The TUBO effusion source

Instead of a wire filament, the TUBO source uses a tubular filament that encloses the entire crucible. The electrons providing the ohmic heater current flow along the source axis, up through the heat insulation shields and down through the filament. Like this, the electrical insulators for the heater contacts can be placed well below the hot zone of the source, allowing crucible temperatures that are only limited by the mechanical stability of the refractory materials the source is made from. The antiparallel direction of the heater current suppresses the generation of magnetic fields outside the source. The crucible temperature is controlled via a thermocouple at the bottom of the crucible, providing the highly precise temperature control, stability and reproducibility of a standard effusion source, however in temperature regimes previously only accessible by e-beam evaporation.

The filament can be made conical to achieve hot lip or cold lip operation of the source, depending on the specific requirements of the source material/crucible combination.

For extremely high temperatures, the TUBO e version of the source features an electron emission booster heater. In this configuration, the crucible is held at a positive high voltage so that a fraction of the ohmic heater current leaves the filament and provides electron beam heating of the crucible. The source geometry and electrical design are chosen in such a way that the electron emission booster heater passively follows the filament heater, providing additional power at high temperatures without having to implement a separate e-beam control loop.