-Add new input deck option 'two_stream_neutral' that is an explicitly…

… quasineutral version of the two_stream deck.

-The usual momentum conservation relation is only valid if the plasma is quasineutral (because the implied neutralising species would gain/lose significant momentum)

-Now momentum conservation seems to be working as expected.

example_decks/two_stream_neutral.deck

@@ -0,0 +1,3 @@
+&control
+   problem = 'two_stream_neutral' 
+/

src/user_interaction/ic_module.f90

@@ -27,6 +27,8 @@ SUBROUTINE custom_problem_setup(deck_state, problem)
     SELECT CASE (TRIM(problem))
     CASE('two_stream')
       CALL two_stream_setup(deck_state)
+    CASE('two_stream_neutral')
+      CALL two_stream_neutral_setup(deck_state)
     CASE('one_stream')
       CALL one_stream_setup(deck_state)
     CASE('drift_kin_default')
@@ -164,6 +166,166 @@ SUBROUTINE two_stream_setup(deck_state)
 
   END SUBROUTINE two_stream_setup
 
+  SUBROUTINE two_stream_neutral_setup(deck_state)
+
+    INTEGER, INTENT(IN) :: deck_state
+    REAL(num), PARAMETER :: v_drift = 0.2_num * c
+    REAL(num), PARAMETER :: v_therm = 0.01_num * c
+    REAL(num), PARAMETER :: v_pert = 0.1_num * v_therm
+    REAL(num), PARAMETER :: n0 = 8e11
+    INTEGER, PARAMETER :: ppc = 16
+    REAL(num) :: gamma_drift, temp_x, omega
+    INTEGER :: ix
+    TYPE(particle_species), POINTER :: current_species
+
+    IF (deck_state == c_ds_first) THEN
+      ! Set control variables here
+      nx_global = 64
+      ny_global = 4
+      nz_global = 4
+      x_min = 0.0_num
+      x_max = 2.0_num * pi
+      y_min = x_min
+      ! Could do (x_max * ny_global) / nx_global, but be wary of compilers
+      ! which don't obey precedence implied by parentheses by default
+      ! (e.g. Intel)
+      y_max = x_max * REAL(ny_global, num) / REAL(nx_global, num)
+      z_min = x_min
+      z_max = x_max * REAL(nz_global, num) / REAL(nx_global, num)
+
+      ! Plasma frequency
+      omega = SQRT(n0 * q0 * q0 / epsilon0 / m0)
+      t_end = 30.0_num / omega
+      stdout_frequency = 10
+
+     ! dt_multiplier = 0.5
+
+
+      ! Need to set-up species here
+      NULLIFY(current_species)
+      CALL setup_species(current_species, 'Right')
+
+      ! mass -- MANDATORY
+      current_species%mass = 1.1_num * m0
+
+      ! charge -- MANDATORY
+      current_species%charge = -1.0_num * q0
+
+      ! npart_per_cell
+      current_species%npart_per_cell = ppc
+
+      ! MANDATORY
+      NULLIFY(current_species)
+      CALL setup_species(current_species, 'Left')
+
+      ! mass -- MANDATORY
+      current_species%mass = 1.0_num * m0
+
+      ! charge -- MANDATORY
+      current_species%charge = -1.0_num * q0
+
+      ! npart_per_cell
+      current_species%npart_per_cell = ppc
+
+     NULLIFY(current_species)
+      CALL setup_species(current_species, 'Balance')
+
+      ! mass -- MANDATORY
+      current_species%mass = 100 * m0
+
+      ! charge -- MANDATORY
+      current_species%charge = 1.0_num * q0
+
+      ! npart_per_cell
+      current_species%npart_per_cell = ppc
+
+      RETURN
+    END IF
+
+    ! Calculate gamma_drift
+    ! Strictly should be function of x, but vpert << vdrift
+    gamma_drift = 1.0_num / SQRT(1.0_num - (v_drift / c)**2)
+
+    ! Calculate (1 DoF) temperature
+    temp_x = v_therm**2 * m0 / kb
+
+    !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+    ! Species
+
+    ! MANDATORY (Is the NULLIFY mandatory?)
+    NULLIFY(current_species)
+    CALL setup_species(current_species, 'Right')
+
+    ! density
+    current_species%density = n0
+
+    ! drift_x
+    ! Add on perturbation to seed instability
+    DO ix = 1-ng, nx+ng
+      current_species%drift(ix,:,:,1) = gamma_drift * current_species%mass &
+          * (v_drift + v_pert * SIN(3.0_num * x(ix)))
+    END DO
+
+    ! temp_x
+    current_species%temp(:,:,:,1) = temp_x
+
+    IF (explicit_pic) THEN
+      current_species%is_implicit = .FALSE.
+    ELSE
+      current_species%is_implicit = .TRUE.
+    END IF
+
+    !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+    ! MANDATORY
+    NULLIFY(current_species)
+    CALL setup_species(current_species, 'Left')
+
+    ! density
+    current_species%density = n0
+
+    ! drift_x
+    ! Add on perturbation to seed instability
+    DO ix = 1-ng, nx+ng
+      current_species%drift(ix,:,:,1) = gamma_drift * current_species%mass &
+          * (-v_drift + v_pert * SIN(3.0_num * x(ix)))
+    END DO
+
+    ! temp_x
+    current_species%temp(:,:,:,1) = temp_x
+
+    IF (explicit_pic) THEN
+      current_species%is_implicit = .FALSE.
+    ELSE
+      current_species%is_implicit = .TRUE.
+    END IF
+
+    !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+       ! MANDATORY
+    NULLIFY(current_species)
+    CALL setup_species(current_species, 'Balance')
+
+    ! density
+    current_species%density = 2.0_num * n0
+
+    ! drift_x
+    DO ix = 1-ng, nx+ng
+      current_species%drift(ix,:,:,1) = 0.0_num
+    END DO
+
+    ! temp_x
+    current_species%temp(:,:,:,1) = temp_x
+
+    IF (explicit_pic) THEN
+      current_species%is_implicit = .FALSE.
+    ELSE
+      current_species%is_implicit = .TRUE.
+    END IF
+
+
+  END SUBROUTINE two_stream_neutral_setup
 
 
   SUBROUTINE em_wave_setup(deck_state)