Glossary
Glossary

A dual filament is composed of two heaters that can be independently operated by two PID controlled power supplies.

Dual filament design provides several operation possibilities depending on which operation mode is chosen. Independent operation of the filaments is possible as well as hot lip, top heated or cold lip operation.


dual filament configuration

Sketch of a dual filament configuration with TC positions

The dual filament is being built of a short top filament, which heats the crucible top and the lip area. It is more densely wired than the bottom filament. Its length is about 1/4 of the crucible's length from the orifice. A thermocouple measures the temperature near the top filament. (TC1)

 

About 3/4 of the crucible's height is heated by the bottom filament. The main thermocouple is the bottom thermocouple, which measures the temperature at the bottom of the crucible. (TC2)


In case you want the upper part of the crucible to be hotter than the lower part you can use both filaments in series. Then you have got a simple hot-lipped filament type, by the filament geometry. 

At higher temperatures the heat loss through radiation from the crucible’s orifice gets higher and the filament geometry on its own is not sufficient for heating the crucible's top to higher temperature than the temperature at the bottom.

If higher temperatures are required at the crucible lip, the top filament has to be heated separately. 

In case you want a cooler lip (e.g. Al evaporation) you can use the bottom filament only and have simple cold lip filament type by the filament geometry. 

For the optimum adoption of the dual filament effusion cell application the dual filament power supply PS302-C2 or any similar power supply/controller configuration with two independent PID controllers offers the possibility to adjust the temperatures in the top and in the bottom area in a given range. The range is given by the dual filament design and the heat loss due to radiation from the crucible's orifice and the heat loss due to radiation from the shielding.

power supplies PS1 and PS2
Front panel of the power supply PS302-C2

The current for each filament must be limited to reasonable maximum values or suitable lower values in case of operating on sub maximal temperatures. This way the temperature cannot exceed maximum values by accident. Only change this limit if it is really needed! 

Be aware that any change in the power ratio may alter the feedback characteristics for the thermocouple temperature and may thus require a readjustment of PID parameters. 

Operation of a dual filament cell 

Mode 1

Filaments in series (hot lip operation)

Recommended TC for control loop: Bottom TC2

Use TC 1 as reference only

Use PS2 for top and bottom heater.

 

Mode 2

Top filament only (top heated cell)

Recommended TC for control loop: Bottom TC2

Use TC 1 as reference only

Use PS2 for top heater

 

Mode 3

Bottom filament only (cold lip operation)

Recommended TC for control loop: Bottom TC2

Use TC 1 as reference only

Use PS2 for bottom heater

 

Mode 4

Dual filament operation (independent control of top and bottom temperature)

TC1 with power supply PS1 for top heater

TC2 with power supply PS2 for bottom heater

 

The dual filament mode 4 is the most critical mode, because there are two controllers and the operation of one filament has effects on both thermocouples and feedback loops.

The temperatures cannot be absolutely independent selected. The maximum possible difference between top and bottom temperature can be estimated from its power graph in top heated mode (Mode 2) where only the top filament is connected to voltage and the bottom part (thermocouple TC2) is being heated solely by radiation. 

Alternative filament types are Standard Filament SFCold Lip Filament CL, Tip Filament TF and Hot Lip Filament HL:
Filament Types