Particle escape and local return

BEACH does not evolve plasma outside the box. [particle_boundary] makes each nonperiodic face open, reflect, or redistributed_reflect, and open faces apply ordinary_open_model="escape" | "potential_barrier". The treatment is shared by boundary-reservoir inflow, plane_source, photo_raycast, volume_seed, and deprecated reservoir_face. Periodicity belongs to domain.periodic_axes; particle-boundary tables cannot specify periodic. The fully qualified key is particle_boundary.ordinary_open_model.

1. escape: remove a particle at an open face

[particle_boundary]
x_low = "open"
x_high = "open"
y_low = "open"
y_high = "open"
z_low = "open"
z_high = "open"
ordinary_open_model = "escape"

The particle is removed at the crossing. Its macro charge $qw$ is recorded in the species-resolved escaped_to_infinity ledger term, while surface charge q_elem is unchanged.

See Particle collision and boundary events for simultaneous face crossings and reintegration of the step remainder after reflecting or periodic faces.

2. potential_barrier: decide reflection at a scalar barrier

potential_barrier is a reduced model that uses only scalar potential at the crossing.

[particle_boundary]
ordinary_open_model = "potential_barrier"

[reservoir]
phi_infty = 0.0

For crossing-point potential $\phi_b$ and outward normal speed $v_n>0$, the barrier to infinity is

$$U_b=q(\phi_\infty-\phi_b).$$

If

$$\frac12m v_n^2<U_b\quad\text{and}\quad U_b>0,$$

the normal velocity is reversed and the step remainder is tracked. Otherwise the particle escapes. Tangential velocity is unchanged.

It does not retain an electric field outside the open face, a turning position, flight time, or space charge. A corner that crosses multiple open faces stops as ambiguous_open_corner.

The crossing potential follows the batch-start fixed field used for particle motion and includes the local potential of sim.e0. Because a uniform field has no finite potential at infinity, a configuration that combines sim.e0 with this model must use a consistent effective reservoir reference for reservoir.phi_infty.

Outer barrier at a matching plane

matching_plane_quasistatic applies electron and ion access potentials plus the maximum outer PE barrier returned by the selected response backend at z-high, independently of the ordinary-open model. The outward decision uses the same scalar-energy test, but its reference values update in every fixed-point iteration. A reflected PE contributes to return, a transmitted PE contributes to escape, and BEACH compares their sum with the measured outward crossings.

Return reverses normal velocity immediately at the same z-high position. This quasistatic approximation retains no outer turning position, flight time, or lateral displacement. Do not stack a manual potential_barrier or closed-PE reflection on a matching-plane case. See Quasistatic Matching-Plane Coupling for response values and scope.

Return of closed photoelectrons

For closed PE, set the face matching inject_face to reflect or redistributed_reflect in [particles.species.boundary] for the negative photo_raycast species, and set surface_charge_closure="neutral_return". Both actions reverse normal velocity and preserve tangential velocity, but they treat the return position differently.

Action	Return position
reflect	Preserve the tangential position of the boundary event
redistributed_reflect	For one face, uniformly redistribute both in-plane coordinates over the box span excluding its end guards

At a simultaneous edge or corner event, redistributed_reflect relocates only axes outside the event mask. See Particle collision and boundary events for details. Keeping the global face open lets ambient species with inherit follow the ordinary-open contract. See Photoelectron emission for the configuration and charge-balance requirements.