Allow scaling of kappa in core rings

Done to try and suppress mesh seam fluctuations. This works successfully but the fluctuations appear to be driven my ion momentum after all

include/evolve_pressure.hxx

@@ -95,6 +95,8 @@ private:
   bool low_p_diffuse_perp; ///< Add artificial cross-field diffusion at low electron pressure?
 
   Field3D kappa_par; ///< Parallel heat conduction coefficient
+  BoutReal core_kappa_factor;   ///< Scaling factor on first core ring conduction
+  int core_kappa_scale_rings;   ///< Number of rings to scale conduction by core_kappa_factor
 
   Field3D source, final_source; ///< External pressure source
   Field3D Sp;     ///< Total pressure source

src/evolve_pressure.cxx

@@ -80,6 +80,15 @@ EvolvePressure::EvolvePressure(std::string name, Options& alloptions, Solver* so
     .doc("Flux limiter factor. < 0 means no limit. Typical is 0.2 for electrons, 1 for ions.")
     .withDefault(-1.0);
 
+  core_kappa_factor = options["core_kappa_factor"]
+    .doc("Multiply kappa by this factor in the first core ring. Helpful in avoiding mesh seam fluctuations at low rtol.")
+    .withDefault(1.0);
+
+  core_kappa_scale_rings = options["core_kappa_scale_rings"]
+    .doc("Number of rings to scale kappa by core_kappa_factor. Default is 1.")
+    .withDefault<int>(1);
+
+
   p_div_v = options["p_div_v"]
                 .doc("Use p*Div(v) form? Default, false => v * Grad(p) form")
                 .withDefault<bool>(false);
@@ -343,6 +352,19 @@ void EvolvePressure::finally(const Options& state) {
       }
     }
 
+
+    // Loop over first core ring and multiply kappa by core_kappa_factor
+    for (int ix = mesh->firstX(); ix < mesh->firstX() + core_kappa_scale_rings + 1; ix++) {
+      for (int iy = 0; iy < mesh->LocalNy ; iy++) {
+        for (int iz = 0; iz < mesh->LocalNz; iz++) {
+
+          if (mesh->periodicY(ix)) { // select only periodic, i.e. core region
+            kappa_par(ix, iy, iz) *= core_kappa_factor;
+          }
+        }
+      }
+    }
+
     // Note: Flux through boundary turned off, because sheath heat flux
     // is calculated and removed separately
     ddt(P) += (2. / 3) * FV::Div_par_K_Grad_par(kappa_par, T, false);