171,208 research outputs found
Progress in preparing real-time control schemes for Deuterium-Tritium operation in JET
No full textOperation of a magnetic fusion experiment, such as JET, relies on the availability of real-time (RT) control schemes, which supervise the plasma as it approaches the expected target performance while maintaining the integrity of the machine and its subsystems. At JET, there have been a series of recent efforts since (Lennholm M. et al 2017 Fusion Engineering and Design 123 535–540) to develop and test RT control schemes in preparation for the upcoming Deuterium-Tritium (DT) campaign. When operating JET in DT, each plasma discharge will in fact be a precious resource, being both T and neutron budget limited. Among the developed control schemes, this paper deals with the isotope ratio controller, which will maintain the required 50:50 DT ratio needed to favor nuclear fusion processes; the dud detector (L. Piron et al 2019 Fusion Engineering and Design 146 1364–1368), which will terminate a discharge moving toward controllers for detecting excessive radiation. Moreover, brand-new detectors, also based on machine learning approaches, have been implemented for detecting off-normal events or pre-disruptive states and have been included in the Plasma Event TRiggering and Alarms system (C.I. Stuart et al 2020 SOFT conference). Work is also ongoing to deploy into JET the RAPTOR suite, a RT observer for plasma state monitoring (C. Piron et al 2020 SOFT conference), and to identify control schemes within RAPTOR capabilities, which could contribute to support the development of high performance plasma scenarios
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Machine learning methods for locked-mode predictions in MAST-U plasmas
No full textIn tokamaks, rotating magneto-hydro-dynamic modes frequently decelerate as their amplitude increases. Once a critical threshold in amplitude is reached, these modes stop rotating into a specific toroidal and poloidal position and are commonly named Locked Modes (LMs). The presence of LMs, especially with low toroidal mode numbers, causes degradation of plasma performance, i.e. particle and energy losses, and can lead to a plasma disruption. Several strategies can be adopted when designing the plasma scenario to avoid the onset of these modes, which foresee the use of plasma heating, current drive methods, error field correction and density rising either via gas puffing or pellet injection. Despite the efforts to avoid the onset of LMs, disruption mitigation systems are considered essential during the International Thermonuclear Experimental Reactor (ITER) operations and in the view of the next fusion reactors such as the DEMOnstration Power Plant (DEMO), which can withstand a limited number of unmitigated plasma disruptions. The necessary condition towards the realization of an effective mitigation system, for safe and steady-state operations, is the development of robust and reliable metrics which are capable of predicting with a sufficient time margin the proximity to a LM. Thanks to the large availability of data, from lots of experimental campaigns performed in different experimental fusion devices, Machine Learning (ML) methods show to be a promising tool towards the achievement of this task. In this context, a wide database of MAST-U discharges has been analyzed considering data from multiple diagnostics with the scope of identifying recurrent paths which lead to LM onset. The data gathered have been the starting point for both the training and testing of two ML models, namely, K-Nearest-Neighbor (KNN) and Classification Tree (CT), developed for mode locking prediction. Both algorithms showed to be very reliable in predicting the proximity to a LM, with low percentages of missed and tardy detections. The methodology adopted for data selection, model training, as well as, the assessment of model performance are described in this work
The dud detector: An empirically-based real-time algorithm to save neutron and T budgets during JET DT operation
No full textOperations using deuterium-tritium (DT) mixtures are envisaged in JET device in the forthcoming 2020 experimental campaign. These experiments will offer a unique possibility to study several open issues in support to ITER and DEMO, such as alpha particle heating, and to improve the high plasma performance obtained in the previous DT campaign Keilhacker et al. [1]. During DT operation, each plasma discharge will be a precious resource, being both T and neutron budget limited. It will be thus mandatory to promptly detect and safely terminate those plasma discharges which do not achieve the expected target parameters. A real-time detector of underperforming discharges has been developed for this purpose and it is named the dud detector. The dud detector calculates and monitors the time evolution of plasma performance indicators, which can be used to trigger an alarm and a proper plasma termination. The experience gained on the algorithm's behavior during DD operation will allow the use of this tool during DT operations and can be used as a guide to design dud detectors for ITER and future fusion reactors
Feedback-assisted extension of the tokamak operating space to low safety factora)
No full textPhysics of Plasmas
Volume 21, Issue 7, July 2014, Article number 072107
Feedback-assisted extension of the tokamak operating space to low safety factor (Article)
Hanson, J.M.a , Bialek, J.M.a, Baruzzo, M.b, Bolzonella, T.b, Hyatt, A.W.c, Jackson, G.L.c, King, J.d, La Haye, R.J.c, Lanctot, M.J.c, Marrelli, L.b, Martin, P.b, Navratil, G.A.a, Okabayashi, M.e, Olofsson, K.E.J.a, Paz-Soldan, C.d, Piovesan, P.b, Piron, C.b, Piron, L.b, Shiraki, D.af, Strait, E.J.c, Terranova, D.b, Turco, F.a, Turnbull, A.D.c, Zanca, P.b
a Department of Applied Mathematics and Applied Physics, Columbia University, New York, NY 10027-6900, United States
b Consorzio RFX, Corso Stati Uniti 4, 35127 Padova, Italy
c General Atomics, P.O. Box 85608, San Diego, CA 92186-5608, United States
d Oak Ridge Institute for Science and Education, Oak Ridge, TN 37830-8050, United States
e Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, NJ 08543-0451, United States
f Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, United States
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Abstract
Recent DIII-D and RFX-mod experiments have demonstrated stable tokamak operation at very low values of the edge safety factor q(a) near and below 2. The onset of n = 1 resistive wall mode (RWM) kink instabilities leads to a disruptive stability limit, encountered at q(a) = 2 (limiter plasmas) and q 95 = 2 (divertor plasmas). However, passively stable operation can be attained for q(a) and q95 values as low as 2.2. RWM damping in the q(a) = 2 regime was measured using active MHD spectroscopy. Although consistent with theoretical predictions, the amplitude of the damped response does not increase significantly as the q(a) = 2 limit is approached, in contrast with damping measurements made approaching the pressure-driven RWM limit. Applying proportional gain magnetic feedback control of the n = 1 modes has resulted in stabilized operation with q95 values reaching as low as 1.9 in DIII-D and q(a) reaching 1.55 in RFX-mod. In addition to being consistent with the q(a) = 2 external kink mode stability limit, the unstable modes have growth rates on the order of the characteristic wall eddy-current decay timescale in both devices, and a dominant m = 2 poloidal structure that is consistent with ideal MHD predictions. The experiments contribute to validating MHD stability theory and demonstrate that a key tokamak stability limit can be overcome with feedback. © 2014 AIP Publishing LLC
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Covert reading of letters in a case of global alexia
No full textThis study describes the case of a global alexic patient with a severe reading deficit affecting words, letters and Arabic numbers, following a left posterior lesion. The patient (VA) could not match spoken letters to their graphic form. A preserved ability to recognize shape and canonical orientation of letters indicates intact access to the representation of letters and numbers as visual objects. A relatively preserved ability to match lowercase to uppercase letters suggests partially spared access to abstract letter identities independently
of their visual forms. The patient was also unable to match spoken letters and numbers to their visual form, indicating that she could not access the graphemic representations of letters from their phonological representations. This pattern of performance suggests that the link between graphemic and phonological representations is disrupted in this patient. We hypothesize that VA’ residual reading abilities are supported by the right hemisphere
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