# A pulse leaves a standing wave

Channel: development; package: 2.0.0rc1; source: 2c32b7cf79b796fae825bfa08fc73d9914e61723

Page: https://remilab.ai/rfx/gallery/cavity-standing-wave/
Source: https://github.com/bk-squared/rfx/blob/2c32b7cf79b796fae825bfa08fc73d9914e61723/docs/public/gallery/cavity-standing-wave.mdx

**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.

![Video poster](https://remilab.ai/rfx/gallery/assets/current-visuals/cavity-standing-wave.png)

  [source](https://remilab.ai/rfx/gallery/assets/current-visuals/cavity-standing-wave.mp4)

  [Download the cavity movie](https://remilab.ai/rfx/gallery/assets/current-visuals/cavity-standing-wave.mp4)

## 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](https://remilab.ai/rfx/gallery/boundary-reflection/#run-this-example), then:

```bash
JAX_PLATFORMS=cpu PYTHONPATH=. python field_movies.py \
  --case cavity-standing-wave --output field-movies
```

Change 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](https://github.com/bk-squared/rfx/blob/a9054338616b91bb1364699882c2530fa29a0048/examples/tutorials/resonance_harminv.py). 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](https://remilab.ai/rfx/gallery/assets/current-visuals/cavity-standing-wave.csv)
- [Selected field frames and all probe samples, NPZ](https://remilab.ai/rfx/gallery/assets/current-visuals/cavity-standing-wave.npz)
- [Preflight, source settings, normalization and checksums, JSON](https://remilab.ai/rfx/gallery/assets/current-visuals/cavity-standing-wave.json)
- [Complete runnable generator](https://remilab.ai/rfx/gallery/assets/current-visuals/field_movies.py)

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](https://remilab.ai/rfx/gallery/boundary-reflection/).