The main research interests are listed below, including codes, experimental apparatuses and facilities, and major scientific results.
Codes used and developed by the Fusion and Plasma Physics group
Although regimes exist in current tokamak experiments, in which the plasma is found stable to global MHD modes, the plasma is always in a state of thermodynamic quasi-equilibrium with an abundance of free energy for driving more benign instabilities. These instabilities reduce the confinement time through convection dominated transport (turbulence).
Confinement modes, which are observed to suppress this micro-turbulence (such as the high confinement mode, or H-mode) have been found experimentally. On the other hand, theoretical models of such confinement mode transitions and related phenomena (such as Edge Localised Modes, or ELMs) are still incomplete. To elucidate the physics behind fundamental transport processes in H-mode plasmas the ELMFIRE code has been developed within the Plasma Physics and Fusion group and VTT Technical Research Centre of Finland. The computational algorithm of the gyrokinetic full-f model ELMFIRE is a Lagrangian explicit/implicit predictor-corrector solver for the reduced Boltzmann equation in a magnetized plasma. For example, it has been applied to numerically investigate on transport phenomena in the tokamak, such as ion temperature gradient and trapped electron modes, zonal flows, and neo-classical feedback to turbulence. (For further information, please contact Timo Kiviniemi.)
Particle orbit simulations
ASCOT (Accelerated Simulation of Charged Particle Orbits in a Tokamak) is a Monte Carlo guiding-center orbit following code developed in collaboration with VTT Technical Research Centre of Finland since 1991. It has been applied to numerous problems from studies of relativistic reverse runaway electrons and LH/IC heating/current drive to orbit loss and wall/divertor loading studies and simulations of CX diagnostics in the presence of magnetic field ripple. With the capability to use a fully three-dimensional magnetic equilibrium and first wall, ASCOT is an established tool of European fusion research. For further information, please check ASCOT pages or contact Taina Kurki-Suonio and Seppo Sipilä.
Scrape-off layer and plasma-wall interaction
Controlling and mitigation the interaction of the thermalised plasma with the surrounding wall is one of the most challenging task in building a fusion power plant. Used materials in fusion experiments include carbon (carbon fibre composites, CFCs) and metals (beryllium, molybdenum, tungsten). The amount of power these materials can absorb before being significantly damaged is limited, typically to 10s of MW/m2, set by their thermo-mechanical properties. Hence, the plasma at the very edge – the scrape-off layer – needs to be sufficiently cooled before meeting the material surface. Concomitantly, impurities produced at the material surfaces due to plasma impact can penetrate the region where the fusion process takes place – the core plasma – and thereby incidentally dilute the fuel and cool the core plasma.
Simulations of the processes in the scrape-off requires taking into account an open magnetic field line geometry and a significant neutral and impurity population in the plasmas. Our research focuses on understanding the scrape-off layer utilising several simulation codes developed outside the group and validating their predictions against detailed measurements in JET, ASDEX Upgrade, and other tokamaks. These numerical tools include the following fluid and trace-impurity Monte-Carlo codes:
Experimental plasma-wall interaction research
To validate the code predictions, the group maintains close collaboration with JET, ASDEX Upgrade and other tokamaks in the world, with members actively participating in experiments. The investigations include the physics of divertor detachment, radiative divertors for heat flux mitigation, scrape-off layer flows, and impurity migration within the vacuum chamber. Centre piece of these investigations are dedicated tracer injection experiments using, for example 13C and 14N, and subsequent tile surface analyses to elucidate the transport of these elements from the source to the region of final deposition. The figures below show the measured 13C deposition in ASDEX Upgrade and the ASCOT predictions – mapped onto the 2-D wall – for first deposition. The studies highlight the 3-D nature of impurity transport and deposition – which is unrealistic to be measured – and the importance of flows in the scrape-off layer.
Tile analysis at VTT Research Centre Finland
In collaboration with the Fusion and Plasma Technology team at VTT Research Centre Finland and DIARC Technology Inc., functional marker coatings for selected wall tiles or probes are designed and produced for selected wall tiles or probes of ASDEX Upgrade and JET. Limiter and divertor tiles are removed from the torus of ASDEX Upgrade or JET and surface-analysed using secondary ion mass spectrometry (SIMS). It is a sensitive technique to determine the elemental composition of the studied samples as a function of depth from the surface. In fusion research, we have used SIMS to study depth profiles of beryllium, carbon, and tungsten (main elements in the wall-tile samples) as well as deuterium (principal plasma fuel) at different poloidal locations. Based on this data we have determined the erosion of the tiles, deposition of material on them, and accumulation of deuterium into critical regions. The work is funded by Euratom, Tekes, and EFDA under the Plasma-Wall Interaction Task Force.
For further information, please contact Antti Hakola (Antti.Hakola [at] vtt [dot] fi).
Plasma diagnostics development by Fusion and Plasma Physics
The group performs the following recent and on-going hardware development projects:
NPA detector for JET
Neutral particle analysers (NPA) are used for measuring atom (ie. neutral particle) fluxes escaping from plasma. Atoms are first ionised by a thin (~30 nm) carbon foil and then separated spatially in energy and ion species through a combined action of parallel magnetic and electric fields. Presently, the ions are detected by thin CsI(Tl) scintillators coupled to photomultiplier tubes.
In the NPA detector upgrade project, thin silicon detectors have been developed using Silicon-on-Insulator (SOI) technology. The aim with the upgrade is to improve background rejection under heavy neutron and gamma background, allow simultaneous measurements of alpha particles and deuterons on a single detector and to improve count rate and operational reliability. (For further information, please contact Marko Santala.)
NPA DAQ for ASDEX Upgrade
The ASDEX Upgrade neutral particle analysers have the same operational principle as the ones for JET. The ionisation is done in a gas stripping cell, and the detectors are channeltrons.
The data-acquisition system (DAQ) for the neutral particle analyser at ASDEX Upgrade tokamak was recently retooled. The Fusion and Plasma Physics group had key role in the work. The system is maintained by the group. (For further information, please contact Simppa Äkäslompolo.)
Collaboration with Experimental Research Institutes
Since the Plasma Physics and Fusion group does not maintain its own fusion device on the Otaniemi campus, we work in close cooperation with the JET tokamak at the Culham Science Centre site in the United Kingdom, with the ASDEX Upgrade tokamak at the Institute for Plasma Physics in Garching, Germany, the Institute of Climate and Energy Research in Jülich, Germany, the Ioffe Institute in St. Petersburg, Russia, and various other sites around the world. The ITER facility, currently under construction at the CEA site in Cadarache, France, will be the most important fusion experiment in the 2020s, aiming to reach reactor-relevant plasma conditions and to demonstrate the feasibility of steady-state operation for 100s of seconds.
The Joint European Torus, JET
JET is the largest experimental fusion device in the world. It is jointly operated by the European fusion associations and funded by EU/EURATOM. Several group members participate in the JET programme, in particular during plasma operational periods, and several researchers are seconded for extended periods up to a few years. JET has been successfully operated up to conditions relevant in reactors, including operation with deuterium-tritium, providing a scientific basis for ITER and future fusion power plant. In 2009-10, the carbon-based plasma-facing components inside the vacuum vessel were replaced by beryllium and tungsten to mitigate tritium retention. Our primary contributions are in developing, maintaining and validation computational models for interpretation and extrapolation of experiments. Several group members utilise JET to obtain experimental data to validate computer simulations. For example, the ASCOT and EDGE2D/EIRENE codes are used to calculate the wall loads due to both fast and thermal particles to understand images like the one given below. Our group is also involved in developing Neutral Particle Analysers (NPAs) in a fusion-relevant environment.
The ASDEX Upgrade tokamak is a large fusion device in Garching, Germany, run by the Max-Planck-Institut for Plasmaphysics. Despite somewhat smaller than JET, the tokamak has unique heating systems providing large amounts of non-inductive heating and thus reaching reactor relevant power density. Its plasma-facing components are entirely made of tungsten to establish high-performance plasmas in a device with reactor-relevant materials. ASDEX Upgrade comprised an excellent diagnostic system making is suitable for validation of plasma computer simulations. The Fusion and Plasma Physics group works in close collaboration with the ASDEX Upgrade team in fast particle and edge physics. For example, we have been using one of the edge spectrometer systems to measure the plasma flow in the scrape-off layer for input to and validation of the SOLPS and DIVIMP codes (see figure below). Members of the Fusion and Plasma Physics group frequently visit ASDEX Upgrade for up to four weeks at a time.
Ioffe-Institute St. Petersburg, Russia
The Ioffe institute is a research institute in St. Petersburg hosting several experimental plasma physics devices, such as the FT2 tokamak. Advanced diagnostics, in particular for plasma fluctuations, allow direct comparison and validation of simulations with the ELMFIRE code. The collaboration includes annual meetings between the groups as well as research visits for extended periods.
ITER is the first tokamak to be built to reach burning-plasma conditions relevant to future fusion reactors. It is the primary focus of the world-wide fusion effort in magnetic confinement system, jointly carried out by seven international partners. The device is currently (in 2013) under construction at the CEA site in Cadarache, southern France, and expected to be completed by the beginning of the 2020s. First deuterium-tritium plasmas are anticipated by the mid 2020s.