Tommy Ahlgren

Tommy Ahlgren
University of Helsinki | HY · Department of Physics

PhD

About

88
Publications
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Publications

Publications (88)
Article
In nuclear fusion reactors, plasmas are heated to very high temperatures of more than 100 million kelvin and, in so-called tokamaks, they are confined by magnetic fields in the shape of a torus. Light nuclei, such as deuterium and tritium, undergo a fusion reaction that releases energy, making fusion a promising option for a sustainable and clean e...
Article
Alpha particles with energies on the order of megaelectronvolts will be the main source of plasma heating in future magnetic confinement fusion reactors. Instead of heating fuel ions, most of the energy of alpha particles is transferred to electrons in the plasma. Furthermore, alpha particles can also excite Alfvénic instabilities, which were previ...
Article
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Solute hydrogen can cause many damaging processes in the lattices of metals, such as deformation of the material, which can take place in large scales through blistering and embrittlement. Especially in nuclear fusion applications, the trapped hydrogen isotope Tritium in the reactor wall materials can pose a radiological safety hazard. Techniques f...
Article
Full-text available
The application of mean-field rate theory equations have proven to be a versatile method in simulating defect dynamics and temporal changes in the microstructure of materials. The reliability and usefulness of the method, however, depends critically on the defect interaction parameters used. In this study, we show that the main interaction paramete...
Article
Full-text available
Conditioning of a metal structure in a high-voltage system is the progressive development of resistance to vacuum arcing over the operational life of the system. This is, for instance, seen during the initial operation of radio frequency (rf) cavities in particle accelerators. It is a relevant topic for any technology where breakdown limits perform...
Article
Full-text available
A global heat flux model based on a fractional derivative of plasma pressure is proposed for the heat transport in fusion plasmas. The degree of the fractional derivative of the heat flux, α, is defined through the power balance analysis of the steady state. The model was used to obtain the experimental values of α for a large database of the Joint...
Article
Full-text available
Future fusion reactors use a D–T plasma mixture as fuel. A fraction of hydrogen species can escape the plasma confinement and hit the first wall. Hydrogen isotope exchange, a process in which trapped T atoms are replaced with lighter hydrogen isotopes D or H, is a potential method to minimize radioactive T retention in the wall materials. The prese...
Article
Following the IAEA Technical Meeting on 'Advanced Methodologies for the Analysis of Materials in Energy Applications Using Ion Beam Accelerators', this paper reviews the current status of ion beam analysis (IBA) techniques and some aspects of ion-induced radiation damage in materials for the field of materials relevant to fusion. Available faciliti...
Article
Results from two different sets of JET experiments are presented. In experiments in which toroidicity-induced Alfven eigenmodes (TAEs) localized at different radial locations had the same frequencies and toroidal mode numbers, the occurrence of enhanced losses after the excitation of TAEs in the core of the plasma was observed. On the contrary, enh...
Preprint
Full-text available
The mean-field rate equations have proven to be a versatile method in simulating defect dynamics and temporal changes in the micro-structure of materials. However, the reliability and usefulness of the method depends critically on the defect interaction parameters used. In this study, we show that the sink strength depends also on the detrapping or...
Preprint
Full-text available
Conditioning of a metal structure in a high-voltage system is the progressive development of resistance to vacuum arcing over the operational life of the system. This is, for instance, seen during the initial operation of radio frequency (rf) cavities in particle accelerators. It is a relevant topic for any technology where breakdown limits perform...
Article
Full-text available
Effect of ELMs on fuel retention at the bulk W target of JET ITER-Like Wall was studied with multi-scale calculations. Plasma input parameters were taken from ELMy H-mode plasma experiment. The energetic intra-ELM fuel particles get implanted and create near-surface defects up to depths of few tens of nm, which act as the main fuel trapping sites d...
Article
Reliable and accurate knowledge of the physical properties of elementary point defects is crucial for predictive modeling of the evolution of radiation damage in materials employed in harsh conditions. We have applied positron annihilation spectroscopy to directly detect mono-vacancy defects created in tungsten through particle irradiation at cryog...
Poster
Full-text available
The paper has been published in Nuclear Materials and Energy 19, 397 (2019). DOI: 10.1016/j.nme.2019.03.013.
Article
Full-text available
The JET ITER-Like Wall experiment, with its all-metal plasma-facing components, provides a unique environment for plasma and plasma-wall interaction studies. These studies are of great importance in understanding the underlying phenomena taking place during the operation of a future fusion reactor. Present work summarizes and reports the plasma fue...
Article
Full-text available
Properties of small vacancy clusters in tungsten were studied with first-principles calculations. The binding and formation energies of the vacancy clusters increase with the cluster size. Dynamic characteristics of a di-vacancy were specified between room temperature and 700 K with lattice kinetic Monte Carlo calculations, which were parametrised...
Article
The provision of a particle and power exhaust solution which is compatible with first-wall components and edge-plasma conditions is a key area of present-day fusion research and mandatory for a successful operation of ITER and DEMO. The work package plasma-facing components (WP PFC) within the European fusion programme complements with laboratory e...
Article
Full-text available
The 2014–2016 JET results are reviewed in the light of their significance for optimising the ITER research plan for the active and non-active operation. More than 60 h of plasma operation with ITER first wall materials successfully took place since its installation in 2011. New multi-machine scaling of the type I-ELM divertor energy flux density to...
Article
The sink strength is an important parameter for the mean-field rate equations to simulate temporal changes in the micro-structure of materials. However, there are noteworthy discrepancies between sink strengths obtained by the Monte Carlo and analytical methods. In this study, we show the reasons for these differences. We present the equations to e...
Article
The fuel outgassing efficiency from plasma-facing components exposed in JET-ILW has been studied at ITER-relevant baking temperatures. Samples retrieved from the W divertor and Be main chamber were annealed at 350 and 240 °C, respectively. Annealing was performed with thermal desoprtion spectrometry (TDS) for 0, 5 and 15 h to study the deuterium re...
Article
Full-text available
Hydrogen induced vacancy formation in metals and metal alloys has been of great interest during the past couple of decades. The main reason for this phenomenon, often referred to as the superabundant vacancy formation, is the lowering of vacancy formation energy due to the trapping of hydrogen. By means of thermodynamics, we study the equilibrium v...
Article
Full-text available
The interaction of fusion reactor plasma with the material of the first wall involves a complex multitude of interlinked physical and chemical effects. Hence, modern theoretical treatment of it relies to a large extent on multiscale modelling, i.e. using different kinds of simulation approaches suitable for different length and time scales in conne...
Article
Full-text available
The diffusion of monovacancies in tungsten is studied computationally over a wide temperature range from 1300 K until the melting point of the material. Our modelling is based on Molecular Dynamics technique and Density Functional Theory. The monovacancy migration barriers are calculated using nudged elastic band method for nearest and next-nearest...
Article
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We report a hydrogen induced vacancy formation mechanism in tungsten based on classical molecular dynamics simulations. We demonstrate the vacancy formation in tungsten due to the presence of hydrogen associated directly with a stable hexagonal self-interstitial cluster as well as a linear crowdion. The stability of different self-interstitial stru...
Article
The current magnetic confinement nuclear fusion power reactor concepts going beyond ITER are based on assumptions about the availability of materials with extreme mechanical, heat, and neutron load capacity. In Europe, the development of such structural and armour materials together with the necessary production, machining, and fabrication technolo...
Article
Particle irradiation produces defects which trap hydrogen isotopes and impurities in nuclear reactor materials. However, a comprehensive understanding of the basic mechanisms, and the final outcome of this process is still lacking. Here the evolution of defects, hydrogen, and impurities in tungsten during and after deuterium irradiation is simulate...
Article
All the recent DEMO design studies for helium cooled divertors utilize tungsten materials and alloys, mainly due to their high temperature strength, good thermal conductivity, low erosion, and comparably low activation under neutron irradiation. The long-term objective of the EFDA fusion materials programme is to develop structural as well as armor...
Article
We compare optical and structural properties of silicon-rich silicon oxide (SiOx, x ∼ 1.8) films obtained by ion implantation and molecular beam deposition (MBD). Before annealing, amorphous clusters (≥2 nm) are present in the MBD samples whereas these are absent for ion implantation, and the absorption at 488 nm is much stronger for MBD. Upon anne...
Article
Full-text available
First-principles calculations were used in determining the binding and trapping properties of hydrogen to point defects in tungsten. Hydrogen zero-point vibrations were taken into account. It was concluded that the monovacancy can hold up to five hydrogen atoms at room temperature. The hydrogen was found to distort the self-interstitial atom config...
Article
Full-text available
First principle calculations were used to study the hydrogen migration properties in bulk bcc tungsten. Hydrogen has low solubility in tungsten and occupies the tetrahedral interstitial site with an energy difference of 0.38 eV compared to the octahedral interstitial site. The hydrogen diffusion coefficient was evaluated using the harmonic transiti...
Article
Full-text available
A reactive interatomic bond-order potential for bcc tungsten is presented. Special attention in the potential development was given for obtaining accurate formation and migration energies for point defects, making the potential useful in atomic scale simulations of point and extended defects. The potential was used to calculate binding energies and...
Article
Full-text available
The first-principles calculations were used to study the hydrogen energetics on the (100) tungsten (√2×√2)R45° surface. Two equilibrium sites for H at the surface are identified, with a low migration barrier from the energetically clearly higher long bridge site to the short bridge site. At low coverages, the majority of H surface diffusion events...
Article
Full-text available
We report in situ electron microscopy observations of the plastic deformation of individual nanometer-sized Au, Pt, W, and Mo crystals. Specifically designed graphitic cages that contract under electron irradiation are used as nanoscopic deformation cells. The correlation with atomistic simulations shows that the observed slow plastic deformation i...
Article
Full-text available
We would like to discuss the role that 1 MeV tritons produced in deuterium–deuterium fusion reactions might play in a long-pulse or steady-state fusion reactor. Albeit a small minority in quantity compared to the fuel tritium, the fusion tritons have significantly longer penetration length in materials and can have detrimental consequences for the...
Article
Full-text available
Deuterium retention in the implantation-induced defects in polycrystalline tungsten has been studied. Deuterium was implanted with different energies and concentrations of retained D were analysed with secondary ion mass spectrometry and nuclear reaction analysis. Annealings were carried out at four pre-determined temperatures corresponding to four...
Article
Tungsten has been proposed for first wall material in thermonuclear reactors, where its behaviour in the presence of hydrogen containing plasma irradiation at elevated temperatures is of key interest. Deuterium induced defects in polycrystalline tungsten have been studied. Deuterium was implanted into tungsten samples and retained D-concentrations...
Article
Full-text available
Flaking of carbon film was studied by depositing hydrogen doped carbon films on tungsten and molybdenum substrates. Topographies of the carbon coatings were analyzed with scanning electron microscopy and three-dimension profilometer. Secondary ion mass spectrometry and Raman spectra analysis was used to determine the hydrogen concentration and the...
Article
A Monte Carlo method using ion packets is introduced. The method presented is optimal for the low probability scattering simulations of multiple and plural backscattering effects. Non-Rutherford cross sections are easily implemented in the framework of this model.
Article
At the end of C4 campaign at JET, a 1% SiH4/99% D2 mixture and pure 13CH4 were injected into the torus from the outer divertor wall and from the top of the vessel, respectively, in order to study material transport and scrape-off layer (SOL) flows. A set of MkIIGB tiles was removed during the 2001 shutdown for surface analysis. The tiles were analy...
Article
Six Joint European Torus (JET) divertor tiles were coated with tungsten marker stripes for erosion/deposition studies. The average thickness was 3.4 μm and the film was uniform within 5% across and within 10% along the stripe. Small amounts of impurities C, Fe, Ni and Cr were found and the in-plane stress was 965 MPa. Resistance to high heat loads...
Article
The atomic layer deposition (ALD) technique has been used to deposit different types of hafnium and zirconium silicates. The technique allows controlling the material thickness and quality due to atomic/molecular layer-by-layer growth mechanism. The films were deposited on 200 mm Si(1 0 0) substrates. Both thickness and Hf/Si or Zr/Si ratio were va...
Article
Full-text available
The interstitial to substitutional nitrogen atoms as a function of concentration in GaNAs were determined by nuclear reaction analysis utilizing the 14N(d,p)15N and 14N(d,α)12C reactions using ion channeling technique. The GaNAs films with mean nitrogen concentration between 0.3% and 3%, measured with secondary ion mass spectrometry and time-of-fli...
Article
Annealing behaviour of deuterium in carbon films with different Si contents deposited by a pulsed arc discharge method in deuterium atmosphere was studied. The concentration profiles of D were measured by secondary ion mass spectrometry and elastic recoil detection techniques. A model taking into account diffusion, detrapping and trapping of D is u...
Article
Full-text available
The stopping cross sections of the III-V semiconductor materials GaN and InP for 0.3-2.5 MeV 1H have been studied by the Rutherford backscattering technique. The data are given with an estimated uncertainty better than 2% and the agreement with earlier data existing for InP below 500 keV is good. A commonly used model ZBL-85 predicts the data corre...
Article
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Positron-annihilation measurements and nuclear reaction analysis [utilizing the <sup> 14 </sup> N (d,p)<sup>15</sup> N and <sup>14</sup> N (d, He )<sup>12</sup> C reactions] in conjunction with Rutherford backscattering spectrometry in the channeling geometry were used to study the defects in as-grown Ga(In)NAs materials grown by molecular beam epi...
Article
We prepared InGaN layers on GaN/sapphire substrates using rf-MBE. Photoluminescence (PL) from these layers, grown at different temperatures TS, shows that there is a strong tendency of GaN to form a separate phase as TS is increased from 600°C to 650°C. Concomitant with the phase separation, the PL from the InGaN phase broadens, which indicates tha...
Article
Full-text available
The diffusion of arsenic has been studied in Ge, implanted with 1.1015, 120-keV 75As+ ions/cm2. The implanted samples were subjected to annealing in argon atmosphere in the temperature range of 450-550 °C. The annealing times varied between 0.5 and 91 h. The As concentration profiles were measured by secondary ion mass spectrometry. A Fermi-level-d...
Article
Annealing behavior of implanted silicon in amorphous carbon films deposited by a pulsed arc discharge method was studied. Raman spectroscopy was used to characterize the changes in the bonding structure after annealing. The concentration profiles of Si were measured by secondary ion mass spectrometry (SIMS). The obtained diffusion coefficients resu...
Article
Full-text available
Diffusion of deuterium in diamondlike carbon films with different Si contents deposited by a pulsed arc discharge method in deuterium atmosphere was studied. The concentration profiles of D were measured by secondary-ion-mass spectrometry and elastic-recoil-detection techniques. A model is proposed to describe the experimental depth profiles. Diffu...
Article
Full-text available
We study Si–C films (Si content from 0 to 33 at. %) prepared with a pulsed arc discharge method. The structural modifications introduced by annealing up to 1100 °C, irradiation with 53 MeV 127I10+ ions, and deposition onto heated substrates are characterized by Raman spectroscopy. For all the treatments, the structural modifications decrease when t...
Article
Full-text available
Diffusion of As into Ge from a GaAs overlayer deposited on p-type Ge substrates has been studied by means of secondary ion mass spectrometry. A concentration-dependent diffusion of As atoms was observed in addition to the concentration-independent diffusion of Ga and As atoms. The concentration dependence is explained by a Fermi-level-dependent dif...
Article
Full-text available
The annealing behaviour of aluminium has been studied in single-crystal InP implanted with 40 and 120 keV 27Al+ ions. The implantation doses were 1×1015 and 1×1016cm−2. The aluminium concentration profiles were determined by two techniques, Secondary ion mass spectrometry (SIMS) and the nuclear resonance broadening technique (NRB) which was used fo...
Article
The lattice location of diffused gold and platinum in zinc selenide (ZnSe) epitaxial layers was studied using the Rutherford backscattering (RBS) channeling technique. Thin Au and Pt films were evaporated onto ZnSe samples. The Au/ZnSe samples were annealed at 525°C and the residual Au film was removed by etching. Channeling angular scan measuremen...
Article
Full-text available
A method for determining correct depth profiles from samples with rough surfaces is presented. The method combines Rutherford backscattering spectrometry with atomic force microscopy. The topographical information obtained by atomic force microscopy is used to calculate the effect of the surface roughness on the backscattering spectrum. As an examp...
Chapter
Deuterium retention, solubility and out-diffusion have been studied in silicon doped carbon films produced by physical vapor deposition. The deuterium concentration profiles were measured by the time-of-flight elastic recoil detection analysis technique and secondary ion mass spectrometry. The D retention and solubility were measured in D implanted...
Article
Full-text available
The erosion of carbon by intensive hydrogen bombardment has been recently shown to decrease sharply at very high fluxes (~1019 ions/cm2 s). This effect cannot be explained by standard sputtering or erosion models, yet understanding it is central for selection of fusion reactor divertor materials, and formulation of sputtering models for high-flux c...