Slovenian Fusion Association

We are interested in processes involving neutral hydrogen molecules (H₂, HD, D₂) which occur on plasma facing surfaces and in the edge plasma of tokamak reactors. These molecules are typically vibrationally excited what influences respective reaction cross sections. Special experimental technique has been developed for vibrational spectroscopy of molecules and ion beam analytical technique ERDA is used for characterising hydrogen (H and D) content on and beneath the material surface. The following activities are currently under way:
- Detailed study of processes leading to the emission of neutral vibrationally excited hydrogen molecules (H₂ and D₂) from high Z materials (mainly tungsten) used for the plasma facing components and for the reactor wall in tokamaks.
- Study of a role of vibrationally excited hydrogen molecules (H₂ and D₂) in chemical erosion of carbon deposits or in modification of properties of carbon layers.
- Emission spectroscopy of liner magnetized hydrogen plasma in order to study the influence of wall-created molecules on Fulcher band emission.
- Modelling of particle transport in neutral hydrogen including vibrationally excited states by taking into account binary collisions and surface interactions with the aim to help understanding and evaluation of data from experiments.

Collaboration: EFDA, Plasma-Wall Interaction Task Force, Association EURATOM – FZJ, Association EURATOM – IPP.
Contact: iztok.cadez@ijs.si

Synthesis and characterization of hydrogenated carbon deposits

During operation the first-wall material in tokamaks and other high-temperature plasma reactors is subject to ion bombardment. The first wall material in future ITER tokamak will be berilium, while the divertor will be constructed from carbon-fibre composite (CFC) and tungsten (W). The deposits formed in the remote parts of the divertor will therefore consist of C, H, Be and W. The sputtered material is deposited on different components of plasma reactors. In order to develop a suitable method for occasional removal of future ITER deposits (to prevent hydrogen retention), a study of possible structure of model deposits has to be performed. Our study is focused on hydogenated carbon coating. We used two different deposition techniques for preparation of a-C:H films on suitable substrates. The first one is a sputter deposition of graphite using thermoinic arc. The composition of the films can be changed by varying various discharge parameters (i.e. argon-hydrogen pressure, discharge power, substrate temperature, deposition rate, substrate-target distance). The second deposition technique is based on anode layer source. Using hydro-carbons (acetilenium), hydrogenated cabon coatings can be deposited. For characterization of such coatings we used the ERDA, XPS, AFM, SEM analytical techniques as well as methods for measuring roughness, hardness, adhesion and internal stresses.

Contact: peter.panjan@ijs.si, miha.cekada@ijs.si

The divertor is one of the most important high-heat-flux components in a fusion reactor. About 15 % of the total thermal power gained from the fusion reaction need to be removed by divertor, which results in an extremely high heat flux of about 10 MW/m2 applied on a relatively small divertor target surface. Helium cooled divertor, based on modular design concept has been investigated within the EU power plant conceptual study (PPCS). Although helium has much worse heat transfer characteristics than water, it is the most appropriate coolant from the safety point of view due to its chemical and neutronic inertness. Furthermore, for fusion reactors, helium coolant can be used also in a gas turbine cycle for power conversion. Within this project an advanced modular divertor concept with multiple helium jet cooling is investigated. The reference design, based on jet-to-wall impingement cooling has been developed at the Furschungszentrum Karlsruhe (FZK). The main heat transfer and flow characteristics to meet design requirements are to increase the heat removal capability of the divertor and to minimize the pumping power for the coolant. In this project numerical simulations of reference design will be performed, systematically varying the main input and model parameters as well as design variants. Detailed numerical predictions of heat and stress loads in the divertor structures and pressure drop in the helium flow will help to ensure that the design constraints and material limits are not exceeded. The main results should lead towards the optimal design from the thermal-hydraulic and thermo-mechanical point of view.

Application of ion beam analytical methodes to the studies of plasma wall interaction in tokamaks

Collaboration: Associations EURATOM – FZJ, EURATOM - IPP
Contact: primoz.pelicon@ijs.si

Deuterium retention and release from metal surfaces - a complementary method to nuclear tritium methods.

Determination of fuel retention in metallic materials relevant for ITER

For accurate prediction of tritium inventory in metal walls exposed to gaseous tritium it is of great importance to deal with realistic experimental data, mainly to ensure safe handling and decommissioning of future fusion reactors. So far, only two large machines operated in the regime of using tritium-deuterium fuel (TFTR and JET). It means that most of the data needed for modelling has to be obtained on small experimental equipment with a limited accuracy when scaled to large machines.
There are several channels of non-fused tritium "leakage" from plasma, like: dusting, high energy burial into the bulk of the first wall and gas phase entering the subsurface of all inner surfaces of the UHV chamber after the plasma ignition. This area in ITER will be in the order of 1000 m2. Reports on small experimental devices show that after the exposure of metal surface to molecular tritium gas, tritiated water is reported to be the prevalent gas species that is released afterwards. This fact is indeed unexpected for well outgassed materials as has been often proved in the UHV practice. Experimental data obtained with tritium in small devices may thus differ substantially from fusion reactor data where surface water is presumable removed during conditioning of the walls.
In our experiments, deuterium is applied instead of tritium in fusion reactors as well as in small test units offering a great simplification since non nuclear equipment can be used. Results taken by deuterium still enable recording several parameters that can be then applied in calculations related to tritium. Standard methods in atomic physics are in general less sensitive than nuclear methods. Anyhow, an extremely well prepared UHV chamber with a calibrated mass spectrometer enable to gain the sensitivity to the level of tritium methods. Beside the molecular D2, all other deuterium containing molecules can be determined.
The scope of the proposed activities is to measure and thus quantify the reaction rates (adsorption, absorption, desorption, etc.) of gaseous deuterium with ITER relevant materials, stainless steel, tungsten, beryllium and carbon based materials. Deuterium pressures of interest are below 10 Pa which is the domain where techniques based on the UHV practice are exclusively needed.

Physics: Integrated Tokamak Modelling TF

Investigation of boundary conditions for fusion plasmas and their implementation in existing and future simulation codes

Investigation of the fluid transport models of tokamak boundary plasma and their implementation in existing and future simulation codes

Collaboration: Associations Euratom-ÖAW, Euratom-IPP Garching, Euratom - UKAEA
Contact: mladen.stanojevic@lecad.uni-lj.si

Collaboration in WG DEMO reactor

The work plan of the project was mainly collaboration and participation of the principal investigator in the DEMO Working Group, contributing in the topics related to the conventional NPP technology. The project was incorporated in the DWG activities, with the DEMO conceptual design as the main goal in 2005. The plan of the project was fully accomplished. Dr. Ravnik was nominated as the DWG member. He participated at two DWG meetings in Garching (in March and September). He reviewed the DEMO conceptual design report in the chapters on reactor safety and nuclear waste treatment. His observations stimulated discussion on nuclear waste categorization and treatment, in particular on the problem of long lived waste disposal. He prepared detailed review of international regulations and practice of low and intermediate level waste disposal and presented it at the September meeting of DWG. The presentation was accepted with interest and contributed to further development of the project.

Development of functional material for insulating flow channel inserts

SiC/SiC composites have been recognised as an attractive materials for application in fusion reactor in particular due to their outstanding mechanical properties at extreme conditions. After a period of development of the material for structural application, where high mechanical strength and reliability, gas impermeability and high thermal conductivity were target properties, the focus has been directed recently into functional application. According to the HCLL concept (C), SiC/SiC composites with tailored properties should serve as a thermal and electrical barrier in flow channels inserts. The expected properties of the functional material differ from the properties of the European reference material for structural applications: beside structural stability under high thermal loading and hermeticity, as the main characteristics low through-thickness electric and thermal conductivity are expected.
The aim of the work is to present appropriateness of wet ceramic processing for fabrication of SiC-based composites with expected electrical and thermal properties and to improve the manufacturing process in a way to bring the material with suitable electrical properties to meet also the requirements for mechanical properties and hermeticity. The target properties of the material to be developed are:
- through thickness thermal conductivity: <2 W/mK at 500 °C
- through thickness electrical conductivity: <1 S/m, in-plane < 1000 S/m
- leak tightness

Collaboration: ENEA (FN spa), TUW, Imperial College London,
Contact: sasa.novak@ijs.si, goran.drazic@ijs.si

Hydrogen - Deuterium molecule wall interaction - VEVOF