Browsing by Author "Kiviniemi, Timo"
Now showing 1 - 14 of 14
- Results Per Page
- Sort Options
- 3D Fokker-Planck Code for Numerical Simulation of Tokamaks
Helsinki University of Technology | Master's thesis(1996) Kiviniemi, Timo - Effect of impurities on neoclassical radial electric field in plasma
Perustieteiden korkeakoulu | Bachelor's thesis(2012-10-09) Pyy, Tuuli - ELMFIRE predictions of the bootstrap current profile in the JET pedestal region
Perustieteiden korkeakoulu | Master's thesis(2020-03-17) Virtanen, AnttiMagnetic 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. - Investigating geodesic acoustic modes on the TEXTOR tokamak via gyrokinetic plasma simulations
Perustieteiden korkeakoulu | Master's thesis(2014) Niskala, Paavo EinoWhen designing and developing tokamak nuclear fusion devices, confinement of the plasma is of utmost importance. This confinement is greatly affected by turbulence that enhances transport of particles out of the main plasma. Thus learning to understand and control plasma turbulence and phenomena related to it is a priority. Geodesic acoustic modes are one of these phenomena. According to theory, these oscillations of the electric potential and their effects may serve to regulate turbulence. We investigate geodesic acoustic modes with TEXTOR tokamak parameters using a plasma simulation code ELMFIRE. Based on the gyrokinetic model, ELMFIRE simulates turbulence and mechanisms related to it. Amongst other quantities, the code outputs the electric potential in the plasma. Studying the fluctuations of the electric field caused by this potential using Fourier and correlation analysis, properties of geodesic acoustic modes are investigated. As a result of this analysis, ELMFIRE results are on average within 20 percent of theoretical estimates for the frequency of the oscillations. Simulations indicate that the radial propagation speed of the fluctuations scales as a function of ion mass. Anti-correlation of the radial electric field fluctuations and particle transport diffusion coefficient suggests the physical mechanism of geodesic acoustic modes suppressing turbulence is seen in the simulations. Clear phase differences between radial electric field, diffusion coefficient and thermal conductivity are found. For further analysis, a more direct comparison between experimental work is proposed. - LH-transition initiation and dynamics in a conventional tokamak
A4 Artikkeli konferenssijulkaisussa(2019-11-26) Abdullina, Gulnara; Askinazi, Leonid; Belokurov, Alexander; Bulanin, Victor; Chôné, Laurent; Gurchenko, Alexei; Gusakov, Evgenii; Kiviniemi, Timo; Kornev, Vladimir; Krikunov, Sergei; Kouprienko, Denis; Lashkul, Sergei; Lebedev, Sergei; Leerink, Susan; Niskala, Paavo; Petrov, Alexander; Tukachinsky, Alexander; Yashin, Alexander; Zhubr, NikolaiRadial electric field shear is crucial for turbulence suppression and transition to the H-mode, although the high shear value alone may not be sufficient for the LH-transition initiation. Temporal and spatial parameters of shear perturbation, particle source and turbulence parameters are the main factors responsible for LH-transition initiation. Different plasma discharge scenarios in two Ioffe Institute conventional tokamaks are analyzed using the model of plasma density and ion temperature evolution to clear up the role of aforementioned factors. - Linear plasma instabilities in FT-2 tokamak
Perustieteiden korkeakoulu | Bachelor's thesis(2011) Niskala, Paavo - Neoclassical fluxes in a tokamak and the role of ion orbit loss in the L-H transition
Helsinki University of Technology | Licentiate thesis(1998) Kiviniemi, Timo - Numerical diagnostics in gyrokinetic particle simulations
Helsinki University of Technology | Master's thesis(2009) Korpilo, TuomasTo monitor the plasma state and to investigate different phenomena occurring in the plasma, fusion test reactors are equipped with diagnostic systems. Increasing knowledge of plasma behavior in reactor conditions is important for improving the performance of fusion reactors as well as for optimizing plasma parameters and operation. Like test reactors, different diagnostic tools are also included in the numerical programs modelling fusion plasmas. The purpose of the diagnostic tools is to describe the plasma by a variety of variables and to facilitate the validity study of the models used in the programs. This work aims to present the diagnostic variables embedded in the gyro kinetic code ELMFIRE as well as describe their calculation algorithms. The interest is also in the bootstrap current and in the Fourier transform of the data produced by the code. The study of the bootstrap current is part of the process necessary for validating ELMFIRE results. In the work, we compare the current density profile from a suitable ELMFIRE simulation with theoretical models of the bootstrap current. The resulting profiles are in good agreement with each other. The Fourier transform is used for investigating the plasma modes. In this work, we propose an alternative Fourier transform approach for the conventional Fourier transform that considers the real spacing between the data points. The presented approach is observed to produce smoother power spectra compared to the conventional Fourier transform. - Numerical simulation of neoclassical currents, parallel viscosity, and radial current balance in tokamak plasmas
Doctoral dissertation (article-based)(2001-08-10) Kiviniemi, TimoOne of the principal problems en route to a fusion reactor is that of insufficient plasma confinement, which has lead to both theoretical and experimental research into transport processes in the parameter range relevant for fusion energy production. The neoclassical theory of tokamak transport is well-established unlike the theory of turbulence driven anomalous transport in which extensive progress has been made during last few years. So far, anomalous transport has been dominant in experiments, but transport may be reduced to the neoclassical level in advanced tokamak scenarios. This thesis reports a numerical study of neoclassical fluxes, parallel viscosity, and neoclassical radial current balance in tokamaks. Neoclassical parallel viscosity and particle fluxes are simulated over a wide range of collisionalities, using the fully kinetic five-dimensional neoclassical orbit-following Monte Carlo code ASCOT. The qualitative behavior of parallel viscosity derived in earlier analytic models is shown to be incorrect for high poloidal Mach numbers. This is because the poloidal dependence of density was neglected. However, in high Mach number regime, it is the convection and compression terms, rather than the parallel viscosity term, that are shown to dominate the momentum balance. For fluxes, a reasonable agreement between numerical and analytical results is found in the collisional parameter regime. Neoclassical particle fluxes are additionally studied in the banana regime using the three-dimensional Fokker-Planck code DEPORA, which solves the drift-kinetic equation with finite differencing. Limitations of the small inverse aspect ratio approximation adopted in the analytic theory are addressed. Assuming that the anomalous transport is ambipolar, the radial electric field and its shear at the tokamak plasma edge can be solved from the neoclassical radial current balance. This is performed both for JET and ASDEX Upgrade tokamaks using the ASCOT code. It is shown that shear high enough for turbulence suppression can be driven at the Low (L) to High (H) transition conditions without taking into account anomalous processes. In agreement with experiments, simulations indicate a higher threshold temperature for the L–H transition in JET than in ASDEX Upgrade. The parametric dependence of the shear on temperature, density, and magnetic field, however, is similar for both devices. In agreement with some theoretical models and experimental observations, the results also suggest that the critical shear for strong turbulence suppression in JET should be lower than in ASDEX Upgrade. - Sähköinen kauppa osana mikroyrittäjien liiketoimintamallia kuluttajakaupassa
School of Economics | Master's thesis(2011) Kiviniemi, TimoTutkielman tavoitteet Tämän tutkielman tavoitteena oli selvittää sähköisessä kuluttajatavarakaupassa toimivien mikroyrittäjien liiketoimintamalleja mukaan lukien palvelukonsepti, ansaintalogiikka, arvoverkosto ja monikanavaisuus. Palvelukonseptin osalta selvitettiin erityisesti tuotevalikoiman sopivuutta verkkokauppaan, yhtenä erityistapauksena elintarvikemyynnin logistiset haasteet. Ansaintalogiikan osalta kartoitettiin muun muassa eri työvaiheiden vaatimia henkilöresursseja ja uusasiakashankintaa. Arvoverkostosta pyrittiin lisäämään ymmärrystä erityisesti raha- ja tavaravirtojen liikkumiseen liittyvistä kumppaneista. Lisäksi selvitettiin mitä muita kanavia kautta yrittäjät palvelevat asiakkaitaan. Lähdeaineisto ja tutkimusmenetelmät Tutkielman teoreettisessa osuudessa tarkasteltiin eri teoreettisia malleja koskien mikroyrittäjyyttä, sähköistä kauppaa, liiketoimintamalleja ja monikanavaisuutta. Näistä malleista koottiin tutkielmalle teoreettinen viitekehys. Empiirinen osuus toteutettiin laadullisena tutkimuksena jonka aineisto kerättiin haastattelemalla mikroyrittäjiä jotka myyvät fyysisiä tavaroita verkkokaupan välityksellä. Teemahaastatteluista on koottu kuusi tapauskuvausta sekä teemoittelemalla tehty analyysi. Keskeiset tutkimustulokset Keskeisin havainto palvelukonseptin osalta oli, että useimmat haastatellut yritykset keskittyivät joko melko kapeaan asiakassegmenttiin tai tiettyyn erikoistuneeseen tarpeeseen. Ansaintalogiikassa selkein havainto koski hakukoneiden keskeisyyttä uusasiakashankinnassa. Haastateltujen mikroyritysten arvoverkostossa tärkeässä osassa olivat suurempien yritysten tarjoamat verkkokauppa-, logistiikka- ja maksujen välityspalvelut. Logistiikka oli keskeinen haaste erityisesti ruokamyynnissä. Varsinkin kapeaan asiakassegmenttiin erikoistuneet yrittäjät pyrkivät palvelemaan omaa segmenttiään useita eri kanavia pitkin. - Simulating the effect of a poloidally localized neutral particle source on bootstrap current in the pedestal region
Perustieteiden korkeakoulu | Bachelor's thesis(2020-01-18) Systä, Heikki - Simulation of transport in the FT-2 tokamak up to the electron scale with GENE
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-01) Janhunen, Salomon; Merlo, Gabriele; Gurchenko, Alexey; Gusakov, Evgeniy; Jenko, Frank; Kiviniemi, TimoPrior experimental work on the FT-2 tokamak has observed electron density fluctuations at electron Larmor radius scales using the enhanced scattering (ES) diagnostic (Gusakov et al 2006 Plasma Phys. Control. Fusion 48 A371-6, Gurchenko and Gusakov 2010 Plasma Phys. Control. Fusion 52 124035). Gyrokinetic GENE simulations of conditions at the upper hybrid resonance layer probed by the ES diagnostic show the presence of the anticipated turbulence from the electron temperature gradient (ETG) driven instability in linear and nonlinear simulations. Ion-scale turbulence is responsible for majority of the transport via trapped electron modes, while impurities act to merge the spectrum of the ion and the electron scale instabilities into a continuum. The linear spectrum at electron scales is characterized by maximal growth rate at a significant ballooning angle θ 0, and at ion scales the turbulence is broad in the ballooning angle distribution. The neoclassical shearing rate obtained from GENE breaks symmetry in nonlinear simulations of ETG turbulence, which manifests itself as an asymmetric turbulence spectrum. The electron density fluctuation spectrum obtained with GENE corresponds well to the ES measurement at electron scales, as do the fluxes obtained from the ion-scale simulations. - Sources of transport in TEXTOR tokamak plasma edge
Perustieteiden korkeakoulu | Bachelor's thesis(2013-03-15) Simbierowicz, Slawomir - Study of the triggering mechanism of internal transport barriers in a gyrokinetic simulation for FT-2 tokamak
Perustieteiden korkeakoulu | Bachelor's thesis(2016-11-11) Grigoriadis, Benjamin