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1. Accelerated Bayesian inference of plasma profiles with self-consistent MHD equilibria at W7-X via neural networks

Author

Merlo, Andrea, Pavone, Andrea, Böckenhoff, Daniel, Pasch, Ekkehard, Fuchert, Golo, Brunner, Kai Jakob, Rahbarnia, Kian, Schilling, Jonathan, Höfel, Udo, Kwak, Sehyun, Svensson, Jakob, Pedersen, Thomas Sunn, and team, the W7-X

Subjects
Physics - Plasma Physics
Abstract

High-$\langle \beta \rangle$ operations require a fast and robust inference of plasma parameters with a self-consistent MHD equilibrium. Precalculated MHD equilibria are usually employed at W7-X due to the high computational cost. To address this, we couple a physics-regularized NN model that approximates the ideal-MHD equilibrium with the Bayesian modeling framework Minerva. We show the fast and robust inference of plasma profiles (electron temperature and density) with a self-consistent MHD equilibrium approximated by the NN model. We investigate the robustness of the inference across diverse synthetic W7-X plasma scenarios. The inferred plasma parameters and their uncertainties are compatible with the parameters inferred using the VMEC, and the inference time is reduced by more than two orders of magnitude. This work suggests that MHD self-consistent inferences of plasma parameters can be performed between shots., Comment: 18 pages, 6 figures

Published
2023
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2. D-Mag: a laboratory for studying plasma physics and diagnostics in strong magnetic fields

Author

Jagielski, Bartholomaeaus, Wenzel, Uwe, Pedersen, Thomas Sunn, Melzer, Andre, Pandey, Arun, Mackel, Felix, and team, the Wendelstein 7-X

Subjects
Physics - Instrumentation and Detectors, Physics - Applied Physics
Abstract

We have set up a diagnostic magnet (D-Mag) laboratory for a wide range of applications in plasma physics. It consists of a superconducting magnet for field strengths of up to 5.9 T. The main purpose is to provide an experimental environment for the development of plasma diagnostics for nuclear fusion studies and the investigation of dusty plasmas in strong magnetic fields. We describe in the article the setup and operation of the D-Mag. Some applications are presented for the development of plasma diagnostics, such as neutral pressure gauges and Langmuir probes that have to be operated in strong magnetic fields. Among the examples is the test of the long-pulse capability and stability of the diagnostic pressure gauge (DPG) for the ITER device., Comment: Superconducting Magnet - laboratory for diagnostic development and tests in strong and variable magnetic fields of up to 6 Tesla

Published
2023
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3. Physics-regularized neural network of the ideal-MHD solution operator in Wendelstein 7-X configurations

Author

Merlo, Andrea, Böckenhoff, Daniel, Schilling, Jonathan, Lazerson, Samuel Aaron, Pedersen, Thomas Sunn, and team, the W7-X

Subjects
Physics - Plasma Physics, Physics - Computational Physics
Abstract

The computational cost of constructing 3D magnetohydrodynamic (MHD) equilibria is one of the limiting factors in stellarator research and design. Although data-driven approaches have been proposed to provide fast 3D MHD equilibria, the accuracy with which equilibrium properties are reconstructed is unknown. In this work, we describe an artificial neural network (NN) that quickly approximates the ideal-MHD solution operator in Wendelstein 7-X (W7-X) configurations. This model fulfils equilibrium symmetries by construction. The MHD force residual regularizes the solution of the NN to satisfy the ideal-MHD equations. The model predicts the equilibrium solution with high accuracy, and it faithfully reconstructs global equilibrium quantities and proxy functions used in stellarator optimization. We also optimize W7-X magnetic configurations, where desiderable configurations can be found in terms of fast particle confinement. This work demonstrates with which accuracy NN models can approximate the 3D ideal-MHD solution operator and reconstruct equilibrium properties of interest, and it suggests how they might be used to optimize stellarator magnetic configurations., Comment: 47 pages, 28 figures, published in Nuclear Fusion

Published
2022
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4. Proof of concept of a fast surrogate model of the VMEC code via neural networks in Wendelstein 7-X scenarios

Author

Merlo, Andrea, Böckenhoff, Daniel, Schilling, Jonathan, Höfel, Udo, Kwak, Sehyun, Svensson, Jakob, Pavone, Andrea, Lazerson, Samuel Aaron, Pedersen, Thomas Sunn, and Team, W7-X

Subjects
Physics - Plasma Physics
Abstract

In magnetic confinement fusion research, the achievement of high plasma pressure is key to reaching the goal of net energy production. The magnetohydrodynamic (MHD) model is used to self-consistently calculate the effects the plasma pressure induces on the magnetic field used to confine the plasma. Such MHD calculations serve as input for the assessment of a number of important physics questions. The VMEC code is the most widely used to evaluate 3D ideal-MHD equilibria, as prominently present in stellarators. However, considering the computational cost, it is rarely used in large-scale or online applications. Access to fast MHD equilbria is a challenging problem in fusion research, one which machine learning could effectively address. In this paper, we present artificial neural network (NN) models able to quickly compute the equilibrium magnetic field of W7-X. Magnetic configurations that extensively cover the device operational space, and plasma profiles with volume averaged normalized plasma pressure $\langle \beta \rangle$ ($\beta$ = $\frac{2 \mu_0 p}{B^2}$) up to 5% and non-zero net toroidal current are included in the data set. By using convolutional layers, the spectral representation of the magnetic flux surfaces can be efficiently computed with a single network. To discover better models, a Bayesian hyper-parameter search is carried out, and 3D convolutional neural networks are found to outperform feed-forward fully-connected neural networks. The achieved normalized root-mean-squared error ranges from 1% to 20% across the different scenarios. The model inference time for a single equilibrium is on the order of milliseconds. Finally, this work shows the feasibility of a fast NN drop-in surrogate model for VMEC, and it opens up new operational scenarios where target applications could make use of magnetic equilibria at unprecedented scales., Comment: 24 pages, 16 figures. Accepted for publication to Nuclear Fusion

Published
2021
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5. Experimental confirmation of efficient island divertor operation and successful neoclassical transport optimization in Wendelstein 7-X

Author

Pedersen, Thomas Sunn and Team, the W7-X

Subjects
Physics - Plasma Physics
Abstract

We present recent highlights from the most recent operation phases of Wendelstein 7-X, the most advanced stellarator in the world. Stable detachment with good particle exhaust, low impurity content, and energy confinement times exceeding 100 ms, have been maintained for tens of seconds. Pellet fueling allows for plasma phases with reduced ITG turbulence, and during such phases, the overall confinement is so good (energy confinement times often exceeding 200 ms) that the attained density and temperature profiles would not have been possible in less optimized devices, since they would have had neoclassical transport losses exceeding the heating applied in W7-X. This provides proof that the reduction of neoclassical transport through magnetic field optimization is successful. W7-X plasmas generally show good impurity screening and high plasma purity, but there is evidence of longer impurity confinement times during turbulence-suppressed phases.

Published
2021

6. Heat and particle flux detachment with stable plasma conditions in the Wendelstein 7-X stellarator fusion experiment

Author

Jakubowski, Marcin, König, Ralf, Schmitz, Oliver, Feng, Yuhe, Krychowiak, Maciej, Otte, Matthias, Reimold, Felix, Dinklage, Andreas, Drewelow, Peter, Effenberg, Florian, Gao, Yu, Niemann, Holger, Schlisio, Georg, Pavone, Andrea, Pedersen, Thomas Sunn, Wenzel, Uwe, Zhang, Daihong, Brezinsek, Sebastijan, Bozhenkov, Sergey, Brunner, Kai Jakob, Carralero, Daniel, Hammond, Ken, Fuchert, Golo, Knauer, Jens, Langenberg, Andreas, Laqua, Heinrich, Marsen, Stefan, Pablant, Novimir, Rahbarnia, Kian, Rudichshauser, Lukas, Stange, Torsten, Wurden, Glen, and team, W7-X

Subjects
Physics - Plasma Physics, Nuclear Experiment
Abstract

Reduction of particle and heat fluxes to plasma facing components is critical to achieve stable conditions for both the plasma and the plasma material interface in magnetic confinement fusion experiments. A stable and reproducible plasma state in which the heat flux is almost completely removed from the material surfaces was discovered recently in the Wendelstein 7-X stellarator experiment. At the same time also particle fluxes are reduced such that material erosion can be mitigated. Sufficient neutral pressure was reached to maintain stable particle exhaust for density control in this plasma state. This regime could be maintained for up to 28 seconds with a minimum feedback control., Comment: Aimed at Physical Review Letters

Published
2020

8. Erosion and deposition investigations on Wendelstein 7-X first wall components for the first operation phase in divertor configuration

Author

Dhard, Chandra Prakash, Mayer, Matej, Brezinsek, Sebastijan, Masuzaki, Suguru, Motojima, Gen, König, Ralf, Pedersen, Thomas Sunn, Neu, Rudolf, Hathiramani, Dag, Krause, Marco, Ehrke, Gunnar, Ruset, Cristian, Schwarz-Selinger, Thomas, Balden, Martin, Burwitz, Vassily Vadimovitch, Coenen, Jan Willem, Linsmeier, Christian, Naujoks, Dirk, Neubauer, Olaf, Rack, Michael, Tokitani, Masayuki, Oelmann, Jannis, Li, Cong, Rasinski, Marcin, Höschen, Daniel, and Yajima, Miyuki

Published
2019
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10. Demonstration of reduced neoclassical energy transport in Wendelstein 7-X

Author

BEIDLER, Craig, SMITH, H. M., ALONSO, Juan Arturo, ANDREEVA, T., BALDZUHN, J., BEURSKENS, Marc, BORCHARDT, Matthias, BOZHENKOV, Sergey, BRUNNER, Kai Jakob, DAMM, Hannes, DREVLAK, M., FORD, Oliver P., FUCHERT, Golo, GEIGER, Joachim, HELANDER, P., HERGENHAHN, Uwe, HIRSCH, M., Höfel, Udo, KAZAKOV, Yevgen, KLEIBER, Ralf, KRYCHOWIAK, Maciej, KWAK, Sehyun, LANGENBERG, Andreas, LAQUA, Heinrich, NEUNER, U., PABLANT, Novimir, PASCH, E., PAVONE, Andrea, PEDERSEN, Thomas Sunn, RAHBARNIA, Kian, SCHILLING, Jonathan, SCOTT, Evan, STANGE, Torsten, SVENSSON, J., THOMSEN, Henning, TURKIN, Yuriy, WARMER, Felix, WOLF, Robert, ZHANG, Daihong, the W7-X Team, BEIDLER, Craig, SMITH, H. M., ALONSO, Juan Arturo, ANDREEVA, T., BALDZUHN, J., BEURSKENS, Marc, BORCHARDT, Matthias, BOZHENKOV, Sergey, BRUNNER, Kai Jakob, DAMM, Hannes, DREVLAK, M., FORD, Oliver P., FUCHERT, Golo, GEIGER, Joachim, HELANDER, P., HERGENHAHN, Uwe, HIRSCH, M., Höfel, Udo, KAZAKOV, Yevgen, KLEIBER, Ralf, KRYCHOWIAK, Maciej, KWAK, Sehyun, LANGENBERG, Andreas, LAQUA, Heinrich, NEUNER, U., PABLANT, Novimir, PASCH, E., PAVONE, Andrea, PEDERSEN, Thomas Sunn, RAHBARNIA, Kian, SCHILLING, Jonathan, SCOTT, Evan, STANGE, Torsten, SVENSSON, J., THOMSEN, Henning, TURKIN, Yuriy, WARMER, Felix, WOLF, Robert, ZHANG, Daihong, and the W7-X Team

Abstract

Research on magnetic confinement of high-temperature plasmas has the ultimate goal of harnessing nuclear fusion for the production of electricity. Although the tokamak1 is the leading toroidal magnetic-confinement concept, it is not without shortcomings and the fusion community has therefore also pursued alternative concepts such as the stellarator. Unlike axisymmetric tokamaks, stellarators possess a three-dimensional (3D) magnetic field geometry. The availability of this additional dimension opens up an extensive configuration space for computational optimization of both the field geometry itself and the current-carrying coils that produce it. Such an optimization was undertaken in designing Wendelstein 7-X (W7-X)2, a large helical-axis advanced stellarator (HELIAS), which began operation in 2015 at Greifswald, Germany. A major drawback of 3D magnetic field geometry, however, is that it introduces a strong temperature dependence into the stellarator’s non-turbulent ‘neoclassical’ energy transport. Indeed, such energy losses will become prohibitive in high-temperature reactor plasmas unless a strong reduction of the geometrical factor associated with this transport can be achieved; such a reduction was therefore a principal goal of the design of W7-X. In spite of the modest heating power currently available, W7-X has already been able to achieve high-temperature plasma conditions during its 2017 and 2018 experimental campaigns, producing record values of the fusion triple product for such stellarator plasmas3,4. The triple product of plasma density, ion temperature and energy confinement time is used in fusion research as a figure of merit, as it must attain a certain threshold value before net-energy-producing operation of a reactor becomes possible1,5. Here we demonstrate that such record values provide evidence for reduced neoclassical energy transport in W7-X, as the plasma profiles that produced these results could not have been obtained in stellarators lackin, source:Beidler, C.D., Smith, H.M., Alonso, A. et al. Demonstration of reduced neoclassical energy transport in Wendelstein 7-X. Nature 596, 221–226 (2021). https://doi.org/10.1038/s41586-021-03687-w, source:https://doi.org/10.1038/s41586-021-03687-w, identifier:0000-0002-4395-239X

Published
2023

11. Physics-regularized neural network of the ideal-MHD solution operator in Wendelstein 7-X configurations

Author

Merlo, Andrea, Böckenhoff, Daniel, Schilling, Jonathan, Lazerson, Samuel Aaron, Pedersen, Thomas Sunn, W7-X Team, and W7-X Team, Max Planck Institute for Plasma Physics, Max Planck Society

Subjects
Plasma Physics (physics.plasm-ph), surrogate models, Technology, Nuclear and High Energy Physics, ideal-MHD, FOS: Physical sciences, Wendelstein 7-X, Computational Physics (physics.comp-ph), Condensed Matter Physics, neural networks, ddc:600, Physics - Computational Physics, Physics - Plasma Physics
Abstract

The computational cost of constructing 3D magnetohydrodynamic (MHD) equilibria is one of the limiting factors in stellarator research and design. Although data-driven approaches have been proposed to provide fast 3D MHD equilibria, the accuracy with which equilibrium properties are reconstructed is unknown. In this work, we describe an artificial neural network (NN) that quickly approximates the ideal-MHD solution operator in Wendelstein 7-X (W7-X) configurations. This model fulfils equilibrium symmetries by construction. The MHD force residual regularizes the solution of the NN to satisfy the ideal-MHD equations. The model predicts the equilibrium solution with high accuracy, and it faithfully reconstructs global equilibrium quantities and proxy functions used in stellarator optimization. We also optimize W7-X magnetic configurations, where desiderable configurations can be found in terms of fast particle confinement. This work demonstrates with which accuracy NN models can approximate the 3D ideal-MHD solution operator and reconstruct equilibrium properties of interest, and it suggests how they might be used to optimize stellarator magnetic configurations., 47 pages, 28 figures, published in Nuclear Fusion

Published
2023
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12. Experimental confirmation of efficient island divertor operation and successful neoclassical transport optimization in Wendelstein 7-X

Author

The Wendelstein 7-X Team, Pedersen, Thomas Sunn, Abramovic, I., de Baar, M., Proll, J., Pueschel, M.J., Warmer, F., The Wendelstein 7-X Team, Pedersen, Thomas Sunn, Abramovic, I., de Baar, M., Proll, J., Pueschel, M.J., and Warmer, F.

Abstract

We present recent highlights from the most recent operation phases of Wendelstein 7-X, the most advanced stellarator in the world. Stable detachment with good particle exhaust, low impurity content, and energy confinement times exceeding 100 ms, have been maintained for tens of seconds. Pellet fueling allows for plasma phases with reduced ion-temperature-gradient turbulence, and during such phases, the overall confinement is so good (energy confinement times often exceeding 200 ms) that the attained density and temperature profiles would not have been possible in less optimized devices, since they would have had neoclassical transport losses exceeding the heating applied in W7-X. This provides proof that the reduction of neoclassical transport through magnetic field optimization is successful. W7-X plasmas generally show good impurity screening and high plasma purity, but there is evidence of longer impurity confinement times during turbulence-suppressed phases.

Published
2022

13. Experimental confirmation of efficient island divertor operation and successful neoclassical transport optimization in Wendelstein 7-X

Author

Pedersen, Thomas Sunn, Abramovic, I., Agostinetti, P., Torres, M. Agredano, Äkäslompolo, S., Belloso, J. Alcuson, Aleynikov, P., Aleynikova, K., Alhashimi, M., Ali, A., Allen, N., Alonso, A., Anda, G., Andreeva, T., Angioni, C., Arkhipov, A., Arnold, A., Asad, W., Ascasibar, E., Aumeunier, M. H., Avramidis, K., Aymerich, E., Baek, S. G., Bähner, J., Baillod, A., Balden, M., Baldzuhn, J., Ballinger, S., Banduch, M., Bannmann, S., Navarro, A. Bañón, Barbui, T., Beidler, C., Belafdil, C., Bencze, A., Benndorf, A., Beurskens, M., Biedermann, C., Biletskyi, O., Blackwell, B., Blatzheim, M., Bluhm, T., Böckenhoff, D., Bongiovi, G., Borchardt, M., Borodin, D., Boscary, J., Bosch, H., Bosmann, T., Böswirth, B., Böttger, L., Bottino, A., Bozhenkov, S., Brakel, R., Brandt, C., Bräuer, T., Braune, H., Brezinsek, S., Brunner, K., Buller, S., Burhenn, R., Bussiahn, R., Buttenschön, B., Buzás, A., Bykov, V., Calvo, I., Mata, K. Camacho, Caminal, I., Cannas, B., Cappa, A., Carls, A., Carovani, F., Carr, M., Carralero, D., Carvalho, B., Casas, J., Castano-Bardawil, D., Castejon, F., Chaudhary, N., Chelis, I., Chomiczewska, A., Coenen, J. W., Cole, M., Cordella, F., Corre, Y., Crombe, K., Cseh, G., Csillag, B., Damm, H., Day, C., de Baar, M., De la Cal, E., Degenkolbe, S., Demby, A., Denk, S., Dhard, C., Di Siena, A., Dinklage, A., Dittmar, T., Dreval, M., Drevlak, M., Drewelow, P., Drews, P., Dunai, D., Edlund, E., Effenberg, F., Ehrke, G., Endler, M., Ennis, D. A., Escoto, F. J., Estrada, T., Fable, E., Fahrenkamp, N., Fanni, A., Faustin, J., Fellinger, J., Feng, Y., Figacz, W., Flom, E., Ford, O., Fornal, T., Frerichs, H., Freundt, S., Fuchert, G., Fukuyama, M., Füllenbach, F., Gantenbein, G., Gao, Y., Garcia, K., Regaña, J. M.García, García-Cortés, I., Gaspar, J., Gates, D. A., Geiger, J., Geiger, B., Giudicotti, L., González, A., Goriaev, A., Gradic, D., Grahl, M., Graves, J. P., Green, J., Grelier, E., Greuner, H., Groß, S., Grote, H., Groth, M., Gruca, M., Grulke, O., Grün, M., Arnaiz, J. Guerrero, Günter, S., Haak, V., Haas, M., Hacker, P., Hakola, A., Hallenbert, A., Hammond, K., Han, X., Hansen, S. K., Harris, J. H., Hartfuß, H., Hartmann, D., Hathiramani, D., Hatzky, R., Hawke, J., Hegedus, S., Hein, B., Heinemann, B., Helander, P., Henneberg, S., Hergenhahn, U., Hidalgo, C., Hindenlang, F., Hirsch, M., Höfel, U., Hollfeld, K. P., Holtz, A., Hopf, D., Höschen, D., Houry, M., Howard, J., Huang, X., Hubeny, M., Hudson, S., Ida, K., Igitkhanov, Y., Igochine, V., Illy, S., Ionita-Schrittwieser, C., Isobe, M., Jabłczynska, M., Jablonski, S., Jagielski, B., Jakubowski, M., van Vuuren, A. Jansen, Jelonnek, J., Jenko, F., Jensen, T., Jenzsch, H., Junghanns, P., Kaczmarczyk, J., Kallmeyer, J., Kamionka, U., Kandler, M., Kasilov, S., Kazakov, Y., Kennedy, D., Kharwandikar, A., Khokhlov, M., Kiefer, C., Killer, C., Kirschner, A., Kleiber, R., Klinger, T., Klose, S., Knauer, J., Knieps, A., Köchl, F., Kocsis, G., Kolesnichenko, Ya I., Könies, A., König, R., Kontula, J., Kornejew, P., Koschinsky, J., Kozulia, M. M., Krämer-Flecken, A., Krampitz, R., Krause, M., Krawczyk, N., Kremeyer, T., Krier, L., Kriete, D. M., Krychowiak, M., Ksiazek, I., Kubkowska, M., Kuczynski, M., Kühner, G., Kumar, A., Kurki-Suonio, T., Kwak, S., Landreman, M., Lang, P. T., Langenberg, A., Laqua, H., Laube, R., Lazerson, S., Lewerentz, M., Li, C., Liang, Y., Linsmeier, Ch, Lion, J., Litnovsky, A., Liu, S., Lobsien, J., Loizu, J., Lore, J., Lorenz, A., Losada, U., Louche, F., Lunsford, R., Lutsenko, V., Machielsen, M., Mackel, F., Maisano-Brown, J., Maj, O., Makowski, D., Manduchi, G., Maragkoudakis, E., Marchuk, O., Marsen, S., Martines, E., Martinez-Fernandez, J., Marushchenko, M., Masuzaki, S., Maurer, D., Mayer, M., McCarthy, K. J., McCormack, O., McNeely, P., Meister, H., Mendelevitch, B., Mendes, S., Merlo, A., Messian, A., Mielczarek, A., Mishchenko, O., Missal, B., Mitteau, R., Moiseenko, V. E., Mollen, A., Moncada, V., Mönnich, T., Morisaki, T., Moseev, D., Motojima, G., Mulas, S., Mulsow, M., Nagel, M., Naujoks, D., Naulin, V., Neelis, T., Neilson, H., Neu, R., Neubauer, O., Neuner, U., Nicolai, D., Nielsen, S. K., Niemann, H., Nishiza, T., Nishizawa, T., Nührenberg, C., Ochoukov, R., Oelmann, J., Offermanns, G., Ogawa, K., Okamura, S., Ölmanns, J., Ongena, J., Oosterbeek, J., Otte, M., Pablant, N., Alvarez, N. Panadero, Pandey, A., Pasch, E., Pavlichenko, R., Pavone, A., Pawelec, E., Pechstein, G., Pelka, G., Perseo, V., Peterson, B., Pilopp, D., Pingel, S., Pisano, F., Plöckl, B., Plunk, G., Pölöskei, P., Pompe, B., Popov, A., Porkolab, M., Proll, J., Pueschel, M. J., Puiatti, M. E., Sitjes, A. Puig, Purps, F., Rahbarnia, K., Rasinski, M., Rasmussen, J., Reiman, A., Reimold, F., Reisner, M., Reiter, D., Richou, M., Riedl, R., Riemann, J., Riße, K., Roberg-Clark, G., Rohde, V., Romazanov, J., Rondeshagen, D., Rong, P., Rudischhauser, L., Rummel, T., Rummel, K., Runov, A., Rust, N., Ryc, L., Salembier, P., Salewski, M., Sanchez, E., Satake, S., Satheeswaran, G., Schacht, J., Scharff, E., Schauer, F., Schilling, J., Schlisio, G., Schmid, K., Schmitt, J., Schmitz, O., Schneider, W., Schneider, M., Schneider, P., Schrittwieser, R., Schröder, T., Schröder, M., Schroeder, R., Schweer, B., Schwörer, D., Scott, E., Shanahan, B., Sias, G., Sichta, P., Singer, M., Sinha, P., Sipliä, S., Slaby, C., Sleczka, M., Smith, H., Smoniewski, J., Sonnendrücker, E., Spolaore, M., Spring, A., Stadler, R., Stanczak, D., Stange, T., Stepanov, I., Stephey, L., Stober, J., Stroth, U., Strumberger, E., Suzuki, C., Suzuki, Y., Svensson, J., Szabolics, T., Szepesi, T., Szücs, M., Tabarés, F. L., Tamura, N., Tancetti, A., Tantos, C., Terry, J., Thienpondt, H., Thomsen, H., Thumm, M., Travere, J. M., Traverso, P., Tretter, J., Trier, E., Mora, H. Trimino, Tsujimura, T., Turkin, Y., Tykhyi, A., Unterberg, B., van Eeten, P., van Milligen, B. Ph, van Schoor, M., Vano, L., Varoutis, S., Vecsei, M., Vela, L., Velasco, J. L., Vervier, M., Vianello, N., Viebke, H., Vilbrandt, R., Vogel, G., Vogt, N., Volkhausen, C., von Stechow, A., Wagner, F., Wang, E., Wang, H., Warmer, F., Wauters, T., Wegener, L., Wegner, T., Weir, G., Wenzel, U., White, A., Wilde, F., Wilms, F., Windisch, T., Winkler, M., Winter, A., Winters, V., Wolf, R., Wright, A. M., Wurden, G. A., Xanthopoulos, P., Xu, S., Yamada, H., Yamaguchi, H., Yokoyama, M., Yoshinuma, M., Yu, Q., Zamanov, M., Zanini, M., Zarnstorff, M., Zhang, D., Zhou, S., Zhu, J., Zhu, C., Zilker, M., Zocco, A., Zohm, H., Zoletnik, S., Zsuga, L., Pedersen, Thomas Sunn, Abramovic, I., Agostinetti, P., Torres, M. Agredano, Äkäslompolo, S., Belloso, J. Alcuson, Aleynikov, P., Aleynikova, K., Alhashimi, M., Ali, A., Allen, N., Alonso, A., Anda, G., Andreeva, T., Angioni, C., Arkhipov, A., Arnold, A., Asad, W., Ascasibar, E., Aumeunier, M. H., Avramidis, K., Aymerich, E., Baek, S. G., Bähner, J., Baillod, A., Balden, M., Baldzuhn, J., Ballinger, S., Banduch, M., Bannmann, S., Navarro, A. Bañón, Barbui, T., Beidler, C., Belafdil, C., Bencze, A., Benndorf, A., Beurskens, M., Biedermann, C., Biletskyi, O., Blackwell, B., Blatzheim, M., Bluhm, T., Böckenhoff, D., Bongiovi, G., Borchardt, M., Borodin, D., Boscary, J., Bosch, H., Bosmann, T., Böswirth, B., Böttger, L., Bottino, A., Bozhenkov, S., Brakel, R., Brandt, C., Bräuer, T., Braune, H., Brezinsek, S., Brunner, K., Buller, S., Burhenn, R., Bussiahn, R., Buttenschön, B., Buzás, A., Bykov, V., Calvo, I., Mata, K. Camacho, Caminal, I., Cannas, B., Cappa, A., Carls, A., Carovani, F., Carr, M., Carralero, D., Carvalho, B., Casas, J., Castano-Bardawil, D., Castejon, F., Chaudhary, N., Chelis, I., Chomiczewska, A., Coenen, J. W., Cole, M., Cordella, F., Corre, Y., Crombe, K., Cseh, G., Csillag, B., Damm, H., Day, C., de Baar, M., De la Cal, E., Degenkolbe, S., Demby, A., Denk, S., Dhard, C., Di Siena, A., Dinklage, A., Dittmar, T., Dreval, M., Drevlak, M., Drewelow, P., Drews, P., Dunai, D., Edlund, E., Effenberg, F., Ehrke, G., Endler, M., Ennis, D. A., Escoto, F. J., Estrada, T., Fable, E., Fahrenkamp, N., Fanni, A., Faustin, J., Fellinger, J., Feng, Y., Figacz, W., Flom, E., Ford, O., Fornal, T., Frerichs, H., Freundt, S., Fuchert, G., Fukuyama, M., Füllenbach, F., Gantenbein, G., Gao, Y., Garcia, K., Regaña, J. M.García, García-Cortés, I., Gaspar, J., Gates, D. A., Geiger, J., Geiger, B., Giudicotti, L., González, A., Goriaev, A., Gradic, D., Grahl, M., Graves, J. P., Green, J., Grelier, E., Greuner, H., Groß, S., Grote, H., Groth, M., Gruca, M., Grulke, O., Grün, M., Arnaiz, J. Guerrero, Günter, S., Haak, V., Haas, M., Hacker, P., Hakola, A., Hallenbert, A., Hammond, K., Han, X., Hansen, S. K., Harris, J. H., Hartfuß, H., Hartmann, D., Hathiramani, D., Hatzky, R., Hawke, J., Hegedus, S., Hein, B., Heinemann, B., Helander, P., Henneberg, S., Hergenhahn, U., Hidalgo, C., Hindenlang, F., Hirsch, M., Höfel, U., Hollfeld, K. P., Holtz, A., Hopf, D., Höschen, D., Houry, M., Howard, J., Huang, X., Hubeny, M., Hudson, S., Ida, K., Igitkhanov, Y., Igochine, V., Illy, S., Ionita-Schrittwieser, C., Isobe, M., Jabłczynska, M., Jablonski, S., Jagielski, B., Jakubowski, M., van Vuuren, A. Jansen, Jelonnek, J., Jenko, F., Jensen, T., Jenzsch, H., Junghanns, P., Kaczmarczyk, J., Kallmeyer, J., Kamionka, U., Kandler, M., Kasilov, S., Kazakov, Y., Kennedy, D., Kharwandikar, A., Khokhlov, M., Kiefer, C., Killer, C., Kirschner, A., Kleiber, R., Klinger, T., Klose, S., Knauer, J., Knieps, A., Köchl, F., Kocsis, G., Kolesnichenko, Ya I., Könies, A., König, R., Kontula, J., Kornejew, P., Koschinsky, J., Kozulia, M. M., Krämer-Flecken, A., Krampitz, R., Krause, M., Krawczyk, N., Kremeyer, T., Krier, L., Kriete, D. M., Krychowiak, M., Ksiazek, I., Kubkowska, M., Kuczynski, M., Kühner, G., Kumar, A., Kurki-Suonio, T., Kwak, S., Landreman, M., Lang, P. T., Langenberg, A., Laqua, H., Laube, R., Lazerson, S., Lewerentz, M., Li, C., Liang, Y., Linsmeier, Ch, Lion, J., Litnovsky, A., Liu, S., Lobsien, J., Loizu, J., Lore, J., Lorenz, A., Losada, U., Louche, F., Lunsford, R., Lutsenko, V., Machielsen, M., Mackel, F., Maisano-Brown, J., Maj, O., Makowski, D., Manduchi, G., Maragkoudakis, E., Marchuk, O., Marsen, S., Martines, E., Martinez-Fernandez, J., Marushchenko, M., Masuzaki, S., Maurer, D., Mayer, M., McCarthy, K. J., McCormack, O., McNeely, P., Meister, H., Mendelevitch, B., Mendes, S., Merlo, A., Messian, A., Mielczarek, A., Mishchenko, O., Missal, B., Mitteau, R., Moiseenko, V. E., Mollen, A., Moncada, V., Mönnich, T., Morisaki, T., Moseev, D., Motojima, G., Mulas, S., Mulsow, M., Nagel, M., Naujoks, D., Naulin, V., Neelis, T., Neilson, H., Neu, R., Neubauer, O., Neuner, U., Nicolai, D., Nielsen, S. K., Niemann, H., Nishiza, T., Nishizawa, T., Nührenberg, C., Ochoukov, R., Oelmann, J., Offermanns, G., Ogawa, K., Okamura, S., Ölmanns, J., Ongena, J., Oosterbeek, J., Otte, M., Pablant, N., Alvarez, N. Panadero, Pandey, A., Pasch, E., Pavlichenko, R., Pavone, A., Pawelec, E., Pechstein, G., Pelka, G., Perseo, V., Peterson, B., Pilopp, D., Pingel, S., Pisano, F., Plöckl, B., Plunk, G., Pölöskei, P., Pompe, B., Popov, A., Porkolab, M., Proll, J., Pueschel, M. J., Puiatti, M. E., Sitjes, A. Puig, Purps, F., Rahbarnia, K., Rasinski, M., Rasmussen, J., Reiman, A., Reimold, F., Reisner, M., Reiter, D., Richou, M., Riedl, R., Riemann, J., Riße, K., Roberg-Clark, G., Rohde, V., Romazanov, J., Rondeshagen, D., Rong, P., Rudischhauser, L., Rummel, T., Rummel, K., Runov, A., Rust, N., Ryc, L., Salembier, P., Salewski, M., Sanchez, E., Satake, S., Satheeswaran, G., Schacht, J., Scharff, E., Schauer, F., Schilling, J., Schlisio, G., Schmid, K., Schmitt, J., Schmitz, O., Schneider, W., Schneider, M., Schneider, P., Schrittwieser, R., Schröder, T., Schröder, M., Schroeder, R., Schweer, B., Schwörer, D., Scott, E., Shanahan, B., Sias, G., Sichta, P., Singer, M., Sinha, P., Sipliä, S., Slaby, C., Sleczka, M., Smith, H., Smoniewski, J., Sonnendrücker, E., Spolaore, M., Spring, A., Stadler, R., Stanczak, D., Stange, T., Stepanov, I., Stephey, L., Stober, J., Stroth, U., Strumberger, E., Suzuki, C., Suzuki, Y., Svensson, J., Szabolics, T., Szepesi, T., Szücs, M., Tabarés, F. L., Tamura, N., Tancetti, A., Tantos, C., Terry, J., Thienpondt, H., Thomsen, H., Thumm, M., Travere, J. M., Traverso, P., Tretter, J., Trier, E., Mora, H. Trimino, Tsujimura, T., Turkin, Y., Tykhyi, A., Unterberg, B., van Eeten, P., van Milligen, B. Ph, van Schoor, M., Vano, L., Varoutis, S., Vecsei, M., Vela, L., Velasco, J. L., Vervier, M., Vianello, N., Viebke, H., Vilbrandt, R., Vogel, G., Vogt, N., Volkhausen, C., von Stechow, A., Wagner, F., Wang, E., Wang, H., Warmer, F., Wauters, T., Wegener, L., Wegner, T., Weir, G., Wenzel, U., White, A., Wilde, F., Wilms, F., Windisch, T., Winkler, M., Winter, A., Winters, V., Wolf, R., Wright, A. M., Wurden, G. A., Xanthopoulos, P., Xu, S., Yamada, H., Yamaguchi, H., Yokoyama, M., Yoshinuma, M., Yu, Q., Zamanov, M., Zanini, M., Zarnstorff, M., Zhang, D., Zhou, S., Zhu, J., Zhu, C., Zilker, M., Zocco, A., Zohm, H., Zoletnik, S., and Zsuga, L.

Abstract

We present recent highlights from the most recent operation phases of Wendelstein 7-X, the most advanced stellarator in the world. Stable detachment with good particle exhaust, low impurity content, and energy confinement times exceeding 100 ms, have been maintained for tens of seconds. Pellet fueling allows for plasma phases with reduced ion-temperature-gradient turbulence, and during such phases, the overall confinement is so good (energy confinement times often exceeding 200 ms) that the attained density and temperature profiles would not have been possible in less optimized devices, since they would have had neoclassical transport losses exceeding the heating applied in W7-X. This provides proof that the reduction of neoclassical transport through magnetic field optimization is successful. W7-X plasmas generally show good impurity screening and high plasma purity, but there is evidence of longer impurity confinement times during turbulence-suppressed phases.

Published
2022

14. TOROIDAL SYSTEMS

Author

Pedersen, Thomas Sunn, primary, Stoneking, Matthew, additional, and Saitoh, Haruhiko, additional

Published
2016
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17. Proof of concept of a fast surrogate model of the VMEC code via neural networks in Wendelstein 7-X scenarios

Author

W7-X Team, Merlo, Andrea, Böckenhoff, Daniel, Schilling, Jonathan, Höfel, Udo, Kwak, Sehyun, Svensson, Jakob, Pavone, Andrea, Lazerson, Samuel Aaron, Pedersen, Thomas Sunn, Gantenbein, Gerd, Huber, Martina, Illy, Stefan, Jelonnek, John, Kobarg, Thorsten, Lang, Rouven, Leonhardt, Wolfgang, Mellein, Daniel, Papenfuß, Daniel, Scherer, Theo, Thumm, Manfred, Wadle, Simone, and Weggen, Jörg

Subjects
Technology, ddc:600
Abstract

In magnetic confinement fusion research, the achievement of high plasma pressure is key to reaching the goal of net energy production. The magnetohydrodynamic (MHD) model is used to self-consistently calculate the effects the plasma pressure induces on the magnetic field used to confine the plasma. Such MHD calculations—usually done computationally—serve as input for the assessment of a number of important physics questions. The variational moments equilibrium code (VMEC) is the most widely used to evaluate 3D ideal-MHD equilibria, as prominently present in stellarators. However, considering the computational cost, it is rarely used in large-scale or online applications (e.g. Bayesian scientific modeling, real-time plasma control). Access to fast MHD equilibria is a challenging problem in fusion research, one which machine learning could effectively address. In this paper, we present artificial neural network (NN) models able to quickly compute the equilibrium magnetic field of Wendelstein 7-X. Magnetic configurations that extensively cover the device operational space, and plasma profiles with volume-averaged normalized plasma pressure ⟨β⟩ (β = $\frac{2{\mu }_{0}p}{{B}^{2}}$) up to 5% and non-zero net toroidal current are included in the data set. By using convolutional layers, the spectral representation of the magnetic flux surfaces can be efficiently computed with a single network. To discover better models, a Bayesian hyper-parameter search is carried out, and 3D convolutional NNs are found to outperform feed-forward fully-connected NNs. The achieved normalized root-mean-squared error, the ratio between the regression error and the spread of the data, ranges from 1% to 20% across the different scenarios. The model inference time for a single equilibrium is on the order of milliseconds. Finally, this work shows the feasibility of a fast NN drop-in surrogate model for VMEC, and it opens up new operational scenarios where target applications could make use of magnetic equilibria at unprecedented scales.

Published
2021

18. Demonstration of reduced neoclassical energy transport in Wendelstein 7-X

Author

BEIDLER, Craig, SMITH, H. M., ALONSO, Juan Arturo, ANDREEVA, T., BALDZUHN, J., BEURSKENS, Marc, BORCHARDT, Matthias, BOZHENKOV, Sergey, BRUNNER, Kai Jakob, DAMM, Hannes, DREVLAK, M., FORD, Oliver P., FUCHERT, Golo, GEIGER, Joachim, HELANDER, P., HERGENHAHN, Uwe, HIRSCH, M., Höfel, Udo, KAZAKOV, Yevgen, KAZAKOV, Yevgen O., KLEIBER, Ralf, KRYCHOWIAK, Maciej, KWAK, Sehyun, LANGENBERG, Andreas, LAQUA, Heinrich, NEUNER, U., PABLANT, Novimir, PASCH, E., PAVONE, Andrea, PEDERSEN, Thomas Sunn, RAHBARNIA, Kian, SCHILLING, Jonathan, SCOTT, Evan, STANGE, Torsten, SVENSSON, J., THOMSEN, Henning, TURKIN, Yuriy, WARMER, Felix, WOLF, Robert, ZHANG, Daihong, the W7-X Team, BEIDLER, Craig, SMITH, H. M., ALONSO, Juan Arturo, ANDREEVA, T., BALDZUHN, J., BEURSKENS, Marc, BORCHARDT, Matthias, BOZHENKOV, Sergey, BRUNNER, Kai Jakob, DAMM, Hannes, DREVLAK, M., FORD, Oliver P., FUCHERT, Golo, GEIGER, Joachim, HELANDER, P., HERGENHAHN, Uwe, HIRSCH, M., Höfel, Udo, KAZAKOV, Yevgen, KAZAKOV, Yevgen O., KLEIBER, Ralf, KRYCHOWIAK, Maciej, KWAK, Sehyun, LANGENBERG, Andreas, LAQUA, Heinrich, NEUNER, U., PABLANT, Novimir, PASCH, E., PAVONE, Andrea, PEDERSEN, Thomas Sunn, RAHBARNIA, Kian, SCHILLING, Jonathan, SCOTT, Evan, STANGE, Torsten, SVENSSON, J., THOMSEN, Henning, TURKIN, Yuriy, WARMER, Felix, WOLF, Robert, ZHANG, Daihong, and the W7-X Team

Abstract

0000-0002-4395-239X, Research on magnetic confinement of high-temperature plasmas has the ultimate goal of harnessing nuclear fusion for the production of electricity. Although the tokamak1 is the leading toroidal magnetic-confinement concept, it is not without shortcomings and the fusion community has therefore also pursued alternative concepts such as the stellarator. Unlike axisymmetric tokamaks, stellarators possess a three-dimensional (3D) magnetic field geometry. The availability of this additional dimension opens up an extensive configuration space for computational optimization of both the field geometry itself and the current-carrying coils that produce it. Such an optimization was undertaken in designing Wendelstein 7-X (W7-X)2, a large helical-axis advanced stellarator (HELIAS), which began operation in 2015 at Greifswald, Germany. A major drawback of 3D magnetic field geometry, however, is that it introduces a strong temperature dependence into the stellarator’s non-turbulent ‘neoclassical’ energy transport. Indeed, such energy losses will become prohibitive in high-temperature reactor plasmas unless a strong reduction of the geometrical factor associated with this transport can be achieved; such a reduction was therefore a principal goal of the design of W7-X. In spite of the modest heating power currently available, W7-X has already been able to achieve high-temperature plasma conditions during its 2017 and 2018 experimental campaigns, producing record values of the fusion triple product for such stellarator plasmas3,4. The triple product of plasma density, ion temperature and energy confinement time is used in fusion research as a figure of merit, as it must attain a certain threshold value before net-energy-producing operation of a reactor becomes possible1,5. Here we demonstrate that such record values provide evidence for reduced neoclassical energy transport in W7-X, as the plasma profiles that produced these results could not have been obtained in stellarators lackin

Published
2021

22. First divertor physics studies in Wendelstein 7-X

Author

Pedersen, Thomas Sunn, König, R, Jakubowski, M, Krychowiak, M, Gradic, D., Killer, C., Niemann, H., Szepesi, Tamás, Wenzel, Uwe, Ali, A., Blackwell, Boyd, Pedersen, Thomas Sunn, König, R, Jakubowski, M, Krychowiak, M, Gradic, D., Killer, C., Niemann, H., Szepesi, Tamás, Wenzel, Uwe, Ali, A., and Blackwell, Boyd

Abstract

The Wendelstein 7-X (W7-X) optimized stellarator fusion experiment, which went into operation in 2015, has been operating since 2017 with an un-cooled modular graphite divertor. This allowed first divertor physics studies to be performed at pulse energies up to 80 MJ, as opposed to 4 MJ in the first operation phase, where five inboard limiters were installed instead of a divertor. This, and a number of other upgrades to the device capabilities, allowed extension into regimes of higher plasma density, heating power, and performance overall, e.g. setting a new stellarator world record triple product. The paper focuses on the first physics studies of how the island divertor works. The plasma heat loads arrive to a very high degree on the divertor plates, with only minor heat loads seen on other components, in particular baffle structures built in to aid neutral compression. The strike line shapes and locations change significantly from one magnetic configuration to another, in very much the same way that codes had predicted they would. Strike-line widths are as large as 10 cm, and the wetted areas also large, up to about 1.5 m2, which bodes well for future operation phases. Peak local heat loads onto the divertor were in general benign and project below the 10 MW m−2 limit of the future water-cooled divertor when operated with 10 MW of heating power, with the exception of low-density attached operation in the high-iota configuration. The most notable result was the complete (in all 10 divertor units) heat-flux detachment obtained at high-density operation in hydrogen.

Published
2019

23. APEX – Newly implemented functionalities towards the first magnetically confined electron-positron pair plasma

Author

Singer, Markus, primary, Hugenschmidt, Christoph, additional, Stenson, Eve V., additional, Hergenhahn, Uwe, additional, Horn-Stanja, Juliane, additional, Nissl, Stefan, additional, Pedersen, Thomas Sunn, additional, Saitoh, Haruhiko, additional, Dickmann, Marcel, additional, Stoneking, Matthew R., additional, Danielson, James R., additional, and Surko, Clifford M., additional

Published
2019
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24. Understanding baffle overloads observed in high-mirror configuration on Wendelstein 7-X.

Author

Gao, Yu, Feng, Yuhe, Jakubowski, Marcin W., Geiger, Joachim, Endler, Michael, Dhard, Chandra Prakash, Biedermann, Christoph, Naujoks, Dirk, Pedersen, Thomas Sunn, König, Ralf, Drewelow, Peter, Pisano, Fabio, Sitjes, Aleix Puig, Niemann, Holger, Bozhenkov, Sergey, Lazerson, Samuel, Otte, Matthias, Fellinger, Joris, Zhu, Jiawu, and Krychowiak, Maciej

Subjects
FUSION reactor divertors, HIGH temperature plasmas, HEATING load, MAGNITUDE (Mathematics), TILES
Abstract

The operational regimes in the first divertor campaign of W7-X were limited by unexpectedly high heat loads on certain baffle tiles. In the high-mirror configuration, a permanent hotspot on the baffle plate was detected by the thermographic system, which was further confirmed during the post-campaign inspections of the plasma-facing components. The maximum heat load on three baffle tiles reached about 4.5 MW m−2, which was almost an order of magnitude above its designed value of 0.5 MW m−2. The paper presents a detailed analysis to understand how the baffle plate, which is originally designed to screen the recycling neutrals and is therefore hidden from the hot plasma, can receive such a high heat load - a level that is expected for target plates. Three main causes have been identified: 1) the three baffle tiles are radially only about 5 mm away from a main heat channel towards a target, which is much shorter than the radial power decay length of several centimeters derived from the thermographic measurements in this region; 2) the three baffle tiles are intersected with field-lines of about 30 m length, which are long enough to collect considerable amount of heat via cross-field transport; 3) the baffle tiles have locally large grazing angles. In addition, the analysis method shown in this paper is of general use to estimate the heat flows in shadow areas of targets and thus to evaluate heat loads on the potentially critical components occurring in these areas, especially required for the design of complex 3D divertors like the one in W7-X. [ABSTRACT FROM AUTHOR]

Published
2020
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25. Progress of the APEX experiment for creation of an electron-positron pair plasma

Author

Hergenhahn, Uwe, primary, Horn-Stanja, Juliane, additional, Nissl, Stefan, additional, Pedersen, Thomas Sunn, additional, Saitoh, Haruhiko, additional, Stenson, Eve V., additional, Stoneking, Matthew R., additional, Dickmann, Marcel, additional, Hugenschmidt, Christoph, additional, Singer, Markus, additional, Danielson, James R., additional, and Surko, Clifford M., additional

Published
2018
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26. Algorithm development for safe operation of the Wendelstein 7-X divertor

Author

Koch, Alexander W. (Prof. Dr. Dr. h.c.), Pedersen, Thomas Sunn (Prof. Dr.), Rodatos, Alexander, Koch, Alexander W. (Prof. Dr. Dr. h.c.), Pedersen, Thomas Sunn (Prof. Dr.), and Rodatos, Alexander

Abstract

In the frame of this thesis an algorithm to monitor the state of actively cooled plasma facing components at the Wendelstein 7-X divertor is developed. The interface between the plasma facing divertor tiles and the water cooled copper structure can develop cracks if the divertor is exposed to high heat fluxes. Task of the algorithm is to detect and monitor such defect interfaces while under the harsh conditions of the fusion environment during the experiments., Im Rahmen dieser Arbeit wurde ein Algorithmus entwickelt, welcher den Zustand aktiv gekühlter Bauteile des Divertors am Wendelstein 7-X überwacht. Das Interface zwischen den Divertorkacheln und der wassergekühlten Kupferstruktur kann bei hoher Wärmebelastung Risse ausbilden. Aufgabe des Algorithmus ist die Erkennung und Überwachung dieser Defekte während des Experimentbetriebs. Störende Einflüsse, wie veränderliche Oberflächenschichten oder Plasmastrahlung, wurden ebenfalls betrachtet.

Published
2017

27. Error field measurement, correction and heat flux balancing on Wendelstein 7-X a

Author

Lazerson, Samuel A., primary, Otte, Matthias, additional, Jakubowski, Marcin, additional, Israeli, Ben, additional, Wurden, Glen A., additional, Wenzel, Uwe, additional, Andreeva, Tamara, additional, Bozhenkov, Sergey, additional, Biedermann, Christoph, additional, Kocsis, Gábor, additional, Szepesi, Tamás, additional, Geiger, Joachim, additional, Pedersen, Thomas Sunn, additional, and Gates, David, additional

Published
2017
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28. Progress of the APEX Experiment for Creation of an Electron-Positron Pair Plasma.

Author

Hergenhahn, Uwe, Horn-Stanja, Juliane, Nissl, Stefan, Pedersen, Thomas Sunn, Haruhiko Saitoh, Stenson, Eve V., Stoneking, Matthew R., Dickmann, Marcel, Hugenschmidt, Christoph, Singer, Markus, Danielson, James R., and Surko, Clifford M.

Subjects
ELECTRON-positron plasmas, POSITRONIUM, ASTROPHYSICS, NUCLEAR reactors, POSITRON beams, MAGNETIC dipoles
Abstract

Electron-positron pair plasmas are an unexplored state of matter predicted to have properties intriguing for plasma physics as well as astrophysics. Here we described recent progress in the APEX collaboration dedicated to the production of a cold, confined electron-positron plasma in the laboratory. We focus on methods to inject positrons produced externally into a magnetic dipole trap, and to manipulate the ensuing trapped positron cloud. These experiments are carried out at the NEPOMUC positron beamline of the FRM II research reactor. Recent progress in producing more intense positron beams is briefly discussed. [ABSTRACT FROM AUTHOR]

Published
2018
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29. Confirmation of the topology of the Wendelstein 7-X magnetic field to better than 1:100,000

Author

Houry, M, Pedersen, Thomas Sunn, Otte, Matthias, Lazerson, Samuel A., Helander, Per, Bozhenkov, Sergey A., Biedermann, Christoph, Klinger, Thomas, Wolf, Robert, Bosch, Hans Stephan, Blackwell, Boyd, Wright, Adelle, Houry, M, Pedersen, Thomas Sunn, Otte, Matthias, Lazerson, Samuel A., Helander, Per, Bozhenkov, Sergey A., Biedermann, Christoph, Klinger, Thomas, Wolf, Robert, Bosch, Hans Stephan, Blackwell, Boyd, and Wright, Adelle

Abstract

Fusion energy research has in the past 40 years focused primarily on the tokamak concept, but recent advances in plasma theory and computational power have led to renewed interest in stellarators. The largest and most sophisticated stellarator in the world, Wendelstein 7-X (W7-X), has just started operation, with the aim to show that the earlier weaknesses of this concept have been addressed successfully, and that the intrinsic advantages of the concept persist, also at plasma parameters approaching those of a future fusion power plant. Here we show the first physics results, obtained before plasma operation: that the carefully tailored topology of nested magnetic surfaces needed for good confinement is realized, and that the measured deviations are smaller than one part in 100,000. This is a significant step forward in stellarator research, since it shows that the complicated and delicate magnetic topology can be created and verified with the required accuracy

Published
2016

31. Magnetic surface visualizations in the Columbia Non-neutral Torus

Author

Brenner, Paul W., Pedersen, Thomas Sunn, Berkery, John W., Marksteiner, Quinn R., and Hahn, Michael S.

Subjects
Plasma diagnostics -- Methods, Plasma devices -- Design and construction, Plasma devices -- Magnetic properties, Magnetism -- Research, Plasma (Ionized gases) -- Properties, Business, Chemistry, Electronics, Electronics and electrical industries
Abstract

Visualizations of magnetic surfaces are a valuable diagnostic in the Columbia Non-neutral Torus (CNT). The CNT is a compact stellarator, which is currently being used to study non-neutral plasmas confined on magnetic surfaces. The full 3-D shapes of magnetic surfaces created by CNT's simple four circular coil geometry are readily visualized by using an electron beam and neutral gas. These visualizations are useful for probe alignment and the confirmation of the magnetic surface topology, and they were necessary for the recent installation of a conducting boundary conforming to the last closed magnetic surface. Index Terms--Magnetic surfaces, non-neutral, stellarator, visualization.

Published
2008

33. Edge plasma phenomena in the Alcator C-Mod tokamak measured by high resolution X-ray imaging diagnostics

Author

Robert S. Granetz and Miklos Porkolab., Massachusetts Institute of Technology. Dept. of Physics., Pedersen, Thomas Sunn, 1970, Robert S. Granetz and Miklos Porkolab., Massachusetts Institute of Technology. Dept. of Physics., and Pedersen, Thomas Sunn, 1970

Abstract

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 2000., Includes bibliographical references (p. 203-206)., In this thesis high resolution soft x-ray measurements from the Alcator C-Mod plasma edge are presented for a variety of different plasma conditions. These measurements provide radial profiles of the soft x-ray emissivity with 1.5 mm resolution or better, and temporal resolution down to 12 [mu]s. These profiles show a distinct and very narrow pedestal shape in H-mode, indicative of the H-mode transport barrier. The soft x-ray emissivity pedestal at the outboard edge is typically 10 mm inside the last closed flux surface, near the top of the electron density and temperature pedestals. Modelling shows that the inward shift of the x-ray pedestal implies an inward shift of the impurity density pedestal. This inward shift is explained by an inward impurity pinch located in the region of strong electron density gradient, as predicted by neoclassical impurity transport theory. Calculations using the impurity transport code MIST support the existence of a neoclassical-like inward pinch. Changes in the soft x-ray pedestal width can be interpreted as changes in the edge impurity diffusion coefficient. We find several scaling laws of the edge diffusion coefficient with various plasma parameters in EDA H-mode. A second array views the top of the plasma. The x-ray emissivity measured with this array also shows a distinct and narrow pedestal in H-mode. However, it is located significantly closer to the separatrix and is often narrower. Both of these differences increase with the safety factor at the edge, q95 . Thus, there is a significant poloidal asymmetry in the impurity density in the H-mode edge region, which increases with q95 . Therefore, the impurity transport in the H-mode edge is highly two-dimensional. The strong poloidal asymmetries measured show some quantitative agreement with theories developed to explain poloidal impurity asymmetries. However, none of the theories are strictly applicable to the Alcator C-Mod edge, and they all significantly underestimate the actua, by Thomas Sunn Pedersen., Ph.D.

Published
2005

50. Confinement of pure electron plasmas in the CNT stellarator

Author

Pedersen, T. Sunn, primary, Berkery, J. W., additional, Boozer, A. H., additional, Marksteiner, Q. R., additional, Brenner, P. W., additional, Hahn, M., additional, de Gevigney, B. Durand, additional, Martin, X. Sarasola, additional, Danielson, James R., additional, and Pedersen, Thomas Sunn, additional

Published
2009
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