HIP Theory Programme:

Radiation Damage in Particle Accelerator Materials

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Activities

Modelling of ''rf breakdown''

Recently we have initiated the modelling of "rf breakdown", i.e. spontanous sparking from surfaces induced by high-gradient electric fields in CLIC components. We propose a multiscale model to encompass stepwise all the processes involved in the electrical breakdown: starting from triggering of plasma formation near a metal surface, the plasma development and the eventual damage of the surface as a result of plasma discharge. The first step includes a development of an hybrid atomistic-electrodynamic simulation model of metal surfaces. This model combines forces calculated using classical electrodynamics for atoms near surfaces with equilibrium thermodynamic interaction models for metal atoms. It enables us to track the development of surface morphology under the electric field. However, the molecular dynamics time span does not allow for long term evolution processes which would lead to the growth of surface asperities. The presence of a tip on the surface is believed to explain the enormous field enhancement near the surface observed in the experiment. To understand the process of appearing of surface protrusions we also inititated the studies of the development of extended structural defects which can be found in an industrial metal. In parallel, in collaboration with group of Ralf Schneider from MPIPP, Greifswald, Germany, we carry out the simulations of 1D and 2D plasma models to enable the simulation of surface damage.

Radiation damage in detectors

The study of radiation damage in Si is a well-established field, but suffers from significant quantitative uncertainties. In particular, the overall level of primary damage produced by atomic recoils has, despite decades of study, still an uncertainty of about a factor of 2. Since this quantity lays the baseline for all further damage buildup, including that affecting the performance of particle detectors, knowing it well is of great importance. We have already completed a project where the primary level of radiation damage at low energies was calculated on a more fundamental level of theory than before, namely density-functional theory molecular dynamics. Additionally, we will study the effect of clustered defects of pure Si and defects involving oxygen on the properties of Si detectors, again using quantum-mechanical density functional theory.

Nanophotonics materials

Nanophotonics within conventional Si technology forms the third main research topic. Recent results indicate that optical memories can be constructed from Si nanocrystals (Si-nc) in SiO2 host, providing storage of information with high density and very long lifetime. This work has been initiated within the the Academy of Finland FinNano consortium ``OPNA''. Within this project we examine the structure of Si, Ge, and Au nanocrystals in amorphous silica matrix, with particular emphasis on understanding the interface structure between the nanocrystal and the silica, and the laser annealing of the clusters. The range of ion energies ranges from several MeV to hundreds of MeV to simulate the track formation in amorphous silica, responsible for the elongation of gold nanoparticles.