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Magnetic order in two-dimensional islands of spherical 21 nm cobalt-ferrite (CoFe2O4) nanoparticles is studied by magnetic force microscopy and spectroscopy. Images obtained at a temperature of 105 K clearly reveal the presence of repulsive and attractive areas on top of the islands. Monte Carlo simulations on hexagonal arrangements of nanoparticles have been carried out and are consistent with the experimental findings. The simulations show that the magnetization patterns are determined by a competition between interactions among the nanoparticle magnetic dipole moments and alignment of the individual moments along the easy axes of magnetization. At an elevated temperature of 500 K, it will be shown that dipole-dipole interactions lead to short-range ferromagnetic order, which fluctuates rapidly in time due to thermal excitations. Upon reduction of the temperature to 105 K, the dipole moments are forced toward the easy axes of magnetization, thereby increasing the amount of disorder.

Nuclear fusion has the potential for large-scale sustainable energy production. Currently, the most promising fusion reactor concept is a tokamak. Scientists and engineers from all over the world are collaborating on building the next-generation fusion reactor: ITER. A critical component of the ITER design is the exhaust, the divertor. The material of choice for the divertor is tungsten in order to be able to withstand the extreme heat and particle fluxes that it experiences during operation. One of the main challenges for ITER operation is the tritium retention in the reactor wall. For safety and efficiency reasons, only a limited in vessel tritium inventory is allowed. High energy neutrons, produced in the fusion process, will create damage in the tungsten crystal lattice and enhance the tritium retention in tungsten. This thesis describes the investigation of deuterium retention in radiation damaged tungsten after high-flux plasma exposure. Our results suggest that tritium retention in the divertor of ITER will not be problematic, even when damage creation by neutrons is included.

The Dutch Institute for Fundamental Energy Research (DIFFER) has built the new experimental research facility Magnum-PSI. In Magnum-PSI, Plasma Surface Interaction (PSI) research for the nuclear fusion reactor ITER and reactors beyond ITER will be carried out. As such, it is essential that the experiment can access the high-density, lowtemperature regime relevant for the ITER divertor strike zones (electron density ne ~ 1020 – 1021 m-3 and electron temperature Te ~ 1 – 5 eV). Magnum-PSI is a linear plasma generator in which the interaction between a large (up to 10 cm) diameter plasma beam with a target material can be investigated. It will have a steady state high flux of 1023 - 1025 ions m-2 s-1 at a plasma temperature in the eV range confined by a magnetic field of 2.5 T. In this way, molecules and dust particles that come off the surface are confined and remain part of the plasma-surface interaction system. The work described in this thesis forms the basis for the design of the new machine and shows its excellent performance. The design makes sure that the ITER relevant regime of PSI can be accessed while having sufficient diagnostic access to the machine. Key elements are a low neutral gas density in the target region and a continuous magnetic field of sufficient magnitude. The neutral gas contribution to the surface interaction region coming directly from the source should be much lower than the plasma flux, and the magnetic field should be strong enough to guide the ionized particles from the source to the target and to confine particles of a certain size which are ionized after coming off the surface. The demands for a steady state magnetic field without limiting the diagnostic access to the machine have led to the design of a superconducting magnet with a 1.3 m warm bore and 16 radial access ports. This design is the result of a conceptual design study to optimize the following parameters: field strength and homogeneity, diagnostic transparency, inner diameter, ramp-up and down times, safety and costs (capital and running costs). The most cost effective cooling solution turned out to be a recondensing system, where the coils are submerged in liquid helium while the evaporated helium is stored in a gas balloon to be re-condensed by cryocoolers. The stray field of this large magnet is shielded by iron walls surrounding the experimental area. The dimensions and positions of these walls have been determined with a nonlinear finite element solver. The vacuum system must be capable of reaching the required neutral pressure in the target region while the plasma source puts in large quantities of gas to reach the high fluxes. This demand has led to the design of a 3 stage differentially pumped vacuum system, where the vacuum vessel consists of separate chambers divided by skimmers and pumped with their own pumping stations. In a comparative study between roots pumps, vapor boosters, cryopumps and turbo pumps, the roots pumps were identified as being the most reliable, economic and user friendly pump solution for these high gas loads. The pumps have been commissioned and the measured pump speeds are comparable to the calculated values. It was shown that the chosen pump solution is capable of reaching its requirements. Since the flow in Magnum-PSI is mostly in the rarified regime, a direct simulation Monte Carlo (DSMC) method was used to model the flows. Two stage differential pumping was modeled and applied in the linear plasma devices Pilot-PSI and PLEXIS. Both simulations and experiments showed that the optimum skimmer position depends on the position of the shock and therefore shifts for different gas parameters. In Magnum-PSI, a moveable plasma source is implemented which makes it possible to change the distance between source and skimmer for different operating conditions. Using DSMC simulations, the shape of the skimmer has been designed in such a way that it effectively stops neutrals from flowing to the next chamber, while having a minimum impact on the shock structure. During the commissioning phase of Magnum-PSI, the non-magnetized expansion of 5 Pa m3 s-1 (3 slm) argon in the differentially pumped vacuum vessel at low background pressures was studied. The behavior of the neutral component was studied with Rayleigh scattering measurements and DSMC simulations. Thomson scattering and double Langmuir probe measurements were performed on the ionized fraction. It was found that the electrons and neutral particles are not completely coupled in the shock front. The neutral fraction shows clear signs of invasion from hotter background gas, causing the average temperature and density to increase before the shock. This is also evident from the ionization ratio, which has been determined in front of and behind the first skimmer. This study has validated the design of the machine and has helped to understand the behavior of the gas flow. Due to a significant delay in the delivery of the superconducting magnet, Magnum-PSI has started operation as a pulsed machine using conventional electromagnets. To accommodate the smaller diameter coils, one of the vacuum chambers is temporarily removed leading to a two stage differentially pumped vacuum system. This results in a somewhat higher neutral pressure in the target region. The coils provide a magnetic field in the source region of 1.9 T for 6 seconds. Lower field settings with correspondingly longer pulse lengths are available. Due to the expanding magnetic field in the target region, the plasma beam expands a factor of 2.3 in diameter before it reaches the target. First experiments have been carried out on 5 Pa m3 s-1 (3 slm) magnetized hydrogen and deuterium plasma beams. Thomson scattering 25 mm in front of the target has yielded electron densities up to 3.6×1020 m-3 and electron temperatures up to 3.7 eV in a 25 mm (FWHM) diameter beam. Exposure of a tungsten target has led to average heat and particle fluxes well in excess of 4 MW m-2 and 1024 m-2 s-1 respectively. Using pressure and calorimetric measurements, it was shown that the plasma surface interaction is mainly determined by the incoming ion flux and not by neutrals coming directly from the source. The application of a differentially pumped vacuum vessel has led to an unprecedented high ionization efficiency of the plasma source. These first results validated the design of Magnum-PSI and have shown that ITER relevant conditions have been achieved.

This thesis involves a fundamental study of two-dimensional arrays of magnetic nanoparticles using non-contact Atomic Force Microscopy, Magnetic Force Microscopy, and Atomic Force Spectroscopy. The goal is to acquire a better understanding of the interactions between magnetic nanoparticles and the resulting configuration of their magnetic moments. We have studied two systems: 20-nm magnetite (Fe3O4) and 21-nm cobalt ferrite (CoFe2O4) nanoparticles, capped with oleic acid and oleylamine, deposited using drop-casting technique on HOPG (Highly Oriented Pyrolytic Graphite). We present an analytical model to interpret the experimental spectroscopy curves (frequency shift as a function of the tip-sample distance) in terms of force, to enable a quantitative interpretation of the experimental results. In addition, a numerical model based on the Metropolis Monte Carlo method has been developed to calculate the configuration of magnetic moments with minimum energy. The energy consists of dipolar energy and anisotropy energy. The simulations have shown that local energy minima are present that correspond to different configurations of magnetic moments. Because of the thermal energy, the system may jump in between these energy minima. Considering a specific configuration of moments, MFM images were simulated taking the dipolar interactions between the nanoparticles and the tip into account. Spectroscopy curves were calculated for two cases: namely with and without the influence of the tip. The magnetite nanoparticles present a relatively large magnetic moment and exhibit strong dipole-dipole interactions and small, negligible, anisotropy. The observed repulsion between the tip and the nanoparticles at the side of the nanoparticle islands shows that dipolar coupling between the particles causes blocking of their magnetic moments. The experimental observations agree well with the numerical calculations, which show that the magnetic moments arrange themselves in flux-closure structures. However, the magnetite nanoparticle 2D systems can be considered soft-magnetic: their moments are strongly influenced by the field of the tip. This has been observed experimentally and confirmed by simulations for the considered magnetic moment of the tip. The cobalt-ferrite nanoparticles present a similar magnetic moment as the magnetite nanoparticles. The anisotropy, however, increases significantly with decreasing temperature, which leads to blocking of the moments even in the strong field of the tip. As a consequence, attractive and repulsive areas were observed in the MFM image above the islands of nanoparticles at low temperatures. In our simulation we have assumed arbitrary orientations of the easy axes of the particles. Simulation results reveal short-range order of the magnetic moments, consistent with the experimental results.