Particle update

BEACH stores particle position and velocity at the same time level. Each step first constructs a next-time candidate, then commits the trajectory only as far as the earliest mesh or box event.

(xⁿ, vⁿ)
    │ sample the field once at a predicted midpoint
    ▼
Boris velocity update + trapezoidal position update
    │
    ▼
candidate (xⁿ⁺¹, vⁿ⁺¹)
    │ compare mesh hit and box-face crossing
    ├─ mesh first ─────── absorb
    ├─ open face first ── escape or scalar-barrier reflection
    └─ reflect/periodic ─ re-integrate the remaining time

Stage	Details
Field sample, Boris rotation, and position update	Boris particle update
Triangle collision, box faces, periodic images, and event ordering	Particle collision and boundary events
Absorption and element charge delta after an event	How surfaces charge
Escape or potential-barrier reflection at an open face	Choose where particles enter, Particle escape and return

State committed by one step

A step ends in a next-time state, surface absorption, box escape, or an incomplete status. Reflect, periodic, and scalar-barrier reflection keep the particle alive and advance the remaining time with the same update method.

For a mesh hit, the hit position and element index are committed and the candidate endpoint is discarded. The absorbed charge does not immediately change the field. It is accumulated in a thread-local element delta and committed at the end of the batch.

Step limit

A particle advances at most sim.max_step times while waiting for absorption or escape. A particle still unclassified at the limit is counted as survived_max_step, not silently reassigned to absorption or escape.

sim.dt is one particle-step interval, and sim.max_step * sim.dt is the maximum tracked time for one particle. batch_duration, in contrast, connects particle supply to surface-charge updates and is not the particle-step interval.

First checks

• halve sim.dt and check stability of trajectories, hit elements, and absorbed counts
• verify that survived_max_step is too small to affect the conclusion
• exercise trajectories crossing a periodic seam, corner, or post-reflection remainder
• reconcile absorbed, escaped, and unresolved particle counts with the charge ledger

See Configuration parameters for settings and Inspect Output Files for result categories.

Code reference

• Step candidates and event ordering: bem_particle_stepper.f90
• Batch loop tracking particles: bem_simulator_loop.f90
• Step regression tests: test_particle_stepper.f90