A modified design of the thermal assembly is presented for the directional solidification or transparent alloys that eliminates the radial temperature gradient and minimizes the curvature of the interface, An additional booster heater is designed, and the position of the heater is shown to be critical in obtaining a flat interface, A full-scale numerical calculation. carried out for succinonitrile-0.5wt% Salol, shows that the interface concavity can be reduced gradually by placing the booster heater just above the cold end and by adjusting the temperature of the booster heater while keeping the hot and cold zone temperatures fixed. Experimental measurements of temperatures at the wall and at the center have been carried out systematically by using two calibrated thermocouples. and the observed thermal profiles have been shown to strongly support the numerical prediction. When a macroscopically flat interface is obtained, it is shown that columnar growth away from the ampoule wall can be observed and photographed. The effects of thermal gradient and the temperature setting of the booster heater on the planarity of the interface are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.