Growth of Bose-Einstein Condensates from Thermal Vapor

We report on a quantitative study of the growth process of $^{87}$Rb Bose-Einstein condensates. By continuous evaporative cooling we directly control the thermal cloud from which the condensate grows. We compare the experimental data with the results of a theoretical model based on quantum kinetic theory. We find quantitative agreement with theory for the situation of strong cooling, whereas in the weak cooling regime a distinctly different behaviour is found in the experiment.