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Hellsten, Torbjörn
Publications (10 of 11) Show all publications
Ström, P., Petersson, P., Rubel, M., Bergsåker, H., Bykov, I., Frassinetti, L., . . . et al., . (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 analyzed 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 micrometer were indicated, but typical values were on the order of a few hundred nanometers. 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 micrometer 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
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-240616 (URN)10.1016/j.jnucmat.2018.11.027 (DOI)000458897100020 ()2-s2.0-85060313456 (Scopus ID)
Note

QC 20190125

Available from: 2018-12-20 Created: 2018-12-20 Last updated: 2019-08-08Bibliographically approved
Trier, E., Frassinetti, L., Fridström, R., Garcia Carrasco, A., Hellsten, T., Johnson, T., . . . Zuin, M. (2019). ELM-induced cold pulse propagation in ASDEX Upgrade. Plasma Physics and Controlled Fusion, 61(4), Article ID 045003.
Open this publication in new window or tab >>ELM-induced cold pulse propagation in ASDEX Upgrade
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2019 (English)In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 61, no 4, article id 045003Article in journal (Refereed) Published
Abstract [en]

In ASDEX Upgrade, the propagation of cold pulses induced by type-I edge localized modes (ELMs) is studied using electron cyclotron emission measurements, in a dataset of plasmas with moderate triangularity. It is found that the edge safety factor or the plasma current are the main determining parameters for the inward penetration of the T-e perturbations. With increasing plasma current the ELM penetration is more shallow in spite of the stronger ELMs. Estimates of the heat pulse diffusivity show that the corresponding transport is too large to be representative of the inter-ELM phase. Ergodization of the plasma edge during ELMs is a possible explanation for the observed properties of the cold pulse propagation, which is qualitatively consistent with non-linear magneto-hydro-dynamic simulations.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2019
Keywords
ELMs, MHD instabilities, stochastic field, magnetic islands, cold pulse
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-245121 (URN)10.1088/1361-6587/aaf9c3 (DOI)000458986000002 ()
Note

QC 20190315

Available from: 2019-03-15 Created: 2019-03-15 Last updated: 2019-05-20Bibliographically approved
Blanken, T. C., Frassinetti, L., Fridström, R., Garcia-Carrasco, A., Hellsten, T., Jonsson, T., . . . Dori, V. (2019). Real-time plasma state monitoring and supervisory control on TCV. Nuclear Fusion, 59(2), Article ID 026017.
Open this publication in new window or tab >>Real-time plasma state monitoring and supervisory control on TCV
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2019 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 59, no 2, article id 026017Article in journal (Refereed) Published
Abstract [en]

In ITER and DEMO, various control objectives related to plasma control must be simultaneously achieved by the plasma control system (PCS), in both normal operation as well as off-normal conditions. The PCS must act on off-normal events and deviations from the target scenario, since certain sequences (chains) of events can precede disruptions. It is important that these decisions are made while maintaining a coherent prioritization between the real-time control tasks to ensure high-performance operation. In this paper, a generic architecture for task-based integrated plasma control is proposed. The architecture is characterized by the separation of state estimation, event detection, decisions and task execution among different algorithms, with standardized signal interfaces. Central to the architecture are a plasma state monitor and supervisory controller. In the plasma state monitor, discrete events in the continuous-valued plasma state arc modeled using finite state machines. This provides a high-level representation of the plasma state. The supervisory controller coordinates the execution of multiple plasma control tasks by assigning task priorities, based on the finite states of the plasma and the pulse schedule. These algorithms were implemented on the TCV digital control system and integrated with actuator resource management and existing state estimation algorithms and controllers. The plasma state monitor on TCV can track a multitude of plasma events, related to plasma current, rotating and locked neoclassical tearing modes, and position displacements. In TCV experiments on simultaneous control of plasma pressure, safety factor profile and NTMs using electron cyclotron heating (ECI I) and current drive (ECCD), the supervisory controller assigns priorities to the relevant control tasks. The tasks are then executed by feedback controllers and actuator allocation management. This work forms a significant step forward in the ongoing integration of control capabilities in experiments on TCV, in support of tokamak reactor operation.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2019
Keywords
real-time control, supervisory control, control of tokamak plasmas
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-242971 (URN)10.1088/1741-4326/aaf451 (DOI)000455823900003 ()
Note

QC 20190212

Available from: 2019-02-12 Created: 2019-02-12 Last updated: 2019-02-12Bibliographically approved
Tierens, W., Frassinetti, L., Hellsten, T., Petersson, P., Fridström, R., Garcia Carrasco, A., . . . et al., . (2019). Validation of the ICRF antenna coupling code RAPLICASOL against TOPICA and experiments. Nuclear Fusion, 59(4), Article ID 046001.
Open this publication in new window or tab >>Validation of the ICRF antenna coupling code RAPLICASOL against TOPICA and experiments
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2019 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 59, no 4, article id 046001Article in journal (Refereed) Published
Abstract [en]

In this paper we validate the finite element code RAPLICASOL, which models radiofrequency wave propagation in edge plasmas near ICRF antennas, against calculations with the TOPICA code. We compare the output of both codes for the ASDEX Upgrade 2-strap antenna, and for a 4-strap WEST-like antenna. Although RAPLICASOL requires considerably fewer computational resources than TOPICA, we find that the predicted quantities of experimental interest (including reflection coefficients, coupling resistances, S- and Z-matrix entries, optimal matching settings, and even radiofrequency electric fields) are in good agreement provided we are careful to use the same geometry in both codes.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2019
Keywords
ICRF, finite elements, simulation
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-243928 (URN)10.1088/1741-4326/aaf455 (DOI)000456197200001 ()
Note

QC 20190212

Available from: 2019-02-12 Created: 2019-02-12 Last updated: 2019-02-12Bibliographically approved
Vallejos, P., Hellsten, T. & Jonsson, T. (2018). A numerical tool based on FEM and wavelets to account for spatial dispersion in ICRH simulations. In: Journal of Physics: Conference Series. Paper presented at 2018 Joint Varenna-Lausanne International Workshop on the Theory of Fusion Plasmas, 27 August 2018 through 31 August 2018. Institute of Physics Publishing (1)
Open this publication in new window or tab >>A numerical tool based on FEM and wavelets to account for spatial dispersion in ICRH simulations
2018 (English)In: Journal of Physics: Conference Series, Institute of Physics Publishing , 2018, no 1Conference paper, Published paper (Refereed)
Abstract [en]

Modeling of Ion Cyclotron Resonance Heating (ICRH) is difficult because of spatial dispersion. Numerical methods based on finite element or finite difference have difficulties in handling spatial dispersive effects, because the response is non-local. Fourier spectral methods can handle spatial dispersion, however, these methods have difficulties in handling the complex geometries outside the plasma domain and tend to produce dense matrices that are time consuming to invert. In this study, we investigate the potential of a new numerical method for solving the spatially dispersive wave equation based on FEM and wavelets. The spatially dispersive terms in the wave equation are evaluated using wavelets, and its contribution is represented as an induced current density in the wave equation. The wave equation is then solved using a finite element scheme, where the induced current density is represented as an inhomogeneous term and added using a fixed point iteration scheme. The method is applied to a case of one dimensional fast wave minority heating, including the up- and downshift in the parallel wave number, where we show that convergence can be obtained in a few iterations.

Place, publisher, year, edition, pages
Institute of Physics Publishing, 2018
Keywords
Cyclotron resonance, Finite element method, Fusion reactions, Iterative methods, Numerical methods, Wave equations, Complex geometries, Dispersive waves, Finite element schemes, Fixed point iteration, Ion cyclotron resonance heating, Numerical tools, Spatial dispersion, Spectral methods, Dispersion (waves)
National Category
Mathematics
Identifiers
urn:nbn:se:kth:diva-247044 (URN)10.1088/1742-6596/1125/1/012020 (DOI)2-s2.0-85058276573 (Scopus ID)
Conference
2018 Joint Varenna-Lausanne International Workshop on the Theory of Fusion Plasmas, 27 August 2018 through 31 August 2018
Note

QC 20190625

Available from: 2019-06-25 Created: 2019-06-25 Last updated: 2019-06-25Bibliographically approved
Hellsten, T. (2017). A statistical model of the wave field in a bounded domain. Physics of Plasmas, 24(2), Article ID 022122.
Open this publication in new window or tab >>A statistical model of the wave field in a bounded domain
2017 (English)In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 24, no 2, article id 022122Article in journal (Refereed) Published
Abstract [en]

Numerical simulations of plasma heating with radiofrequency waves often require repetitive calculations of wave fields as the plasma evolves. To enable effective simulations, bench marked formulas of the power deposition have been developed. Here, a statistical model applicable to waves with short wavelengths is presented, which gives the expected amplitude of the wave field as a superposition of four wave fields with weight coefficients depending on the single pass damping, as. The weight coefficient for the wave field coherent with that calculated in the absence of reflection agrees with the coefficient for strong single pass damping of an earlier developed heuristic model, for which the weight coefficients were obtained empirically using a full wave code to calculate the wave field and power deposition. Antennas launching electromagnetic waves into bounded domains are often designed to produce localised wave fields and power depositions in the limit of strong single pass damping. The reflection of the waves changes the coupling that partly destroys the localisation of the wave field, which explains the apparent paradox arising from the earlier developed heuristic formula that only a fraction a(s)(2)(2-a(s)) and not as of the power is absorbed with a profile corresponding to the power deposition for the first pass of the rays. A method to account for the change in the coupling spectrum caused by reflection for modelling the wave field with ray tracing in bounded media is proposed, which should be applicable to wave propagation in non-uniform media in more general geometries.

Place, publisher, year, edition, pages
AMER INST PHYSICS, 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-205147 (URN)10.1063/1.4977053 (DOI)000396012900028 ()2-s2.0-85013944777 (Scopus ID)
Note

QC 20170412

Available from: 2017-04-12 Created: 2017-04-12 Last updated: 2017-11-29Bibliographically approved
Tholerus, E., Johnson, T. & Hellsten, T. (2017). FOXTAIL: Modeling the nonlinear interaction between Alfven eigenmodes and energetic particles in tokamaks. Computer Physics Communications, 214, 39-51
Open this publication in new window or tab >>FOXTAIL: Modeling the nonlinear interaction between Alfven eigenmodes and energetic particles in tokamaks
2017 (English)In: Computer Physics Communications, ISSN 0010-4655, E-ISSN 1879-2944, Vol. 214, p. 39-51Article in journal (Refereed) Published
Abstract [en]

FOXTAIL is a new hybrid magnetohydrodynamic-kinetic code used to describe interactions between energetic particles and Alfven eigenmodes in tokamaks with realistic geometries. The code Simulates the nonlinear dynamics of the amplitudes of individual eigenmodes and of a set of discrete markers in five dimensional phase space representing the energetic particle distribution. Action angle coordinates of the equilibrium system are used for efficient tracing of energetic particles, and the particle acceleration by the wave fields of the eigenmodes is Fourier decomposed in the same angles. The eigenmodes are described using temporally constant eigenfunctions with dynamic complex amplitudes. Possible applications of the code are presented, e.g., making a quantitative validity evaluation of the one-dimensional bump-on-tail approximation of the system. Expected effects of the fulfillment of the Chirikov criterion in two-mode scenarios have also been verified.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2017
Keywords
Magnetohydrodynamic waves, Tokamaks, Fast particle effects, Nonlinear dynamics
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-205427 (URN)10.1016/j.cpc.2017.01.007 (DOI)000397358400005 ()2-s2.0-85011005108 (Scopus ID)
Funder
Swedish Research Council, 621-2011-5387
Note

QC 20170522

Available from: 2017-05-22 Created: 2017-05-22 Last updated: 2017-06-08Bibliographically approved
Nave, M. F., Kirov, K., Bernardo, J., Brix, M., Ferreira, J., Giroud, C., . . . Parra, F. (2017). The effect of lower hybrid waves on JET plasma rotation. Nuclear Fusion, 57(3), Article ID 034002.
Open this publication in new window or tab >>The effect of lower hybrid waves on JET plasma rotation
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2017 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 57, no 3, article id 034002Article in journal (Refereed) Published
Abstract [en]

This paper reports on observations of rotation in JET plasmas with lower hybrid current drive. Lower hybrid (LH) has a clear impact on rotation. The changes in core rotation can be either in the co- or counter-current directions. Experimental features that could determine the direction of rotation were investigated. Changes from co- to counter-rotation as the q-profile evolves from above unity to below unity suggests that magnetic shear could be important. However, LH can drive either co- or counter-rotation in discharges with similar magnetic shear and at the same plasma current. It is not clear if a slightly lower density is significant. A power scan at fixed density, shows a lower hybrid power threshold around 3 MW. For smaller LH powers, counter rotation increases with power, while for larger powers a trend towards co-rotation is found. The estimated counter-torque from the LH waves, would not explain the observed angular frequencies, neither would it explain the observation of co-rotation.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2017
Keywords
current drive, lower hybrid, rotation, tokamak, Electric discharges, Shear flow, Tokamak devices, Angular frequencies, Current drives, Direction of rotation, Lower hybrid current drive, Lower hybrid power, Lower Hybrid waves, Magnetoplasma
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-207989 (URN)10.1088/1741-4326/aa4e54 (DOI)2-s2.0-85013498569 (Scopus ID)
Note

QC 20170607

Available from: 2017-06-07 Created: 2017-06-07 Last updated: 2017-06-07Bibliographically approved
Vallejos, P., Johnson, T. & Hellsten, T. (2016). An iterative method to include spatial dispersion for waves in nonuniform plasmas using wavelet decomposition. Paper presented at 29 August 2016 through 2 September 2016. Journal of Physics, Conference Series, 775(1), Article ID 012016.
Open this publication in new window or tab >>An iterative method to include spatial dispersion for waves in nonuniform plasmas using wavelet decomposition
2016 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 775, no 1, article id 012016Article in journal (Refereed) Published
Abstract [en]

A novel method for solving wave equations with spatial dispersion is presented, suitable for applications to ion cyclotron resonance heating. The method splits the wave operator into a dispersive and a non-dispersive part. The latter can be inverted with e.g. finite element methods. The spatial dispersion is evaluated using a wavelet representation of the dielectric kernel and added by means of iteration. The method has been successfully tested on a low frequency kinetic Alfvén wave with second order Larmor radius effects in a nonuniform plasma slab.

Keywords
Cyclotron resonance, Finite element method, Fusion reactions, Iterative methods, Wave equations, Wavelet decomposition, Ion cyclotron resonance heating, Larmor radius effects, Nonuniform plasma, Second orders, Solving wave equations, Spatial dispersion, Wave operators, Wavelet representation, Dispersion (waves)
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-201866 (URN)10.1088/1742-6596/775/1/012016 (DOI)2-s2.0-85009809630 (Scopus ID)
Conference
29 August 2016 through 2 September 2016
Note

QC 20170308

Available from: 2017-03-08 Created: 2017-03-08 Last updated: 2017-11-29Bibliographically approved
Sharapov, S. E., Hellsten, T., Kiptily, V. G., Craciunescu, T., Eriksson, J., Fitzgerald, M., . . . Zoita, V. (2016). Fusion product studies via fast ion D-D and D-He-3 fusion on JET. Paper presented at 14th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems, 2015, Vienna, AUSTRIA. Nuclear Fusion, 56(11), Article ID 112021.
Open this publication in new window or tab >>Fusion product studies via fast ion D-D and D-He-3 fusion on JET
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2016 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 56, no 11, article id 112021Article in journal (Refereed) Published
Abstract [en]

Dedicated fast ion D-D and D-He-3 fusion experiments were performed on JET with carbon wall (2008) and ITER-like wall (2014) for testing the upgraded neutron and energetic ion diagnostics of fusion products. Energy spectrum of D-D neutrons was the focus of the studies in pure deuterium plasmas. A significant broadening of the energy spectrum of neutrons born in D-D fast fusion was observed, and dependence of the maximum D and D-D neutron energies on plasma density was established. Diagnostics of charged products of aneutronic D-He-3 fusion reactions, 3.7 MeV alpha-particles similar to those in D-T fusion, and 14.6 MeV protons, were the focus of the studies in D-He-3 plasmas. Measurements of 16.4 MeV gamma-rays born in the weak secondary branch of D(He-3, gamma)Li-5 reaction were used for assessing D-He-3 fusion power. For achieving high yield of D-D and D-He-3 reactions at relatively low levels of input heating power, an acceleration of D beam up to the MeV energy range was used employing 3rd harmonic (f = 3f(CD)) ICRH technique. These results were compared to the techniques of D beam injection into D-He-3 mixture, and He-3-minority ICRH in D plasmas.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2016
Keywords
fusion, neutrons, ICRH, NBI, JET, Deuterium, Helium-3
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-200769 (URN)10.1088/0029-5515/56/11/112021 (DOI)000391393900021 ()
Conference
14th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems, 2015, Vienna, AUSTRIA
Note

QC 20170207

Available from: 2017-02-07 Created: 2017-02-02 Last updated: 2017-11-29Bibliographically approved
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