Current-run teaching example · actual FDTD snapshots · 2D TMz · single mesh
A closed conducting boundary traps the field. A carrier-centered pulse excites the rectangle; after the drive becomes quiet, its field pattern continues to change sign with nearly fixed nodal lines. Watch the entire domain, then follow the green probe in the trace below.
From geometry to field
| Model detail | Value |
|---|---|
| Cross-section | 80 × 60 mm vacuum rectangle with PEC walls |
| Solver | Uniform Yee, 2D TMz, 1 mm cells |
| Excitation | ModulatedGaussian current source at the center, bandwidth 0.35 |
| Carrier choice | Analytic lowest TMz cavity mode: c/2 × sqrt(1/a² + 1/b²) |
| Probe | Ez at (55, 30) mm |
| Recording | 1,600 steps, one actual snapshot every 8 steps |
The analytic expression chooses the source carrier; it is not a resonance frequency extracted from this run. A point source may excite other modes. This movie does not prove a pure mode or measure a frequency error.
The color scale is fixed through the movie: signed Ez / Eref, where Eref
is the largest absolute recorded field outside a 4 mm source neighborhood.
Source pixels can saturate. Coordinates and frame times come directly from
Result.snapshot_axes; these are time-stepped fields, not a harmonic phasor
animated afterward.
Reproduce and change it
Use the pinned solver installation and generator download in the boundary example, then:
JAX_PLATFORMS=cpu PYTHONPATH=. python field_movies.py \ --case cavity-standing-wave --output field-moviesChange the rectangle’s aspect ratio to change its field pattern and source carrier together. Moving the source away from the center changes which modes it excites. Recalculate the model after either edit.
For a follow-up calculation that actually extracts modal frequencies, read the resonance tutorial. Do not estimate a physical Q from the finite window here: vacuum and PEC are lossless, so this is not a lossy ring-down measurement.
Data and scope
- Probe CSV, seconds and V/m
- Selected field frames and all probe samples, NPZ
- Preflight, source settings, normalization and checksums, JSON
- Complete runnable generator
The download retains every 16th actual movie frame with its native spatial
coordinates and time stamps. Local regeneration retains all frames. Preflight
reported no findings for this vacuum model. Solver source: a9054338; full SHA,
generator hash and CPU environment are in the manifest.
This is an introductory 2D illustration with one mesh and one record length. Finite output and correct snapshot axes were checked. Mesh convergence, modal frequency accuracy and physical Q were not measured. For the distinction between open and closed boundaries, compare the pulse reflection example.