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10 results on '"Sheikh, U.A."'

1. Overview of the TCV tokamak experimental programme

Author

Reimerdes, H., primary, Agostini, M., additional, Alessi, E., additional, Alberti, S., additional, Andrebe, Y., additional, Arnichand, H., additional, Balbin, J., additional, Bagnato, F., additional, Baquero-Ruiz, M., additional, Bernert, M., additional, Bin, W., additional, Blanchard, P., additional, Blanken, T.C., additional, Boedo, J.A., additional, Brida, D., additional, Brunner, S., additional, Bogar, C., additional, Bogar, O., additional, Bolzonella, T., additional, Bombarda, F., additional, Bouquey, F., additional, Bowman, C., additional, Brunetti, D., additional, Buermans, J., additional, Bufferand, H., additional, Calacci, L., additional, Camenen, Y., additional, Carli, S., additional, Carnevale, D., additional, Carpanese, F., additional, Causa, F., additional, Cavalier, J., additional, Cavedon, M., additional, Cazabonne, J.A., additional, Cerovsky, J., additional, Chandra, R., additional, Chandrarajan Jayalekshmi, A., additional, Chellaï, O., additional, Chmielewski, P., additional, Choi, D., additional, Ciraolo, G., additional, Classen, I.G.J., additional, Coda, S., additional, Colandrea, C., additional, Dal Molin, A., additional, David, P., additional, de Baar, M.R., additional, Decker, J., additional, Dekeyser, W., additional, de Oliveira, H., additional, Douai, D., additional, Dreval, M., additional, Dunne, M.G., additional, Duval, B.P., additional, Elmore, S., additional, Embreus, O., additional, Eriksson, F., additional, Faitsch, M., additional, Falchetto, G., additional, Farnik, M., additional, Fasoli, A., additional, Fedorczak, N., additional, Felici, F., additional, Février, O., additional, Ficker, O., additional, Fil, A., additional, Fontana, M., additional, Fransson, E., additional, Frassinetti, L., additional, Furno, I., additional, Gahle, D.S., additional, Galassi, D., additional, Galazka, K., additional, Galperti, C., additional, Garavaglia, S., additional, Garcia-Munoz, M., additional, Geiger, B., additional, Giacomin, M., additional, Giruzzi, G., additional, Gobbin, M., additional, Golfinopoulos, T., additional, Goodman, T., additional, Gorno, S., additional, Granucci, G., additional, Graves, J.P., additional, Griener, M., additional, Gruca, M., additional, Gyergyek, T., additional, Haelterman, R., additional, Hakola, A., additional, Han, W., additional, Happel, T., additional, Harrer, G., additional, Harrison, J.R., additional, Henderson, S., additional, Hogeweij, G.M.D., additional, Hogge, J.-P., additional, Hoppe, M., additional, Horacek, J., additional, Huang, Z., additional, Iantchenko, A., additional, Innocente, P., additional, Insulander Björk, K., additional, Ionita-Schrittweiser, C., additional, Isliker, H., additional, Jardin, A., additional, Jaspers, R.J.E., additional, Karimov, R., additional, Karpushov, A.N., additional, Kazakov, Y., additional, Komm, M., additional, Kong, M., additional, Kovacic, J., additional, Krutkin, O., additional, Kudlacek, O., additional, Kumar, U., additional, Kwiatkowski, R., additional, Labit, B., additional, Laguardia, L., additional, Lammers, J.T., additional, Laribi, E., additional, Laszynska, E., additional, Lazaros, A., additional, Linder, O., additional, Linehan, B., additional, Lipschultz, B., additional, Llobet, X., additional, Loizu, J., additional, Lunt, T., additional, Macusova, E., additional, Marandet, Y., additional, Maraschek, M., additional, Marceca, G., additional, Marchetto, C., additional, Marchioni, S., additional, Marmar, E.S., additional, Martin, Y., additional, Martinelli, L., additional, Matos, F., additional, Maurizio, R., additional, Mayoral, M.-L., additional, Mazon, D., additional, Menkovski, V., additional, Merle, A., additional, Merlo, G., additional, Meyer, H., additional, Mikszuta-Michalik, K., additional, Molina Cabrera, P.A., additional, Morales, J., additional, Moret, J.-M., additional, Moro, A., additional, Moulton, D., additional, Muhammed, H., additional, Myatra, O., additional, Mykytchuk, D., additional, Napoli, F., additional, Nem, R.D., additional, Nielsen, A.H., additional, Nocente, M., additional, Nowak, S., additional, Offeddu, N., additional, Olsen, J., additional, Orsitto, F.P., additional, Pan, O., additional, Papp, G., additional, Pau, A., additional, Perek, A., additional, Pesamosca, F., additional, Peysson, Y., additional, Pigatto, L., additional, Piron, C., additional, Poradzinski, M., additional, Porte, L., additional, Pütterich, T., additional, Rabinski, M., additional, Raj, H., additional, Rasmussen, J.J., additional, Rattá, G.A., additional, Ravensbergen, T., additional, Ricci, D., additional, Ricci, P., additional, Rispoli, N., additional, Riva, F., additional, Rivero-Rodriguez, J.F., additional, Salewski, M., additional, Sauter, O., additional, Schmidt, B.S., additional, Schrittweiser, R., additional, Sharapov, S., additional, Sheikh, U.A., additional, Sieglin, B., additional, Silva, M., additional, Smolders, A., additional, Snicker, A., additional, Sozzi, C., additional, Spolaore, M., additional, Stagni, A., additional, Stipani, L., additional, Sun, G., additional, Tala, T., additional, Tamain, P., additional, Tanaka, K., additional, Tema Biwole, A., additional, Terranova, D., additional, Terry, J.L., additional, Testa, D., additional, Theiler, C., additional, Thornton, A., additional, Thrysøe, A., additional, Torreblanca, H., additional, Tsui, C.K., additional, Vaccaro, D., additional, Vallar, M., additional, van Berkel, M., additional, Van Eester, D., additional, van Kampen, R.J.R., additional, Van Mulders, S., additional, Verhaegh, K., additional, Verhaeghe, T., additional, Vianello, N., additional, Villone, F., additional, Viezzer, E., additional, Vincent, B., additional, Voitsekhovitch, I., additional, Vu, N.M.T., additional, Walkden, N., additional, Wauters, T., additional, Weisen, H., additional, Wendler, N., additional, Wensing, M., additional, Widmer, F., additional, Wiesen, S., additional, Wischmeier, M., additional, Wijkamp, T.A., additional, Wünderlich, D., additional, Wüthrich, C., additional, Yanovskiy, V., additional, Zebrowski, J., additional, and EUROfusion MST1 Team, the, additional

Published
2022
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3. Results on quiescent and post-disruption runaway electrons mitigation experiments at Frascati Tokamak Upgrade

Author

Carnevale D., Buratti P., Bin W., Bombarda F., Boncagni L., Paz-Soldan C., Duval B., Ceccuzzi S., Calacci L., Baruzzo M., Cappelli M., Castaldo C., Centioli C., Cianfarani C., Coda S., Cordella F., D'Arcangelo O., Decker J., Esposito E., Gabellieri L., Galperti C., Galeani S., Garavaglia S., Ghillardi G., Granucci G., Lehnen M., Liuzza D., Martinelli F., Mazzotta C., Napoli F., Nardon E., Oliva F., Panaccione L., Passeri M., Possieri C., Pucella G., Ramogida G., Romano A., Sassano M., Sheikh U.A., Tudisco O., the FTU Team, and EUROfusion MST1 Team

Subjects
mitigation experiments, quiescent and post-disruption runaway electrons, Frascati Tokamak Upgrade
Abstract

Analyses of experimental data collected in the last FTU campaign provide interesting results on Runaway Electrons (REs) suppression by means of large (wrt FTU plasma volume) deuterium pellets on RE quiescent discharges, mainly inducing bursts of MHD activity that expel the RE seeds. This phenomenon was found to be very reproducible on discharges at 0.5 MA and 5.3 T. Pellets injected during current ramp-ups are also shown to lead to a complete suppression of the RE seed; however, they triggered disruptions as well. These promising results on pellet cleaning effects for different RE quiescent scenarios are discussed. Avalanche multiplication of REs after single pellet injection on 0.36 MA RE quiescent discharges is reported. We provide quantitative indications of RE mitigation effects in terms of increased Connor-Hastie (critical) electrical field due to Anomalous Doppler Instabilities (ADI) for RE quiescent scenarios and post-disruption RE beams. Analysis of large fan-like instabilities on post-disruption RE beams, that seem correlated to the applied negative electrical field and the background density drops, revealed their strong capability to dissipate the RE energy increasing their pitch angle, expelling some of them, and transferring a large fraction of their magnetic energy to the background plasma. Then, ADI can lead to efficient RE energy dissipation (on FTU) as well as full conversion from runaway into thermal electrons (on TCV), indicating a new possible strategy for RE mitigation by controlling the onset of large fan-like instabilities. We also show that short pulses of ECRH, combined with D2 gas puffing, generate a density increase that gives way to a density decrease, when ECRH is switched off, triggering ADI and opening the path to a possible alternative RE mitigation strategy.

Published
2021

4. Tomographic reconstruction of the runaway distribution function in TCV using multispectral synchrotron images

Author

the TCV team, EUROfusion MST1 Team, Wijkamp, T.A., Perek, A., Decker, J., Duval, B., Hoppe, M., Papp, G., Sheikh, U.A., Classen, I.G.J., Jaspers, R.J.E., the TCV team, EUROfusion MST1 Team, Wijkamp, T.A., Perek, A., Decker, J., Duval, B., Hoppe, M., Papp, G., Sheikh, U.A., Classen, I.G.J., and Jaspers, R.J.E.

Abstract

Synchrotron radiation observed in a quiescent Tokamak Configuration Variable (TCV) runaway discharge is studied using filtered camera images targeting three distinct wavelength intervals. Through the tomographic simultaneous algebraic reconstruction technique (SART) procedure the high momentum, high pitch angle part of the spatial and momentum distribution of these relativistic particles is reconstructed. Experimental estimates of the distribution are important for verification and refinement of formation-, decay- and transport-models underlying runaway avoidance and mitigation strategy design. Using a test distribution it is demonstrated that the inversion procedure provides estimates accurate to within a few tens of percent in the region of phase-space contributing most to the synchrotron image. We find that combining images filtered around different parts of the emission spectrum widens the probed part of momentum-space and reduces reconstruction errors. Next, the SART algorithm is used to obtain information on the spatiotemporal runaway momentum distribution in a selected TCV discharge. The momentum distribution is found to relax towards an avalanche-like exponentially decaying profile. Anomalously high pitch angles and a radial profile increasing towards the edge are found for the most strongly emitting particles in the distribution.Pitch angle scattering by toroidal magnetic field ripple is consistent with this picture. An alternative explanation is the presence of high frequency instabilities in combination with the formation of a runaway shell at the edge of the plasma.

Published
2021

6. Progress in disruption prevention for ITER

Author

Strait, E.J., primary, Barr, J.L., additional, Baruzzo, M., additional, Berkery, J.W., additional, Buttery, R.J., additional, de Vries, P.C., additional, Eidietis, N.W., additional, Granetz, R.S., additional, Hanson, J.M., additional, Holcomb, C.T., additional, Humphreys, D.A., additional, Kim, J.H., additional, Kolemen, E., additional, Kong, M., additional, Lanctot, M.J., additional, Lehnen, M., additional, Lerche, E., additional, Logan, N.C., additional, Maraschek, M., additional, Okabayashi, M., additional, Park, J.K., additional, Pau, A., additional, Pautasso, G., additional, Poli, F.M., additional, Rea, C., additional, Sabbagh, S.A., additional, Sauter, O., additional, Schuster, E., additional, Sheikh, U.A., additional, Sozzi, C., additional, Turco, F., additional, Turnbull, A.D., additional, Wang, Z.R., additional, Wehner, W.P., additional, and Zeng, L., additional

Published
2019
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7. Disruption mitigation efficiency and scaling with thermal energy fraction on ASDEX Upgrade.

Author

Sheikh, U.A., David, P., Ficker, O., Bernert, M., Brida, D., Dibon, M., Duval, B., Faitsch, M., Maraschek, M., Papp, G., Pautasso, G., Sozzi, C., team, the AUG, and team, EUROfusion MST1

Subjects
*HEAT, *RADIATION, *THERMAL efficiency, *MAGNETIC declination, *GAS injection
Abstract

Disruption mitigation remains a critical and unresolved issue for ITER. Measurement uncertainties preventing quantification of a system's efficacy remains a significant hurdle in producing and validating a viable disruption mitigation system. This study addresses this issue through the creation of a dataset on the ASDEX Upgrade tokamak targeted at developing analysis techniques to quantify mitigation that are applicable irrespective of the disruption mitigation system being studied. These experiments used a range of thermal energy fractions to address concerns for ITER from JET massive gas injection (MGI) experiments showing a decrease in efficacy with increasing thermal energy fraction. The dataset produced in this study used MGI valves in two toroidal locations. The high resolution foil bolometers and AXUV diode arrays were used to infer radiation emission profiles at varying toroidal distances from the injection location. The fueling efficiency of the two systems was found to be comparable and toroidal asymmetries over the entire disruption were found to be negligible. The AXUV diodes were cross-calibrated with the foil bolometers and used to estimate the thermal energy radiated and magnetic energy coupled to the vessel structure and coils. It was estimated that 75–95% of the thermal energy was radiated and an almost constant 60% of the total magnetic energy was coupled. Radiated energy fractions of 0.8–1.0 were calculated and no decrease as a function of thermal energy fraction was found. A ± 20% variation in the coupled magnetic energy was explored and it did not alter this trend. [ABSTRACT FROM AUTHOR]

Published
2020
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8. Runaway electron synchrotron radiation in a vertically translated plasma.

Author

Hoppe, M., Papp, G., Wijkamp, T., Perek, A., Decker, J., Duval, B., Embreus, O., Fülöp, T., Sheikh, U.A., team, the TCV, and team, the EUROfusion MST1

Subjects
SYNCHROTRON radiation, PREDICTION theory, ELECTRONS, MAGNETIC fields, SYNCHROTRONS, TOKAMAKS
Abstract

Synchrotron radiation observed from runaway electrons (REs) in tokamaks depends upon the position and size of the RE beam, the RE energy and pitch distributions, as well as the location of the observer. We show experimental synchrotron images of a vertically moving RE beam sweeping past the detector in the Tokamak ŕ Configuration Variable (TCV) tokamak and compare it with predictions from the synthetic synchrotron diagnostic Soft. This experimental validation lends confidence to the theory underlying the synthetic diagnostics which are used for benchmarking theoretical models of and probing runaway dynamics. We present a comparison of synchrotron measurements in TCV with predictions of kinetic theory for runaway dynamics in uniform magnetic fields. We find that to explain the detected synchrotron emission, significant non-collisional pitch angle scattering as well as radial transport of REs would be needed. Such effects could be caused by the presence of magnetic perturbations, which should be further investigated in future TCV experiments. [ABSTRACT FROM AUTHOR]

Published
2020
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10. Impact of the new TCV baffled divertor upgrade on pedestal structure and performance

Author

Sheikh, U.A., Dunne, M., Frassinetti, L., Labit, B., Blanchard, P., Duval, B.P., Février, O., Galassi, D., Merle, A., Reimerdes, H., Theiler, C., TCV Team, and EUROfusion MST1 Team

Abstract

A new set of carbon tiles, neutral beam heating optics and gas baffles were installed on TCV during thebaffled divertor upgrade in early 2019. The installation of the baffles allows a deconvolution of the roles ofmain chamber and divertor neutral pressure on the H-mode pedestal structure. This physical barrier allowsrelatively high neutral pressures to be constrained to the divertor, thus preventing neutrals from entering themain chamber and potentially degrading core confinement. This study presents the experimentally measuredand modelled pedestal heights and structure for a series of H-mode discharges prior to and after this upgrade.Increased pedestal performance at high divertor neutral pressure was observed after the baffled divertorupgrade. This was consistent across all triangularities and outer target locations investigated and is attributedto higher pedestal top temperatures being maintained at high gas injection rates. ASTRA simulations indicatedbeam heating power coupled to the plasma did not significantly vary after the baffled divertor upgrade oras a function of divertor neutral gas pressure. Analysis of the pedestal structure exposed a strong correlationbetween pedestal performance and the density pedestal position prior to and after the baffled divertor upgrade.The baffled divertor upgrade limited the outward shift of the density pedestal, thus maintaining higher pedestalperformance at high divertor neutral pressures. Stability analysis indicated the majority of discharges studiedwere within 25% of the stability boundary. No correlation was found between the distance from the stabilityboundary and pedestal performance or structure. Comparison with the EPED1 model indicated that TCVdischarges do not have a fixed dependence between pedestal𝛽𝜃and pedestal width. A large variation inthe EPED1 relating parameter was observed and found to vary with the density pedestal position.

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