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Hellsten, Torbjörn
Publications (9 of 9) Show all publications
Lawson, K. D., Bergsåker, H., Bykov, I., Frassinetti, L., Garcia Carrasco, A., Hellsten, T., . . . et al., . (2019). Population modelling of the He II energy levels in tokamak plasmas: I. Collisional excitation model. Paper presented at ARK REH, 1982, ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, V49, P545. JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 52(4), Article ID 045001.
Open this publication in new window or tab >>Population modelling of the He II energy levels in tokamak plasmas: I. Collisional excitation model
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2019 (English)In: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, Vol. 52, no 4, article id 045001Article in journal (Refereed) Published
Abstract [en]

Helium is widely used as a fuel or minority gas in laboratory fusion experiments, and will be present as ash in DT thermonuclear plasmas. It is therefore essential to have a good understanding of its atomic physics. To this end He II population modelling has been undertaken for the spectroscopic levels arising from shells with principal quantum number n = 1-5. This paper focuses on a collisional excitation model; ionisation and recombination will be considered in a subsequent article. Heavy particle collisional excitation rate coefficients have been generated to supplement the currently-available atomic data for He II, and are presented for proton, deuteron, triton and alpha-particle projectiles. The widely-used criterion for levels within an n shell being populated in proportion to their statistical weights is reassessed with the most recent atomic data, and found not to apply to the He II levels at tokamak densities (10(18)-10(21) m(-3)). Consequences of this and other likely sources of errors are quantified, as is the effect of differing electron and ion temperatures. Line intensity ratios, including the so-called 'branching ratios' and the fine-structure beta(1), beta(2), beta(3), and gamma ratios, are discussed, the latter with regard to their possible use as diagnostics.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2019
Keywords
He II, population modelling, tokamak plasmas, spectral line intensity ratios
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-243932 (URN)10.1088/1361-6455/aaf703 (DOI)000456847500001 ()
Conference
ARK REH, 1982, ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, V49, P545
Note

QC 20190212

Available from: 2019-02-12 Created: 2019-02-12 Last updated: 2019-02-12Bibliographically 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
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
Hellsten, T. & Scheffel, J. (1984). Continuous Double Adiabatic Spectrum in Toroidal Plasmas. Physica Scripta, 30, 78
Open this publication in new window or tab >>Continuous Double Adiabatic Spectrum in Toroidal Plasmas
1984 (English)In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 30, p. 78-Article in journal (Refereed) Published
Abstract [en]

The continuous spectrum of an anisotropic and axisymmetric toroidal plasma is investigated using the double adiabatic theory. The continuum is given by an eigenvalue problem of a fourth order system of ordinary differential equations. In contrast to the magnetohydrodynamic continuum the double adiabatic continuum may become unstable. The stability depends upon the parallel and perpendicular pressure distributions along the field lines. In absence of a toroidal magnetic field, the fourth order system decouples into two second order differential equations for which specific stability criteria are derived.

National Category
Natural Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-240877 (URN)
Note

QCR 20190108

Available from: 2019-01-04 Created: 2019-01-04 Last updated: 2019-01-08Bibliographically approved
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