With this energy, the center of the silicon charge quickly heats to temperatures approaching 2000 Centigrade. At this temperature, atoms of silicon begin to evaporate from its surface.
The edges of the charge are cooled by the copper hearth. Thus the molten Si is contained inside a shell of solid Si, preventing the liquid from dissolving the copper hearth (yes, molten Si will immediately dissolve metal!).
This saves the hearth AND ensures a pure evaporated "beam" of silicon atoms.
