Interior Number Density Reconstruction

It is often desirable to know the number density at some point within the vacuum chamber (for example, along a beam line). There are two ways to compute the number density within a domain in COMSOL Multiphysics.

The first method is to add a Number Density Reconstruction feature to the model. COMSOL Multiphysics then computes the number density for each species (which is available as the dependent variable N) on the feature in addition to on the exterior surfaces of the model at the time of solving.


	This method is recommended for large regions, such as surfaces and domains, where the calculation of the number density is computationally intensive, or along curved lines that are explicitly drawn within the geometry (such lines are more difficult to define in results processing).

The Free Molecular Flow Interface also defines a number density operator, which is called in the following manner:

comp1.fmf.nop_name(x,y,z) (3D)

comp1.fmf.nop_name(x,y) (2D)

comp1.fmf.nop_name(r,z) (2D axisymmetric)

where name is the dependent variable name for a given species. The default is G, so comp1.fmf.nop_G(x,y,z) would be the default operator to compute the number density in 3D.


	If multiple components or Free Molecular Flow interfaces are added to the model, change comp1 and fmf to the corresponding component and physics interface names.

This operator computes the number density at the time it is called, and consequently if a number of computations are required (for example, for plotting the number density on a surface) it takes a long time to return the result (the physics interface is unresponsive during this time).


	This method is recommended if the number density at a point or on a straight line not drawn in the geometry is required.


	The number density operator and the number density reconstruction features produce unreliable results adjacent to the exterior surfaces of a Molecular Flow domain. In this region the number density on the surface is much more accurate.