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Hellesen, Carl
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Publications (10 of 385) Show all publications
Sias, G., Cecconello, M., Klimek, I., Wodniak, I., Yadykin, D., Andersson Sundén, E., . . . Zychor, I. (2019). A locked mode indicator for disruption prediction on JET and ASDEX upgrade. Fusion engineering and design, 138, 254-266
Open this publication in new window or tab >>A locked mode indicator for disruption prediction on JET and ASDEX upgrade
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2019 (English)In: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 138, p. 254-266Article in journal (Refereed) Published
Abstract [en]

The aim of this paper is to present a signal processing algorithm that, applied to the raw Locked Mode signal, allows us to obtain a disruption indicator in principle exploitable on different tokamaks. A common definition of such an indicator for different machines would facilitate the development of portable systems for disruption prediction, which is becoming of increasingly importance for the next tokamak generations. Moreover, the indicator allows us to overcome some intrinsic problems in the diagnostic system such as drift and offset. The behavior of the proposed indicator as disruption predictor, based on crossing optimized thresholds of the signal amplitude, has been analyzed using data of both JET and ASDEX Upgrade experiments. A thorough analysis of the disruption prediction performance shows how the indicator is able to recover some missed and tardy detections of the raw signal. Moreover, it intervenes and corrects premature or even wrong alarms due to, e.g., drifts and/or offsets.

Keywords
Tokamak, Disruption prediction, Locked mode signal, Disruption indicators, Feature extraction
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-377710 (URN)10.1016/j.fusengdes.2018.11.021 (DOI)000457663100032 ()
Note

For complete list of authors see http://dx.doi.org/10.1016/j.fusengdes.2018.11.021

Available from: 2019-03-08 Created: 2019-03-08 Last updated: 2019-03-08Bibliographically approved
Pau, A., Andersson Sundén, E., Binda, F., Cecconello, M., Conroy, S., Dzysiuk, N., . . . Zychor, I. (2019). A machine learning approach based on generative topographic mapping for disruption prevention and avoidance at JET. Nuclear Fusion, 59(10), Article ID 106017.
Open this publication in new window or tab >>A machine learning approach based on generative topographic mapping for disruption prevention and avoidance at JET
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2019 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 59, no 10, article id 106017Article in journal (Refereed) Published
Abstract [en]

The need for predictive capabilities greater than 95% with very limited false alarms are demanding requirements for reliable disruption prediction systems in tokamaks such as JET or, in the near future, ITER. The prediction of an upcoming disruption must be provided sufficiently in advance in order to apply effective disruption avoidance or mitigation actions to prevent the machine from being damaged. In this paper, following the typical machine learning workflow, a generative topographic mapping (GTM) of the operational space of JET has been built using a set of disrupted and regularly terminated discharges. In order to build the predictive model, a suitable set of dimensionless, machine-independent, physics-based features have been synthesized, which make use of 1D plasma profile information, rather than simple zero-D time series. The use of such predicting features, together with the power of the GTM in fitting the model to the data, obtains, in an unsupervised way, a 2D map of the multi-dimensional parameter space of JET, where it is possible to identify a boundary separating the region free from disruption from the disruption region. In addition to helping in operational boundaries studies, the GTM map can also be used for disruption prediction exploiting the potential of the developed GTM toolbox to monitor the discharge dynamics. Following the trajectory of a discharge on the map throughout the different regions, an alarm is triggered depending on the disruption risk of these regions. The proposed approach to predict disruptions has been evaluated on a training and an independent test set and achieves very good performance with only one tardive detection and a limited number of false detections. The warning times are suitable for avoidance purposes and, more important, the detections are consistent with physical causes and mechanisms that destabilize the plasma leading to disruptions.

Keywords
disruption prevention and avoidance, machine learning, artificial intelligence, dimensionless physics-based indicators, unsupervised learning and clustering of high-dimensional spaces, disruption classification and causes, JET tokamak real-time opearation and control
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-399009 (URN)10.1088/1741-4326/ab2ea9 (DOI)000482571700001 ()
Note

For complete list of authors see http://dx.doi.org/10.1088/1741-4326/ab2ea9

Available from: 2019-12-13 Created: 2019-12-13 Last updated: 2019-12-13Bibliographically approved
Garcia, J., Andersson Sundén, E., Binda, F., Cecconello, M., Conroy, S., Ericsson, G., . . . Zychor, I. (2019). A new mechanism for increasing density peaking in tokamaks: improvement of the inward particle pinch with edge E x B shearing. Plasma Physics and Controlled Fusion, 61(10), Article ID 104002.
Open this publication in new window or tab >>A new mechanism for increasing density peaking in tokamaks: improvement of the inward particle pinch with edge E x B shearing
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2019 (English)In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 61, no 10, article id 104002Article in journal (Refereed) Published
Abstract [en]

Developing successful tokamak operation scenarios, as well as confident extrapolation of present-day knowledge requires a rigorous understanding of plasma turbulence, which largely determines the quality of the confinement. In particular, accurate particle transport predictions are essential due to the strong dependence of fusion power or bootstrap current on the particle density details. Here, gyrokinetic turbulence simulations are performed with physics inputs taken from a JET power scan, for which a relatively weak degradation of energy confinement and a significant density peaking is obtained with increasing input power. This way physics parameters that lead to such increase in the density peaking shall be elucidated. While well-known candidates, such as the collisionality, previously found in other studies are also recovered in this study, it is furthermore found that edge E x B shearing may adopt a crucial role by enhancing the inward pinch. These results may indicate that a plasma with rotational shear could develop a stronger density peaking as compared to a non-rotating one, because its inward convection is increased compared to the outward diffusive particle flux as long as this rotation has a significant on E x B flow shear stabilization. The possibly significant implications for future devices, which will exhibit much less torque compared to present day experiments, are discussed.

Keywords
turbulence, transport, plasma
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-399008 (URN)10.1088/1361-6587/ab31a4 (DOI)000482602800002 ()
Note

For complete list of authors see http://dx.doi.org/10.1088/1361-6587/ab31a4

Available from: 2019-12-13 Created: 2019-12-13 Last updated: 2019-12-13Bibliographically approved
Zanca, P., Andersson Sundén, E., Binda, F., Cecconello, M., Conroy, S., Dzysiuk, N., . . . Zychor, I. (2019). A power-balance model of the density limit in fusion plasmas: application to the L-mode tokamak. Nuclear Fusion, 59(12), Article ID 126011.
Open this publication in new window or tab >>A power-balance model of the density limit in fusion plasmas: application to the L-mode tokamak
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2019 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 59, no 12, article id 126011Article in journal (Refereed) Published
Abstract [en]

A power-balance model, with radiation losses from impurities and neutrals, gives a unified description of the density limit (DL) of the stellarator, the L-mode tokamak, and the reversed field pinch (RFP). The model predicts a Sudo-like scaling for the stellarator, a Greenwald- like scaling, alpha I-p(8/9), for the RFP and the ohmic tokamak, a mixed scaling, alpha (PIp4/9)-I-4/9, for the additionally heated L-mode tokamak. In a previous paper (Zanca et al 2017 Nucl. Fusion 57 056010) the model was compared with ohmic tokamak, RFP and stellarator experiments. Here, we address the issue of the DL dependence on heating power in the L-mode tokamak. Experimental data from high-density disrupted L-mode discharges performed at JET, as well as in other machines, arc taken as a term of comparison. The model fits the observed maximum densities better than the pure Greenwald limit.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2019
Keywords
magnetohydrodynamics, transport, radiation
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-399086 (URN)10.1088/1741-4326/ab3b31 (DOI)000488059900001 ()
Funder
EU, Horizon 2020, 633053
Note

For complete list of authors see http://dx.doi.org/10.1088/1741-4326/ab3b31

Available from: 2019-12-16 Created: 2019-12-16 Last updated: 2019-12-16Bibliographically approved
Pamela, S., Andersson Sundén, E., Binda, F., Cecconello, M., Conroy, S., Dzysiuk, N., . . . Zychor, I. (2019). A wall-aligned grid generator for non-linear simulations of MHD instabilities in tokamak plasmas. Computer Physics Communications, 243, 41-50
Open this publication in new window or tab >>A wall-aligned grid generator for non-linear simulations of MHD instabilities in tokamak plasmas
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2019 (English)In: Computer Physics Communications, ISSN 0010-4655, E-ISSN 1879-2944, Vol. 243, p. 41-50Article in journal (Refereed) Published
Abstract [en]

Block-structured mesh generation techniques have been well addressed in the CFD community for automobile and aerospace studies, and their applicability to magnetic fusion is highly relevant, due to the complexity of the plasma-facing wall structures inside a tokamak device. Typically applied to non-linear simulations of MHD instabilities relevant to magnetically confined fusion, the JOREK code was originally developed with a 2D grid composed of isoparametric bi-cubic Bezier finite elements, that are aligned to the magnetic equilibrium of tokamak plasmas (the third dimension being represented by Fourier harmonics). To improve the applicability of these simulations, the grid-generator has been generalised to provide a robust extension method, using a block-structured mesh approach, which allows the simulations of arbitrary domains of tokamak vacuum vessels. Such boundary-aligned grids require the adaptation of boundary conditions along the edge of the new domain. Demonstrative non-linear simulations of plasma edge instabilities are presented to validate the robustness of the new grid, and future potential physics applications for tokamak plasmas are discussed. The methods presented here may be of interest to the wider community, beyond tokamak physics, wherever imposing arbitrary boundaries to quadrilateral finite elements is required.

Keywords
Fusion, Tokamak, MHD, Instability, ELM, Grid
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-399011 (URN)10.1016/j.cpc.2019.05.007 (DOI)000474316900005 ()
Note

For complete list of authors see http://dx.doi.org/10.1016/j.cpc.2019.05.007

Available from: 2019-12-13 Created: 2019-12-13 Last updated: 2019-12-13Bibliographically approved
Murari, A., Andersson Sundén, E., Binda, F., Cecconello, M., Conroy, S., Ericsson, G., . . . Zychor, I. (2019). Adaptive learning for disruption prediction in non-stationary conditions. Nuclear Fusion, 59(8), Article ID 086037.
Open this publication in new window or tab >>Adaptive learning for disruption prediction in non-stationary conditions
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2019 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 59, no 8, article id 086037Article in journal (Refereed) Published
Abstract [en]

For many years, machine learning tools have proved to be very powerful disruption predictors in tokamaks. On the other hand, the vast majority of the techniques deployed assume that the input data is independent and is sampled from exactly the same probability distribution for the training set, the test set and the final real time deployment. This hypothesis is certainly not verified in practice, since the experimental programmes evolve quite rapidly, resulting typically in ageing of the predictors and consequent suboptimal performance. This paper describes various adaptive training strategies that have been tested to maintain the performance of disruption predictors in non-stationary conditions. The proposed approaches have been implemented using new ensembles of classifiers, explicitly developed for the present application. The improvements in performance are unquestionable and, given the difficulties encountered so far in translating predictors from one device to another, the proposed adaptive methods from scratch can therefore be considered a useful option in the arsenal of alternatives envisaged for the next generation of devices, particularly at the very beginning of their operation.

Keywords
disruptions, machine learning predictors, adaptive training, de-learning, obsolescence, ensembles of classifiers
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-398830 (URN)10.1088/1741-4326/ab1ecc (DOI)000474298800006 ()
Note

For complete list of authors see http://dx.doi.org/10.1088/1741-4326/ab1ecc

Available from: 2019-12-13 Created: 2019-12-13 Last updated: 2019-12-13Bibliographically approved
Henderson, S. S., Andersson Sundén, E., Binda, F., Cecconello, M., Conroy, S., Dzysiuk, N., . . . Zychor, I. (2019). An assessment of nitrogen concentrations from spectroscopic measurements in the JET and ASDEX upgrade divertor. Nuclear Materials and Energy, 18, 147-152
Open this publication in new window or tab >>An assessment of nitrogen concentrations from spectroscopic measurements in the JET and ASDEX upgrade divertor
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2019 (English)In: Nuclear Materials and Energy, E-ISSN 2352-1791, Vol. 18, p. 147-152Article in journal (Refereed) Published
Abstract [en]

The impurity concentration in the tokamak divertor plasma is a necessary input for predictive scaling of divertor detachment, however direct measurements from existing tokamaks in different divertor plasma conditions are limited. To address this, we have applied a recently developed spectroscopic N II line ratio technique for measuring the N concentration in the divertor to a range of H-mode and L-mode plasma from the ASDEX Upgrade and JET tokamaks, respectively. The results from both devices show that as the power crossing the separatrix, P-sep, is increased under otherwise similar core conditions (e.g. density), a higher N concentration is required to achieve the same detachment state. For example, the N concentrations at the start of detachment increase from approximate to 2% to approximate to 9% as P-sep, is increased from approximate to 2.5 MW to approximate to 7 MW. These results tentatively agree with scaling law predictions (e.g. Goldston et al.) motivating a further study examining the parameters which affect the N concentration required to reach detachment. Finally, the N concentrations from spectroscopy and the ratio of D and N gas valve fluxes agree within experimental uncertainty only when the vessel surfaces are fully-loaded with N.

Keywords
Impurity, Nitrogen, Divertor, Concentration, Spectroscopy, Tokamak
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-398542 (URN)10.1016/j.nme.2018.12.012 (DOI)000460107500026 ()
Note

For complete list of authors see http://dx.doi.org/10.1016/j.nme.2018.12.012

Available from: 2019-12-09 Created: 2019-12-09 Last updated: 2019-12-09Bibliographically approved
Ström, P., Petersson, P., Rubel, M. J., Fortuna-Zaleśna, E., Widdowson, A. & Sergienko, G. (2019). Analysis of deposited layers with deuterium and impurity elements on samples from the divertor of JET with ITER-like wall. Journal of Nuclear Materials, 516, 202-213
Open this publication in new window or tab >>Analysis of deposited layers with deuterium and impurity elements on samples from the divertor of JET with ITER-like wall
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2019 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 516, p. 202-213Article in journal (Refereed) Published
Abstract [en]

Inconel-600 blocks and stainless steel covers for quartz microbalance crystals from remote corners in the JET-ILW divertor were studied with time-of-flight elastic recoil detection analysis and nuclear reaction analysis to obtain information about the areal densities and depth profiles of elements present in deposited material layers. Surface morphology and the composition of dust particles were examined with scanning electron microscopy and energy-dispersive X-ray spectroscopy. The analysed components were present in JET during three ITER-like wall campaigns between 2010 and 2017. Deposited layers had a stratified structure, primarily made up of beryllium, carbon and oxygen with varying atomic fractions of deuterium, up to more than 20%. The range of carbon transport from the ribs of the divertor carrier was limited to a few centimeters, and carbon/deuterium co-deposition was indicated on the Inconel blocks. High atomic fractions of deuterium were also found in almost carbon-free layers on the quartz microbalance covers. Layer thicknesses up to more than 1 mu m were indicated, but typical values were on the order of a few hundred nm. Chromium, iron and nickel fractions were less than or around 1% at layer surfaces while increasing close to the layer-substrate interface. The tungsten fraction depended on the proximity of the plasma strike point to the divertor corners. Particles of tungsten, molybdenum and copper with sizes less than or around 1 mu m were found. Nitrogen, argon and neon were present after plasma edge cooling and disruption mitigation. Oxygen-18 was found on component surfaces after injection, indicating in-vessel oxidation. Compensation of elastic recoil detection data for detection efficiency and ion-induced release of deuterium during the measurement gave quantitative agreement with nuclear reaction analysis, which strengthens the validity of the results.

Keywords
Fusion, Tokamak, Plasma-wall interactions, ToF-ERDA, NRA, SEM
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-379019 (URN)10.1016/j.jnucmat.2018.11.027 (DOI)000458897100020 ()
Funder
Swedish Foundation for Strategic Research , RIF14-0053Swedish Research Council, 821-2012-5144Swedish Research Council, 2015-04884Swedish Research Council, 2017-00643
Note

Authors listed as Contributor / bidragsgivare above are part of EUROfusion Consortium, JET, Culham Science Centre, UK.

Available from: 2019-03-12 Created: 2019-03-12 Last updated: 2019-04-24Bibliographically approved
Drenik, A., Andersson Sundén, E., Binda, F., Cecconello, M., Conroy, S., Dzysiuk, N., . . . Zychor, I. (2019). Analysis of the outer divertor hot spot activity in the protection video camera recordings at JET. Fusion engineering and design, 139, 115-123
Open this publication in new window or tab >>Analysis of the outer divertor hot spot activity in the protection video camera recordings at JET
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2019 (English)In: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 139, p. 115-123Article in journal (Refereed) Published
Abstract [en]

Hot spots on the divertor tiles at JET result in overestimation of the tile surface temperature which causes unnecessary termination of pulses. However, the appearance of hot spots can also indicate the condition of the divertor tile surfaces. To analyse the behaviour of the hot spots in the outer divertor tiles of JET, a simple image processing algorithm is developed. The algorithm isolates areas of bright pixels in the camera image and compares them to previously identified hot spots. The activity of the hot spots is then linked to values of other signals and parameters in the same time intervals. The operation of the detection algorithm was studied in a limited pulse range with high hot spot activity on the divertor tiles 5, 6 and 7. This allowed us to optimise the values of the controlling parameters. Then, the wider applicability of the method has been demonstrated by the analysis of the hot spot behaviour in a whole experimental campaign.

Keywords
JET, ITER-like wall, Plasma-wall interaction, Image analysis
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-378736 (URN)10.1016/j.fusengdes.2018.12.079 (DOI)000458939100016 ()
Note

For complete list of authors see http://dx.doi.org/10.1016/j.fusengdes.2018.12.079

Available from: 2019-03-11 Created: 2019-03-11 Last updated: 2019-03-11Bibliographically approved
Romazanov, J., Andersson Sundén, E., Binda, F., Cecconello, M., Conroy, S., Dzysiuk, N., . . . Zychor, I. (2019). Beryllium global erosion and deposition at JET-ILW simulated with ERO2.0. Nuclear Materials and Energy, 18, 331-338
Open this publication in new window or tab >>Beryllium global erosion and deposition at JET-ILW simulated with ERO2.0
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2019 (English)In: Nuclear Materials and Energy, E-ISSN 2352-1791, Vol. 18, p. 331-338Article in journal (Refereed) Published
Abstract [en]

The recently developed Monte-Carlo code ERO2.0 is applied to the modelling of limited and diverted discharges at JET with the ITER-like wall (ILW). The global beryllium (Be) erosion and deposition is simulated and compared to experimental results from passive spectroscopy. For the limiter configuration, it is demonstrated that Be self-sputtering is an important contributor (at least 35%) to the Be erosion. Taking this contribution into account, the ERO2.0 modelling confirms previous evidence that high deuterium (D) surface concentrations of up to similar to 50% atomic fraction provide a reasonable estimate of Be erosion in plasma-wetted areas. For the divertor configuration, it is shown that drifts can have a high impact on the scrape-off layer plasma flows, which in turn affect global Be transport by entrainment and lead to increased migration into the inner divertor. The modelling of the effective erosion yield for different operational phases (ohmic, L- and H-mode) agrees with experimental values within a factor of two, and confirms that the effective erosion yield decreases with increasing heating power and confinement.

Keywords
Beryllium, Erosion, ER02.0, JET ITER-like wall
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-398544 (URN)10.1016/j.nme.2019.01.015 (DOI)000460107500056 ()
Note

For complete list of authors see http://dx.doi.org/10.1016/j.nme.2019.01.015

Available from: 2019-12-10 Created: 2019-12-10 Last updated: 2019-12-10Bibliographically approved
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