Split Bfield advance in two for SemiImplicit_EM evolve (#5483)

This PR splits the BField advance for the SemiImplicit_EM evolve scheme
into two half dt advances, consistent with the explicit evolve scheme.
This makes it such that the B field at write time is at the same
instance in time as the electric field, as stated in the documentation.

Regression/Checksum/benchmarks_json/test_1d_semi_implicit_picard.json

@@ -1,7 +1,7 @@
 {
   "lev=0": {
-    "Bx": 3559.0541122456157,
-    "By": 1685.942868827529,
+    "Bx": 3625.566538877196,
+    "By": 1684.1769211109035,
     "Bz": 0.0,
     "Ex": 796541204346.5195,
     "Ey": 961740397927.6577,

Source/FieldSolver/ImplicitSolvers/SemiImplicitEM.cpp

@@ -61,7 +61,7 @@ void SemiImplicitEM::OneStep ( amrex::Real  a_time,
     // Set the member time step
     m_dt = a_dt;
 
-    // Fields have Eg^{n}, Bg^{n-1/2}
+    // Fields have Eg^{n}, Bg^{n}
     // Particles have up^{n} and xp^{n}.
 
     // Save up and xp at the start of the time step
@@ -70,9 +70,9 @@ void SemiImplicitEM::OneStep ( amrex::Real  a_time,
     // Save Eg at the start of the time step
     m_Eold.Copy( FieldType::Efield_fp );
 
-    // Advance WarpX owned Bfield_fp to t_{n+1/2}
-    m_WarpX->EvolveB(m_dt, DtType::Full);
-    m_WarpX->ApplyMagneticFieldBCs();
+    // Advance WarpX owned Bfield_fp from t_{n} to t_{n+1/2}
+    m_WarpX->EvolveB(0.5_rt*m_dt, DtType::FirstHalf);
+    m_WarpX->FillBoundaryB(m_WarpX->getngEB(), true);
 
     const amrex::Real half_time = a_time + 0.5_rt*m_dt;
 
@@ -92,6 +92,10 @@ void SemiImplicitEM::OneStep ( amrex::Real  a_time,
     m_E.linComb( 2._rt, m_E, -1._rt, m_Eold );
     m_WarpX->SetElectricFieldAndApplyBCs( m_E );
 
+    // Advance WarpX owned Bfield_fp from t_{n+1/2} to t_{n+1}
+    m_WarpX->EvolveB(0.5_rt*m_dt, DtType::SecondHalf);
+    m_WarpX->FillBoundaryB(m_WarpX->getngEB(), true);
+
 }
 
 void SemiImplicitEM::ComputeRHS ( WarpXSolverVec&  a_RHS,

Source/FieldSolver/ImplicitSolvers/WarpXImplicitOps.cpp

@@ -101,7 +101,7 @@ WarpX::UpdateMagneticFieldAndApplyBCs( ablastr::fields::MultiLevelVectorField co
         amrex::MultiFab::Copy(*Bfp[2], *a_Bn[lev][2], 0, 0, ncomps, a_Bn[lev][2]->nGrowVect());
     }
     EvolveB(a_thetadt, DtType::Full);
-    ApplyMagneticFieldBCs();
+    FillBoundaryB(guard_cells.ng_alloc_EB, WarpX::sync_nodal_points);
 }
 
 void
@@ -111,14 +111,8 @@ WarpX::FinishMagneticFieldAndApplyBCs( ablastr::fields::MultiLevelVectorField co
     using warpx::fields::FieldType;
 
     FinishImplicitField(m_fields.get_mr_levels_alldirs(FieldType::Bfield_fp, 0), a_Bn, a_theta);
-    ApplyMagneticFieldBCs();
-}
-
-void
-WarpX::ApplyMagneticFieldBCs()
-{
-    FillBoundaryB(guard_cells.ng_alloc_EB, WarpX::sync_nodal_points);
     ApplyBfieldBoundary(0, PatchType::fine, DtType::Full);
+    FillBoundaryB(guard_cells.ng_alloc_EB, WarpX::sync_nodal_points);
 }
 
 void

Source/WarpX.H

@@ -145,7 +145,6 @@ public:
     void SetElectricFieldAndApplyBCs ( const WarpXSolverVec&  a_E );
     void UpdateMagneticFieldAndApplyBCs ( ablastr::fields::MultiLevelVectorField const&  a_Bn,
                                           amrex::Real  a_thetadt );
-    void ApplyMagneticFieldBCs ();
     void SpectralSourceFreeFieldAdvance ();
     void FinishMagneticFieldAndApplyBCs ( ablastr::fields::MultiLevelVectorField const&  a_Bn,
                                           amrex::Real  a_theta );