Space Charge Limited Emission
Use the Space Charge Limited Emission multiphysics coupling () to model the space charge limited emission of electrons from a surface.
In order to use the Space Charge Limited Emission node, Specify current must be selected from the Particle release specification list in the coupled Charged Particle Tracing interface.
Settings
The Label is the default multiphysics coupling name.
The Name is used primarily as a scope prefix for variables defined by the coupling node. Refer to such variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different coupling nodes or physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.
The default Name (for the first multiphysics coupling in the model) is scle1.
Coupled Interfaces
The Source should be an instance of a physics interface that solves for electric potential, such as the Electrostatics interface. The Destination must be an instance of the Charged Particle Tracing interface.
If a physics interface is deleted and then added to the model again, and in order to reestablish the coupling, you need to choose the physics interface again from the Source or Destination lists. This is applicable to all multiphysics coupling nodes that would normally default to the once present physics interface. See Multiphysics Modeling Workflow in the COMSOL Multiphysics Reference Manual.
Space Charge Limited Emission
Enter values or expressions for the following:
•
Electric potential at cathode Vc (SI unit: V). The default value is 0.
•
Number of particles per release N (dimensionless). The default value is 1000.
•
Position offset os (SI unit: m). The default value is 1 mm.
The Space Charge Limited Emission node sets the electric potential at the selected boundaries by assuming that these boundaries are a short distance away from a cathode at the specified electric potential Vc. The selected boundaries, also called the emission surface, are usually at an electric potential slightly greater than Vc. The region between the cathode and the emission surface has thickness os. The potential in this buffer region is assumed to follow Child’s Law (that is, the four-thirds power law).
If the emission surface has electric potential less than Vc, the particles will not be released.
Theory for the Space Charge Limited Emission Node