Search

An overview of recent results obtained at the tokamak ASDEX Upgrade (AUG) is given. A work flow for predictive profile modelling of AUG discharges was established which is able to reproduce experimental H-mode plasma profiles based on engineering parameters only. In the plasma center, theoretical predictions on plasma current redistribution by a dynamo effect were confirmed experimentally. For core transport, the stabilizing effect of fast ion distributions on turbulent transport is shown to be important to explain the core isotope effect and improves the description of hollow low-Z impurity profiles. The L–H power threshold of hydrogen plasmas is not affected by small helium admixtures and it increases continuously from the deuterium to the hydrogen level when the hydrogen concentration is raised from 0 to 100%. One focus of recent campaigns was the search for a fusion relevant integrated plasma scenario without large edge localised modes (ELMs). Results from six different ELM-free confinement regimes are compared with respect to reactor relevance: ELM suppression by magnetic perturbation coils could be attributed to toroidally asymmetric turbulent fluctuations in the vicinity of the separatrix. Stable improved confinement mode plasma phases with a detached inner divertor were obtained using a feedback control of the plasma β. The enhanced Dα H-mode regime was extended to higher heating power by feedback controlled radiative cooling with argon. The quasi-coherent exhaust regime was developed into an integrated scenario at high heating power and energy confinement, with a detached divertor and without large ELMs. Small ELMs close to the separatrix lead to peeling-ballooning stability and quasi continuous power exhaust. Helium beam density fluctuation measurements confirm that transport close to the separatrix is important to achieve the different ELM-free regimes. Based on separatrix plasma parameters and interchange-drift-Alfvén turbulence, an analytic model was derived that reproduces the experimentally found important operational boundaries of the density limit and between L- and H-mode confinement. Feedback control for the X-point radiator (XPR) position was established as an important element for divertor detachment control. Stable and detached ELM-free phases with H-mode confinement quality were obtained when the XPR was moved 10 cm above the X-point. Investigations of the plasma in the future flexible snow-flake divertor of AUG by means of first SOLPS-ITER simulations with drifts activated predict beneficial detachment properties and the activation of an additional strike point by the drifts. 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. Peer Reviewed "Article signat per més de 50 autors/es: U. Stroth, D. Aguiam, E. Alessi, C. Angioni, N. Arden, R. Arredondo Parra, V. Artigues, O. Asunta, M. Balden, V. Bandaru, A. Banon-Navarro, K. Behler, A. Bergmann, M. Bergmann, J. Bernardo, M. Bernert, A. Biancalani, R. Bielajew, R. Bilato, G. Birkenmeier, T. Blanken, V. Bobkov, A. Bock, T. Body, T. Bolzonella, N. Bonanomi, A. Bortolon, B. Böswirth, C. Bottereau, A. Bottino, H. van den Brand, M. Brenzke, S. Brezinsek, D. Brida, F. Brochard, C. Bruhn, J. Buchanan, A. Buhler, A. Burckhart, Y. Camenen, B. Cannas, P. Cano Megias, D. Carlton, M. Carr, P. Carvalho, C. Castaldo, M. Cavedon, C. Cazzaniga, C. Challis, A. Chankin, C. Cianfarani, F. Clairet, S. Coda, R. Coelho, J.W. Coenen, L. Colas, G. Conway, S. Costea, D. Coster, T. Cote, A.J. Creely, G. Croci, D.J. Cruz Zabala, G. Cseh, A. Czarnecka, I. Cziegler, O. D'Arcangelo, A. Dal Molin, P. David, C. Day, M. de Baar, P. de Marné, R. Delogu, S. Denk, P. Denner, A. Di Siena, J.J. Dominguez Palacios Durán, D. Dunai, A. Drenik, M. Dreval, R. Drube, M. Dunne, B.P. Duval, R. Dux, T. Eich, S. Elgeti, A. Encheva, K. Engelhardt, B. Erdös, I. Erofeev, B. Esposito, E. Fable, M. Faitsch, U. Fantz, M. Farnik, H. Faugel, F. Felici, O. Ficker, S. Fietz, A. Figueredo, R. Fischer, O. Ford, L. Frassinetti, M. Fröschle, G. Fuchert, J.C. Fuchs, H. Fünfgelder, S. Futatani, K. Galazka, J. Galdon-Quiroga, D. Gallart Escolà, A. Gallo10, Y. Gao11, S. Garavaglia3, M. Garcia Muñoz16, B. Geiger21, L. Giannone1, S. Gibson32, L. Gil2, E. Giovannozzi18, S. Glöggler1, M. Gobbin8, J. Gonzalez Martin, T. Goodman, G. Gorini, T. Görler, D. Gradic, G. Granucci, A. Gräter, H. Greuner, M. Griener, M. Groth, A. Gude, L. Guimarais, S. Günter, G. Haas, A.H. Hakola, C. Ham, T. Happel, N. den Harder, G. Harrer, J. Harrison, V. Hauer, T. Hayward-Schneider, B. Heinemann, T. Hellsten, S. Henderson, P. Hennequin, A. Herrmann, E. Heyn, F. Hitzler, J. Hobirk, K. Höfler, J.H. Holm, M. Hölzl, C. Hopf, L. Horvath, T. Höschen, A. Houben, A. Hubbard, A. Huber, K. Hunger, V. Igochine, M. Iliasova, T. Ilkei, K. Insulander Björk, C. Ionita-Schrittwieser, I. Ivanova-Stanik, W. Jacob, N. Jaksic, F. Janky, A. Jansen van Vuuren, A. Jardin, F. Jaulmes, F. Jenko, T. Jensen, E. Joffrin, A. Kallenbach, S. Kálvin, M. Kantor, A. Kappatou, O. Kardaun, J. Karhunen4, C.-P. Käsemann, S. Kasilov, A. Kendl, W. Kernbichler, E. Khilkevitch, A. Kirk, S. Kjer Hansen, V. Klevarova, G. Kocsis, M. Koleva, M. Komm, M. Kong, A. Krämer-Flecken, K. Krieger, A. Krivska, O. Kudlacek, T. Kurki-Suonio, B. Kurzan, B. Labit, K. Lackner, F. Laggner, A. Lahtinen, P.T. Lang, P. Lauber, N. Leuthold, L. Li, J. Likonen, O. Linder, B. Lipschultz, Y. Liu, A. Lohs, Z. Lu, T. Luda di Cortemiglia, N.C. Luhmann, T. Lunt, A. Lyssoivan, T. Maceina, J. Madsen, A. Magnanimo, H. Maier, J. Mailloux, R. Maingi, O. Maj, E. Maljaars, P. Manas, A. Mancini, A. Manhard, P. Mantica, M. Mantsinen, P. Manz, M. Maraschek, C. Marchetto, L. Marrelli, P. Martin, A. Martitsch, F. Matos, M. Mayer, M.-L. Mayoral, D. Mazon, P.J. McCarthy, R. McDermott, R. Merkel, A. Merle, D. Meshcheriakov, H. Meyer, D. Milanesio, P. Molina Cabrera, F. Monaco, M. Muraca, F. Nabais, V. Naulin, R. Nazikian, R.D. Nem, A. Nemes-Czopf, G. Neu, R. Neu, A.H. Nielsen, S.K. Nielsen, T. Nishizawa, M. Nocente, J.-M. Noterdaeme, I. Novikau, S. Nowak, M. Oberkofler, R. Ochoukov, J. Olsen, F. Orain, F. Palermo, O. Pan, G. Papp, I. Paradela Perez, A. Pau, G. Pautasso, C. Paz-Soldan, P. Petersson, P. Piovesan, C. Piron, U. Plank, B. Plaum, B. Plöck, V. Plyusnin, G. Pokol, E. Poli, L. Porte, T. Pütterich, M. Ramisch, J. Rasmussen, G. Ratta, S. Ratynskaia, G. Raupp, D. Réfy, M. Reich1, F. Reimold, D. Reiser, M. Reisner, D. Reiter, T. Ribeiro, R. Riedl, J. Riesch, D. Rittich, J.F. Rivero Rodriguez, G. Rocchi, P. Rodriguez-Fernandez, M. Rodriguez-Ramos, V. Rohde, G. Ronchi, A. Ross, M. Rott, M. Rubel, D.A. Ryan, F. Ryter, S. Saarelma, M. Salewski, A. Salmi, O. Samoylov, L. Sanchis Sanchez, J. Santos, O. Sauter, G. Schall, K. Schlüter, K. Schmid, O. Schmitz, P.A. Schneider, R. Schrittwieser, M. Schubert, C. Schuster, T. Schwarz-Selinger, J. Schweinzer, E. Seliunin, A. Shabbir, A. Shalpegin, S. Sharapov, U. Sheikh, A. Shevelev, G. Sias, M. Siccinio, B. Sieglin, A. Sigalov, A. Silva, C. Silva, D. Silvagni, J. Simpson, S. Sipilä, E. Smigelskis, A. Snicker, E. Solano, C. Sommariva, C. Sozzi, G. Spizzo, M. Spolaore, A. Stegmeir, M. Stejner, J. Stober, E. Strumberge1, G. Suarez Lopez, H.-J. Sun, W. Suttrop, E. Sytova, T. Szepesi, B. Tál, T. Tala, G. Tardini, M. Tardocchi, D. Terranova, M. Teschke, E. Thorén, W. Tierens, D. Told, W. Treutterer, G. Trevisan, E. Trier, M. Tripský, M. Usoltceva, M. Valisa, M. Valovic, M. van Zeeland, F. Vannini, B. Vanovac, P. Varela, S. Varoutis, N. Vianello, J. Vicente, G. Verdoolaege, T. Vierle, E. Viezzer, I. Voitsekhovitch, U. von Toussaint, D. Wagner, X. Wang, M. Weiland, A.E. White, M. Willensdorfer, B. Wiringer, M. Wischmeier, R. Wolf, E. Wolfrum, Q. Yang, Q. Yu, R. Zagórski, I. Zammuto, T. Zehetbauer, W. Zhang, W. Zholobenko, M. Zilker, A. Zito, H. Zohm, S. Zoletnik and the EUROfusion MST1 Team "

Visualization of measurements together with experimental settings is a general subject in experiments analysis. The complex engineering design, 3D geometry, and manifold of diagnostics in larger fusion research experiments justify the development of special analysis and visualization programs. Novel ASDEX Upgrade (AUG) software tools bring together virtual navigation through 3D device models and advanced play-back and interpretation of video streams from plasma discharges. A third little tool allows the web-based platform independent observation of real-time diagnostic signals. While all three tools stem from spontaneous development ideas and are not considered mission critical for the operation of a fusion device, they with time and growing completeness shaped up as valuable helpers to visualize acquired data in fusion research. A short overview on the goals, the features, and the design as well as the operation of these tools is given in this paper.

The ASDEX Upgrade (AUG) programme, jointly run with the EUROfusion MST1 task force, continues to significantly enhance the physics base of ITER and DEMO. Here, the full tungsten wall is a key asset for extrapolating to future devices. The high overall heating power, flexible heating mix and comprehensive diagnostic set allows studies ranging from mimicking the scrape-off-layer and divertor conditions of ITER and DEMO at high density to fully non-inductive operation (q95 = 5.5, ) at low density. Higher installed electron cyclotron resonance heating power 6 MW, new diagnostics and improved analysis techniques have further enhanced the capabilities of AUG. Stable high-density H-modes with MW m−1 with fully detached strike-points have been demonstrated. The ballooning instability close to the separatrix has been identified as a potential cause leading to the H-mode density limit and is also found to play an important role for the access to small edge-localized modes (ELMs). Density limit disruptions have been successfully avoided using a path-oriented approach to disruption handling and progress has been made in understanding the dissipation and avoidance of runaway electron beams. ELM suppression with resonant magnetic perturbations is now routinely achieved reaching transiently . This gives new insight into the field penetration physics, in particular with respect to plasma flows. Modelling agrees well with plasma response measurements and a helically localised ballooning structure observed prior to the ELM is evidence for the changed edge stability due to the magnetic perturbations. The impact of 3D perturbations on heat load patterns and fast-ion losses have been further elaborated. Progress has also been made in understanding the ELM cycle itself. Here, new fast measurements of and Er allow for inter ELM transport analysis confirming that Er is dominated by the diamagnetic term even for fast timescales. New analysis techniques allow detailed comparison of the ELM crash and are in good agreement with nonlinear MHD modelling. The observation of accelerated ions during the ELM crash can be seen as evidence for the reconnection during the ELM. As type-I ELMs (even mitigated) are likely not a viable operational regime in DEMO studies of 'natural' no ELM regimes have been extended. Stable I-modes up to have been characterised using -feedback. Core physics has been advanced by more detailed characterisation of the turbulence with new measurements such as the eddy tilt angle—measured for the first time—or the cross-phase angle of and fluctuations. These new data put strong constraints on gyro-kinetic turbulence modelling. In addition, carefully executed studies in different main species (H, D and He) and with different heating mixes highlight the importance of the collisional energy exchange for interpreting energy confinement. A new regime with a hollow profile now gives access to regimes mimicking aspects of burning plasma conditions and lead to nonlinear interactions of energetic particle modes despite the sub-Alfvénic beam energy. This will help to validate the fast-ion codes for predicting ITER and DEMO. 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. "Article signat per més de 100 autors/es: H. Meyer, for the AUG Team: D. Aguiam, C. Angioni, C.G. Albert, N. Arden, R. Arredondo Parra, O. Asunta, M. de Baar, M. Balden, V. Bandaru, K. Behler, A. Bergmann, J. Bernardo, M. Bernert, A. Biancalani, R. Bilato, G. Birkenmeier, T.C. Blanken, V. Bobkov, A. Bock, T. Bolzonella, A. Bortolon, B. Böswirth, C. Bottereau, A. Bottino, H. van den Brand, S. Brezinsek, D. Brida, F. Brochard, C. Bruhn, J. Buchanan, A. Buhler, A. Burckhart, Y. Camenen, D. Carlton, M. Carr, D. Carralero, C. Castaldo, M. Cavedon, C. Cazzaniga, S. Ceccuzzi, C. Challis, A. Chankin, S. Chapman, C. Cianfarani, F. Clairet, S. Coda, R. Coelho, J.W. Coenen, L. Colas, G.D. Conway, S. Costea, D.P. Coster, T.B. Cote, A. Creely, G. Croci, G. Cseh, A. Czarnecka, I. Cziegler, O. D'Arcangelo, P. David, C. Day, R. Delogu, P. de Marné, S.S. Denk, P. Denner, M. Dibon, A. Di Siena, D. Douai, A. Drenik, R. Drube, M. Dunne, B.P. Duval, R. Dux, T. Eich, S. Elgeti, K. Engelhardt, B. Erdös, I. Erofeev, B. Esposito, E. Fable, M. Faitsch, U. Fantz, H. Faugel, I. Faust, F. Felici, J. Ferreira, S. Fietz, A. Figuereido, R. Fischer, O. Ford, L. Frassinetti, S. Freethy, M. Fröschle, G. Fuchert, J.C. Fuchs, H. Fünfgelder, K. Galazka, J. Galdon-Quiroga, A. Gallo, Y. Gao, S. Garavaglia, A. Garcia-Carrasco, M. Garcia-Muñoz, B. Geiger, L. Giannone, L. Gil, E. Giovannozzi, C. Gleason-González, S. Glöggler, M. Gobbin, T. Görler, I. Gomez Ortiz, J. Gonzalez Martin, T. Goodman, G. Gorini, D. Gradic, A. Gräter, G. Granucci, H. Greuner, M. Griener, M. Groth, A. Gude, S. Günter, L. Guimarais, G. Haas, A.H. Hakola, C. Ham, T. Happel, N. den Harder, G.F. Harrer, J. Harrison, V. Hauer, T. Hayward-Schneider, C.C. Hegna, B. Heinemann, S. Heinzel, T. Hellsten, S. Henderson, P. Hennequin, A. Herrmann, M.F. Heyn, E. Heyn, F. Hitzler, J. Hobirk, K. Höfler, M. Hölzl, T. Höschen, J.H. Holm, C. Hopf, W.A. Hornsby, L. Horvath, A. Houben, A. Huber, V. Igochine, T. Ilkei, I. Ivanova-Stanik, W. Jacob, A.S. Jacobsen, F. Janky, A. Jansen van Vuuren, A. Jardin, F. Jaulmes, F. Jenko, T. Jensen, E. Joffrin, C.-P. Käsemann, A. Kallenbach, S. Kálvin, M. Kantor, A. Kappatou, O. Kardaun, J. Karhunen, S. Kasilov,, Y. Kazakov, W. Kernbichler, A. Kirk, S. Kjer Hansen, V. Klevarova, G. Kocsis, A. Köhn, M. Koubiti, K. Krieger, A. Krivska, A. Krämer-Flecken, O. Kudlacek, T. Kurki-Suonio, B. Kurzan, B. Labit, K. Lackner, F. Laggner, P.T. Lang, P. Lauber, A. Lebschy, N. Leuthold, M. Li, O. Linder, B. Lipschultz, F. Liu, Y. Liu, A. Lohs, Z. Lu, T. Luda di Cortemiglia, N.C. Luhmann, R. Lunsford, T. Lunt, A. Lyssoivan, T. Maceina, J. Madsen, R. Maggiora, H. Maier, O. Maj, J. Mailloux, R. Maingi, E. Maljaars, P. Manas, A. Mancini, A. Manhard, M.-E. Manso, P. Mantica, M. Mantsinen, P. Manz, M. Maraschek, C. Martens, P. Martin, L. Marrelli, A. Martitsch, M. Mayer, D. Mazon, P.J. McCarthy, R. McDermott, H. Meister, A. Medvedeva, R. Merkel, A. Merle, V. Mertens, D. Meshcheriakov, O. Meyer, J. Miettunen, D. Milanesio, F. Mink, A. Mlynek, F. Monaco, C. Moon, F. Nabais, A. Nemes-Czopf, G. Neu, R. Neu, A.H. Nielsen, S.K. Nielsen, V. Nikolaeva, M. Nocente, J.-M. Noterdaeme, I. Novikau, S. Nowak, M. Oberkofler, M. Oberparleiter, R. Ochoukov, T. Odstrcil, J. Olsen, F. Orain, F. Palermo, O. Pan, G. Papp, I. Paradela Perez, A. Pau, G. Pautasso, F. Penzel, P. Petersson, J. Pinzón Acosta, P. Piovesan, C. Piron, R. Pitts, U. Plank, B. Plaum, B. Ploeckl, V. Plyusnin, G. Pokol, E. Poli, L. Porte, S. Potzel, D. Prisiazhniuk, T. Pütterich, M. Ramisch, J. Rasmussen, G.A. Rattá, S. Ratynskaia, G. Raupp, G.L. Ravera, D. Réfy, M. Reich, F. Reimold, D. Reiser, T. Ribeiro, J. Riesch, R. Riedl, D. Rittich, J.F. Rivero-Rodriguez, G. Rocchi, M. Rodriguez-Ramos, V. Rohde, A. Ross1, M. Rott, M. Rubel, D. Ryan, F. Ryter, S. Saarelma, M. Salewski, A. Salmi, L. Sanchis-Sanchez, J. Santos, O. Sauter, A. Scarabosio, G. Schall, K. Schmid, O. Schmitz, P.A. Schneider, R. Schrittwieser, M. Schubert, T. Schwarz-Selinger, J. Schweinzer, B. Scott, T. Semer, E. Seliunin, M. Sertoli, A. Shabbir, A. Shalpegin, L. Shao, S. Sharapov, G. Sias, M. Siccinio, B. Sieglin, A. Sigalov, A. Silva, C. Silva, D. Silvagni, P. Simon, J. Simpson, E. Smigelskis, A. Snicker, C. Sommariva, C. Sozzi, M. Spolaore, A. Stegmeir, M. Stejner, J. Stober, U. Stroth, E. Strumberger, G. Suarez, H.-J. Sun, W. Suttrop, E. Sytova, T. Szepesi, B. Tál, T. Tala, G. Tardini, M. Tardocchi, M. Teschke, D. Terranova, W. Tierens, E. Thorén, D. Told, P. Tolias, O. Tudisco, W. Treutterer, E. Trier, M. Tripský, M. Valisa, M. Valovic, B. Vanovac, D. van Vugt, S. Varoutis, G. Verdoolaege, N. Vianello, J. Vicente, T. Vierle, E. Viezzer, U. von Toussaint, D. Wagner, N. Wang, X. Wang, M. Weiland, A.E. White, S. Wiesen, M. Willensdorfer, B. Wiringer, M. Wischmeier, R. Wolf, E. Wolfrum, L. Xiang, Q. Yang, Z. Yang, Q. Yu, R. Zagórski, I. Zammuto, W. Zhang, M. van Zeeland, T. Zehetbauer, M. Zilker, S. Zoletnik, H. Zohm and the EUROfusion MST1 Team55

In this paper the capability of a localized laser dispersing technique for changing the material microstructure and the surface topology of steels is discussed. The laser implantation named technique bases on a discontinuous dispersing of ceramic particles into the surface of steels by using pulsed laser radiation. As ceramic particles TiC, WC and TiB2 are used, substrate material is high-alloyed cold working steel (X153CrMoV12). The influence of the laser parameters pulse length and pulse intensity was investigated in a comprehensive parameter study. The gained surface topology and microstructure were evaluated by optical microscopy, energy dispersive X-ray spectroscopy (EDX) and white light interferometry; mechanical properties were analyzed by micro hardness measurement. The experiments reveal that the alignment of separated, elevated, dome-shaped spots on the steel surface is feasible. The geometrical properties as well as the mechanical properties are highly controllable by the laser parameters. The laser implanted spots show a mostly crack-free and pore-free bonding to the substrate material as well as a significant increase of micro hardness.

This report summarizes the contributions presented at the 12th IAEA technical meeting on control, data acquisition and remote participation for fusion research, held in Daejeon, Republic of Korea, 13–17 May 2019. The meeting brought together more than 70 experts from nuclear fusion research sites worldwide. The main topics of the meeting were plasma control; machine control, monitoring, safety and remote manipulation; data acquisition and signal processing; database techniques for information storage and retrieval; advanced computing and massive data analysis; remote participation and virtual laboratory; and fast network technology and its application.

In fusion research the ability to generate and sustain high performance fusion plasmas gains more and more importance. Optimal combinations of magnetic shape, temperature and density profiles as well as the confinement time are identified as advanced regimes. Safe operation in such regimes will be crucial for the success of ITER and later fusion reactors. The operational space, on the other hand, is characterized by nonlinear dependencies between plasma parameters. Various MHD limits must be avoided in order to minimize the risk of a disruption. Sophisticated feedback control schemes help to tackle this challenge. But these in turn require detailed information on plasma state in time to allow proper reaction. Control system and diagnostic systems therefore must establish a symbiotic relationship to carry out such schemes. Today, all major fusion devices implement such a concept. An implementation of such a concept with sustained integration is presented using the example of ASDEX Upgrade. It covers data communication via a real-time network, synchronization mechanisms for data-driven algorithm execution as well as operational aspects and exception handling for failure detection and recovery. A modular distributed software framework offers standardized user algorithm interfaces, automated workflow procedures and the application of various computer and network hardware components. Designed with a special focus on reliability, robustness and flexibility, it is a sound base for exploring ITER-relevant plasma regimes and control strategies.

A universal time to digital converter (UTDC) and a digital I/O (DIO) card have been purpose-built for the new ASDEX Upgrade control system, the Thomson scattering diagnostic, a renovation of the magnetic measurements, and a couple of other diagnostics requiring renewed timing or real-time features. The principal features of these cards are presented and it is shown how synergy in hardware and software development was achieved. Examples of actual usage in diagnostics are given. Measurements of the real-time behaviour of cPCI-based diagnostics show low jitter of the Solaris operating system and highly reliable operation under moderate real-time conditions. Future applications of this family of cards are foreseen.

Significant progress has been made on ASDEX Upgrade during the last two years in the basic understanding of transport, in the extension of the improved H-mode in parameter space and towards an integrated operating scenario and in the development of control methods for major performance limiting instabilities. The important features were the understanding of particle transport and the control of impurity accumulation based on it, the satisfactory operation with predominantly tungsten-clad walls, the improved H-mode operation over density ranges and for temperature ratios covering (non-simultaneously) the ITER requirements on ν*, n/nGW and Te/Ti, the ELM frequency control by pellet injection and the optimization of NTM suppression by DC-ECCD through variation of the launching angle. From these experiments an integrated scenario has emerged which extrapolates to a 50% improvement in n T τ or a 30% reduction of the required current when compared with the ITER base-line assumptions, with moderately peaked electron and controllable high-Z density profiles.

ASDEX Upgrade is being equipped with a new real-time plasma control and data acquisition system and a novel time system. Major components were implemented and installed. While much work for performance optimisation and application programming remains to be done, commissioning of the new system parallel to experiment operation is being prepared. Commissioning of the new system will be done step-by-step. To facilitate testing the old and new control systems share all input signals. Switching between old and new system can be performed within 60 min: 23 fibre optics for output of actuator commands and input triggers must be connected to the active system and minor modifications done to interface the machine protection. Commissioning phases include background listening, technical discharges and full plasma operation. With the strategy chosen we minimize risk to the machine and reduce interference with ongoing experiment campaigns.

ASDEX upgrade investigates the integration of confinement, stability and exhaust issues into an operating scenario for ITER and a future fusion reactor. Since commissioned in 1990 the systems used to feedback control plasma position and shape as well as performance have continuously been enhanced. To overcome performance limitations and improve connectivity and steady state capability, a new plasma control system is being implemented. For the new system, adequate and reliable communication mechanisms are essential to integrate the realtime discharge control and data acquisition. We present communication methods and the process organisation of the new system and show that the new concept allows easy performance scaling. We demonstrate how existing periphery and new realtime diagnostics interface to control applications. This facilitates the realisation of novel and sophisticated control tasks combining multiple diagnostics and actuators for common physical goals.

For the new generation of intelligent controllers for plasma diagnostics, discharge control and long-pulse experiment control a new time system supporting steady state real-time operation has been devised. A central unit counts time at nanosecond resolution, covering more than the experiment lifetime. The broadcast time information serves local units to perform application functions such as current time readout, trigger generation and sample time measurement. Time is treated as a precisely measured quantity like other physical quantities. Tagging all detected events and sampled values with measured times as [value; time]-entities facilitates real-time data analysis, steady state protocolling and time-sorted archiving.

Fast data acquisition with direct transfer to memory is facilitated by the S-LINK interface protocol, developed at CERN, the European Organization for Nuclear Research. The need for customized hardware development reduces to application-specific front-end interfaces. Two applications of S-LINK data acquisition systems for microwave Doppler reflectometry and electron cyclotron emission radiometry at the ASDEX Upgrade nuclear fusion experiment are presented.

ASDEX Upgrade is being equipped with new plasma controllers and advanced diagnostics. These will be integrated into a distributed system, exchanging process data and plasma state information via a real-time network. Infrastructure tasks on dedicated nodes represent a functional framework for embedding of applications. Any type of hardware or operating systems may implement these, as long as the general rules on I/O timing and signal exchange methods are observed. To avoid temporal uncertainties in the measurement process a new time system was designed. Time becomes a measurement quantity sampled with data and process values.

The identification of MHD equilibrium states on the ASDEX Upgrade tokamak is a prerequisite for interpreting measurements from a wide range of diagnostics which are correlated with the shape of the plasma. The availability in realtime of plasma parameters related to the MHD state is crucial for controlling the experiment. Function Parameterization is used as a standard tool to determine the position, shape, and other global parameters of the plasma as well as the MHD equilibrium flux surfaces. The recently developed interpretive equilibrium code CLISTE now enables the calculation of MHD equilibria on an intershot timescale. These calculations are parallelized by the use of a Message Passing Interface (MPI).

ASDEX Upgrade today delivers approximately 25 GBytes of data per week. To manage this demand, which in fact is a growth by a factor of two in the last 2 years, several improvements to the data acquisition (DAQ) system have been made to avoid bottlenecks and to enhance the usability. Modifications were done to the diagnostic clients to speed up the storage of big diagnostic files to the central analysis server. The diagnostic synchronization server has been modified to handle wait requests not only for raw but for any level of evaluated data files. The central analysis server has been upgraded to deliver the power to do synoptic data analysis on up to 500 MBytes/shot on a single multiprocessor machine in shared memory. Additionally a cluster of ten workstations for parallel applications has been built up for MHD equilibrium calculations and other CPU-intensive tasks. The Andrew File System (AFS) archive servers have been upgraded to more disk capacity, a redundant storage architecture and faster network connections. However, as a basis for these improvements the network backbone and the server connections have been moved from FDDI to Gigabit-Ethernet and single workstation connections from Ethernet to Fast-Ethernet. Performance analysis results give an impression of the achieved improvements. Other projects in conjunction with the DAQ system at ASDEX Upgrade are the ‘hotlink’ interface system development for the Soft-X-Ray and Mirnov-Probes diagnostics and the ‘S-link’ development for an enhanced electron cyclotron emission (ECE) diagnostic. Both will serve as prototypes for future real-time diagnostics, which shall be able to deliver processed data in real-time to other systems — especially experiment control — to achieve a possibly better experiment performance.

Based on present achievements in tokamak control and considering requirements for reactor-oriented research, a next generation discharge control system is presented. Decomposition of control functions mapped onto dedicated controllers and diagnostics gives a distributed system integrated with a common real-time network to publish process and reference values. Basic mechanisms for cycle administration, data exchange and process management of the prototype system currently being implemented are shown.

Preprogrammed discharge control method and static experiment scheduling traditionally used with short-pulse experiment operation cycles are not compatible with requirements aiming at (near) steady state operation. An event-driven discharge control method is needed to handle deviations from the experiment programme without terminating the discharge. A dynamic experiment scheduling method is required to insert or take away experiments in a running discharge. To support these methods to operate the machine, data acquisition needs a close integration into the control, supporting calculation and exchange of processed quantities and state information, and into the experiment operation, supporting self-monitoring and exchange of resource availability and configuration information. These functions must be performed in parallel with the concurrent execution of the data acquisitions front-end control, data sampling, data processing and reduction and archiving.

The data acquisition environment of the ASDEX Upgrade fusion experiment was designed in the late 1980s to handle a predicted quantity of 8 Mbytes of data per discharge. After 7 years of operation a review of the whole data acquisition and analysis environment shows what remains of the original design ideas. Comparing the original 15 diagnostics with the present set of 250 diagnostic datasets generated per shot shows how the system has grown. Although now a vast accumulation of functional parts, the system still works in a stable manner and is maintainable. The underlying concepts affirming these qualities are modularity and compatibility. Modularity ensures that most parts of the system can be modified without affecting others. Standards for data structures and interfaces between components and methods are the prerequisites which make modularity work. The experience of the last few years shows that, besides the standards achieved, new, mainly real-time, features are needed: real-time event recognition allowing reaction to complex changing conditions; real-time wavelet analysis allowing adapted sampling rates; real-time data exchange between diagnostics and control; real-time networks allowing flexible computer coupling to permit interplay between different components; object-oriented programming concepts and databases are required for readily adaptable software modules. A final assessment of our present data processing situation and future requirements shows that modern information technology methods have to be applied more intensively to provide the most flexible means to improve the interaction of all components on a large fusion device.

It has been a while since it had been reported on the status of ASDEX Upgrade data acquisition (DAQ) and data management environment. An update on changes, expansions, and enhancements applied in the last years will be given. The acquired amount of data per shot increased from 4 GiB to 40 GiB in eight years. Network, storage, and archive challenges have been managed by stepwise improvements. New DAQ techniques have been introduced to replace outdated technologies. Real-time diagnostics speed-up data provisioning and contribute to feedback control. Information technology applied to ASDEX Upgrade is under permanent change. Recent and future steps are outlined.

In this presentation an overview will be given about laser beam welding of aluminium. Different aspects regarding process parameters, metallurgical aspects, weld seam properties and possible applications will be discussed.

The completely detached high confinement (CDH) regime established recently in high power ASDEX-Upgrade single null divertor discharges is described. The standard CDH mode scenario with feedback controlled external deuterium and neon puffing is analysed in detail with respect to its divertor detachment and confinement properties. Impurity transport and pumping effects and the density operation windows of the CDH mode are outlined. First results with other impurity radiators (N,Ar) are given. Finally the CDH mode operation space is discussed in terms of main chamber and scrape-off layer plus divertor radiation and possible consequences for future fusion experiments are outlined.

The ASDEX Upgrade programme is directed towards physics input to critical elements of the ITER design and the preparation of ITER operation, as well as addressing physics issues for a future DEMO design. After the finalization of the tungsten coating of the plasma facing components, the re-availability of all flywheel-generators allowed high-power operation with up to 20 MW heating power at I p up to 1.2 MA. Implementation of alternative ECRH schemes (140 GHz O2- and X3-mode) facilitated central heating above n e = 1.2 × 1020 m−3 and low q 95 operation at B t = 1.8 T. Central O2-mode heating was successfully used in high P/R discharges with 20 MW total heating power and divertor load control with nitrogen seeding. Improved energy confinement is obtained with nitrogen seeding both for type-I and type-III ELMy conditions. The main contributor is increased plasma temperature, no significant changes in the density profile have been observed. This behaviour may be explained by higher pedestal temperatures caused by ion dilution in combination with a pressure limited pedestal and hollow nitrogen profiles. Core particle transport simulations with gyrokinetic calculations have been benchmarked by dedicated discharges using variations of the ECRH deposition location. The reaction of normalized electron density gradients to variations of temperature gradients and the T e/T i ratio could be well reproduced. Doppler reflectometry studies at the L–H transition allowed the disentanglement of the interplay between the oscillatory geodesic acoustic modes, turbulent fluctuations and the mean equilibrium E × B flow in the edge negative E r well region just inside the separatrix. Improved pedestal diagnostics revealed also a refined picture of the pedestal transport in the fully developed H-mode type-I ELM cycle. Impurity ion transport turned out to be neoclassical in between ELMs. Electron and energy transport remain anomalous, but exhibit different recovery time scales after an ELM. After recovery of the pre-ELM profiles, strong fluctuations develop in the gradients of n e and T e. The occurrence of the next ELM cannot be explained by the local current diffusion time scale, since this turns out to be too short. Fast ion losses induced by shear Alfvén eigenmodes have been investigated by time-resolved energy and pitch angle measurements. This allowed the separation of the convective and diffusive loss mechanisms.

ASDEX Upgrade is a poloidal divertor experiment very similar to ITER with respect to magnetic field properties and especially to the plasma boundary geometry. A large part of the programme on ASDEX Upgrade is therefore dedicated to investigating and optimizing reactor-relevant plasma boundary issues. This contribution is concerned with H-mode studies and edge physics in general. The L/H-power threshold, H-mode confinement, details of ELM dynamic and 'dithering' L/H-transitions will be discussed. The edge physics investigations are concerned with model calculations and validations, the characteristic 'approach' to the density limit (DL), target plate sputtering and the influence of hydrocarbons, asymmetries and drifts depending on the direction of the toroidal magnetic field, and finally first results of high-Z material experiments.

We discuss the circuit design of a digital multiradian phase detector that measures the phase difference between two 10 kHz square wave TTL signals and provides the result as a binary number. The phase resolution of the circuit is 1/64 period and its dynamic range is 256 periods. This circuit has been developed for fusion plasma interferometry with submillimeter waves on the ASDEX Upgrade tokamak. The results from interferometric density measurement are discussed and compared to those obtained with the previously used phase detectors, especially with respect to the occurrence of phase jumps. It is illustrated that the new phase measurement provides a powerful tool for automatic real-time validation of the measured density, which is important for feedback algorithms that are sensitive to spurious density signals.

The ASDEX Upgrade tokamak employs a 60-channel electron cyclotron emission (ECE) radiometer diagnostic for the measurement of radial electron temperature profiles of the plasma. The data acquisitio...

To improve the plasma position and shape control system in ASDEX Upgrade, it is constantly expanded by the integration of data acquisition systems of important and interesting diagnostics. Usually the main responsibility of data acquisition systems is to collect data and subsequently put it into a data base archive system from where it is later analysed. The obligation now is also to simultaneously process the acquired data with an appropriate algorithm and send the pre-processed data in real-time to the plasma control system during a discharge. To achieve this requirements the used hardware must provide enough processing power and the operating system has to meet some real-time constraints. To avoid the burden of using proprietary real-time operating systems the trend is to use Open Source variants mainly based on Linux. Some of these solutions also allow us doing real-time capable communication using standard Ethernet hardware. By way of an example the implementation process of a prototype of a real-time data acquisition system based on a multi-core processor and Xenomai is demonstrated. Other possible solutions like Realtime Linux and their differences to Xenomai which we propose as the most sophisticated real-time framework for Linux are discussed in this paper.

This addendum applies to papers authored by contributors from the Axially Symmetric Divertor Experiment (ASDEX) Upgrade Team published in Review of Scientific Instruments. This addendum provides the full list of ASDEX Upgrade Team contributors and their affiliations.

____

ASDEX Upgrade has recently finished its transition towards an all-W divertor tokamak, by the exchange of the last remaining graphite tiles to W-coated ones. The plasma start-up was performed without prior boronization. It was found that the large He content in the plasma, resulting from DC glow discharges for conditioning, leads to a confinement reduction. After the change to D glow for inter-shot conditioning, the He content quickly dropped and, in parallel, the usual H-Mode confinement with H factors close to one was achieved. After the initial conditioning phase, oxygen concentrations similar to that in previous campaigns with boronizations could be achieved. Despite the removal of all macroscopic carbon sources, no strong change in C influxes and C content could be observed so far. The W concentrations are similar to the ones measured previously in discharges with old boronization and only partial coverage of the surfaces with W. Concomitantly it is found that although the W erosion flux in the divertor is larger than the W sources in the main chamber in most of the scenarios, it plays only a minor role for the W content in the main plasma. For large antenna distances and strong gas puffing, ICRH power coupling could be optimized to reduce the W influxes. This allowed a similar increase of stored energy as yielded with comparable beam power. However, a strong increase of radiated power and a loss of H-Mode was observed for conditions with high temperature edge plasma close to the antennas. The use of ECRH allowed keeping the central peaking of the W concentration low and even phases of improved H-modes have already been achieved.

ASDEX Upgrade was equipped with a distributed real-time (RT) control and diagnostic system. Application processes can be freely mapped onto controllers. They communicate through RT data exchanged via a shared memory network. Processes run self-organized with a data-driven scheme, i.e. a process executes when all required data has become available and produced data drive other processes waiting for these. The process chain starts periodically through a time-synchronous cycle master process to deterministically execute closed-loop control. Generic processes were implemented for feedback of plasma position and shape, and for performance control with fuelling and heating systems, for evaluation and monitoring of plasma quantities and Tokamak components, and generation of reference values in RT. Upon commissioning the system was speed-optimized to run a 1.6 ms cycle. Methods to exchange data and time information in RT and operate distributed data-driven processes work efficiently and reliably. The ability to freely map processes to computing nodes and RT data to generic processes provides outstanding configurational flexibility for optimizing system performance or supporting in situ replay and simulation runs. In the computation of reference values in RT acting on underlying feedback controllers, demonstrated for soft-landing, we see great potential to counteract instabilities or optimize pulses.

Today's data acquisition systems and control at the ASDEX Upgrade fusion experiment communicate during the setup of an experimental discharge only. While control executes the discharge based on recipes and a fixed number of input and output channels, diagnostic measurements are done under a predefined schema. Considerable benefits for scenario specific discharge execution are expected to be drawn from a real-time integration of data acquisition and discharge control systems: Information from the control system can be used for the real-time adaptation of the data acquisitions configuration, such as channel selection, time resolution, signal filtering, analysis process selection and protocolling functions. Information from data acquisition processed from the full set of input channels and diagnostic-specific knowledge can be used to select and fine-tune control configurations. Access to the data acquisition systems as external information sources potentially reduces the complexity of the discharge controls peripheral hardware and preprocessing software. Information exchange among these systems on a peer-to-peer basis allows freely to communicate the results. This is preferable to a master-slave concept where strict communication protocols have to be observed. In this paper we present the current work on the integration of discharge control and data acquisition at ASDEX Upgrade and give an outlook on future developments.

Recent results from ASDEX Upgrade are presented. An improved understanding of energy and particle transport emerges in terms of a 'critical gradient' model for the temperature gradients. Coupling this to particle diffusion explains most of the observed behaviour of the density profiles, in particular the finding that strong central heating reduces the tendency for density profile peaking. Internal transport barriers with Te and Ti in excess of 20 keV (but not simultaneously) have been achieved. By shaping the plasma, a regime with small type II ELMs has been established. Here, the maximum power deposited on the target plates was greatly reduced at constant average power. Also, an increase of the ELM frequency by injection of shallow pellets was demonstrated. ELMfree operation is possible in the QH-mode regime previously found in DIII-D which has also been established on ASDEX Upgrade. Regarding stability, a regime with benign NTMs was found. During ECCD stabilisation of NTMs, bN could be increased well above the usual onset level without a reappearance of the NTM. ECRH and ECCD have also been used to control the sawtooth repetition frequency at a moderate fraction of the total heating power. The inner wall of the ASDEX Upgrade vessel has increasingly been covered with tungsten without detrimental effects on plasma performance. Regarding scenario integration, a scenario with a large fraction of noninductively driven current ( 50%), but without internal tranport barrier has been established. It combines improved confinement (tE=tITER98 1:2) and stability (bN 3:5) at high Greenwald fraction (ne=nGW 0:85) in steady state and with type II ELMy edge

On ASDEX Upgrade a concept for real-time stabilization of NTMs has been realized and successfully applied to (3,2)- and (2,1)-NTMs. Since most of the work has meanwhile been published elsewhere, a short summary with the appropriate references is given. Limitations, deficits and future extensions of the system are discussed. In a second part the recent work on using modulated ECCD for NTM stabilisation is described in some detail. In these experiments ECCD power is modulated according to a magnetic footprint of the rotating NTM. In agreement with earlier results it could be shown that O-point heating reduces the necessary average power for stabilisation whereas X-point heating hampers stabilisation. Although this modulated scheme is not relevant for routine NTM stabilisation on ASDEX Upgrade it may be mandatory for ITER or DEMO. On ASDEX Upgrade it has been re-developed to demonstrate the usage of a FAst DIrectional Switch to continously heat the O-point of the rotating island with only one gyrotron switching between two launchers which target the mode at locations separated in phase by 180 degrees as described in [1].

Increasing requirements concerning taking care of resources, reduction of emission and increase of the recycling rate result in using more and more Aluminium especially in transportation industries. Aluminium alloys are characterised by low specific weight, relative high specific strength capabilities, good properties regarding machining including welding and by typically high corrosion resistance. Because of these characteristics Aluminium alloys in principle are very useful for light weight constructions /1/. Laser beam welding of Aluminium is a relative new technology which now has been developed to a state of the art which allows to use it in industrial applications. Some first applications are for example steering wheels, gasoline or oil filter boxes, transmission parts or small pressure components for passenger cars /2/. Beside such applications which are characterised by rotational symmetric geometries actual developments are dealing with flexible use of laser beam welding in light weight constructions. Main aspects in these developments are product-, functional- and production oriented investigations and adaptations for 3d- or big components or structures. Typical examples, based on quality function deployment procedure, are Alcar bodies or components.Increasing requirements concerning taking care of resources, reduction of emission and increase of the recycling rate result in using more and more Aluminium especially in transportation industries. Aluminium alloys are characterised by low specific weight, relative high specific strength capabilities, good properties regarding machining including welding and by typically high corrosion resistance. Because of these characteristics Aluminium alloys in principle are very useful for light weight constructions /1/. Laser beam welding of Aluminium is a relative new technology which now has been developed to a state of the art which allows to use it in industrial applications. Some first applications are for example steering wheels, gasoline or oil filter boxes, transmission parts or small pressure components for passenger cars /2/. Beside such applications which are characterised by rotational symmetric geometries actual developments are dealing with flexible use of laser beam welding in light weight construct...

Feedback-controlled puffing of neon and deuterium has been applied to control the edge-localized-mode behavior and the target plate power deposition during high-power H -mode discharges in ASDEX Upgrade. A regime has been found in which more than 90% of the heating power is lost through radiation and divertor detachment occurs, without deterioration of the energy confinement. The plasma remains in the H mode, exhibiting small-amplitude, high-frequency ELM's, which do not penetrate to the target plates in the strike zone region.

Die Prozesentwicklung zum Laserstrahlschweisen von Aluminium Werkstoffen hat gezeigt, das eine wesentlich genauere Anpassung des Arbeitsgasgemisches aus Ar und He an die Laserleistung, Strahlintensitat und die Bearbeitungsgeschwindigkeit erforderlich ist, um eine optimale Nahtqualitat zu erreichen, als bei Stahl. Die besten Ergebnisse hinsichtlich Nahtqualitat und Energieeinkopplung werden erzielt, wenn der Prozes nahe am Bereich der Plasmaabschirmung gefuhrt wird. Eine hohe Prozessicherheit und damit eine gesicherte Bearbeitungsqualitat kann mittels zeitaufgeloster Analyse und Kontrolle des laserinduzierten Plasmas erreicht werden.

Zur Erweiterung des Werkstoffspektrums und der Erschliesung neuer Einsatzfelder fur das Laserstrahlschweisen werden Ergebnisse von Untersuchungen der Schweiseignung von Nichteisenmetallen, wie Reintitan und Nickelbasiswerkstoffen, dargestellt. Sie gewahren Einblick in die Metallurgie der Verbindung und erlauben Aussagen uber die Gebrauchseigenschaften geschweister Bauteile. Anwendungsgrenzen sowie neue Entwicklungs- und Einsatzmoglichkeiten werden aufgezeigt.

In the case of welding with CO2-laser the overall efficiency will be between 2% and 5%. Using a Nd:YAG-laser the efficiency is only between 1% and 2%. Compared to that, metal arc welding has a very high efficiency and the arc welding machine has a more than 10 times lower price than a laser system. Besides several distinctions concerning process and welding results laser beam and arc metal welding are characterized by different geometries of the melt pool. In the case of arc metal welding the weld seam is defined by a lower ratio of seam depths to widths. In laser welding this ratio is quite high. These distinctions are influencing e. g. geometrical tolerance requirements, gaps bridging ability or the heat input which is necessary to solve the defined welding problem.Both welding processes are governed by plasma phenomena. The paper describes the plasma phenomena and how it is possible to combine both processes to reach a higher efficiency. By combing arc and laser beam welding in an optimal way, on the one hand the welding speed could be increased and on the other hand the same welding depths could be reached with a reduced laser power.In overlap welding as well as in case of gaps the combination allows to get a bigger seam width and that means a bigger gap between the workpieces could be excepted. Welding results on steel sheets in overlap and butt joints are discussed with regard to process parameter for CO2- and Nd:YAG-laser welding combined with the arc metal welding process.In the case of welding with CO2-laser the overall efficiency will be between 2% and 5%. Using a Nd:YAG-laser the efficiency is only between 1% and 2%. Compared to that, metal arc welding has a very high efficiency and the arc welding machine has a more than 10 times lower price than a laser system. Besides several distinctions concerning process and welding results laser beam and arc metal welding are characterized by different geometries of the melt pool. In the case of arc metal welding the weld seam is defined by a lower ratio of seam depths to widths. In laser welding this ratio is quite high. These distinctions are influencing e. g. geometrical tolerance requirements, gaps bridging ability or the heat input which is necessary to solve the defined welding problem.Both welding processes are governed by plasma phenomena. The paper describes the plasma phenomena and how it is possible to combine both processes to reach a higher efficiency. By combing arc and laser beam welding in an optimal way, on the o...

The first plasma in ASDEX Upgrade (AUG) was produced on 21stMarch '91 after more than 8 years of design, manufacture, and assembly. During the first operational period until August '92, with 1949 shots, the control and safety systems were commissioned in parallel with plasma discharges. Most discharges were run with He, but also H and D discharges were executed up to a plasma current of 800 kA at Bo = 2 T. With additional heating by ICRH, of up to 2.2 MW power and up to 2 s duration, the H-mode was obtained at a threshold of 1.2 MW after boronization. Discharges with line-densities of up to ne = 4*1019m−3and central electron temperatures of up to Teo =1.5 keV could adequately be diagnosed (ne-, Te-profiles, radiated power, ELM and MHD-activities, impurity influx …). With a CCD-camera the plasma boundary and critical in vessel components were observed. The magnetic diagnostics provided input for elaborate feedback control and reconstruction of the flux surfaces. The main effort of the experimental program was put on establishing the SN-divertor configuration, investigating the vertical n = 0 instability (VDE) and ICR heating scenarios. This paper describes the control systems, discharge essentials, vessel conditioning and the specific VDE behaviour.

Beim Laserstrahlschweisen mit hohen Prozesgeschwindigkeiten entstehen ausere Nahtfehler (Humping), die durch vorwiegend periodisch auftretende Aufwurfe der Schmelze gekennzeichnet sind und eine technische Verwendbarkeit der Schweisnaht ausschliesen. In diesem Beitrag sollen die Einflusse der masgebenden Prozesparameter untersucht und die Ursachen der Humping-Instabilitat mittels Hochgeschwindigkeits-Aufnahmen der Schmelzbadoberflache gezeigt werden. Es wird ein Verfahren der variablen Strahlformung zur Eliminierung oder zumindest Verschiebung der Nahtfehler zu hoheren Schweisgeschwindigkeiten vorgestellt.

The availability of high power lasers with average power levels above 5 kW and high beam quality triggered a number of industrial applications of lasers. Especially the process of deep penetration welding made the use of lasers attractive in applications where low distortion and small heat affected zones are required. In deep penetration welding a channel, called keyhole, supported by the pressure of evaporating material is formed by the laser beam, which allows the radiation of the laser a direct access to deep layers of the material. This is in contrast to conventional welding, where the heat either has to diffuse from the surface to the bulk material by heat conduction or the heat must be allowed to reach deeper layers by pro-viding a direct access using a preformed chamfer which has to be closed by filler material. By moving the laser beam relative to the material, the keyhole is driven parallel to the surface. The material molten at the front of the keyhole flows around the keyhole and resolidifies in its wake. The diameter of the keyhole and the thickness of the fluid layer necessary to support the flow around the keyhole determines the minimum width of the molten area and the width of the heat affected zone and hence the minimum energy per unit length of the seam if deep penetration laser welding is used.

Due to the material properties like high reflectivity high thermal conductivity low ionisation energy aluminium is weldable by CO -Lasers only under appropiate conditions. Possibilities to increase the energy coupling will be discussed in this paper. The suitable application of assist gas is one possibility. Hereby it is necessary to accomodate the composition of the assist gas supply to the process parameters (beam intensity, focal length, welding speed, material composition and so on). With optimized parameters it is possible to weld aluminium sheets of thicknesses > 6mm.