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Jari Likonen

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I'm a material scientist working with plasma facing components of a fusion reactor.

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Papers by Jari Likonen

Impact of ICRF on the scrape-off layer and on plasma wall interactions: From present experiments to fusion reactor

Nuclear Materials and Energy, 2019

Recent achievements in studies of the effects of ICRF (Ion Cyclotron Range of Frequencies) power ... more Recent achievements in studies of the effects of ICRF (Ion Cyclotron Range of Frequencies) power on the SOL (Scrape-Off Layer) and PWI (Plasma Wall Interactions) in ASDEX Upgrade (AUG), Alcator C-Mod, and JET-ILW are reviewed. Capabilities to diagnose and model the effect of DC biasing and associated impurity production at active antennas and on magnetic field connections to antennas are described. The experiments show that ICRF near-fields can lead not only to E × B convection, but also to modifications of the SOL density, which for Alcator C-Mod are limited to a narrow region near antenna. On the other hand, the SOL density distribution along with impurity sources can be tailored using local gas injection in AUG and JET-ILW with a positive effect on reduction of impurity sources. The technique of RF image current cancellation at antenna limiters was successfully applied in AUG using the 3-strap AUG antenna and extended to the 4-strap Alcator C-Mod field-aligned antenna. Multiple observations confirmed the reduction of the impact of ICRF on the SOL and on total impurity production when the ratio of the power of the central straps to the total antenna power is in the range 0.6 < P cen / P total < 0.8. Near-field calculations indicate that this fairly robust technique can be applied to the ITER ICRF antenna, enabling the mode of operation with reduced PWI. On the contrary, for the A2 antenna in JET-ILW the technique is hindered by RF sheaths excited at the antenna septum. Thus, in order to reduce the effect of ICRF power on PWI in a future fusion reactor, the antenna design has to be optimized along with design of plasmafacing components.

First Mirrors Test in JET for ITER: An overview of optical performance and surface morphology

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2010

Metallic mirrors will be essential components of all optical systems for plasma diagnosis in ITER... more Metallic mirrors will be essential components of all optical systems for plasma diagnosis in ITER. A comprehensive First Mirror Test (FMT) has been carried out in the JET tokamak. The exposure was followed by a characterisation of mirrors and their carriers. The main results obtained by optical and surface analysis methods are: (i) reflectivity of all tested mirrors is degraded either by erosion with charge exchange neutrals or by the formation of thick deposits; (ii) deuterium and carbon are the main elements detected on all mirror surfaces and the presence of beryllium is also found in several cases; (iii) thick deposits show columnar structure; (iv) bubble-like structures are detected in deposits; their formation is a probable reason for deposits' disintegration and peeling-off; (v) the deposition in channels in the divertor cassettes is pronounced at the very entrance and then it sharply decreases with the distance from the plasma, l$5-7 mm.

Modeling of Carbon Migration During JET Injection Experiments

JET has performed two dedicated carbon migration experiments on the final run day of separate cam... more JET has performed two dedicated carbon migration experiments on the final run day of separate campaigns (2001 and 2004) using 13 CH 4 methane injected into repeated discharges. The EDGE2D/NIMBUS code modelled the carbon migration in both experiments. This paper describes this modelling and identifies a number of important migration pathways: 1. deposition and erosion near the injection location, 2. migration through the main chamber SOL, 3. migration through the private flux region aided by E×B drifts, and 4. neutral migration originating near the strike points. In H-Mode, type I ELMs are calculated to influence the migration by enhancing erosion during the ELM peak and increasing the longrange migration immediately following the ELM. The erosion/re-deposition cycle along the outer target leads to a multistep migration of 13 C towards the separatrix which is called "walking". This walking created carbon neutrals at the outer strike point and led to 13 C deposition in the private flux region. Although several migration pathways have been identified, quantitative analyses are hindered by experimental uncertainty in divertor leakage, and the lack of measurements at locations such as gaps and shadowed regions.

Effect of Hydrogen on Flaking of Carbon Films on Mo and W

Physica Scripta, 2004

The quantum physics of light is a most fascinating field. Here I present a very personal viewpoin... more The quantum physics of light is a most fascinating field. Here I present a very personal viewpoint, focusing on my own path to quantum entanglement and then on to applications. I have been fascinated by quantum physics ever since I heard about it for the first time in school. The theory struck me immediately for two reasons: (1) its immense mathematical beauty, and (2) the unparalleled precision to which its predictions have been verified again and again. Particularly fascinating for me were the predictions of quantum mechanics for individual particles, individual quantum systems. Surprisingly, the experimental realization of many of these fundamental phenomena has led to novel ideas for applications. Starting from my early experiments with neutrons, I later became interested in quantum entanglement, initially focusing on multi-particle entanglement like GHZ states. This work opened the experimental possibility to do quantum teleportation and quantum hyper-dense coding. The latter became the first entanglement-based quantum experiment breaking a classical limitation. One of the most fascinating phenomena is entanglement swapping, the teleportation of an entangled state. This phenomenon is fundamentally interesting because it can entangle two pairs of particles which do not share any common past. Surprisingly, it also became an important ingredient in a number of applications, including quantum repeaters which will connect future quantum computers with each other. Another application is entanglement-based quantum cryptography where I present some recent long-distance experiments. Entanglement swapping has also been applied in very recent so-called loophole-free tests of Bell's theorem. Within the physics community such loophole-free experiments are perceived as providing nearly definitive proof that local realism is untenable. While, out of principle, local realism can never be excluded entirely, the 2015 achievements narrow down the remaining possibilities for local realistic explanations of the quantum phenomenon of entanglement in a significant way. These experiments may go down in the history books of science. Future experiments will address particularly the freedom-of-choice loophole using cosmic sources of randomness. Such experiments confirm that unconditionally secure quantum cryptography is possible, since quantum cryptography based on Bell's theorem can provide unconditional security. The fact that the experiments were loophole-free proves that an eavesdropper cannot avoid detection in an experiment that correctly follows the protocol. I finally discuss some recent experiments with single-and entangled-photon states in higher dimensions. Such experiments realized quantum entanglement between two photons, each with quantum numbers beyond 10 000 and also simultaneous entanglement of two photons where each carries more than 100 dimensions. Thus they offer the possibility of quantum communication with more than one bit or qubit per photon. The paper concludes discussing Einstein's contributions and viewpoints of quantum mechanics. Even if some of his positions are not supported by recent experiments, he has to be given credit for the fact that his analysis of fundamental issues gave rise to developments which led to a new information technology. Finally, I reflect on some of the lessons learned by the fact that

Material erosion and migration in tokamaks

Plasma Physics and Controlled Fusion, 2005

The issue of first wall and divertor target lifetime represents one of the greatest challenges fa... more The issue of first wall and divertor target lifetime represents one of the greatest challenges facing the successful demonstration of integrated tokamak burning plasma operation, even in the case of the planned next step device, ITER, which will run at a relatively low duty cycle in comparison to future fusion power plants. Material erosion by continuous or transient plasma ion and neutral impact, the susbsequent transport of the released impurities through and by the plasma and their deposition and/or eventual re-erosion constitute the process of migration. Its importance is now recognized by a concerted research effort throughout the international tokamak community, comprising a wide variety of devices with differing plasma configurations, sizes and plasmafacing component material. No single device, however, operates with the first wall material mix currently envisaged for ITER, and all are far from the ITER energy throughput and divertor particle fluxes and fluences. This paper aims to review the basic components of material erosion and migration in tokamaks, illustrating each by way of examples from current research and attempting to place them in the context of the next step device. Plans for testing an ITER-like first wall material mix on the JET tokamak will also be briefly outlined.

Global migration of impurities in tokamaks

Plasma Physics and Controlled Fusion, 2013

Migration of impurities in tokamaks has been studied with the help of tracer-injection ( 13 C and... more Migration of impurities in tokamaks has been studied with the help of tracer-injection ( 13 C and 15 N) experiments in JET and ASDEX Upgrade since 2001. We have identified a common pattern for the migrating particles: SOL flows drive impurities from the low-field side towards the high-field side of the vessel. Migration is also sensitive to the density and magnetic configuration of the plasma, and strong local variations in the resulting deposition patterns require 3D treatment of the migration process. Moreover, re-erosion of the deposited particles has to be taken into account to properly describe the migration process during steady-state operations of the reactor.

Migration and deposition of13C in the full-tungsten ASDEX Upgrade tokamak

Plasma Physics and Controlled Fusion, 2010

The migration of carbon in low-density, low-confinement plasmas of ASDEX Upgrade was studied by i... more The migration of carbon in low-density, low-confinement plasmas of ASDEX Upgrade was studied by injecting 13 C into the main chamber of the torus at the end of the 2007 experimental campaign. A selection of standard tungsten-coated lower-divertor and main-chamber tiles as well as a complete set of lower-divertor tiles with an uncoated poloidal marker stripe were removed from one poloidal cross section and analyzed using secondary ion mass spectrometry (SIMS). The poloidal deposition profiles of 13 C on both the tungsten-coated tiles and on the uncoated graphite areas of the marker tiles were measured and compared. For the W-coated lower-divertor tiles, 13 C was deposited mainly on the high-field side tiles, while barely detectable amounts of 13 C were observed on low-field side samples. In contrast, on the uncoated marker stripes the deposition was equally pronounced in the high-field and low-field side divertor. The marker-tile results are in agreement with those obtained from graphite tiles after the 2003 and 2005 13 C experiments in ASDEX Upgrade. In the case of W-coated tiles, the 13 C measurements were complemented by determining the total amount of deposited carbon ( 12 C) on the tiles, which also shows strong deposition at the inner parts of the lower divertor. The estimated deposition of 13 C on W at the divertor areas was less than 1.5% of the injected amount of 13 C atoms. The 13 C analyses of the main-chamber tiles and small silicon samples mounted in remote areas revealed significant deposition in the upper divertor, in upper parts of the heat shield, in the limiter region close to the injection valve, and below the roof baffle. Approximately 8% of the injected 13 C is estimated to have accumulated in these regions. Possible reasons for the different deposition patterns on W and on graphite in different regions of the torus are discussed.

Investigation of tungsten coatings on graphite and CFC

Physica Scripta, 2007

Deuterium Inventory in Tore Supra: status of post-mortem analyses

Physica Scripta, 2009

A dedicated study on fuel retention has been launched in Tore Supra which included a D wallloadin... more A dedicated study on fuel retention has been launched in Tore Supra which included a D wallloading campaign and the dismantling of the main limiter (Deuterium Inventory in Tore Supra, DITS project). This paper presents new results of post mortem analyses performed on selected limiter tiles. Scanning electron microscopy shows thick layers with poloidally oriented tip-shaped structures in deposition zones. In the erosion zone deposits inside the open porosities of the bulk material of the tiles were found. Raman microscopy indicates that hard deuterated amorphous carbon layers are deposited on the limiter tiles and on their gap sides. Secondary ion mass spectrometry D depth profiles show an increase of the D content near the bottom of the gap, in agreement with previous results. The campaign markers ( 13 C and 11 B) are found in only 2 out of 7 measurements in the deposited layers. Finally, the agreement of the SIMS data with previous NRA measurements is discussed, especially discrepancies possibly due to the non-uniformity of the samples.

The deactivation of nitrogen acceptors in ZnSxSe1−x and MgyZn1−ySxSe1−x studied by combining positron annihilation, SIMS, and CV measurements

Physica B: Condensed Matter, 1999

We show that quantitative information on the electrical deactivation of doping can be obtained by... more We show that quantitative information on the electrical deactivation of doping can be obtained by combining the results of positron annihilation, secondary ion-mass spectrometry and capacitance}voltage measurements. We have applied this method to study the nitrogen doping in ZnS V Se \V and Mg W Zn \W S V Se \V and we conclude that in addition to isolated acceptor impurities N\ 1 , nitrogen atoms are also situated at high concentrations in compensating donors, most probably in (Zn N 1 )> and (V 1 N 1

Observation of Ga vacancies and negative ions in undoped and Mg-doped GaN bulk crystals

Physica B: Condensed Matter, 1999

Gallium vacancies and negative ions are observed in GaN bulk crystals by applying positron lifeti... more Gallium vacancies and negative ions are observed in GaN bulk crystals by applying positron lifetime spectroscopy. The concentration of Ga vacancies decreases with increasing Mg doping, as expected from the behavior of the < % formation energy as a function of the Fermi level. The concentration of negative ions correlates with that of Mg impurities determined by secondary ion mass spectrometry. We thus attribute the negative ions to Mg\ % . The negative charge of Mg suggests that Mg doping converts n-type GaN to semi-insulating mainly due to the electrical compensation of O> , donors by Mg\ % acceptors.

Gas balance and fuel retention in fusion devices

Nuclear Fusion, 2007

The effect of non-axisymmetric wall geometry on 13C transport in ASDEX Upgrade

Nuclear Fusion, 2012

We present the first results of 3D simulations of global 13C transport in ASDEX Upgrade (AUG) ind... more We present the first results of 3D simulations of global 13C transport in ASDEX Upgrade (AUG) indicating that the deposition profile of 13C exhibits toroidal asymmetry in the main chamber. In 2007, the migration of carbon in AUG was studied with a methane (13CH4) injection experiment (A. Hakola et al and the ASDEX Upgrade Team 2010 Plasma Phys. Control. Fusion 52 065006). The total amount of deposited 13C was estimated by assuming toroidally symmetric deposition. Remarkably, the total number of deposited atoms was observed to be less than 10% of the number of injected atoms. The experiment has been simulated with the 3D orbit-following Monte Carlo code ASCOT using both a realistic 3D wall geometry of AUG and a 3D magnetic field with toroidal ripple. The simulations indicate that the non-axisymmetric wall geometry causes notable toroidal asymmetry in the deposition profile in the outer (low-field side) midplane region which can provide a partial explanation for the missing carbon inf...

The deuterium inventory in ASDEX Upgrade

Nuclear Fusion, 2007

Carbon erosion and deposition in the ASDEX Upgrade divertor was investigated using a poloidal sec... more Carbon erosion and deposition in the ASDEX Upgrade divertor was investigated using a poloidal section of marked divertor tiles and silicon samples below the divertor structure. The whole inner divertor is a net carbon deposition area, while a large fraction of the outer divertor is erosion dominated and the roof baffle tiles show a complicated distribution of erosion and deposition areas. In total, 43.7 g B+C were redeposited, of which 88% were deposited on tiles and 9% in remote areas (below roof baffle, on vessel wall structures). Identified carbon sources in the main chamber are too low by a factor of ten to explain the observed carbon divertor deposition, but carbon erosion is observed at the outer divertor tiles. Deuterium is trapped mainly on the surfaces of the inner divertor tiles. The long term retention in codeposited hydrocarbon layers is about 3% of the total deuterium fuel input.

Beryllium accumulation at the inner divertor of JET

Journal of Nuclear Materials, 2005

MkIIGB divertor tiles exposed in JET for the 1998-2001 and 1999-2001 campaigns have been used to ... more MkIIGB divertor tiles exposed in JET for the 1998-2001 and 1999-2001 campaigns have been used to assess the amount of beryllium and carbon deposited at the inner divertor wall. Total amount of Be at the inner divertor tiles was determined and integrated toroidally. Results were compared with data obtained with optical spectroscopy and good agreement was obtained. The amount of deposited C was computed from the amount of deposited Be assuming that the Be/C ratio arriving in the divertor is the same as the Be/C ratio in the main chamber. On the basis of this analysis we would expect there to be $0.4 kg of C deposited. This gives an average C deposition rate lower than during the MkIIA phase.

Carbon erosion and deposition on the ASDEX Upgrade divertor tiles

Journal of Nuclear Materials, 2005

Carbon deposition and erosion were measured on ASDEX Upgrade divertor tiles and below the roof ba... more Carbon deposition and erosion were measured on ASDEX Upgrade divertor tiles and below the roof baffle during the operation period 2002/2003. The inner divertor is a net carbon deposition area, while a large fraction of the outer divertor is erosion dominated and the roof baffle tiles show a complicated distribution of erosion and deposition areas. In total, 43.7 g B+C were redeposited, of which 88% were deposited on tiles and 9% in remote areas (below roof baffle, on vessel wall structures). 0.6 g C was pumped out as volatile hydrocarbon molecules. Carbon sources in the main chamber are too low by a factor of more than ten to explain the observed carbon divertor deposition. Carbon erosion is observed at the outer divertor strike point tiles, but it is arguable if material can be transported from the outer strike point to the inner divertor.

Erosion and deposition in the JET MkII-SRP divertor

Journal of Nuclear Materials, 2007

Carbon-13 labelled methane was injected into the outer divertor during a series of H-mode dischar... more Carbon-13 labelled methane was injected into the outer divertor during a series of H-mode discharges on the last day of operations with the JET MkII-SRP divertor. Tiles from around the vessel were removed during the subsequent shutdown and surface deposits were analysed by IBA techniques and SIMS. First attempts to model the pattern of 13 C deposition using EDGE2D are reported. Erosion of W markers at the outer divertor was observed, with implications for the ITERlike wall experiment planned for JET, whilst thin film growth in the same region has been followed by the effect on infrared measurements. The composition of thick films deposited at the inner divertor during the MkII-SRP campaign, and the migration to the inner corner of the divertor observed by a quartz micro-balance, provide further information on divertor transport.

Structural studies of deposited layers on JET MkII-SRP inner divertor tiles

Journal of Nuclear Materials, 2007

Deposited layers formed on JET inner divertor tiles during 1998-2004 and 2001-2004 campaigns have... more Deposited layers formed on JET inner divertor tiles during 1998-2004 and 2001-2004 campaigns have been investigated using secondary ion mass spectrometry (SIMS), Rutherford Backscattering (RBS) and optical microscopy. The thickness of the deposit decreases from the top of vertical tile 1 to the bottom and then increases on vertical tile 3 reaching $60 lm. There are even thicker deposits on the small sloping section of the floor tile 4 that can be accessed by the plasma at the inner divertor legs. Deposited films on divertor inner wall tiles are enriched in Be indicating chemical erosion of C and a multi-step transport of C to the shadowed area on floor tile 4. The films have generally a layered and globular structure in the areas with plasma contact.

Multi machine scaling of fuel retention in 4 carbon dominated tokamaks

Journal of Nuclear Materials, 2011

In order to benchmark predictions for the in vessel tritium inventory in ITER, a survey of fuel r... more In order to benchmark predictions for the in vessel tritium inventory in ITER, a survey of fuel retention measured in 4 carbon dominated tokamaks (TEXTOR, ASDEX Upgrade in the 2002-2003 carbon configuration, Tore Supra and JET) was performed, showing retention rates from ~1 g D/h in TEXTOR (L mode, limiter machine) up to ~6-12 g D/h in AUG (H mode, divertor machine). A simple scaling used for ITER predictions is applied for comparison with experimental values : 1) estimate of wall fluxes, 2) estimate of the gross carbon erosion, 3) estimate of the net erosion/redeposition assuming a redeposition fraction and 4) estimate of the retention rate using D/C ratio scalings. The validity of each step is discussed, showing that this approach yields the right order of magnitude, but tends to underestimate the experimental values unless a high wall flux, a low local redeposition fraction and/or a high D/C ratio are used.

Global transport of light elements boron and carbon in the full-W ASDEX Upgrade

Journal of Nuclear Materials, 2011

Transport of carbon and boron has been investigated in the full-W ASDEX Upgrade after experimenta... more Transport of carbon and boron has been investigated in the full-W ASDEX Upgrade after experimental campaigns with (2008) and without (2007) boronizations. For this purpose, poloidal deposition profiles of the two elements on tungsten and graphite regions of lowerdivertor tiles have been determined. Carbon is mainly deposited in the inner divertor -80-90% of the determined 12 C and 13 C inventories on W -while boron shows a much more symmetric deposition profile. In the unboronized machine, the boron inventories are a factor of 10 smaller than in the boronized case and result from residual boron atoms left in the torus prior to the 2007 campaign. Both carbon and boron are deposited more efficiently and/or show less erosion on graphite than on tungsten, particularly in the outer divertor. For 13 C, the difference is 10-100 in favor of graphite. This is most probably caused by a higher re-erosion from tungsten surfaces.