Browsing by Author "Groth, Mathias"

Filter results by typing the first few letters
Now showing 1 - 20 of 64
  • ASCOT simulations of global 13C transport in ASDEX Upgrade in the presence of resonant magnetic perturbations
    (2014-10-12) Kontula, Joona
     Perustieteiden korkeakoulu | Bachelor's thesis
  • ASCOT simulations of ion cyclotron heated fast ion losses in the ASDEX Upgrade tokamak
    (2019-10-21) Luotonen, Peetu
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Assessment of Prediction Models For Plasma Conditions In a Magnetic-Confinement Fusion Reactor
    (2022-07-27) Mäkelä, Tuomas
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Assessment of the two-point model using SOLPS-ITER
    (2018-12-17) af Björkesten, Emil
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Assessment of the UEDGE fluid neutral molecular model using the kinetic neutral model EIRENE
    (2021-10-19) Myllynen, Luukas
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Atomic processes leading to asymmetric divertor detachment in KSTAR L-mode plasmas
    (2018-11-02) Park, Jae Sun; Groth, Mathias; Pitts, Richard; Bak, Jun Gyo; Thatipamula, S. G.; Juhn, June Woo; Hong, Suk Ho; Choe, Wonho
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The experimentally observed in/out detachment asymmetry in KSTAR L-mode plasmas with deuterium (D) fueling and carbon walls has been investigated with the SOLPS-ITER code to understand its mechanism and identify important atomic processes in the divertor region. The simulations show that the geometrical combination of a vertical, inner target with a short poloidal connection from the X-point to the target and a much longer outer divertor leg on an inclined target lead to neutral accumulation towards the outer target, driving the outer target detachment at lower upstream density than is required for the inner target. This is consistent with available Langmuir probe measurements at both target plates, although the inner target profile is poorly resolved in these plasmas and further experiments with corroborating diagnostics are required to confirm this finding. The pressure and power loss factors defined in the two-point model (Stangeby 2018 Plasma Phys. Control. Fusion 60 4; Kotov and Reiter 2009 Plasma Phys. Control. Fusion 51 115002; Stangeby and Sang 2017 Nucl. Fusion 57 056007; Moulton et al 2017 Plasma Phys. Control. Fusion 59 6) of the divertor scrape-off layer (SOL) and the sources contributing to the loss factors are calculated through post-processing of the SOLPS-ITER results. The momentum losses are mainly driven by plasma-neutral interaction and the power losses by plasma-neutral interaction and carbon radiation. The presence of carbon impurities in the simulation enhances the pressure and power dissipation compared to the pure D case. Carbon radiation is a strong power loss channel which cools the plasma, but its effect on the pressure balance is indirect. Reduction of the electron temperature indirectly increases the momentum loss and increasing the volumetric reaction rates which are responsible for the loss of momentum. As a result, the addition of carbon saturates the momentum and power losses in the flux tube at lower upstream densities, reducing the roll-over threshold of the upstream density. The relative strengths of the various mechanisms contributing to momentum and power loss depend on the radial distance of the SOL flux tubes from the separatrix (near/far SOL) and the target (inner/outer target). This is related to the strong D2 molecule accumulation near the outer strike point, which makes the deuterium gas density at the outer target 2-10 times higher than that at the inner target. A large portion of the recycled neutral particles from both targets reach and accumulate in the outer SOL, which is predominantly attributed to the target inclination and gap structure between the central and outboard divertors and hence to the impact of geometry. The accumulated neutrals enhance the reactions involving D2, which causes momentum and power loss.
  • Combination of micro-macro and spatially hybrid fluid-kinetic approach for hydrogenic plasma edge neutrals
    (2022-06) Horsten, Niels; Groth, Mathias; Dekeyser, Wouter; Van Uytven, Wim; Carli, Stefano; , JET Contributors
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    A new hybrid fluid-kinetic approach for the hydrogenic neutrals (atoms and molecules) in the plasma edge is presented. The hybrid approach combines a fully kinetic model for the atoms in the low-collisional regions near the vessel wall, and for the molecules in the whole plasma edge domain, with a micro-macro approach for atoms originating from recycling at the divertor targets, volumetric recombination, and dissociation of molecules. With the micro-macro approach, the originally scattering-dominated collision term due to charge-exchange collisions in the kinetic equation is transformed to an absorption-dominated term, while a large part of the neutral population is treated through a fluid approach. For JET L-mode plasmas, the premature termination of Monte Carlo particle trajectories in the hybrid approach leads to a reduction of the CPU time by approximately a factor 3 for a high-recycling case and by approximately a factor 11 for a partially detached case compared with a simulation with fully kinetic neutrals and the same amount of particles. For coupled fluid plasma - hybrid neutral simulations - the hybrid approach predicts the plasma divertor target profiles with a maximum hybrid-kinetic discrepancy of approximately 30%.
  • Comparison of fluid and quasi-kinetic transport models for divertor retention of tungsten in JET
    (2016-02-03) Karjalainen, Mikko
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Comparison of OEDGE and EDGE2D-EIRENE predictions of the scrape-off layer conditions for attached plasmas
    (2024-08-01) Rikala, Vesa Pekka; Groth, Mathias; Kumpulainen, Henri; Rees, David
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Predictions of the scrape-off layer (SOL) plasma conditions from the 2D multi-fluid code EDGE2D-EIRENE are compared to the OEDGE 1D fluid code for JET low-confinement mode (L-mode) plasmas. In the low-recycling divertor conditions (divertor target plasma collisionality (Formula presented.)), OEDGE and EDGE2D-EIRENE agree on the electron temperature, the ion temperature, and the electron density within 10% in the low-field side (LFS) divertor X-point region (divertor SOL) and within 25% in the LFS midplane region. In high-recycling conditions ((Formula presented.)), the predictions of both codes agree on the electron temperature within 20%, while differences in the ion temperature and electron density increase to as high as 50% in the divertor SOL.
  • Comparison of the scrape-off layer two-point model for deuterium and helium plasmas in JET ITER-like wall low-confinement plasma conditions
    (2024-08-01) Rees, David; Sissonen, Joona; Groth, Mathias; Rikala, Vesa Pekka; Kumpulainen, Henri; Thomas, Beth; Brix, Mathias
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The hydrogenic two-point model (H-2PM), an analytical model for the scrape-off layer that predicts a common electron and ion temperature and density along a flux tube from a target temperature and density, is adapted to a single-species helium (He) model, preserving the 2PM assumptions and analytical nature. Across a range of densities and heating powers, the predicted (Formula presented.) is within (Formula presented.) of upstream measurements by high-resolution Thomson scattering (HRTS) for low-confinement He plasmas performed in JET ITER-like wall (JET-ILW). For high-recycling conditions, He-2PM predictions of (Formula presented.) were within 10% of the Li-beam diagnostic measurements, excluding the near-SOL, when assuming (Formula presented.), suggesting the divertor SOL was not fully ionised in the JET-ILW He plasmas.
  • Confinement analysis of ohmic FT-2 tokamak plasmas with the ELMFIRE first principle code
    (2015-11-30) Fontell, Mathias
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Confinement of ion cyclotron resonance heated ions in a 5 MA/1.8 T ITER hydrogen plasma
    (2022-12-08) Lehti, Petteri
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Development of a field line tracing tool for thermal power load analysis
    (2022-06-26) Hyvärinen, Otso Juhani
     Perustieteiden korkeakoulu | Bachelor's thesis
  • E × B Flux Driven Detachment Bifurcation in the DIII-D Tokamak
    (2018-08-14) Järvinen, Aaro; Allen, S.L.; Eldon, D.; Fenstermacher, M.E.; Groth, Mathias; Hill, David N; Leonard, A.W.; McLean, A.G.; Porter, G.D.; Rognlien, T.D.; Samuell, C. M.; Wang, Huiqian
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    A bifurcative step transition from low-density, high-temperature, attached divertor conditions to high-density, low-temperature, detached divertor conditions is experimentally observed in DIII-D tokamak plasmas as density is increased. The step transition is only observed in the high confinement mode and only when the B × ∇B drift is directed towards the divertor. This work reports for the first time a theoretical explanation and numerical simulations that qualitatively reproduce this bifurcation and its dependence on the toroidal field direction. According to the model, the bifurcation is primarily driven by the interdependence of the E × B-drift fluxes, divertor electric potential structure, and divertor conditions. In the attached conditions, strong potential gradients in the low field side (LFS) divertor drive E × B-drift flux towards the high field side divertor, reinforcing low density, high temperature conditions in the LFS divertor leg. At the onset of detachment, reduction in the potential gradients in the LFS divertor leg reduce the E × B-drift flux as well, such that the divertor plasma evolves nonlinearly to high density, strongly detached conditions. Experimental estimates of the E × B-drift fluxes, based on divertor Thomson scattering measurements, and their dependence on the divertor conditions are qualitatively consistent with the numerical predictions. The implications for divertor power exhaust and detachment control in the next step fusion devices are discussed.
  • Effect of inter-molecular inter-actions on ASDEX Upgrade subdivertor conditions: An analysis of EIRENE simulation results
    (2019-10-25) Ihalainen, Erkko
     Perustieteiden korkeakoulu | Bachelor's thesis
  • ELMFIRE predictions of the bootstrap current profile in the JET pedestal region
    (2020-03-17) Virtanen, Antti
     Perustieteiden korkeakoulu | Master's thesis
    Magnetic confinement of a plasma in a tokamak fusion reactor requires sufficient plasma current drive to sustain the magnetic field configuration. In present devices, the necessary current is mainly driven by external means. Non-inductive, self-generated plasma current called bootstrap current is considered critical in any future tokamak-based designs. The bootstrap current develops in a plasma due to the presence of a radial plasma pressure gradient. Consequently, significant bootstrap current drive arises in particular in a steep pedestal that forms in high confinement mode plasma scenarios. The bootstrap current also plays an important role in pedestal stability. In this thesis, the bootstrap current profile is studied in the pedestal region numerically with both neoclassical and turbulent simulations using plasma and magnetic parameters present in the JET tokamak. The simulations are performed with the gyrokinetic plasma simulation code ELMFIRE in the low-collisionality regime. The neoclassical simulation results are compared to two different analytical estimates. From the turbulence simulations, the effect of electrostatic instabilities on the bootstrap current profile is investigated by comparing the results to neoclassical predictions. The neoclassical bootstrap current simulations are found to agree with the analytical estimates within a few percent. No large deviation between the two analytical models is observed for the low-collisionality regime, and both models match the simulation results within numerical accuracy, even when approaching the limit where the neoclassical approximations start to break down. However, discrepancies as large as 20% between the numerical simulation and the analytical estimates by the models are introduced when the collision grid used by ELMFIRE is made sparser, resulting in inaccuracy in the collision operator. The turbulence simulations show a strong effect of turbulent fluctuations on the plasma current which reduces the current density in comparison to the neoclassical predictions. Computational restrictions prevent further quantitative investigations and analyses.
  • Environmental grading of materials as an eco-design tool
    (2020-08-18) Bernardi, Louison
     Perustieteiden korkeakoulu | Master's thesis
    The main goal of this master thesis is to develop an Eco-design tool for the aeronautical sectors comparing the traditional aeronautical materials (aluminium vs carbon composite). This Eco-design tool assesses the environmental performance of materials into grades and those grades are implement in traditional design tool. The environmental grades are determined using a life cycle assessment (LCA) approach. The method used to answer those question is: i. performing an environmental assessment of the selected materials, ii. translating the environmental assessment into grades and iii. evaluating the LCA uncertainties of the obtained grade thanks to characterization and numerical simulations. First, the comparison of materials environmental impacts during the design process relies on environmental assessment of materials. This environmental assessment is performed using the LCA method and the LCA software GaBi. The scope on the LCA is limited to the manufacturing of the final materials from virgin and (if possible) secondary materials. The impact assessment methods used are ReCiPe 2016 and Environmental Footprint 2.0. The environmental assessment gives the environmental impacts on 16 impacts categories including acidification, human toxicity, climate change or energy resource. The raw results of the conducted LCA are not on an easy-accessible format and that is the reason why the LCA results are transformed into environmental grades. Thus, the impacts of a material are translated into grade by normalizing the environmental assessment results and a weighting method. The normalization allows to make the different impacts dimensionless and thus be able to combine the different impact into a grade. The weighting method provides weighting factors for each impact category and allows to obtain the final environmental grade. The weighting factors represent the importance of each impact category, these factors were determined by Castenalli (Castellani et al., 2016). Finally, the uncertainties are determined by applying standard deviation analysis to the critical parameters and Monte Carlo simulations.
  • ERO2.0 modelling of medium-Z impurity sources in JET
    (2022-10-14) Virtanen, Pyry
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Estimation of ITER FILD fast ion fluxes using ASCOT simulations
    (2023-08-22) Hyvärinen, Otso Juhani
     Perustieteiden korkeakoulu | Master's thesis
    One of the main goals of the ITER project is to reach Q = 10 operation. At these conditions, alpha particles will be the dominant source of plasma heating. Therefore, it is essential to understand how well these particles are confined. The fast ion loss detector (FILD) will be installed to test new models for the fast alpha particle losses. The objective of this thesis is to show that the ITER FILD is expected to measure fusion-born alpha particle fluxes larger than the background noise in Ip = 15 MA H-mode baseline. Furthermore, the effect of the neutral beam injector (NBI) heated deuterium ions on the FILD signal was studied. Velocity-integrated pinhole fluxes were analyzed from the ASCOT simulation with two different schemes. A uniform flux approximation resulted in a signal strength two orders of magnitude larger than the threshold for fusion-born alpha particles. A more accurate estimation method, voxel sum estimate, resulted in three orders of magnitude larger estimated pinhole flux compared to the threshold for fusion-born alpha particles. For NBI deuterium ions, the voxel sum estimate gave a factor of 500 larger than the threshold deuterium ion flux. However, neither of estimates include analysis of the velocity-space distribution of the incoming particle flux, so the estimations of only pinhole fluxes exceed the observable signals from the scintillator plate corresponding to the pinhole fluxes. The velocity-space of the incoming fluxes response on the scintillator plate was analyzed with the FILDSIM code. The results show that, when NBI is turned on, low-energy response (gyroradius < 6 cm) is dominated by the NBI, and fusion-born alpha particle response cannot be identified. However, higher energy response is caused by only fusion-born alpha particles, and low-energy response is observed if NBI is turned off. The magnitude of the peak fusion-born alpha particle response from the scintillator plate indicated that signal from fusion-born alpha particles is observable as low as 4% of the nominal fusion power.