Add check for conduction

Author: marc-flex

CHANGELOG.md

@@ -21,6 +21,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `ModeSimulation.plot()` method that plots the mode simulation plane by default, or the containing FDTD simulation if any of ``x``, ``y``, or ``z`` is passed. 
 - Enable singularity correction at PEC and lossy metal edges.
 - New `VolumeMesher` simulation type and associated `VolumeMeshMonitor` and `VolumeMesherData`, which can be used to run the unstructured meshing for a `HeatChargeSimulation` separately before running the solver.
+- The current validator for Conduction simulations has been modified so that it checks that in a Conduction simulation there is at least one structure defined with `ChargeConductorMedium` in the `charge` field of a `MultiPhysicsMedium`. This is necessary to ensure simulations are properly set up in the back-end since it relies on the `conductivity` field of `ChargeConductorMedium` and not that of `Medium`.
 
 ### Changed
 - Switched to an analytical gradient calculation for spatially-varying pole-residue models (`CustomPoleResidue`).

tests/test_components/test_heat_charge.py

@@ -1364,7 +1364,7 @@ def place_box(center_offset):
             structures=[
                 td.Structure(
                     geometry=td.Box(size=(1, 1, 1), center=shifted_center),
-                    medium=td.Medium(),
+                    medium=td.MultiPhysicsMedium(charge=td.ChargeConductorMedium(conductivity=1)),
                 )
             ],
             boundary_spec=[
@@ -1409,7 +1409,10 @@ def place_box(center_offset):
 )
 def test_sim_structure_extent(box_size, log_level):
     """Ensure we warn if structure extends exactly to simulation edges."""
-    box = td.Structure(geometry=td.Box(size=box_size), medium=td.Medium(permittivity=2))
+    box = td.Structure(
+        geometry=td.Box(size=box_size),
+        medium=td.MultiPhysicsMedium(charge=td.ChargeConductorMedium(conductivity=1)),
+    )
 
     with AssertLogLevel(log_level):
         _ = td.HeatChargeSimulation(
@@ -2040,3 +2043,15 @@ def test_heat_conduction_simulations():
     with pytest.raises(pd.ValidationError):
         # This should error since the conduction simulation doesn't have a monitor
         _ = sim.updated_copy(monitors=[temp_monitor])
+
+    # test error if structures defined with Medium instead of MultiPhysicsMedium
+    with pytest.raises(pd.ValidationError):
+        struct_error = struct1.updated_copy(medium=td.Medium(conductivity=1))
+        _ = sim.updated_copy(structures=[struct_error])
+
+    # test error if structures aren't conducting
+    with pytest.raises(pd.ValidationError):
+        struct_error = struct1.updated_copy(
+            medium=struct1.medium.updated_copy(charge=td.ChargeInsulatorMedium)
+        )
+        _ = sim.updated_copy(structures=[struct_error])

tidy3d/components/tcad/simulation/heat_charge.py

@@ -835,6 +835,19 @@ def check_conduction_sim(cls, values):
                                 "has been defined. This is not supported in Conduction simulations."
                             )
 
+            # make sure that at least one structure has appropriate charge medium
+            ValidConductionMediums = ChargeConductorMedium
+            structures = values.get("structures")
+            if all(isinstance(s.medium, Medium) for s in structures):
+                raise SetupError(
+                    "Conduction simulations must be defined using 'MultiPhysicsMedium' but none have been defined."
+                )
+            if not any(isinstance(s.medium.charge, ValidConductionMediums) for s in structures):
+                raise SetupError(
+                    "Conduction simulations require at least one structure with a 'ChargeConductorMedium' "
+                    "but none have been defined."
+                )
+
         return values
 
     @pd.root_validator(skip_on_failure=True)