Title: THERMAL VACANCIES AND THERMAL EXPANSION.

The effect of radiation on the thermal conductivity of alumina is an important design consideration in the development of microwave windows for ion cyclotron resonance heating (ICRH) systems for the heating of plasmas in fusion reactors. Several recent papers have addressed this question at higher temperatures and the present report extends the calculation of the effect of point defects to low temperatures. This extension of the calculations to low temperatures is of interest because it has been proposed to cool these windows to liquid nitrogen temperatures in order to take advantage of the much higher thermal conductivity of alumina atmore » these temperatures.« less

The volume expansion of NaCl heavily irradiated at room temperature with penetrating x rays was studied with a photoelastic technique. In the course of the study a new effect was found: the coefficient of thermal expansion of NaCl is increased by irradiation. The magnitude of the fractional increase in expansion coefficient is about fourteen times the concentration of radiation induced defects, as measured by the volume expansion. Typically, this increase is a fraction of 1 per cent. Its algebraic sign and approximate magnitude is predicted on general thermodynamic grounds. The increase in expansion coefficient seems to be correlated only withmore » the total concentration of point lattice defects, and is rather insensitive to their electronic state or spatial distribution. The growth and annealing of the volume expansion have been measured, and correlated with the growth and annealing of the optical absorption. The volume expansion increases linearly with time for short irradiation times at a rate consistent with that implied by the F center growth curve on the basis of Mitchell's interpretation. The annealing of the volume expansion and optical absorption is in general agreement with the earlier results of Kobayashi. Interpretation of the annealing data is difficult if the Varley mechanism of vacancy generation is assumed. There is some evidence that the initial annealing of the volume expansion is limited by diffusion of negative ion vacancies. An interesting surface feature, apparently connected with the radiation, was observed on several samples. (auth)« less