Importing three-dimensional ANSYS field maps

Ansys offers two element types for three-dimensional electrostatic problems: the quadratic curved tetrahedron and the brick. Only field maps based on the former, known as SOLID123, can be imported in Garfield++ (using the class ComponentAnsys123).

In the following we briefly recap the main steps in a typical Ansys script. For a more detailed discussion see the GEM tutorial.

Clear earlier calculations and enter the pre-processor.
```
FINISH
/CLEAR,START
/PREP7
```
If using the GUI, enter the preferences and ensure that the only discipline selected within the "Electromagnetic" group is "Electric". The equivalent commands are
```
KEYW,PR_ELMAG,1
KEYW,MAGELC,1
```
Disable the p-method solution options. This option leads to elements of higher polynomial order, for which Garfield++ does not have shape functions implemented.
```
/PMETH,OFF,1
```
Select the second-order tetrahedron as element.
```
ET,1,SOLID123
```
When defining material properties, be sure that every material has its permittivity set. Choose material numbers starting from 1 and leave no holes in the numbering. Set the permittivity of conductors to a very high value. Set the resistivity of perfect conductors to 0 so as to make them eligible for removal as background. Try to avoid temperature-dependent properties as these can not be used by Garfield to identify materials. For instance, to define a perfect conductor (material 1), a gas (material 2) and a dielectricum (material 3):
```
! Material properties
MP,PERX,1,1e10  ! Metal
MP,RSVX,1,0.0   !
MP,PERX,2,1.0   ! Gas
MP,PERX,3,4.0   ! Permittivity of FR4
```

Create the model. Pay particular particular attention to gluing adjacent volumes where necessary (VGLUE command). For instance, to create a conducting strip on the wall of a block of dielectric material:

metal = 0.2
gas = 2
sub = -1
BLOCK, -10, -5, -10, 10,     0, metal ! 1: Wide side strip
BLOCK,  -2, -4, -10, 10,     0, metal ! 2: First signal
BLOCK,  -1,  1, -10, 10,     0, metal ! 3: 2nd signal
BLOCK,   2,  4, -10, 10,     0, metal ! 4: 3rd signal
BLOCK,   5, 10, -10, 10,     0, metal ! 5: Wide side strip
BLOCK, -10, 10, -10, 10,   sub,     0 ! 6: Substrate
BLOCK, -10, 10, -10, 10,     0,   gas ! 7: Gas

! Subtract the strips from the gas
VSBV,  7, 1, , , KEEP   ! 7 -> 8
VSBV,  8, 2, , , KEEP   ! 8 -> 7
VSBV,  7, 3, , , KEEP   ! 7 -> 8
VSBV,  8, 4, , , KEEP   ! 8 -> 7
VSBV,  7, 5, , , KEEP   ! 7 -> 8

! Glue everything together 1 = left wide, 2, 3, 4, 5 = wide, 7 = sub, 8 = gas
VGLUE, ALL

Assign material properties with the VATT command.

VSEL, S, VOLU, , 1, 5 ! Metal strips
VATT, 1, ,1
VSEL, S, VOLU, , 7    ! Gas volume
VATT, 3, ,1
VSEL, S, VOLU, , 8    ! Substrate
VATT, 2, ,1

Set boundary conditions.

! Voltage boundary conditions on the metal
VSEL, S, VOLU, , 1, 5 ! All strips at ground
ASLV, S
DA, ALL, VOLT, 0
ASEL, S, LOC, Z, gas  ! Drift electrode
DA, ALL, VOLT, -1000
ASEL, S, LOC, Z, sub  ! Back plane
DA, ALL, VOLT, 0

Mesh the problem.

! Meshing options
VSEL, S, VOLU, , 8    ! Only mesh the gas
ASLV, S

MSHKEY,0
SMRT, 4
VMESH, 1,8

Then solve the problem.

/SOLU
SOLVE
FINISH

Visualize the solution (optional):

/POST1
/EFACET,1
PLNSOL, VOLT,, 0

Write the solution to files. The potentials at the nodes are written to a file with the PRNSOL command. In the example below, the file name is chosen to be "field.lis".
```
/OUTPUT, field, lis
PRNSOL
/OUTPUT
```
In addition, we need to export the coordinates of the nodes (command NLIST), the nodes constituting the elements (command ELIST), and the material properties (command MPLIST).
```
/OUTPUT, NLIST, lis
NLIST,,,,COORD
/OUTPUT

/OUTPUT, ELIST, lis
ELIST
/OUTPUT

/OUTPUT, MPLIST, lis
MPLIST
/OUTPUT
```

Weighting potentials

In order to compute signals, Garfield++ needs weighting potentials or weighting fields. These are obtained by setting the read-out electrodes to a potential of 1 V and all other electrodes to ground. ComponentAnsys123- requires the weighting fields and the main field map to share the same mesh.

Continuing the above example, we first clear the existing loads.
```
/SOLU
LSCLEAR,ALL
```
We then apply the boundary conditions corresponding to the weighting field of the first (leftmost) narrow strip (volume index 1), i. e. we set the potential of volume 1 to 1 V and the potential of the other metal volumes to 0 V. The potentials on the top side of the gas and the bottom side of the substrate are set to 0 V as well.
```
VSEL, S, VOLU, , 1
VSEL, A, VOLU, , 3, 5
ASLV, S
DA, ALL, VOLT, 0

VSEL, S, VOLU, , 2
ASLV, S
DA, ALL, VOLT, 1

ASEL, S, LOC, Z, gas
DA, ALL, VOLT, 0
ASEL, S, LOC, Z, sub
DA, ALL, VOLT, 0
```

We now solve without meshing again.

VSEL, S, VOLU, , 1, 8
ASLV, S

! Solve the field
SOLVE

Using the PRNSOL command we export the new solution to a file called "weight1.lis". ``` ! Write the solution /POST1 /OUTPUT, weight1, lis PRNSOL /OUTPUT ````

Following the same recipe, we can calculate and save maps of the weighting potential for the other two narrow strips.

Importing main and weighting field maps

To load the ".lis" files exported from Ansys in a ComponentAnsys123 object, we use the function Initialise.

ComponentAnsys123 fm;
fm.Initialise("ELIST.lis", "NLIST.lis", "MPLIST.lis", "field.lis", "mm");

To import the maps of the weighting potentials we use the function SetWeightingField which takes two string arguments: the name of the file that contains the nodal solution, and the identifier we want to give the corresponding readout electrode. `

fm.SetWeightingField("weight1.lis", "strip1");
fm.SetWeightingField("weight2.lis", "strip2");
fm.SetWeightingField("weight3.lis", "strip3");

To make a contour plot of the weighting potential we use the class ViewField.

ViewField fieldView(&fm);
fieldView.SetPlaneXZ();
fieldView.PlotContourWeightingField("strip1", "v");

Source files

The Ansys script discussed above and a simple C++ program to read the field map can be found in the directory Examples/Ansys123 of the Garfield++ source tree. This folder also contains Ansys scripts for modelling a Micromegas and a Triple-GEM.