Nopeiden ionien tuottaman neutraalien hiukkasten vuon simulointi JET-tokamakissa

Neutral particle analysis (NPA) is a diagnostic method for measuring neutral particle fluxes resulting from charge exchange reactions between neutrals and ions in tokamak plasmas. The neutral atoms can escape the plasma, directly conveying information on the density and temperature of the initial ions. This piece of information in turn is crucial for the upcoming JET D-T campaign as well as eventual fusion reactor operation, where precise control of the ratio of different fuel isotopes is required. The measured neutral particle fluxes are, however, distorted by energetic particles such as those resulting from neutral beam injection (NBI) heating. The fluxes from the slowing down beam ions mask the signal from the bulk plasma. This complicates analysis of the main plasma ions, while at the same time providing an opportunity to diagnose the injected ions themselves. This can be used to infer properties of the NBI ion distribution as well as determine possible residual isotopic composition of the beams. In this thesis, the fast ion orbit following code ASCOT, together with other plasma codes available within the JET modelling suite, were used for predictive and interpretive modelling of the distribution and neutral flux of NBI ions in the JET tokamak. The NPA signal was found to be highly dependent on factors such as neutral particle distribution, plasma density and heating scheme, necessitating detailed numerical modelling.Fuusioplasman polttoaineiden seossuhteen määrittäminen ja hallinta on keskeistä tulevaisuuden fuusioreaktoreille. Neutraalien hiukkasten analysointi (neutral particle analysis, NPA) on diagnostiikkamenetelmä, jossa mitataan tokamak-reaktorin plasman emittoimien atomien vuota. Koska nämä hiukkaset ovat varauksettomia, ne voivat paeta tokamakin koossapitävästä magneettikentästä, ja mitatun vuon avulla voidaan määrittää plasman koostumus. Suurienergiset hiukkaset, kuten plasman kuumentamiseen käytettävistä neutraalisuihkuista (neutral beam injection, NBI) syntyvät ionit, voivat hidastuessaan vääristää plasman varauksettomien hiukkasten vuota. Tämä vaikeuttaa plasman termisten polttoaineionien mittaamista, mutta toisaalta mahdollistaa itse neutraalisuihkujen tuottaman nopeiden ionien jakauman ja koostumuksen diagnosoinnin. Tässä diplomityössä suurienergisten hiukkasten radanseurantakoodia ASCOTia sekä joukkoa muita plasmasimulaatiokoodeja käytettiin sekä tulkitseviin että ennustaviin NBI-hiukkasten NPA-simulaatioihin JET-tokamakissa. NPA-signaalin havaittiin riippuvan voimakkaasti neutraalien hiukkasten jakaumasta, plasman tiheydestä sekä käytetystä kuumennusmenetelmästä

An advection-diffusion model for cross-field runaway electron transport in perturbed magnetic fields

Disruption-generated runaway electrons (RE) present an outstanding issue for ITER. The predictive computational studies of RE generation rely on orbit-averaged computations and, as such, they lack the effects from the magnetic field stochasticity. Since stochasiticity is naturally present in post-disruption plasma, and externally induced stochastization offers a prominent mechanism to mitigate RE avalanche, we present an advection-diffusion model that can be used to couple an orbit-following code to an orbit-averaged tool in order to capture the cross-field transport and to overcome the latter's limitation. The transport coefficients are evaluated via a Monte Carlo method. We show that the diffusion coefficient differs significantly from the well-known Rechester-Rosenbluth result. We also demonstrate the importance of including the advection: it has a two-fold role both in modelling transport barriers created by magnetic islands and in amplifying losses in regions where the islands are not present

Mental health by gender-specific occupational groups : Profiles, risks and dominance of predictors

Background: We defined gender-specific profiles of mental ill-health for the main occupational groups using three outcomes; antidepressant use, sickness absence (SA) due to depression, and suicides. We also examined which occupational groups had the highest risk of the outcomes, and compared the importance of their predictors. Methods: From a random register cohort of Finnish working age population, individuals in the six largest occupational groups in 2004 for men and women were included (N = 414 357). We used register data to define the first antidepressant purchase (i.e. use), the first long-term SA spell for depression, and suicide between Jan 1st 2005 and Dec 31st 2014. We assessed the risk of each outcome by occupational group with logistic regression models, and used dominance analysis to compare the relative importance of predictors. Results: In all six occupational groups for women, the prevalence of antidepressant use and SA for depression was higher than in the men's occupational groups. The opposite was observed for suicides. The risk of antidepressant use was lower, but the risk of suicide was 2-times higher among men in low vs. high-skilled occupations. Among women, a lower skill-level was associated with a higher risk of SA due to depression. Gender was the most important predictor of all outcomes. Limitations: We lacked information on history of medication use or health problems prior to follow-up. Conclusions: Gendered occupational status was an underlying factor explaining distinctive mental health profiles in the working population. Occupational class-dependent behavioural patterns related to mental health existed among men.Peer reviewe

Nuorten työntekijöiden sosialisaatio työpaikoilla: Sosiaalisten suhteiden, hyvinvoinnin ja perehdytyksen merkitys

Tässä tutkimusraportissa tarkastelemme ensinnäkin miten uusien työntekijöiden esimies- ja verkostosuhteet ovat yhteydessä heidän työtehtävissä suoriutumiseen sekä työryhmän ja työyhteisön jäsenenä toimimiseen. Lisäksi tarkastelemme muutoksia uusien työntekijöiden esimies- ja verkostosuhteissa. Toiseksi katsomme miten uusien työntekijöiden sosialisaatio on yhteydessä työssä koettuun hyvinvointiin. Tarkastelemme myös uusien työntekijöiden perehdytyskäytäntöjä henkilöstöhallinnon näkökulmasta

H-mode dithering phase studies on ST40

The dithering H-mode phase, characterized by oscillations, is generally observed at input power values close to the L-H transition power threshold and low plasma collisionalities (low electron density and/or high plasma temperature). Measurements to characterize the dithering phase are presented for the low aspect ratio, high magnetic field tokamak, ST40. The dithering phase oscillation frequency is observed between 400 and 800 Hz and demonstrates an inverse relationship with core plasma density. Dithering phase H-modes are documented across a nonlinear, low-density power threshold operational space, with signature low- and high-density branches. The minimum power threshold for dithering H-mode access is measured at a core, line average electron density of 4.7(±0.5) × 10(19) m(−3), close to a predicted value of 4.1(±0.4) × 10(19) m(−3) from multi-machine studies. ASTRA calculated values of power coupled to the ion species, at the dithering H-mode transition, exhibit a similar nonlinear dependence on density. This analysis points to the important contribution of the ion thermal channel to the L-H phase transition. The low-frequency plasma density and D-alpha dithers appear to be accompanied by sudden bursts of magnetohydrodynamic (MHD) activity. A simple model is tested to demonstrate a possible scenario of self-regulation among turbulence, zonal flows, pressure (density) gradient and MHD activities. This article is part of a discussion meeting issue 'H-mode transition and pedestal studies in fusion plasmas'

Validating the simulation of beam-ion charge exchange in MAST Upgrade

Simulation of the impact of charge-exchange (CX) reactions on beam ions in the Mega Amp Spherical Tokamak (MAST) Upgrade was compared to measurements carried out with a fission chamber (neutron fluxes) and a fast ion deuterium-alpha (FIDA) diagnostic. A simple model was developed to reconstruct the outer-midplane neutral density based on measurements of deuterium-alpha emission from edge neutrals, and on Thomson scattering measurements of electron density and temperature. The main computational tools used were the ASCOT orbit-following code and the FIDASIM code for producing synthetic FIDA signals. The neutral density reconstruction agrees qualitatively with SOLPS-ITER modelling and yields a synthetic passive FIDA signal that is consistent with measurement. When CX losses of beam ions are accounted for, predicted neutron emission rates are quantitatively more consistent with measurement. It was necessary to account for CX losses of beam ions in simulations to reproduce the measured passive FIDA signal quantitatively and qualitatively. The results suggest that the neutral density reconstruction is a good approximation, that CX with edge neutrals causes significant beam-ion losses in MAST Upgrade, typically 20% of beam power, and that the ASCOT fast-ion CX model can be used to accurately predict the redistribution and loss of beam ions due to CX

Energetic particles in reactor-relevant plasmas: modelling and validation

Nuclear fusion is a promising future energy source with few carbon dioxide emissions and nearly limitless source of fuel in heavy isotopes of hydrogen. Energetic particles, such as fusion-born alpha particles and neutral beam injected (NBI) fast ions play a vital role in reactor-relevant fusion plasmas, as they are responsible for heating the plasma, but can simultaneously cause localized heat loads and risk of damage on the plasma facing components. In this work, the Monte Carlo orbit-following code ASCOT has been used to simulate fast ions both to validate simulation results with present-day experiments at the JET tokamak, and to predict fast ion losses in next-generation fusion reactors ITER and DEMO. For validation of ASCOT predictions against JET plasmas, synthetic diagnostics were used to compare the simulated fast ion distributions with the neutral particle analyser (NPA) and fast ion loss detector (FILD) measurements. The NPA simulations qualitatively reproduced the experimentally measured slowing-down distributions and fast ion isotope fraction for NBI-injected hydrogen and deuterium ions, while the FILD simulations for fusion product losses were within 10 % of the experimentally observed losses. For predictions in ITER plasmas, simulations with resonant magnetic perturbations showed that including the response of the plasma to the external perturbations is vital, as the response not only affected the magnitude but also the distribution of fast ion losses. For DEMO plasmas, the sensitivity of fast ion losses due to various magnetic perturbations was studied, including the toroidal field ripple and ferritic inserts in various configurations. The design was found to be robust with respect to fast ion confinement and losses. Finally, over the course of this work, a highly parallelized version of the ASCOT code, called ASCOT5, was developed. The new version substantially increased the performance on modern supercomputer hardware as well as improving its maintainability and extensibility.Fuusioenergia on lupaava energiantuotantomenetelmä, joka ei tuota merkittävästi hiilidioksidipäästöjä, ja polttoaineena käytettävät vedyn raskaiden isotoopien varannot ovat runsaat. Suurienergiset hiukkaset, kuten fuusioreaktioissa syntyvät alfahiukkaset ja neutraalisuihkuilla injektoidut (NBI) ionit, ovat merkittävässä roolissa reaktorirelevanteissa plasmoissa. Ne kuumentavat plasmaa ja ylläpitävät fuusiopaloa, mutta saattavat myös vahingoittaa reaktorin rakenteita. Tässä työssä nopeiden ionien radanseurantakoodi ASCOT validoitiin ensin mallintamalla JET-tokamakin plasmapurkauksia, jonka jälkeen sitä käytettiin ennustamaan seuraavan sukupolven ITER- ja DEMO-reaktoreiden nopeiden hiukkasten koossapitoa ja häviöitä. JET-tokamakissa simuloituja nopeiden ionien distribuutioita verrattiin synteettisillä diagnostiikoilla kokeellisiin tuloksiin neutraalihiukkasanalysaattorista (NPA) ja nopeiden hiukkasten häviöitä mittaavasta detektorista (FILD). NPA-simulaatiotulokset toisinsivat kokeellisesti mitatut vety- ja deuterium-ionien hidastumisjakaumat, ja FILD-simulaatiot vastasivat kokeellisesti havaittuja fuusiotuotehäviöitä.ITER-reaktorille tehdyt simulaatiot resonanttien magneettisten häiriöiden kanssa osoittivat, että plasman vasteen huomioiminen on tärkeää, koska se vaikutti niin nopeiden hiukkasten häviöiden määrään kuin jakaumaan reaktorin sisäseinällä. DEMO-reaktorille tutkittiin häviöiden herkkyyttä muun muassa toroidaalisen kentän kelojen ja ferriittisten komponenttien aiheuttamille magneettisille häiriöille. Suunniteltu reaktorigeometria osoittautui kestäväksi nopeiden hiukkasten aiheuttaman tehokuorman kannalta. Työn aikana kehitettiin myös uusi, rinnakkaislaskentaa tehokkaasti hyödyntävä koodiversio ASCOT5. Uusi versio lisäsi suorituskykyä merkittävästi uusimmilla supertietokoneilla sekä paransi koodin ylläpidettävyyttä ja laajennettavuutta