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5.7.3.1. Control Grids

Introducing features such as control grids into fully defined gun models can be challenging. There are a variety of approaches one can take, each having pros and cons.

The traditional approach is to use a Boolean operation:

  1. Draw (or import) the control grid and position it in the model.

  2. Perform a Boolean subtraction of the grid from the original model. This action will disrupt some of your attribute assignments since the model topology has changed.

  3. Apply a conductor boundary condition to all faces of the newly created void that represents the control grid.

This method suffers from several drawbacks including:

  1. The Boolean subtraction impacts the topology of the model, meaning that you need to correct/reapply the boundary conditions.

  2. Picking all the faces is time-consuming and error-prone. Using area picks and other tricks helps tremendously but you still have to be careful.

  3. If you need to import a new control grid, you have to repeat the process.

The preferred approach is to allow the control grid to overlap the original model:

  1. Draw (or import) the control grid and position it in the model.

  2. Choose a conducting material for the grid solid(s). PEC is a good choice.

  3. Apply a conductor boundary condition to all faces of the grid solid(s) by right-clicking the node(s) for the grid solid(s) in the Browser. Select Pick > Pick All Faces from the context menu. On the Structure ribbon, in the Attributes group, click the New Attribute button and select Conductor.

This works because the mesher meshes the volume of the grid, but MICHELLE ignores the non-dielectric elements while maintaining the internal boundary conditions.

To replace one grid with another just delete the existing grid, draw or import the new grid, and repeat the above steps. Alternatively you can use the grid's Include In Simulation parameter to keep multiple grids available and selectively control which one to use.

This method works in both 2D and 3D simulations, and it works for emitters and conductors.

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