Index Introduction The Research at Rijnhuizen Results in 2008 Education, Training, Outreach and Public Information Output Appendix website Rijnhuizen 3.3 | PSI-Lab, an integrated laboratory on plasma surface interaction* Coordinators: Prof. A.W. Kleyn and Prof. N.J. Lopes Cardozo Funding*: FOM Programme 75 Objective: To study the interaction of intense particle or photon fluxes with a material surface in a fundamental approach. An important aim of the investigation is to access the strongly coupled regime, in which the particles that come of the surface are kept in the system and define the plasma-surface interaction. Concrete research areas include: 1. The search for mechanisms to create surfaces that are dynamically stable under intense plasma or radiation bombardment. 2. The physics of plasma jets, in particular those in close contact with a material surface. 3. The physics of dust formation during intense plasma-surface interaction. The FOM-programme 75, ‘PSI-lab, an integrated laboratory on plasma surface interaction’, started on January 1st 2004 and runs until 2015. PSI-lab (Plasma-Surface Interaction lab) is built with the purpose to contribute at a fundamental level to the solution of problems that arise when a surface is subjected to an extreme flux of ions, radicals, or photons, or a mixture of those. The primary area of interest is nuclear fusion: in the fusion reactor the wall in the so-called divertor receives some 10 MW/m2 carried by hydrogen radicals, yet must survive several years without eroding or melting. PSI-lab also covers research on optical elements for lithography, which must survive intense photon fluxes. The plasma-surface interaction at extreme flux densities is also a novel area of fundamental interest per se. Therefore, the programme takes a fundamental approach, with relevance to other areas such as the formation of dust in astrophysical systems. A plasma is a unique source of chemical radicals that allows for tuning chemistry at surfaces with opportunities beyond classical chemistry. However, the plasma-surface system is complex, governed by non-linearities, in particular at high densities. The plasma in front of the surface (composition, temperature, density, etc) is strongly influenced by the plasma-surface interaction. The surface, through radical reactions, erosion and deposition, in particular the deposition of clusters and compounds formed in the plasma, is modified by the plasma. The formation of clusters of molecules and their evolution in the plasma is in itself a process that heavily depends on the PSI conditions, while the clusters in turn influence both the plasma and the surface. Thus, plasma and surface cannot be treated separately. Together they form a strongly coupled physical system in dynamic equilibrium. * supported by the European Fusion Programme (EFP) To study and employ plasma-surface interaction (PSI), four experiments will be built, sharing surface analytical equipment. The centre piece is a worldwide unique linear plasma generator, Magnum-PSI, which will allow access to the strongly coupled regime of PSI. A smaller, already operational facility (Pilot-PSI) is used for source development and testing of diagnostics. A novel, versatile thin film deposition machine (Thin-Film PSI) is employed for the deposition of layered structures for XUV optics. Finally, the Surface-PSI device allows basic PSI studies under well-defined low flux conditions. The latter two experiments are operated within the nSI department, largely in the frame of the FOM-IPP programme XMO. The experiments are complementary, and e.g. layered structures prepared in Thin-Film PSI will be used in erosion studies in Magnum-PSI, and Magnum-PSI is used to study the aging of optical elements developed in the XMO programme. Running in parallel and integrated with the experimental research programme is a strong computational effort. The controlled and well-diagnosed experiments in PSI-lab are ideally suited for the benchmarking of computational models, which in turn can guide further experiments. In FOM programme 75, the emphasis lies on the studies of plasma surface interaction for ITER and fusion reactors beyond ITER. These studies are carried out primarily with experiments in the Magnum-PSI plasma generator, in parallel with numerical modelling studies. Through the modelling, the link to PSI experiments in tokamaks is made, where circumstances are more complex (geometry, influence of hot core plasma) and diagnostic access much worse. The focus in these studies is on the materials carbon (Carbon Fiber Composite) and tungsten, with hydrogen, deuterium and helium as the elements of choice for the plasma. Around the Pilot-PSI and Magnum-PSI devices an extensive network of collaborations is growing, in particular with fusion research institutes in Europe and the USA. These include the partners in the TEC agreement, with PSI-studies evolving into the central theme of this collaboration. 3.3.1 ITER-relevant plasma conditions in Pilot-PSI ITER-Relevant research on material erosion and hydrogen retention in Pilot-PSI Pilot-PSI started as a true pilot experiment to prepare for the design of Magnum-PSI and to develop a scientific basis for the physics of plasma surface interactions at high densities and low temperatures. Meanwhile, it has become a high-performance linear plasma generator in its own right on the basis of its unique, extremely high plasma fluxes. In the previous years, the operation domain of Pilot-PSI was extended so that nowadays high plasma fluxes of 1025 m-2s-1 can routinely be reached at an electron temperature of 1 to 3 eV in front of the target. These fluxes have been exploited to perform unique ITER-relevant experiments on the chemical erosion of carbon. In particular, various reference materials for the ITER divertor were investigated, which yielded that all materials lead to a comparable chemical erosion rate. In addition, the effect of surface temperature on the retention of hydrogen in tungsten was investigated. The samples were analysed with Nuclear Reaction Analysis (NRA) at an Ion Beam Accelerator (IBA) facility in Lisbon. Figure 3.7 shows an overview of a study on the importance of the target surface temperature. It demonstrates that at high surface temperatures of ~1600 K, the retained hydrogen fraction is an order of magnitude lower than at a lower temperature of ~1000 K. Figure 3.7: The left graph shows the results from the 2-D NRA scan of the W target exposed to D plasma for a total of 80 s (~1022 D total fluence). This clearly shows a minimum in retention at the hot center of the target and the highest retention at the cooler edges. The low retention seen at some of the 8 mm off center locations may be a shadowing effect from the target clamping ring. The right graphs show a) the total retained fluence as a function of incident fluence integrated across the entire exposed surface, and b) the retained fraction as a function of total incident fluence. The steep decrease and then flattening of the retained fraction in b) may indicate saturation in the W target. Low temperature plasma physics in Pilot-PSI The rotation of the plasma beam has also been studied in more detail. The rationale for this research was that the rotation is driven by electric fields perpendicular to the magnetic field, which is in turn the consequence of current continuation in the free plasma jet. Such currents are interesting for two reasons. Firstly, it is believed that such a current is responsible for significant power input in the plasma and therefore could be exploited to increase the performance of the plasma source. Secondly, it may lead to net currents flowing into the targets that are exposed to the plasma and thus affect the plasma-surface interaction experiments. Figure 3.8 shows the analysis of the plasma beam rotation in Pilot-PSI, which yielded rotation velocities up to 9 km/s. Figure 3.8: Measurement of the plasma rotation with optical emission spectroscopy. (a) Part of a raw CCD-image of the spectrally as well as spatially resolved Balmer-b light, indicating plasma rotation. The 17 bands of light correspond to the individual optical fibers in a fiber array that covers the entire cross section of the plasma jet. The spectral line appears shifted to the red at the bottom of the plasma jet and to the blue at the top. At the jet edges it is unshifted. The spectral shift reveals rotation of the radiating species. The maximum Doppler shift from the central wavelength value is about 5 pixels. This corresponds to a velocity of (9.3±0.9) km/s. (b) The corrected lateral intensity profile displaying a flat top, pointing to a hollow emission profile.(c) Spectral profiles from, respectively, above, on, and under the jet axis. The off-center profiles are clearly asymmetric, with the direction of asymmetry depending on the lateral position. In the jet center, the profile is symmetric. 3.3.2 Modelling of the Magnum-PSI and Pilot-PSI plasma Parallel to the experimental programme, numerical modelling addresses a number of topics regarding the source, plasma beam, and plasma-surface interaction in Magnum-PSI and Pilot-PSI. The B2-EIRENE code (a hybrid fluid-monte carlo code) is used to study the interaction between the plasma beam and recycling neutrals from the target and the walls. First results were obtained with simulations of Ohmically heated beams, assuming a simplified model for the current distribution. For reasons of compatibility, and to be able to include a self-consistent description of currents, a switch was made to the SOLPS5.1 version of B2-EIRENE. A large effort went into adapting this version to the linear geometry of Pilot and Magnum-PSI. The modelling of plasma wall interactions was done with the HCPARCAS molecular dynamics code. In collaboration with the group Reactive Plasma Processes at the IPP Garching (W. Jacob) the evolution of samples of amorphous hydrogenated carbon (a-C:H) and diamond under extremely high atomic hydrogen fluxes has been studied. Cumulative bombardment shows a negligible flux dependence. This indicates that the experimentally observed reduction in carbon erosion yield with increasing flux is caused by external factors, such as, for example, redeposition of or shielding by erosion products and is not a property inherent to the material. A study of redeposition of hydrocarbons on a-C:H samples has been started. In collaboration with SARA these simulations are run on the DutchGrid. First results of simulations of bombardment of amorphous tungsten carbide show the formation of subsurface molecular hydrogen concentrations that lead to blistering. This phenomenon depends on the tungsten to carbon ratio of the simulated sample. 3.3.3 Modelling of radio-frequency discharges Modelling of dusty plasmas mainly addressed the behaviour of the dust-free central void and its reaction to changes in the applied power. Possibilities to create homogeneous grain distributions were investigated further. Other topics studied were the forces on small grains moving through a plasma containing large grains (relevant for so-called lane formation) and vortices induced by the force field. Kinetic modelling addressed the behaviour of the dust charge in modulated discharges. The modelling of silane-hydrogen radio-frequency discharges concentrated on high-pressure discharges for the deposition of micro-crystalline and amorphous silicon layers. The results of the simulations showed that a frequently used criterion to predict the properties of the deposited material that is based on the emission of hydrogen and silicon is often not reliable. A criterion based on the number of H atoms arriving at the surface per deposited Si atom is more reliable. It also holds in situations with a strong depletion of silane and a strong dilution with hydrogen. 3.3.4 Magnum-PSI: scientific and technical challenges As part of the TEC collaboration and within the framework of Euratom, the FOM Institute Rijnhuizen is building a new machine to study plasma wall interactions. This apparatus, Magnum-PSI, will be world-wide unique and will provide an important new experimental facility in the range of experiments that are available to PSI research for ITER and reactors beyond ITER. The uniqueness of Magnum-PSI lies in its ability to access simultaneously the several aspects of PSI in the combination of which ITER essentially differs from present day experiments: • Large ion fluency and continuous operation, which leads to ‘macroscopic’ modification of plasma-facing surfaces. • High power density (5-10 MW m-2) with low plasma temperature (< 5 eV) such that materials are close to, or at the energy threshold for sputtering, but have high surface temperature and are therefore near their materials limits for stress/strain, etc. • Strong plasma-surface coupling: the high plasma density leads to short mean free paths for dissociation/ionisation of eroded atoms or molecules in comparison with the linear dimensions of the plasma. • Access to plasma diagnostics and in-situ surface analysis. The steady-state high flux of up to 1024 ions m-2s-1 at a plasma temperature in the eV range, magnetic field of 3 T, and large beam diameter make Magnum-PSI a unique experiment, bringing the relevant parameters typically an order of magnitude beyond what is presently available in linear plasma devices, and into the realm of the ITER divertor. It will be the only device so far to enter the strongly coupled regime, in which molecules and dust particles that come off the surface are trapped and remain part of the plasma-surface interaction system, and thus will allow relevant studies of dust formation, re-deposition, migration and hydrogen retention. The steady-state and high flux capability, combined with the large flexibility and easy access, allow post mortem analysis which in present devices normally occurs only every 1-2 years. Design activities in the Magnum-PSI project The major part of the definition and design phase of the Magnum-PSI project has now been completed. In what follows, we summarize some of the important activities that occurred up till now.  We have made the complete design of the Magnum-PSI device. The results can be observed in Figure 3.9. Shown are (from left to right) the source-, heating- and target chamber with pump ducts. The vacuum system is constructed in modules which are movable and placed on a rail system. Next to these, the pumping station for the third stage is shown. The vacuum system is surrounded by the movable superconducting magnet which is placed on a rail. On the right hand side, the target station with target and target manipulator are visible. In the target analysis station, the targets can be analyzed in detail with surface analysis equipment.  In the following sections, we describe some of the highlights in the project in 2008. Vacuum system The Magnum-PSI vacuum system has been designed, manufactured and installed. The pumping systems (turbo molecular and roots blower systems) have been commissioned. The testing of the complete system including the control and part of the data acquisition system has been successfully completed. Figure 3.9: Total overview of the Magnum-PSI experiment with target station and target manipulator. Shown are (from right to left) the source-, heating- and target chamber with pump ducts. Next to these, the pumping station for the third stage is shown. On the left hand side, the target station with target and target manipulator are visible. In the target analysis station, the targets can be analyzed in detail with surface analysis equipment. Superconducting magnet system for Magnum-PSI The magnet has a warm bore of 1300 mm and the axial length is 2450 mm. The magnet system has eight evenly distributed radial access ports of 210 mm diameter, located at two axial positions. These ports allow for good diagnostic access of the experiment. A schematic picture of the superconducting magnet system is given in Figure 3.10. The superconducting magnet system is now under construction at the manufacturer site. Part of the winding of the magnetic coils has been completed. In 2009, the assembled superconducting magnet system will be installed at Rijnhuizen and tested with the Magnum set-up. Plasma beam and additional heating Extensive experiments occurred in the Pilot-PSI experiment on the high flux plasma source and additional heating methods (so-called Ohmic heating and RF heating). The output of the plasma beam (flux density) is within specifications. We will now further concentrate on broadening of the plasma beam diameter to the specified 10 cm.  Extensive testing has occurred on RF heating of the plasma beam. Our theoretical understanding has increased significantly and we are now ready for developing and testing different antennae concepts. The work on RF heating is done in close collaboration with KMS/RMS Brussels and the Politecnico di Torino.  Target Station In FZ-Jülich, the definition and design activities on the target station and manipulator are finalised. The procurement of the system has started and the building activities are ongoing. The analysis chamber has been installed (see Figure 3.11b). The further assembly and testing of the target station will occur in 2009 Figure 3.10: The Magnum-PSI experiment with the 3 T superconducting magnet. Shown are (from left to right) the source-, heating- and target chamber with pump ducts and wave guides inside the superconducting magnet. The plasma beam is guided through two skimmers by the magnetic field to the target plate. A beam dump is inserted in the plasma beam. The target is brought in place after the desired plasma parameters have been established. Control and Data Acquisition We have defined the control and data acquisition system for the Magnum-PSI experiment. An overall functional design of the control and data acquisition system for Magnum-PSI has been made. The system choices are made and the first parts of the control system have been purchased and tested. We have specified the vacuum, cooling and safety requirements and implemented these in the Magnum-PSI control system. Outlook Magnum-PSI will be further constructed and tested at FOM-Rijnhuizen in Hall B. An overview of the future hall B with the Magnum-PSI experiment is displayed in Figure 3.11a. In 2009, the source, magnet and target station system will be installed and tested. A basic Magnum-PSI device with most of the subsystems will be ready in 2010.  Figure 3.11: Overview of the final set-up (top) and pictures of the experiment under construction (bottom).