The heater is often a flat ribbon of refractory metal, arranged parallel to the long axis of the material holder, a design that helps reduce temperature gradients in the important central zone. An evaporation source might be regarded as a further step in the direction of high vapor flux at low vapor pressures. The heater is wound with closer spacing at the ends of the crucible to reduce end effects on the temperature. This source is well suited to conventional evaporation, but can be used as a flash evaporator, converting its contents rapidly into vapor. The term thermal source covers a wide variety of devices that are essentially open heaters, without radiative or insulating shielding or any means of reducing thermal gradients.
Some types have thin coatings of aluminum oxide or are designed to hold oxide crucibles.
Typically, single- or multiplestrand tungsten wire is bent into a shape that suits a particular mounting or the required evaporation pattern. The material is deposited on the wire by melting or from a suitable salt solution. When sufficient current passes through the wire, the material evaporates. This type of source is used only for single materials and is often used only once. Thermal evaporation of larger amounts of material is handled by using a ceramic crucible that sits in a wire filament coiled to fit the crucible's outside dimensions.