improve some comments

src/gray_core.f90

@@ -456,7 +456,7 @@ contains
 
             pow         = p0ray(jk) * exp(-tau) ! residual power: P = P₀exp(-τ)
             ppabs(jk,i) = p0ray(jk) - pow       ! absorbed power: P_abs = P₀ - P
-            dids        = didp * pow * alpha    ! current drive:  dI/ds = dI/dP⋅P⋅α
+            dids        = didp * pow * alpha    ! current driven: dI/ds = dI/dP⋅dP/ds = dI/dP⋅P⋅α
 
             ! current: I = ∫dI/ds⋅ds using the trapezoid rule
             ccci(jk,i) = ccci0(jk) + 0.5_wp_*(dids0(jk) + dids) * dersdst * params%raytracing%dst
@@ -1474,6 +1474,8 @@ contains
       an2s = one - xg - half*xg*yg2*(one + anpl2 + sox*del)/duh
 
       ! ∂(N²s)/∂X
+      ! Note: this term is nonzero for X=0, but it multiplies terms
+      ! proportional to X or ∂X/∂ψ which are zero outside the plasma.
       dan2sdxg  = - half*yg2*(one - yg2)*(one + anpl2 + sox*del)/duh**2    &
                   + sox*xg*anpl2/(del*duh) - one
       ! ∂(N²s)/∂Y

src/gray_params.f90

@@ -10,7 +10,7 @@ module gray_params
     ! From gray_params.data:
     real(wp_) :: alpha, beta        ! Launching angles
     real(wp_) :: power              ! Initial power
-    real(wp_) :: psi, chi           ! Initial polarisation angles
+    real(wp_) :: psi, chi           ! Initial polarisation ellipse parameters
     integer :: iox                  ! Initial wave mode
     integer :: ibeam                ! Beam kind
     character(len=lenfnm) :: filenm ! beamdata.txt filename