| 1. |
- Dejarnac, R., et al.
(författare)
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Numerical evaluation of heat flux and surface temperature on a misaligned JET divertor W lamella during ELMs
- 2014
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 54:12, s. 123011-
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Tidskriftsartikel (refereegranskat)abstract
- A series of experiments has been performed on JET to investigate the dynamics of transient melting due to edge localized modes (ELMs). The experiment employs a deliberately misaligned lamella in one module of the JET bulk tungsten outer divertor, allowing the combination of stationary power flux and ELMs to transiently melt the misaligned edge. During the design of the experiment a number of calculations were performed using 2D particle-in-cell simulations and a heat transfer code to investigate the influence on the deposited power flux of finite Larmor radius effects associated with the energetic ELM ions. This has been performed using parameter scans inside a range of pedestal temperatures and densities to scope different experimentally expected ELM energies. On the one hand, we observe optimistic results, with smoothing of the heat flux due to the Larmor gyration on the protruding side of the lamella which sees the direct parallel flux-the deposited power tends to be lower than the nominal value expected from geometric magnetic field line impact over a distance smaller than 2 Larmor radii, a finding which is always valid during ELMs for such a geometry. On the other hand, the fraction of the flux not reaching the directly wetted side is transferred and spread to the top surface of the lamella. The hottest point of the lamella (corner side/top) does not always benefit from the gain from the Larmor smoothing effect because of an enhanced power deposition from the second contribution.
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| 2. |
- Komm, M., et al.
(författare)
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Simulations of thermionic suppression during tungsten transient melting experiments
- 2017
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Ingår i: Physica Scripta. - : Institute of Physics (IOP). - 0031-8949 .- 1402-4896. ; T170
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Tidskriftsartikel (refereegranskat)abstract
- Plasma-facing components receive enormous heat fluxes under steady state and especially during transient conditions that can even lead to tungsten (W) melting. Under these conditions, the unimpeded thermionic current density emitted from the W surfaces can exceed the incident plasma current densities by several orders of magnitude triggering a replacement current which drives melt layer motion via the J x B force. However, in tokamaks, the thermionic current is suppressed by space-charge effects and prompt re-deposition due to gyro-rotation. We present comprehensive results of particle-in-cell modelling using the 2D3V code SPICE2 for the thermionic emissive sheath of tungsten. Simulations have been performed for various surface temperatures and selected inclinations of the magnetic field corresponding to the leading edge and sloped exposures. The surface temperature dependence of the escaping thermionic current and its limiting value are determined for various plasma parameters; for the leading edge geometry, the results agree remarkably well with the Takamura analytical model. For the sloped geometry, the limiting value is observed to be proportional to the thermal electron current and a simple analytical expression is proposed that accurately reproduces the numerical results.
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| 3. |
- Komm, M., et al.
(författare)
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On thermionic emission from plasma-facing components in tokamak-relevant conditions
- 2017
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Ingår i: Plasma Physics and Controlled Fusion. - : Institute of Physics Publishing (IOPP). - 0741-3335 .- 1361-6587. ; 59:9
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Tidskriftsartikel (refereegranskat)abstract
- The first results of particle-in-cell simulations of the electrostatic sheath and magnetic pre-sheath of thermionically emitting planar tungsten surfaces in fusion plasmas are presented. Plasma conditions during edge localized modes (ELMs) and during inter-ELM periods have been considered for various inclinations of the magnetic field and for selected surface temperatures. All runs have been performed under two assumptions for the sheath potential drop; fixed or floating. The primary focus lies on the evaluation of the escaping thermionic current and the quantification of the suppression due to the combined effects of space-charge and Larmor gyration. When applicable, the results are compared with the predictions of analytical models. The heat balance in the presence of thermionic emission as well as the contribution of the escaping thermionic current to surface cooling are also investigated. Regimes are identified where emission needs to be considered in the energy budget.
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| 4. |
- Komm, M., et al.
(författare)
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Space-charge limited thermionic sheaths in magnetized fusion plasmas
- 2020
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 60:5
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Tidskriftsartikel (refereegranskat)abstract
- A systematic particle-in-cell investigation is reported focusing on the sheaths surrounding hot planar surfaces that are embedded in fusion plasmas with an arbitrary magnetic field inclination angle. The transition to the space-charge limited regime and the subsequent strict limitation of the escaping thermionic current density are demonstrated to be global characteristics of strongly emitting sheaths also in the presence of inclined magnetic fields. A physically transparent semi-empirical expression has been identified for the limited thermionic current as a function of the plasma conditions and inclination angle that is accurate for both the inter-and intra-ELM plasmas of present-day tokamaks.
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| 5. |
- Tolias, Panagiotis, 1984-, et al.
(författare)
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ITER relevant multi-emissive sheaths at normal magnetic field inclination
- 2023
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Ingår i: Nuclear Fusion. - : IOP Publishing Ltd. - 0029-5515 .- 1741-4326. ; 63:2
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Tidskriftsartikel (refereegranskat)abstract
- Reliable modeling of macroscopic melt motion induced by fast transients requires the accurate and computationally efficient description of the emitted current density that escapes to the pre-sheath. The ITER sheaths that surround hot tungsten surfaces during edge-localized modes are characterized by important contributions from secondary electron emission and electron backscattering as well as by the coupling between thermionic emission and field electron emission. Under the guidance of systematic particle-in-cell simulations that incorporate a comprehensive analytical electron emission model, a highly accurate semi-empirical treatment of the escaping electron current has been achieved.
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| 6. |
- Tolias, Panagiotis, 1984-, et al.
(författare)
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Origin and nature of the emissive sheath surrounding hot tungsten tokamak surfaces
- 2020
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Ingår i: Nuclear Materials and Energy. - : Elsevier BV. - 2352-1791. ; 25
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Tidskriftsartikel (refereegranskat)abstract
- The accurate description of the emitted current that escapes from hot tungsten surfaces is essential for reliable predictions of the macroscopic deformation due to melt motion induced by fast transient events. A comprehensive analytical electron emission model is developed and its implementation in the particle-in-cell 2D3V code SPICE2 is discussed. The properties of emissive sheaths of present tokamaks, where thermionic emission is strongly suppressed by space-charge effects and by prompt re-deposition, are reviewed for arbitrary magnetic field inclination angles. The unique characteristics of emissive sheaths that emerge during ITER ELMs, where weakly impeded thermionic emission is coupled with field emission and competes with electron-induced emission, are revealed. The first ITER simulations are reported for normal inclinations.
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