Figure 5.14 comprises a glass chamber that holds the KDP solution, has a diameter of 16.5 cm with a height of 23 cm. For visualization of the concentration field by the schlieren technique, circular optical windows (BK-7, 40 mm diameter, 5 mm thickness, ) are fixed on the glass beaker at opposite ends. A total of eight such windows permit passage of the laser beam for four view angles. Parallelism and straightness of the optical windows are crucial for generating meaningful images, and considerable precautions have been taken in this regard. The Plexiglas tank surrounding the growth chamber is octagonal in plan. It ensures large enough volume for the circulating water to keep the KDP solution at the required temperature level over a considerable period of time. Four heating elements placed diametrically opposite in the outer chamber maintain the temperature of circulating water, and hence the KDP solution. Electrical input to the heating elements is regulated by a programmable temperature controller (Eurotherm). As in the apparatus discussed in Convection Around A Growing Crystal (Figure 5.13) a K-type thermocouple wire fixed to the outer surface of the growth chamber provides the feedback to the controller. Uniformity of temperature within the solution is ascertained by recording temperatures at various locations using 26 gage K-type thermocouples. With this arrangement, it was possible to reduce the temperature of the solution from linearly with time over a time frame of 60 hours.

The salt-solution has an initial temperature that is high enough to keep it from becoming fully saturated. A KDP seed crystal spontaneously crystallized in a second vessel is placed on a glass platform and introduced in the growth cell. When the bulk temperature is lowered, the solution becomes super-saturated with salt that in turn deposits on the crystal. The solution adjacent to the crystal is now close to being saturated with the dissolved salt. A crystal growing from its aqueous solution thus creates a three-dimensional solute distribution in its vicinity. The solutal concentration gradients and the accompanying gradients in density of the solution are responsible for the appearance and evolution of buoyancy-driven convection currents in the growth chamber.