KineticForces - BUGFIX! - Keep the small collision width positive for either rotation sign

src/KineticForces/EnergyIntegration.jl

@@ -53,7 +53,7 @@ end
 Collision frequency ν(x) at normalized energy x = E/T.
 
   - `"zero"`: collisionless (ν = 0)
-  - `"small"`: 1e-5 * we
+  - `"small"`: 1e-5 * |we|, a positive Krook width for either rotation direction
   - `"krook"`: unmodified Krook operator
   - `"harmonic"`: (1 + (l/2)²) * krook * x^(-3/2)
 """
@@ -62,7 +62,7 @@ Collision frequency ν(x) at normalized energy x = E/T.
     nux = if id == 1
         0.0
     elseif id == 2
-        1e-5 * p.we
+        1e-5 * abs(p.we)
     elseif id == 3
         p.nuk
     else

test/runtests_kinetic.jl

@@ -130,6 +130,24 @@
             # qt multiplies by (x - 2.5) = 0.5
             @test val_qt ≈ (x - 2.5) * val_no atol = 1e-14
         end
+
+        @testset "small collisions damp for either rotation sign" begin
+            # Match a positive Krook width for either rotation direction.
+            for we in (2.0, -2.0)
+                p_small = KF.EnergyParams(
+                    1.0, 0.5, we, 0.5, 1.0, 0.0, 1.0, 1,
+                    "small", "maxwellian", 1.0, false
+                )
+                p_krook = KF.EnergyParams(
+                    1.0, 0.5, we, 0.5, 1.0, 1e-5 * abs(we), 1.0, 1,
+                    "krook", "maxwellian", 1.0, false
+                )
+                for x in (0.3, 1.0, 3.0)
+                    @test KF.energy_integrand_scalar(x, p_small) ≈
+                          KF.energy_integrand_scalar(x, p_krook) rtol = 1e-14
+                end
+            end
+        end
     end
 
     # =========================================================================