708 research outputs found
Towards realistic orbit-following simulations of fast ions in ITER
Get PDFOne of the main scientific goals of the international ITER experiment is to provide understanding of burning plasmas, including the behavior of fusion-born alpha particles. These particles form both a potential risk for the first wall and a massive source of free energy in the plasma. Such free energy can drive a multitude of MHD modes, most notably the Alfvénic ones, that can lead to increased transport and even losses of fast ions.
In this work, the alpha particle physics has been studied using kinetic orbit-following Monte Carlo code ASCOT. The code was enhanced with two new physics models. The first model relaxes the usual guiding center (GC) approximation used to save computation time. In some cases, this approximation is not valid and the full gyro motion (FO) has to be resolved. The second model is for fast ion relevant MHD modes and its implementation allows taking into account electromagnetic fields due to these modes.
When the MHD model was used to simulate ITER plasmas, the wall power loads due to fast particles were not found to exceed the design limits of the wall materials even for unrealistically large perturbations. However, redistribution of fast ions was observed to alter the alpha particle heating profile and neutral beam ion (NBI) driven current profile.
Fusion alphas were simulated for the ITER 15 MA scenario using different integration methods. Following the full gyro motion gave slightly larger alpha particle wall power loads than the GC method. Since the FO method uses more than 50 times more CPU than GC integration, a third method was introduced as a compromise between the speed and accuracy: the GC method is used in the plasma core and FO integration is activated in the vicinity of the wall.
Finally, alpha-driven current and torque in ITER were studied using different magnetic field configurations. It was found that, independent of the magnetic configurations, the alpha-driven current is less than 1% of the total plasma current for both 9 MA and 15 MA baseline scenarios. On the contrary, the alpha-driven torque depends on the magnetic field configuration. While in the axisymmetric case the total torque was found to be close to zero, with realistic 3D effects the alpha particles produced substantial torque, about one tenth of that driven by the NBI particles, but in direction opposite to it.Maailman ensimmäisessä fuusioreaktorissa, ITERissä, on tarkoitus oppia ymmärtämään palavien plasmojen fysiikkaa ja siten valmistella tietä kaupalliselle fuusioenergialle. Fuusioreaktioissa syntyvät nopeat alfahiukkaset sekä neutraalisuihkukuumennuksesta (NBI) syntyneet nopeat ionit aiheuttavat riskin ensiseinämän kestävyydelle. Hiukkasten sisältämä vapaa energia mahdollistaa monenlaiset magnetohydrodynaamiset (MHD) epästabilisuudet. Nämä MHD-häiriöt voivat aiheuttaa nopeiden hiukkasten kulkeutumista jopa ulos plasmasta.
Tässä työssä on tutkittu alfa- ja NBI-hiukkasten fysiikkaa käyttäen apuna kineettistä Monte-Carlo menetelmään perustuvaa radanseurantaohjelmistoa nimeltään ASCOT. Koodia on täydennetty numeerisilla malleilla, joista ensimmäinen mahdollistaa johtokeskusmenetelmän (GC) testaamisen ja tarvittaessa korvaa sen. Menetelmässä ratkaistaan hiukkasen gyroliike magneettikentän ympäri (FO). Toinen malli ottaa huomioon tiettyihin MHD ilmiöihin liittyvien sähkömagneettisten häiriöiden vaikutuksen nopeisiin hiukkasiin.
Ensimmäistä mallia sovellettiin ITER:n perusplasmalle (15 MA) ja havaittiin, että FO menetelmällä laskettu alfahiukkasten aiheuttama seinäkuorma on suurempi kuin käytettäessä GC menetelmää. Koska FO menetelmä vaati noin 50 kertaa enemmän laskenta-aikaa, esiteltiin uusi hybridimenetelmä, joka siirtyy käyttämään FO radanseurantaa vain seinämien lähistöllä.
MHD-mallia käytettiin useissa erilaisissa ITER-simuloinneissa. MHD-häiriöt eivät aiheuttaneet missään tapauksessa merkittävää lisäystä nopeiden hiukkasten tuottamiin seinäkuormiin. Sen sijaan MHD-häiriöt vaikuttavat nopeiden hiukkasten jakaumaan plasman sisällä. Tämä uudelleen jakautuminen aiheutti muutoksia sekä alfahiukkasten kuumennusprofiilissa että NBI-hiukkasten ajamassa virtaprofiilissa.
Työssä laskettiin myös alfahiukkasten aiheuttama sähkövirta ja vääntömomentti erilaisten magneettikentän häiriöiden läsnäollessa. Alfahiukkasten synnyttämä virta oli kaikissa tapauksissa alle 1% kokonaisplasmavirrasta. Alfahiukkasten ajaman väännön puolestaan havaittiin riippuvan ulkoisista häiriöistä: aksiaalisesti symmetrisessä tapauksessa väännön komponentit summautuivat nollaan, mutta 3D häiriöiden läsnäollessa alfavääntö oli nollasta poikkeava. Alfaväännön suuruus on kuitenkin kertaluokkaa pienempi kuin NBI-hiukkasten aiheuttama vääntö ja lisäksi vastakkaissuuntainen
How customer knowledge affects exploration: Generating, guiding, and gatekeeping
No full textFunding Information: The authors wish to thank Juha-Antti Lamberg, Jari Ojala, Kalle Pajunen, and Henrikki Tikkanen, as well as the reviewers and audience of the Academy of Management Annual Meeting (Vancouver, 2014), for their constructive feedback on earlier versions of the manuscript. The first author acknowledges the financial support of Foundation for Economic Education, Finland and the Jenny and Antti Wihuri Foundation . Publisher Copyright: © 2021 Elsevier Inc. Copyright: Copyright 2021 Elsevier B.V., All rights reserved.The importance of understanding customers in order to sustain the long-term success of the company has been claimed by academics and practitioners for decades, to the point that the claim has turned into a truism. And still, the role of customer knowledge in organizational renewal, especially via explorative new product development (NPD), remains ambiguous. While existing literature generally emphasizes the value of customer knowledge, critics argue that a strong customer focus can also de-motivate and misguide exploration. This study adds clarity to our understanding of this tension by drawing from an intensive analysis of the corporate archives of a rapidly growing high-tech company. The authors trace the impacts of customer knowledge on twelve explorative NPD projects. The findings reveal three distinct mechanisms through which customer knowledge influences exploration: generating, guiding, and gatekeeping. The impact of customer knowledge on exploration depends on the selective deployment of these mechanisms. The authors further argue that managers should seek to find a fit between the deployment of customer knowledge mechanisms and the exploration project type in order to increase the likelihood of exploration project success.Peer reviewe
Addressing substance use issues in adult social work
No full textThe objective of this thesis was to study the methods that are being used or would need further updating in adult social services when meeting clients with an already existing, or nascent substance abuse problem. As the new social and health care reform has just recently taken place in Finland and the primary focus has shifted to early identification and prevention work, these issues bring the subject of this thesis close to adult social service field.
The chosen method was a descriptive literature review. In theoretical framework the author explains the basic principles of adult social work (in Helsinki, Finland), substance abuse and the multidimensional harms it causes, mental health issues caused by substances, mini- intervention, and motivational interview. Data was collected through governmental and NGO websites, as well as written literature according to the title of thesis.
Thesis aims to find answers to a research question: How to use MI when meeting a client with substance abuse issues in adult social work?
The research showed that there are no simple answers as substance abuse is a broad and challenging subject that oftentimes requires multi-professional teamwork. Motivational interview itself, is a great tool and technique to implement, but it requires additional training and reviewing from the professionals. Studying motivational interviewing techniques should be made possible in Social Service studies, especially for those students who wish to broaden their knowledge in effective and preventative mental health – and substance abuse work
The development of a novel particle transport and thermal-hydraulic calculation chain for the European DEMO
Get PDFA versatile calculation chain has been under development at VTT and Aalto University, featuring the ASCOT Monte Carlo orbit-following suite of codes, the Serpent Monte Carlo particle transport code and the Apros® thermal-hydraulic system code. This project aims to establish a comprehensive analytical environment that can aid researchers at multiple stages during the maturation of DEMO. Plasma product source term profiles have been generated using the ASCOT code, providing input for subsequent neutron transport calculations with Serpent. These studies utilized the CAD-based geometry of the equatorial breeding unit of the Water-Cooled Lithium-Lead (WCLL) breeding blanket. Efforts towards optimization have been made, investigating leakages and scalability of these external source simulations. Preliminary particle transport results were reported along with the methodology of converting the tallied data into Apros-relevant input
Elektronirakenteiden ja positronitilojen mallintaminen (Al,Ga,In)N-puolijohdekerrosrakenteissa
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Scalable Text Mining with Sparse Generative Models
No full textThe information age has brought a deluge of data. Much of this is in text form, insurmountable in scope for humans and incomprehensible in structure for computers. Text mining is an expanding field of research that seeks to utilize the information contained in vast document collections. General data mining methods based on machine learning face challenges with the scale of text data, posing a need for scalable text mining methods.
This thesis proposes a solution to scalable text mining: generative models combined with sparse computation. A unifying formalization for generative text models is defined, bringing together research traditions that have used formally equivalent models, but ignored parallel developments. This framework allows the use of methods developed in different processing tasks such as retrieval and classification, yielding effective solutions across different text mining tasks. Sparse computation using inverted indices is proposed for inference on probabilistic models. This reduces the computational complexity of the common text mining operations according to sparsity, yielding probabilistic models with the scalability of modern search engines.
The proposed combination provides sparse generative models: a solution for text mining that is general, effective, and scalable. Extensive experimentation on text classification and ranked retrieval datasets are conducted, showing that the proposed solution matches or outperforms the leading task-specific methods in effectiveness, with a order of magnitude decrease in classification times for Wikipedia article categorization with a million classes. The developed methods were further applied in two 2014 Kaggle data mining prize competitions with over a hundred competing teams, earning first and second places
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