The onset stage of the electrical breakdown on a broad area metal surfaces under electric field is still unknown, although many theories have been proposed earlier. In many of the theories, it has been common to postulate the existence of a geometric protrusion on the surface that is capable of causing high field enhancement and pre-breakdown electric currents in the vacuum over metal surfaces under electric field. However, such protrusions have never been seen on the metal surface prior to the breakdown.

It has been recently experimentally observed that the average field that the material can tolerate without breakdown is correlated with the crystal structure of the material. This observation hints that some dislocation mechanism could be possibly related to the onset stage of the breakdown event. In this thesis, the following mechanism that can be responsible for the breakdown onset is analyzed. Application of the electric field exerts stress on a metal surface, which can cause the nucleation and mobility of the dislocations, i.e. plasticity. The localized plastic deformation can eventually lead to protrusion growth on the metal surface. Once a protrusion is formed on the surface, the electric field is enhanced on the protrusion site, further enhancing the protrusion growth. A defect such as a void can act as a stress concentrator which changes the otherwise uniform stress field and acts as an initiation site for plastic deformation caused by dislocations.

In this thesis, we have examined the effect of an external stress on a near surface void in conditions which are relevant for the research and design of the accelerating structures of the CLIC collider. A void present at a near surface region of the accelerating structure causes local concentration of the stress induced by the external electric field on the conducting metal surface. The presence of such near surface void was experimentally observed in a metal sample prepared for experimental spark setup. By means of molecular dynamics simulation method we have shown that the stress can cause nucleation and/or movement of dislocations near the void. The mobility of dislocations then leads to formation of a protrusion on the material surface. We analyzed the nucleation of the dislocations in detail and constructed a simplified analytical model that describes the relevant physical factors affecting the nucleation event. Since the shear stress on the slip plane causes the mobility and nucleation of the dislocations, we analyzed the stress distribution on the slip plane between the void and surface by using finite element method and by calculating the atomic level stress with molecular dynamics method. The results were compared also to an analytic solution for a void located deep in the bulk under similar stress. It was found that the nearby surface had significant effect on the stress distribution only when the void depth was less than its diameter. Below this the maximum stress is equal to that for a void located deep in the bulk under similar external stress. The comparison of the finite element results to the atomic level stress revealed that the pre-existing surface stress near the void surface had significant effect on the stress distribution.

In addition to the tensile stress caused by the electric field on the charged metal surface, pulsed surface heating also induces stress in the material surface region under alternating electric field. This cyclic thermal stress is known to cause fatigue and severe deformation of the metal surface. We investigated the condition relevant for yield by calculating atomic level von Mises strain which has been earlier related to dislocation nucleation. The strain concentration caused by the void was 1.9 times the bulk value. In order to see activated slip planes, we exaggerated the compressive stress to the extent that dislocation nucleation could be observed within the timespan allowed by the molecular dynamics simulation method. Dislocations were observed to nucleate at the sites of maximum von Mises strain.

Taken together, the results presented in thesis contribute to the understanding of the stress distributions and possible dislocation related mechanisms under different stressing conditions assuming existence of a stress concentrator, such as a near surface void.

The difference between knowledge and belief is that while knowledge increases monotonically with time, beliefs may at some later point in time turn out to be false. Beliefs may change for various reasons: in belief revision, beliefs are changed when receiving new information about a world that has not changed, while in belief update a change in the world is to be recorded. Different types of change call for different treatments. In belief-change studies, various change types have been characterized by rationality criteria set on each type. The main principles in these criteria are maintaining consistency of beliefs and minimality of change.

When dealing with belief change, our approach is to take knowledge as an integrity constraint that should always hold, and we describe how the rationality criteria should be modified accordingly. In our refined rationality criteria, beliefs that are inconsistent with the knowledge of the knowledge base will never be allowed to enter into the knowledge base.

In the rationality criteria, a common assumption is that the most recent information is the most reliable, and it has therefore been prioritized over the old beliefs. However, this may not be the case in all circumstances. In order to complete the collection of belief-change types, we propose a new, commutative type of change for entering competing evidence into the knowledge base.

The representation theorems that have been given for belief revision indicate that belief revision involves an ordering of disbelief on possible alternative situations, or equivalently, an epistemic entrenchment on logical formulas. A formula less entrenched is more easily given up when eliminating inconsistancies. In view of the changes in the rationality criteria, we also refine the representation theorems.

We introduce two finite representations for knowledge bases, one with a finite ordered set of propositional formulas that are satisfiable but pairwise inconsistent with each other, and the other with a finite list of pairwise inconsistent propositional formulas. Both representations involve dynamic orderings of disbelief that have arisen out of the previous change operations.

We show that for the knowledge base to satisfy the rationality criteria given for belief revision, the dynamic ordering of disbelief in the knowledge base is vital. The representations and the operators that we introduce in this thesis demonstrate how this ordering of disbelief could be dealt with in various operations.

Decade long observations of ice properties were used to construct a statistical model of properties of landfast ice. Temperature was found to be the most important factor determining ice thickness and contribution of snow ice to the ice thickness was determined by the amount of winter time precipitation. Stratigraphy of ice and growth history had influence to the vertical distribution of organisms in the ice cover as snow ice layers and columnar ice layers were found to favor different types of organisms. Thickness of meteoric ice layer, including snow ice and superimposed ice, controlled the albedo of ice cover when no snow cover was on the ice. Based on the observations of fast ice conditions and albedo, the effects of snow thickness and meteoric ice thickness to the albedo of sea ice were formulated as albedo parameterization equations.

The optical properties of sea ice with spectral resolution were studied on the landfast sea ice. Emphasis in these studies was given to optical properties in the ultraviolet and visible wavelengths. Organic matter, dissolved and particulate, was the most important factor determining the ultraviolet properties of sea ice cover. The optical properties in the ultraviolet were also actively modified by the living organisms in the ice cover by producing mycosporine like amino acids (MAAs) in relatively high amounts. MAAs are a family of photoprotective compounds that absorb UV radiation efficiently. At the visible part of spectrum the ice by itself and the thickness of meteoric ice layer were the most important determinants.

Salinity and the initial salt entrapment during ice growth in the Baltic Sea were measured to be less than in the oceans with equal ice growth rates. The turbulent fluxes of heat and salinity under the landfast sea ice were measured to be small.