# Sources (1D FDTD)

A **source** introduces an excitation into the electromagnetic field. In FDTD OpenMP, registered functions add values to $E_z$ after the electric field update in each time step. Without an initial field or a source, a simulation starting from zero remains at zero.

The command-line program provides a [[fdtd-openmp-gaussian-pulse|Gaussian pulse]] and a [[fdtd-openmp-sinusoidal-wave|sinusoidal wave]]. Both default to the middle of the grid. Their parameters can be supplied explicitly:

```bash
--add-pulse=100,40,12
--add-wave=100,1,0.1,0
```

The pulse arguments select the cell, peak step, and width. The wave arguments select the cell, amplitude, frequency, and phase. Multiple source options register multiple excitations.

Injection through `Fdtd_EzSimInjectEz()` is additive, so sources at the same cell contribute to the existing field. It does not replace the field with the waveform value.

The library accepts custom [[fdtd-openmp-source-callbacks|source callbacks]] too. Each receives the simulation and its registered parameters. The callbacks run sequentially in both kernels; the OpenMP kernel assigns them to one thread before all threads proceed to the next step.