How surfaces charge

This page explains where BEACH keeps charge after a particle reaches a triangulated surface. BEACH applies the charge change produced within a batch exactly once, then uses the selected surface_model to determine surface charge for the next batch.

Ordinary choice: use surface_model="insulator" for local charging of an insulating surface. Choose conductor only when a floating conductor in free space should become equipotential.

After reading this page, you should be able to distinguish the two implemented models and identify material effects that are absent from the result.

Feedback between batches

flowchart LR
    field["Calculate field from current surface charge"]
    particles["Track particles in the fixed field"]
    delta["Collect charge from absorption and emission"]
    surface["Apply the surface model"]
    next["Surface charge for the next batch"]

    field --> particles --> delta --> surface --> next --> field

Later particles in the same batch do not see the charge of particles absorbed earlier in that batch. The change becomes final at batch end and first appears in the next batch’s field. Test sensitivity to this lag by following how to choose batch_duration.

Choose a model

surface_model	Treatment at batch end	Suitable target	Main limit
insulator	Retain charge on the hit element	Local charging of an insulator	Does not solve surface conduction or bulk leakage
conductor	Conserve total charge per mesh_id and redistribute element charge to become equipotential	Floating conductor in free space	field_boundary.mode="free" only

dielectric is not implemented. Inputs surface_model="dielectric" and epsilon_r are rejected and are not aliases for the insulator calculation.

Insulator: retain charge at the hit location

insulator adds an absorbed macro-particle’s charge to the triangle it hit and performs no redistribution to other elements. Electrons leave negative charge and positive ions leave positive charge. A particle emitted from a surface leaves reaction charge of the opposite sign at its source.

This model represents charge accumulation on a discretized surface. It does not include:

• lateral surface conduction, finite-resistance relaxation, or leakage into the bulk;
• permittivity interface conditions, polarization charge, or the electric field inside the object;
• general secondary-electron emission or specular / diffuse particle reflection.

If those effects control the time evolution, do not interpret an insulator result as the long-time response of the real material.

Floating conductor: conserve charge and equalize potential

conductor treats elements with one mesh_id as a single floating object. After applying particle charge, it redistributes charge so element-centroid potentials are equal while preserving the object’s total charge. It is not a grounded, fixed-potential boundary.

The current implementation accepts only a free-space field and cannot be combined with periodic fields or an outer matching-plane response. For research use, verify convergence of object potential and surface-charge distribution under mesh refinement.

The linear system, P0-panel influence, parallel reduction, and conserved quantities are separated into surface-charge update numerics.

Difference from a photoelectron closure

surface_charge_closure="neutral_return" is not a material model that conducts charge across a surface. It is a source closure that assigns unresolved closed-photoelectron return to the destination distribution observed in the same batch. See the finite-image periodic2 configuration for its conditions and closed ledger.

Read the output

charge_C in charges.csv is total charge [C] on each triangle. Divide it by element area to obtain surface-charge density. tol_rel monitors the pre/post-batch change; it is not an automatic stopping condition in the current implementation.

See inspect output files for the species-resolved ledger including absorption, emission, and escape. See surface-charge update numerics for equations and implementation ordering.

Where to go next

• Choose a particle source: Choose where particles enter
• Test sensitivity to batch width: How to choose batch_duration
• Inspect photoelectron reaction charge and return: Photoelectron emission and lifecycle
• Search every key and constraint: Input parameters