Title: Computational study of probe diagnostics in high-temperature plasma Author: Jan Lachnitt Department: Department of Surface and Plasma Science Supervisor: prof. RNDr. Rudolf Hrach, DrSc., Department of Surface and Plasma Science Abstract: This work is concerned to the particle computer modelling of the interaction of plasma, especially edge plasma, with immersed solids, especially probes. First, the speed and accuracy of several algorithms of the electrostatic force calculation were compared. One of the algorithms has been newly proposed. Then, a two-dimensional model of the interaction of collision-less plasma with a probe was created. This model has been applied to experimental data from CASTOR tokamak. The crucial point of this work is the creation of a fully three-dimensional particle model. This model has been tested for accuracy and speed and has been parallelized for higher efficiency. Keywords: plasma, probe diagnostics, computational physics, particle modelling

This thesis identifies the obstacles in efficient modelling of interaction of plasma and solid surfaces. It presents an enhanced method of solving the Poisson equation optimized to meet the requirements of the Particle-in-Cell modelling approach. A number of applications are discussed, including models of the plasma-facing wall of the nuclear fusion device, tokamak, and its interaction with particle fluxes driven by the intrinsic magnetic field. Another area of applications covers the modelling of plasma probes deployed to diagnose properties of plasma in various experiments. The thesis also includes the computer library code and instructions enabling a rapid use of the Poisson solver method in a third party computer code which implements the PIC approach in a compatible manner.

The diploma thesis is focused on the study of low-temperature plasma by methods of computational physics, namely particle modelling. The solution focuses on the time- effective execution of simulations, which is concerned, among the other things, with the use of parallelization techniques aimed at maximize shortening of the calculation time. The Langmuir probe was chosen as the most common representative of the probe diagnostics. The main goal of this work was to observe plasma interaction with the embedded solid. Current research on plasma modelling intensively explores dynamic phenomena such as wave propagation or turbulence and instability. For this reason, much of the work is devoted to the dynamics in electropositive and electronegative plasma.

Title: Modelling of plasma-solid interaction Author: Jan Nožka Department: Institute of Theoretical Physics Supervisor: prof. RNDr. Rudolf Hrach, DrSc., Department of Surface and Plasma Science Abstract: This work is devoted to computer modeling of the low-temperature argon plasma discharge. We created one basic particle model and one basic fluid model. Furthermore, we created a model of electron-electron interaction in three dimensions. This model is able to stabilize nonequilibrium electron gas in the expected equilibrium. This model was developed to investigate the influence of electron-electron scattering on the acceleration of electrons above the speed that is sufficient for excitation or ionization of neutral argon atom. At the end of this work there are results that shed a light on the importance of this interaction in comparison with the amount of fast electrons that are present in the plasma due to electric filed field.

This dissertation presents an investigation of the effects of stress, and of absolute and relative particle size, in tests on vertically loaded footings. Two granular materials, namely, a silica rock flour and a Chatelet flint grit, which differed in nominal diameter by a factor of 50 but were otherwise practically similar in all other grain characteristics were used in this work. A comprehensive series of triaxial tests under a wide range of cell pressures was carried out to quantify the stress and absolute particle size effects. Model footing tests were also performed by pushing a rigid circular punch axisymmetrically into the flat surface of a cylindrical soil model either under 1-g (gravity) with surcharge or under elevated g in a centrifuge. The 1-g and centrifuge test series were used to study the scale effects on the surcharge term Ng and the self-weight term Nγ of the Terzaghi bearing capacity equation, respectively. Parameters varied were punch diameter, particle size and surcharge or g level. Two theoretical analyses were attempted based on the finite element method and the method of characteristics. Using the Schofield Soil Model, the finite element analysis can give a reasonable order of magnitude prediction for the settlement of the footing under working load conditions. When the effect of reducing angle of shearing with increasing stress was taken into account together with the change of geometry due to footing penetration, the angles of shearing inferred from the method of characteristics fall within ±20 of those measured in triaxial compression tests. Distortion due to violating the scaling law by not conserving the ratio of particle size to model dimension was not considered to be significant. Distortion due to violating the constitutive soil behaviour by varying the absolute particle size was found to be significant due to differences in grain crushing, but this can be accounted for effectively by the new style of calculations developed in the thesis.

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