# Let a pulse leave, or reflect it back

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

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

**Current-run teaching example · actual FDTD snapshots · 2D TMz · single mesh**

An absorbing boundary represents an open environment. A conducting wall sends
waves back into the model. Start the movie and watch the same pulse in both:
the open box becomes quiet while reflections keep crossing the closed box.

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

  [source](https://remilab.ai/rfx/gallery/assets/current-visuals/boundary-reflection.mp4)

  [Download the field movie](https://remilab.ai/rfx/gallery/assets/current-visuals/boundary-reflection.mp4)

The lower plot is the electric field at the green probe. The vertical cursor
marks the exact simulation time of the frame above. It follows stored field
samples; playback does not run a simulation in your browser.

## Read the model

| Model detail | Value |
|---|---|
| Cross-section | 80 × 60 mm vacuum domain |
| Solver | Uniform Yee, 2D TMz, 1 mm cells |
| Source | Differentiated Gaussian current pulse at (20, 30) mm; `f0=6 GHz`, `bandwidth=0.8` |
| Probe | `Ez` at (55, 30) mm |
| Left boundary | CPML on the outer edges, 16 absorber layers outside the displayed domain |
| Right boundary | PEC on the outer edges |
| Recording | 640 steps, one actual snapshot every 4 steps |

The colors show signed `Ez / Eref`, with one fixed linear scale for both models
and all frames. `Eref` is the largest absolute recorded interior field outside
a 4 mm radius around the current source. Source pixels can saturate; the raw
values remain in the download. This is a 2D field cross-section, not a finite
3D antenna or cavity.

## Run this example

Download the [complete generator](https://remilab.ai/rfx/gallery/assets/current-visuals/field_movies.py).
It uses `Simulation`, `SnapshotSpec`, and the physical coordinates and frame
times in `Result.snapshot_axes`. It also generates the
[dielectric interface](https://remilab.ai/rfx/gallery/dielectric-interface/) and
[cavity standing wave](https://remilab.ai/rfx/gallery/cavity-standing-wave/).

For the solver version used here:

```bash
git clone https://github.com/bk-squared/rfx.git
cd rfx
git checkout a9054338616b91bb1364699882c2530fa29a0048
python -m pip install -e .
curl -fLO https://remilab.ai/rfx/gallery/assets/current-visuals/field_movies.py
# Install ffmpeg using your operating system's package manager.
JAX_PLATFORMS=cpu PYTHONPATH=. python field_movies.py \
  --case boundary-reflection --output field-movies
```

The local output contains the full field arrays, probe CSV, poster, MP4 and
manifest. `--render-only` redraws these saved arrays without solving again.
The script prints preflight findings before each run and refuses preflight
errors. These two vacuum models returned no preflight findings.

## Download and explore

- [Probe CSV, in seconds and V/m](https://remilab.ai/rfx/gallery/assets/current-visuals/boundary-reflection.csv)
- [Selected field arrays and all probe samples, NPZ](https://remilab.ai/rfx/gallery/assets/current-visuals/boundary-reflection.npz)
- [Model, normalization, preflight and checksums, JSON](https://remilab.ai/rfx/gallery/assets/current-visuals/boundary-reflection.json)

The small NPZ contains every 16th movie frame at full spatial resolution,
with its actual coordinate arrays, step numbers and time stamps. Re-running
the generator retains every movie frame locally. Load with
`numpy.load(path, allow_pickle=False)`; fields end in `_ez_V_per_m`, coordinates
in `_x_m`/`_y_m`, and times in `_frame_times_s`.

Try moving the source or changing the domain size, then rerun. Compare when the
first reflected wave reaches the same probe. CPML is a finite numerical
absorber and can produce residual reflections; this example does not measure
its reflection coefficient or an absorption percentage.

## Evidence and limits

Generated on the CPU with RFX solver source `a9054338`. The manifest records
the full source commit, generator hash and environment. The checks confirm
finite fields, recorded frame steps and realized display coordinates. No mesh
refinement or accuracy convergence was performed, and no S-parameter, decay
rate, or performance claim follows from the movie.

Continue with [boundary choices](https://remilab.ai/rfx/support/) or the
[API reference](https://remilab.ai/rfx/api/).