Add callback to write values to external field multifabs (ECP-WarpX#5162

)

* Add callback to write values to external fields

* add external fields after initial E solve

Signed-off-by: roelof-groenewald <regroenewald@gmail.com>

---------

Signed-off-by: roelof-groenewald <regroenewald@gmail.com>

Python/pywarpx/callbacks.py

@@ -25,6 +25,7 @@
 
 Functions can be called at the following times:
 
+* ``loadExternalFields``: during ``WarpX::LoadExternalFields`` to write ``B/Efield_fp_external`` values
 * ``beforeInitEsolve``: before the initial solve for the E fields (i.e. before the PIC loop starts)
 * ``afterinit``: immediately after the init is complete
 * ``beforeEsolve``: before the solve for E fields
@@ -276,6 +277,7 @@ def callfuncsinlist(self, *args, **kw):
 # =============================================================================
 
 callback_instances = {
+    "loadExternalFields": {},
     "beforeInitEsolve": {},
     "afterInitEsolve": {},
     "afterinit": {},
@@ -330,7 +332,7 @@ def clear_all():
         val.clearlist()
 
 
-# =============================================================================
+# ============================================================================
 
 
 def printcallbacktimers(tmin=1.0, lminmax=False, ff=None):
@@ -371,7 +373,17 @@ def printcallbacktimers(tmin=1.0, lminmax=False, ff=None):
             ff.write("\n")
 
 
-# =============================================================================
+# ============================================================================
+
+
+# ----------------------------------------------------------------------------
+def callfromloadExternalFields(f):
+    installcallback("loadExternalFields", f)
+    return f
+
+
+def installloadExternalFields(f):
+    installcallback("loadExternalFields", f)
 
 
 # ----------------------------------------------------------------------------

Source/Initialization/WarpXInitData.cpp

@@ -578,6 +578,11 @@ WarpX::InitData ()
         if (electrostatic_solver_id == ElectrostaticSolverAlgo::LabFrameElectroMagnetostatic) {
             ComputeMagnetostaticField();
         }
+        // Add external fields to the fine patch fields. This makes it so that the
+        // net fields are the sum of the field solutions and any external fields.
+        for (int lev = 0; lev <= max_level; ++lev) {
+            AddExternalFields(lev);
+        }
     }
 
     if (restart_chkfile.empty() || write_diagnostics_on_restart) {
@@ -1013,8 +1018,6 @@ WarpX::InitLevelData (int lev, Real /*time*/)
 
     // load external grid fields into E/Bfield_fp_external multifabs
     LoadExternalFields(lev);
-    // add the external fields to the fine patch fields as initial conditions for the fields
-    AddExternalFields(lev);
 
     if (costs[lev]) {
         const auto iarr = costs[lev]->IndexArray();
@@ -1410,6 +1413,9 @@ WarpX::LoadExternalFields (int const lev)
 #endif
     }
 
+    // Call Python callback which might write values to external field multifabs
+    ExecutePythonCallback("loadExternalFields");
+
     // External particle fields
 
     if (mypc->m_B_ext_particle_s == "read_from_file") {