Papers by Florian Effenberg
Nuclear Materials and Energy, 2024
An integrated modeling framework for investigating the application of solid boron (B) powder inje... more An integrated modeling framework for investigating the application of solid boron (B) powder injection for real-time surface conditioning of plasma-facing components (PFCs) in tokamak environments is presented. Utilizing the DIII-D impurity powder dropper (IPD) setup, this study simulates B powder injection scenarios ranging from milligrams to tens of milligrams per second, corresponding to boron flux rates of 10 20 − 10 21 B/s in standard L-mode conditions. The comprehensive modeling approach combines EMC3-EIRENE for simulating the deuterium plasma background and the Dust Injection Simulator (DIS) for the ablation and transport of the boron powder particles. EMC3 trace impurity fluid modeling results show substantial boron transport to the inboard lower divertor, predominantly influenced by the main ion plasma flow. The dependency on powder particle size (5-250 µm) was found to be insignificant for the scenario considered. The effects of erosion and redeposition were considered to reconcile the discrepancies with experimental observations, which saw substantial deposition on the outer divertor plasma-facing components. For this purpose, the WallDYN3D code was updated to include boron sources within the plasma domain and integrated into the modeling framework. The mixed-material migration modeling shows evolving boron deposition patterns, suggesting the formation of mixed B-C layers or predominantly B coverage depending on the powder mass flow rate. While the modeling outcomes at lower B injection rates tend to align with DIII-D experimental observations, the prediction of near-pure boron layers at higher rates has yet to be experimentally verified in the carbon environment of the DIII-D tokamak. The extensive reach of boron layers found in the modeling suggests the need for modeling that encompasses the entire wall geometry for more accurate experimental correlations. This integrated approach sets a precedent for analyzing and applying real-time in-situ boron coating techniques in advanced tokamak scenarios, potentially extendable to ITER.
Stellarator divertor design is an open area of research. Current approaches include the helical d... more Stellarator divertor design is an open area of research. Current approaches include the helical divertor of LHD and the island divertor of W7-X. These divertor designs will be presented along with non-resonant divertors useful for quasisymmetric devices. Additionally, the advantages and difficulties of each of these divertors will be described. Looking forward, implications for divertors on physics designs of new stellarators will be discussed.
Injection of non-recycling low to mid Z impurity powders has recently been introduced as a new te... more Injection of non-recycling low to mid Z impurity powders has recently been introduced as a new technique to mitigate divertor heat fluxes and improve wall-conditioning at DIII-D, which are both of critical importance for the success of next-step fusion devices. This new technique enables extending the limited number of impurity species and mixtures usable with conventional gas injection. In this paper, the first radiative power exhaust experiments with impurity powder injection at the DIII-D tokamak are presented and discussed. Lithium (Li) and boron (B) powders were dropped with rates of 1-50 mg/s directly into the outer strike point located in the closed small angle slot (SAS) divertor during ELMy H‐mode scenarios in upper single null, forward Bt configuration (Ip~1 MA, Bt=2 T, PNB ~ 6 MW, fELM ~ 80 Hz, ne ~ 3.6-5.0x1019 m-3). The injection caused a substantial reduction in divertor electron temperature, particle fluxes, and heat fluxes, measured by Langmuir probes in the SAS. The...
Nuclear Fusion, 2023
Experiments have been conducted in the DIII-D tokamak to explore the in-situ growth of silicon-ri... more Experiments have been conducted in the DIII-D tokamak to explore the in-situ growth of silicon-rich layers as a potential technique for real-time replenishment of surface coatings on plasma-facing components (PFCs) during steady-state long-pulse reactor operation. Silicon (Si) pellets of 1 mm diameter were injected into low- and high-confinement (L-mode and H-mode) plasma discharges with densities ranging from 3.9–7.5x10^19 m^−3 and input powers ranging from 5.5 to 9 MW. The small Si pellets were delivered with the impurity granule injector at frequencies ranging from 4 to 16 Hz corresponding to mass flow rates of 5–19 mg/s (1–4.2x10^20 Si s^−1 at cumulative amounts of up to 34 mg of Si per five-second discharge. Graphite samples were exposed to the scrape-off layer and private flux region plasmas through the divertor material evaluation system to evaluate the Si deposition on the divertor targets. The Si II emission at the sample correlates with silicon injection and suggests net surface Si-deposition in measurable amounts. Post-mortem analysis showed Si-rich coatings containing silicon oxides, of which SiO2 is the dominant component. No evidence of SiC was found, which is attributed to low divertor surface temperatures. The in-situ and ex-situ analysis found that Si-rich coatings of at least 0.4–1.2 nm thickness have been deposited at 0.4–0.7 nm/s. The technique is estimated to coat a surface area of at least 0.94 m^2 on the outer divertor. These results demonstrate the potential of using real-time material injection to form Si-enriched layers on divertor PFCs during reactor operation.
Bulletin of the American Physical Society, 2016
SWALD TEAM-N seeding discharges have been performed at Wendelstein 7-X during its startup limiter... more SWALD TEAM-N seeding discharges have been performed at Wendelstein 7-X during its startup limiter campaign. In this study, the cooling effects on the local electron temperature Te measured by three diagnostic systems are discussed, which have a defined alignment in the helical SOL topology during the W7-X limiter phase. Radial Te profiles obtained from a thermal helium beam and Te measurements from a reciprocating Langmuir probe system, both located inside the same flux tube as the N injection system, are compared to Te from Langmuir probes installed on a limiter tile, which is not directly connected magnetically to the injection flux channel. This setup enables to study the N cooling effect in the flux tube geometry as an important test which impacts the 3-D topology in a low shear stellarator and the edge cooling symmetry. A clear reduction of Te in the entire SOL has been measured as a reaction to the N seeding and the relative variation in the temperature, however, depends on the actual placing of the diagnostic in the flux tube geometry and on the distances from the N injection point. A direct comparison of the measurements to EMC3-EIRENE modeling will be presented.
Experimental Campaign of Wendelstein 7-X R. König, M. Krychowiak, M. Jakubowski, Y. Feng, O. Schm... more Experimental Campaign of Wendelstein 7-X R. König, M. Krychowiak, M. Jakubowski, Y. Feng, O. Schmitz, F. Effenberg, F. Reimold, S. Brezinsek, M. Otte, G. Anda, T. Barbui, C. Biedermann, S. Bozhenkov, P. Drewelow, M. Endler, D. A Ennis, O. Ford, G. Fuchert, Y. Gao, D. Gradic, K. C. Hammond, J. Harris, M. Hirsch, J. Knauer, P. Kornejew, G. Kocsis, T. Kremeyer, H. Niemann, E. Pasch, V. Perseo, L. Rudischhauser, G. Schlisio, A. Puig Sitjes, T. Sunn Pedersen, F. Pisano, T. Szepesi, E. Wang, T. Wauters, U. Wenzel, V. Winters, D. Zhang, S. Zoletnik and the W7-X team
Bulletin of the American Physical Society, 2016
, W7-X TEAM-A crucial topic for the stellarator W7-X is the power dissipation by impurities for f... more , W7-X TEAM-A crucial topic for the stellarator W7-X is the power dissipation by impurities for future island divertor scenarios. The investigation of the related heat flux distribution and profiles including the radial power fall-off length λ q in the 3D stellarator SOL is less straight forward as in toroidally symmetric tokamaks. Studies with the 3D plasma edge transport code EMC3-Eirene predicted a modulation of plasma parameters with L C and correlated heterogenous heat and particle loads onto the limiters during start-up operation. The relative simple start up geometry at W7-X allows for a detailed analysis of the heat fluxes in separate helical transport channels featuring different ∥ to ⊥ transport ratios. It is shown that the SOL has two characteristic fall off domains-a near SOL and a far SOL domain which both have different power decay lengths. An increase of λ q with L C in the order of 1-1.5cm in the near SOL and 1.8-2.8cm in the far SOL for a power scan in the range of P =0.5-2MW at n LCF S = 2×10 18 m −3 has been found. First comparisons with IR camera data will be discussed.
The magnetic edge topology of the Wendelstein 7-X limiter startup field configuration features se... more The magnetic edge topology of the Wendelstein 7-X limiter startup field configuration features separated magnetic flux tubes of three different target to target connection lengths LC . Simulations are peformed with the 3-D plasma edge fluid and kinetic neutral transport Monte Carlo Code EMC3-EIRENE in order to provide a systematic assessment of the governing mechanisms of the 3-D plasma edge heat transport and its relation to the magnetic topology. The standard limiter configuration is compared with a configuration of increased rotational transform (ιedge = 0.87 → 0.91). It is shown that the configuration with higher ιegde features a re-distribution of LC in the boundary. This change of the magnetic topology causes changes in the plasma profiles and the local limiter heat loads. The levels of parallel heat fluxes q‖ slightly differ dependent on LC and two different decay regimes are found in the near and far scrape off layer (SOL). The characteristic power e-folding length is found ...
Nature Physics, 2018
In the version of this Article originally published, the superscript 6 indicating equally contrib... more In the version of this Article originally published, the superscript 6 indicating equally contributing authors was missing from M. Buzzi. This has now been corrected.
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Papers by Florian Effenberg
Ne seeding allows for sustained enhancement of edge radiation with (frad>80%) a prolonged decay of line emission of several tens of seconds after the end of the injection indicating high recycling of this noble gas at the carbon main plasma facing components (PFCs). In N2 seeded discharges, it is shown that a response of line emission and plasma parameters is in correlation to the puff duration which indicates a higher level of absorption of this seeding gas in the PFCs. Continuous N2 seeding results in global cooling of the scrape-off layer (SOL) and decay of radiation over several seconds after the injection. These results have been obtained by local gas injection into one of the five independent island chains. Zeff increases from 1.2 to 2.1-2.2 at maximum radiation with both seeding gases and reduces after the injection only in case of N2 seeding.
Damping of the counter-streaming SOL island flows is in correlation with a reduction of the divertor particle fluxes in response to impurity seeding. A correlated reduction of density has been compensated by feedback controlled divertor gas fueling.
The controlled reduction of heat fluxes within this complex 3D edge island geometry is a very promising finding concerning detachment optimization in future scenarios with steady-state operation and metallic wall upgrades [2].
The significant features of the 3D distribution of radiative power losses and the effects on heat and particle fluxes have been recovered with EMC3-EIRENE simulations. Simulations with different island geometries and parameter scans have been investigated to resolve the dependencies of the radiative cooling effects and detachment transitions for Ne and N2 seeding.
Acknowledgements:
This work was supported in part by the U.S. Department of Energy (DoE) under grant DE-SC0014210. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
References:
[1] F. Effenberg et al 2019 Nucl. Fusion 59 106020
[2] Y. Feng et al 2016 Nucl. Fusion 56 126011
A systematic set of experiments has been conducted during the first island divertor campaign which show that switching from Neon (Ne) to nitrogen (N2) as seeding gases enables switching from global to more localized edge cooling. In case of Ne seeding significant enhancement of edge radiation with slow decay after end of the injection is observed due to the high recycling properties of this noble gas. The N2 seeded discharges show immediate response of local plasma parameters at the divertor target correlated to the puff duration. Fast Te recovery and drop of Prad after end of the puff suggest a rather low recycling coefficient for this impurity species. These effects are analysed by 3D modeling with EMC3-EIRENE for high and low recycling coefficients.
The impact of the 3D edge magnetic structure on radiation is investigated experimentally by changing island size and connection lengths with the island control coils in the 5/5 configuration for scenarios with ne~1.8e19m-3 at PECRH~2.9MW. A 22ms Ne puff causes enhancement of Prad by ~1.6MW. Application of full control coil currents, Icc=2.5kA, yields a reduction of intrinsic Prad level from ~0.7MW to 0.3MW and an reduced increase of Prad by 1.1MW in response to Ne seeing. The change of island geometry results in a faster decay of total impurity radiation measured by an effective time constant \tau_{Prad}.
The presented findings on power exhaust control by impurity seeding in the W7-X island divertor are the basis for implementing radiative cooling as means to protect plasma facing components as performance levels at this new HELIAS stellarator are rising. With increasing performance, equilibrium effects will impact on the 3D magnetic structure, which is addressed by equilibrium reconstruction with V3FIT and the 3D MHD code HINT. Investigation of the link between the magnetic structure, the appropriate gas species, the injection location and the impurity transport is of critical importance for the high level goal of HELIAS divertor optimization. The experimental and numerical studies presented here represent a first-time consistent exploration of this field in the new island divertor configuration.
The study encompasses a combination of results from a numerical analysis with the coupled 3D fluid plasma edge and kinetic neutral transport Monte-Carlo Code EMC3-EIRENE and experiments during the limiter startup campaign and the first island divertor campaign. In the startup configuration, the limiter positions and their poloidal extension resulted in a helical SOL consisting out of three types of separate helical flux bundles. These distinct magnetic flux tubes allowed a transport analysis based on simple SOL models and a heat flux characterization of power decay lengths q|| and peak heat flux values comparable to methods applied at tokamaks. Experiments with nitrogen (N2) and neon (Ne) injection demonstrated an edge temperature reduction clearly correlated to edge radiation enhancement. The N2 injection showed a fast response in which Te recovered almost entirely after the injection was stopped. Ne featured higher recycling and terminated the discharge due to increased radiation causing radiative instabilities. These experimental results were analyzed with EMC3-EIRENE simulations. N2 was modeled as a gas source neglecting recycling and, in contrast, Ne was sourced from the limiters to incorporate its properties as a fully recycling species.
Initial results from first island divertor experiments at W7-X show that core fueling and refuelling is probably prevented by island neutral screening effects as indicated by predictive modeling [1]. Here, radiative edge cooling is investigated for steering the neutral fueling by controlling the edge temperature. Initial explorations with EMC3-EIRENE indicate, that the location of the gas injection with respect to the island geometry matters. Experiments are commencing about changing the fueling and seeding locations from remote valves to specific SOL locations at the island O-point close to the mid-plane or directly in front of the divertor tiles near the main recycling and erosion zones. Results from fueling experiments aided by radiative edge cooling will be discussed. A focus of this work is the investigation of a correlation of local cooling features to the edge island geometry in the standard island divertor configurations. First, Ne was injected in discharges without using island control coils (Icc=0kA) and showed in response an enhancement of edge radition and correlated drop of divertor target heat fluxes. For Icc=2.5kA the absolute radition level reduces and a faster decay in response of the impurity radiation induced by Ne seeding was observed. These experiments were repeated with N2. First results indicate a lower increase of edge radiation and fast drop of edge radiation in comparison to Ne seeded discharges. A systematic analysis of these experiments based on 3D numerical simulations with EMC3-EIRENE will be presented.
[1] Y. Feng, et al., Nucl. Fusion 126011 (2016) 56
Acknowledgments: This work was supported in part by the U.S. Department of Energy (DoE) under grant DE-SC0014210 and by the EUROfusion Consortium under grant agreement No 633053.
To address these issues and reduce the peak power fluxes and increase the widths of the wetted area, methods of power spreading and power dissipation are currently being developed. A promising technique is radiative power exhaust with seeded impurities. Experiments with neon (Ne) and nitrogen (N2) seeding in the standard island divertor configuration have demonstrated that the island divertor allows for stable plasma operation at enhanced radiative power losses reducing the power fluxes to the divertors by ~70% at the expense of a maximum loss in energy confinement of DtE≈15%. To prepare for long-pulse plasma scenarios seeding experiments have also been performed in configurations mimicking the evolution of the toroidal net-currents and plasma equilibrium effects by carefully adjusting the vacuum magnetic fields. The results show that the edge island geometry strongly determines the power and impurity exhaust and may be carefully adjusted to optimize for more stable and safe high radiative divertor operation. The effects of the magnetic field and impurities on the peak power fluxes and power widths will be discussed. 3D transport modeling with EMC3-EIRENE is used to analyze these effects and resolve the underlying main transport features. In particular, impurity seeding will be discussed with respect to its aim and suitability of detachment control in future long-pulse operation.