5 files changed, 20 insertions, 10 deletions
diff --git a/Source/Evolve/WarpXEvolve.cpp b/Source/Evolve/WarpXEvolve.cpp
index 78f4376eb..3dda534f1 100644
--- a/Source/Evolve/WarpXEvolve.cpp
+++ b/Source/Evolve/WarpXEvolve.cpp
@@ -272,7 +272,7 @@ WarpX::OneStep_nosub (Real cur_time)
     // product species.
     doFieldIonization();
 
-    mypc->doCoulombCollisions();
+    mypc->doCoulombCollisions( cur_time );
 #ifdef WARPX_QED
     mypc->doQEDSchwinger();
 #endif
diff --git a/Source/Particles/Collision/CollisionType.H b/Source/Particles/Collision/CollisionType.H
index 962530973..f1ffbe439 100644
--- a/Source/Particles/Collision/CollisionType.H
+++ b/Source/Particles/Collision/CollisionType.H
@@ -17,6 +17,7 @@ class CollisionType
 public:
     int  m_species1_index;
     int  m_species2_index;
+    int  m_ndt;
     bool m_isSameSpecies;
     amrex::Real m_CoulombLog;
 
@@ -38,7 +39,8 @@ public:
         int const lev, amrex::MFIter const& mfi,
         std::unique_ptr<WarpXParticleContainer>& species1,
         std::unique_ptr<WarpXParticleContainer>& species2,
-        bool const isSameSpecies, amrex::Real const CoulombLog );
+        bool const isSameSpecies, amrex::Real const CoulombLog,
+        int const ndt );
 
 };
 
diff --git a/Source/Particles/Collision/CollisionType.cpp b/Source/Particles/Collision/CollisionType.cpp
index 79962b803..36ce7952d 100644
--- a/Source/Particles/Collision/CollisionType.cpp
+++ b/Source/Particles/Collision/CollisionType.cpp
@@ -26,6 +26,10 @@ CollisionType::CollisionType(
     m_CoulombLog = -1.0;
     pp.query("CoulombLog", m_CoulombLog);
 
+    // number of time steps between collisions
+    m_ndt = 1;
+    pp.query("ndt", m_ndt);
+
     for (int i=0; i<static_cast<int>(species_names.size()); i++)
     {
         if (species_names[i] == collision_species[0])
@@ -57,13 +61,14 @@ using namespace ParticleUtils;
  * @param species1/2 pointer to species container
  * @param isSameSpecies true if collision is between same species
  * @param CoulombLog user input Coulomb logrithm
+ * @param ndt user input number of time stpes between collisions
  *
  */
 void CollisionType::doCoulombCollisionsWithinTile
     ( int const lev, MFIter const& mfi,
     std::unique_ptr<WarpXParticleContainer>& species_1,
     std::unique_ptr<WarpXParticleContainer>& species_2,
-    bool const isSameSpecies, Real const CoulombLog )
+    bool const isSameSpecies, Real const CoulombLog, int const ndt )
 {
 
     if ( isSameSpecies ) // species_1 == species_2
@@ -139,8 +144,7 @@ void CollisionType::doCoulombCollisionsWithinTile
                         indices_1, indices_1,
                         ux_1, uy_1, uz_1, ux_1, uy_1, uz_1, w_1, w_1,
                         q1, q1, m1, m1, Real(-1.0), Real(-1.0),
-                        dt, CoulombLog, dV,
-                        engine);
+                        dt*ndt, CoulombLog, dV, engine );
                 }
             }
         );
@@ -236,8 +240,7 @@ void CollisionType::doCoulombCollisionsWithinTile
                         indices_1, indices_2,
                         ux_1, uy_1, uz_1, ux_2, uy_2, uz_2, w_1, w_2,
                         q1, q2, m1, m2, Real(-1.0), Real(-1.0),
-                        dt, CoulombLog, dV,
-                        engine);
+                        dt*ndt, CoulombLog, dV, engine );
                 }
             }
         );
diff --git a/Source/Particles/MultiParticleContainer.H b/Source/Particles/MultiParticleContainer.H
index 73a8207bd..4a515acf8 100644
--- a/Source/Particles/MultiParticleContainer.H
+++ b/Source/Particles/MultiParticleContainer.H
@@ -122,7 +122,7 @@ public:
                             const amrex::MultiFab& Ex, const amrex::MultiFab& Ey, const amrex::MultiFab& Ez,
                             const amrex::MultiFab& Bx, const amrex::MultiFab& By, const amrex::MultiFab& Bz);
 
-    void doCoulombCollisions ();
+    void doCoulombCollisions (amrex::Real cur_time);
 
     /**
     * \brief This function loops over all species and performs resampling if appropriate.
diff --git a/Source/Particles/MultiParticleContainer.cpp b/Source/Particles/MultiParticleContainer.cpp
index d1dde1837..f986e7dba 100644
--- a/Source/Particles/MultiParticleContainer.cpp
+++ b/Source/Particles/MultiParticleContainer.cpp
@@ -683,12 +683,16 @@ MultiParticleContainer::doFieldIonization (int lev,
 }
 
 void
-MultiParticleContainer::doCoulombCollisions ()
+MultiParticleContainer::doCoulombCollisions ( Real cur_time )
 {
     WARPX_PROFILE("MultiParticleContainer::doCoulombCollisions()");
 
     for( auto const& collision : allcollisions )
     {
+
+        const Real dt = WarpX::GetInstance().getdt(0);
+        if ( int(std::floor(cur_time/dt)) % collision->m_ndt != 0 ) continue;
+
         auto& species1 = allcontainers[ collision->m_species1_index ];
         auto& species2 = allcontainers[ collision->m_species2_index ];
 
@@ -709,7 +713,8 @@ MultiParticleContainer::doCoulombCollisions ()
                 CollisionType::doCoulombCollisionsWithinTile
                     ( lev, mfi, species1, species2,
                       collision->m_isSameSpecies,
-                      collision->m_CoulombLog );
+                      collision->m_CoulombLog,
+                      collision->m_ndt );
 
             }
         }