Helium-tuhkan simuloiminen ITER:ssä

As energetic alpha particles cool down in fusion plasmas, they become helium ash. This helium ash radiates away energy from the plasma, dilutes the deuterium-tritium fuel mix and, thus, decreases the achievable fusion power. Therefore, its efficient removal from the confined plasma is essential for successful operation of the future fusion test reactor ITER. It is also important to prevent the return of the helium back to the confined plasma after is has been first removed from there. In this thesis, simulations were made to examine whether magnetic islands, created by external magnetic perturbations (MPs) due to ELM control coils (ECCs), can be used as a transport barrier to prevent helium from the first wall and divertor from returning to the confined plasma. The issue of helium ash as well as past helium experiments are also reviewed in this work. Along the simulated method, two schemes to improve helium ash removal are also presented in this work. The simulations were carried out with the orbit-following Monte Carlo code ASCOT. Earlier ASCOT-simulations were done mainly for fast ions, so in order to correctly simulate thermal helium ash, ASCOT's numerical integration scheme for stochastic differential equations (SDEs) was updated from the Euler-Maryuama method to the Milstein method. An adaptive time-stepping scheme for stochastic integration was added to the code. The simulations were done for the ITER baseline scenario and showed that the transport barrier effect of the islands was weak. The MPs furthermore enhanced the losses of deuterium and alpha particles more than the losses of helium.Fuusioplasmoissa syntyy energeettisten alfa-hiukkasten jäähtymisen seurauksena niin sanottua helium-tuhkaa. Tämä helium-tuhka säteilee pois plasman energiaa ja rajoittaa saatavaa fuusiotehoa. Näistä syistä on tulevan ITER-testireakrorin onnistuneen toiminnan kannalta tärkeää, että helium-tuhka saadaan tehokkaasti poistettua koossapidetystä plasmasta. Jotta heliumin määrä reaktorissa pysyy riittävän alhaisena, on myös ehkäistävä koossapidetystä plasmasta jo poistuneen heliumin paluu sinne. Tässä työssä tutkittiin, voidaanko helium-tuhkan paluuta reaktorin seinältä koossapidettyyn plasmaan ehkäistä luomalla plasman reunalle magneettisia saaria ulkoisten magneettisten häiriöiden (MP:den) avulla. Työssä myös esitellään helium-tuhkan haittavaikutuksia ja aiempia helium-kokeita. Tutkitun menetelmän lisäksi työssä esitellään kaksi menetelmää heliumin poistamiseksi. Helium-tuhkaa tutkittiin simulaatioilla. Nämä simulaatiot toteutettiin Monte Carlo radanseurantakoodilla ASCOT. ASCOT-simulaatioita on aiemmin tehty lähinnä nopeille ioneille ja onnistuneiden simulaatioiden takaamiseksi termiselle heliumille ASCOT:n numeerinen integraattori päivitettii Euler-Maryuama -menetelmästä Milsteinin menetelmään. Koodiin myös lisättiin uusi adaptiivinen aika-askellus stokastiselle integroinnille. Simulaatiot tehtiin ITER:n perusplasmalle ja havaittiin, että magneettiset saaret ehkäisevät heliumin palautumista koossapidettyyn plasmaan vain hyvin heikosti. Lisäksi MP:t huononsivat deuteriumin ja alfa-hiukkasten koossapitoa enemmän kuin heliumin

N-body simulations of gravitational dynamics

We describe the astrophysical and numerical basis of N-body simulations, both of collisional stellar systems (dense star clusters and galactic centres) and collisionless stellar dynamics (galaxies and large-scale structure). We explain and discuss the state-of-the-art algorithms used for these quite different regimes, attempt to give a fair critique, and point out possible directions of future improvement and development. We briefly touch upon the history of N-body simulations and their most important results.Comment: invited review (28 pages), to appear in European Physics Journal Plu

The physics of streamer discharge phenomena

In this review we describe a transient type of gas discharge which is commonly called a streamer discharge, as well as a few related phenomena in pulsed discharges. Streamers are propagating ionization fronts with self-organized field enhancement at their tips that can appear in gases at (or close to) atmospheric pressure. They are the precursors of other discharges like sparks and lightning, but they also occur in for example corona reactors or plasma jets which are used for a variety of plasma chemical purposes. When enough space is available, streamers can also form at much lower pressures, like in the case of sprite discharges high up in the atmosphere. We explain the structure and basic underlying physics of streamer discharges, and how they scale with gas density. We discuss the chemistry and applications of streamers, and describe their two main stages in detail: inception and propagation. We also look at some other topics, like interaction with flow and heat, related pulsed discharges, and electron runaway and high energy radiation. Finally, we discuss streamer simulations and diagnostics in quite some detail. This review is written with two purposes in mind: First, we describe recent results on the physics of streamer discharges, with a focus on the work performed in our groups. We also describe recent developments in diagnostics and simulations of streamers. Second, we provide background information on the above-mentioned aspects of streamers. This review can therefore be used as a tutorial by researchers starting to work in the field of streamer physics.Comment: 89 pages, 29 figure

DiffCloth: Differentiable Cloth Simulation with Dry Frictional Contact

Cloth simulation has wide applications in computer animation, garment design, and robot-assisted dressing. This work presents a differentiable cloth simulator whose additional gradient information facilitates cloth-related applications. Our differentiable simulator extends a state-of-the-art cloth simulator based on Projective Dynamics (PD) and with dry frictional contact. We draw inspiration from previous work to propose a fast and novel method for deriving gradients in PD-based cloth simulation with dry frictional contact. Furthermore, we conduct a comprehensive analysis and evaluation of the usefulness of gradients in contact-rich cloth simulation. Finally, we demonstrate the efficacy of our simulator in a number of downstream applications, including system identification, trajectory optimization for assisted dressing, closed-loop control, inverse design, and real-to-sim transfer. We observe a substantial speedup obtained from using our gradient information in solving most of these applications