical equilibrium, with parameters typical of L-mode in a medium-sized tokamak.The magnitudes of the torques we find here due to neutrals are 0.5–5 N·m, motivating theapplication of our framework to experimental data, where it will allow us to evaluate theimportance of the momentum flux due to neutrals compared to other transport mechanisms.
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| 15. |
- Omotani, John, 1985, et al.
(författare)
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Edge momentum transport by neutrals: an interpretive numerical framework
- 2017
-
Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 57:6, s. 066048-
-
Tidskriftsartikel (refereegranskat)abstract
- Due to their high cross-field mobility, neutrals can contribute to momentum transport even at the low relative densities found inside the separatrix and they can generate intrinsic rotation. We use a charge-exchange dominated solution to the neutral kinetic equation, coupled to neoclassical ions, to evaluate the momentum transport due to neutrals. Numerical solutions to the drift-kinetic equation allow us to cover the full range of collisionality, including the intermediate levels typical of the tokamak edge. In the edge there are several processes likely to contribute to momentum transport in addition to neutrals. Therefore, we present here an interpretive framework that can evaluate the momentum transport through neutrals based on radial plasma profiles. We demonstrate its application by analysing the neutral angular momentum flux for an L-mode discharge in the ASDEX Upgrade tokamak. The magnitudes of the angular momentum fluxes we find here due to neutrals of 0.6–2 N m are comparable to the net torque on the plasma from neutral beam injection, indicating the importance of neutrals for rotation in the edge.
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| 16. |
- Omotani, John, 1985, et al.
(författare)
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Edge rotation from momentum transport by neutrals
- 2016
-
Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 775:1
-
Konferensbidrag (refereegranskat)abstract
- Due to their high cross field mobility, neutral atoms can have a strong effect on transporteven at the low relative densities found inside the separatrix. We use a charge-exchange dominatedmodel for the neutrals, coupled to neoclassical ions, to calculate momentum transport when it isdominated by the neutrals. We can then calculate self-consistently the radial electric field and predictthe intrinsic rotation in a torque-free plasma. Using a numerical solver for the ion distribution toallow arbitrary collisionality, we investigate the effects of inverse aspect ratio and elongation on plasmarotation. We also calculate the rotation of a trace carbon impurity, to facilitate future comparison toexperiments using charge exchange recombination spectroscopy diagnostics.
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| 17. |
- Omotani, John, 1985, et al.
(författare)
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Momentum transport by neutrals: Effect of kinetic coupling
- 2017
-
Ingår i: 44th EPS Conference on Plasma Physics, EPS 2017.
-
Konferensbidrag (refereegranskat)abstract
- Neutrals couple to the non-Maxwellian as well as (drifting-)Maxwellian parts of the ion distribution function, producing a residual stress that can drive intrinsic rotation. Our modelling predicted a steeper slope of the toroidal rotation profile than observed in dedicated AUG discharges, suggesting the presence of opposing transport mechanisms. Losses from neutrals escaping to the wall which are not captured by the short MFP approximation, neoclassical torque from the toroidal ripple field or turbulent transport are all possibilities. Finally, we did not predict or observe a significant change in the measured rotation profiles when changing the gas fuelling location. This may be due to the modest fuelling possible in the low density discharges used.
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| 18. |
- Omotani, John, 1985, et al.
(författare)
-
Plasma rotation from momentum transport by neutrals in tokamaks
- 2016
-
Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 56:12, s. 124002-
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Tidskriftsartikel (refereegranskat)abstract
- Neutral atoms can strongly influence the intrinsic rotation and radial electric field at the tokamak edge. Here, we present a framework to investigate these effects when the neutrals dominate the momentum transport. We explore the parameter space numerically, using highly flexible model geometries and a state of the art kinetic solver. We find that the most important parameters controlling the toroidal rotation and electric field are the major radius where the neutrals are localized and the plasma collisionality. This offers a means to influence the rotation and electric field by, for example, varying the radial position of the X-point to change the major radius of the neutral peak.
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| 19. |
- Omotani, John, 1985, et al.
(författare)
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The effects of shape and amplitude on the velocity of scrape-off layer filaments
- 2015
-
Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 58:1, s. 014030-
-
Tidskriftsartikel (refereegranskat)abstract
- A complete model of the dynamics of scrape-off layer filaments will be rather complex, including temperature evolution, three dimensional geometry and finite Larmor radius effects. However, the basic mechanism of ExB advection due to electrostatic potential driven by the diamagnetic current can be captured in a much simpler model; a complete understanding of the physics in the simpler model will then aid interpretation of more complex simulations, by allowing the new effects to be disentangled. Here we consider such a simple model, which assumes cold ions and isothermal electrons and is reduced to two dimensions. We derive the scaling with width and amplitude of the velocity of isolated scrape-off layer filaments, allowing for arbitrary elliptical cross-sections, where previously only circular cross-sections have been considered analytically. We also put the scaling with amplitude in a new and more satisfactory form. The analytical results are extensively validated with two dimensional simulations and also compared, with reasonable agreement, to three dimensional simulations having minimal variation parallel to the magnetic field.
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| 20. |
- Pusztai, Istvan, 1983, et al.
(författare)
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Edge rotation as governed by momentum transport due to neutrals
- 2016
-
Ingår i: 58th Annual Meeting of the APS Division of Plasma Physics. ; 61:18, s. TO9.00002-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Neutrals can strongly affect momentum transport even in relatively small concentrations due to their high cross-field mobility. We present a framework to calculate numerically the momentum transport due to charge-exchanging neutrals, in the closed field-line region. We couple a short mean-free-path solution of the neutral kinetic equation to neoclassical ions. We can then determine self-consistently the radial electric field and plasma rotation velocity, assuming that the neutrals dominate the momentum transport. We use the neoclassical solver PERFECT [Landreman et al 2014 PPCF 56 045005] to compute the ion distributions. Numerical solutions allow us to consider the full range of collisionalities; typical experimental parameters fall in the intermediate region that is not well described by analytical limits. We also compute the rotation velocities of minority impurity species, to facilitate experimental comparison of the results. We find that at a fixed collisionality, the important parameter determining the radial electric field and rotation is the major radius where the neutrals are localized. Therefore changes to the location of the peak neutral density, caused by altering the fuelling location or moving the X-point for example, should allow the rotation to be manipulated.
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| 21. |
- Pusztai, Istvan, 1983, et al.
(författare)
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Isotope and mixture effects on neoclassical transport in the pedestal
- 2017
-
Ingår i: 59th Annual Meeting of the APS Division of Plasma Physics, Milwaukee WI, USA. ; 62:12, s. YO4.12-
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The isotope mass scaling of the energy confinement time in tokamak plasmas differs from gyro-Bohm estimates, with implications for the extrapolation from current experiments to D-T reactors. Differences in mass scaling in L-mode and various H-mode regimes suggest that the isotope effect may originate from the pedestal. In the pedestal, sharp gradients render local diffusive estimates invalid, and global effects due to orbit-width scale profile variations have to be taken into account. We calculate neoclassical cross-field fluxes from a radially global drift-kinetic equation using the PERFECT code [Landreman et al. (2014) PPCF 56, 045005], to study isotope composition effects in density pedestals. The relative reduction to the peak heat flux due to global effects as a function of the density scale length is found to saturate at an isotope-dependent value that is larger for heavier ions. We also consider D-T and H-D mixtures with a focus on isotope separation. The ability to reproduce the mixture results via single-species simulations with artificial "DT" and "HD" species has been considered. These computationally convenient single ion simulations give a good estimate of the total ion heat flux in corresponding mixtures.
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| 22. |
- Riva, F, et al.
(författare)
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Blob dynamics in the TORPEX experiment: a multi-code validation
- 2016
-
Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 58:4, s. 044005-
-
Tidskriftsartikel (refereegranskat)abstract
- 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 (Fasoli et al 2006 Phys. Plasmas 13 055902). The five models are implemented in four simulation codes, typically used to simulate the plasma dynamics in the tokamak scrape-off layer, namely BOUT++ (Dudson et al 2009 Comput. Phys. Commun. 180 1467), GBS (Ricci et al 2012 Plasma Phys. Control. Fusion 54 124047), HESEL (Nielsen et al 2015 Phys. Lett. A 379 3097), and TOKAM3X (Tamain et al 2014 Contrib. Plasma Phys. 54 555). Three blobs with different velocities and different stability properties are simulated. The differences observed among the simulation results and the different levels of agreement with experimental measurements are investigated, increasing our confidence in our simulation tools and shedding light on the blob dynamics. The comparisons demonstrate that the radial blob dynamics observed in the three-dimensional simulations is in good agreement with experimental measurements and that, in the present experimental scenario, the two-dimensional model derived under the assumption of ${{k}_{\parallel}}=0$ is able to recover the blob dynamics observed in the three-dimensional simulations. Moreover, it is found that an accurate measurement of the blob temperature is important to perform reliable seeded blob simulations.
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| 23. |
- Walkden, N, et al.
(författare)
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Numerical investigation of isolated filament motion in a realistic tokamak geometry
- 2015
-
Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 55:11, s. 113022-
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a numerical investigation of isolated filament dynamics in a simulation geometry representative of the scrape-off layer (SOL) of the Mega Amp Spherical Tokamak (MAST) previously studied in Walkden et al 2013 (Plasma Phys. Control. Fusion 55 105005). This paper focuses on the evolution of filament cross-sections at the outboard midplane and investigates the scaling of the centre of mass velocity of the filament cross-section with filament width and electron temperature.By decoupling the vorticity equation into even and odd parity components about the centre of the filament in the bi-normal direction parallel density gradients are shown to drive large velocities in the bi-normal (approximately poloidal) direction which scale linearly with electron temperature. In this respect increasing the electron temperature causes a departure of the filament dynamics from two-dimensional (2D) behaviours.Despite the strong impact of three-dimensional effects the radial motion of the filament is shown to be relatively well predicted by 2D scalings. The radial velocity is found to scale positively with both electron temperature and cross-sectional width, suggesting an inertially limited nature. Comparison with the two-region model (Myra et al 2006 Phys. Plasmas 13 112502) achieves reasonable agreement when using a corrected parallel connection length due to the neglect of diamagnetic currents driven in the divertor region of the filament.Analysis of the transport of particles due to the motion of the filament shows that the background temperature has a weak overall effect on the radial particle flux whilst the filament width has a strong effect.
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| 24. |
- Walkden, N. R., et al.
(författare)
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Dynamics of 3D isolated thermal filaments
- 2016
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 58:11
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Tidskriftsartikel (refereegranskat)abstract
- Simulations have been carried out to establish how electron thermal physics, introduced in the form of a dynamic electron temperature, affects isolated filament motion and dynamics in 3D. It is found that thermal effects impact filament motion in two major ways when the pressure perturbation within the filament is supported primarily through a temperature increase as opposed to density: they lead to a strong increase in filament propagation in the bi-normal direction and a significant decrease in net radial propagation. Both effects arise from the temperature dependence of the sheath current which leads to a non-uniform floating potential, with the latter effect supplemented by faster pressure loss. The reduction in radial velocity can only occur when the filament cross-section loses angular symmetry. The behaviour is observed across different filament sizes and suggests that filaments with much larger temperature perturbations than density perturbations are more strongly confined to the near SOL region.
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