Visualizing the simulation results

Output formats

WarpX can write data either in plotfile format (AMReX’s native format), or in openPMD format (a common data schema on top of HDF5 or ADIOS files for particle-in-cell codes).

Note

This is controlled by the parameters warpx.plot_int (AMReX) or the warpx.openpmd_int & warpx.openpmd_backend options in the section Input parameters.

Asynchronous IO

When using the AMReX plotfile format, users can set the amrex.async_out=1 option to perform the IO in a non-blocking fashion, meaning that the simulation will continue to run while an IO thread controls writing the data to disk. This can significantly reduce the overall time spent in IO. This is primarily intended for large runs on supercomputers such as Summit and Cori; depending on the MPI implentation you are using, you may not see a benefit on your workstation.

When writing plotfiles, each rank will write to a separate file, up to some maximum number (by default, 64). This maximum can be adjusted using the amrex.async_out_nfiles inputs parameter. To use asynchronous IO with than amrex.async_out_nfiles MPI ranks, WarpX WarpX must be compiled with the MPI_THREAD_MULTIPLE=TRUE flag. Please see Building/installing WarpX for building instructions.

This section describes some of the tools available to visualize the data:

• Visualization with yt (for plotfiles)
• Visualizing back-transformed diagnostics
• Reduced diagnostics
• Visualization with Visit (for plotfiles)
• PyQt-based visualization GUI: PICViewer (for both plotfiles and openPMD)
• Visualization with openPMD-viewer (for openPMD data)
• Advanced yt visualization, for developers (for plotfiles)
• Out-of-the-box plotting script

In addition, WarpX also has In-Situ Visualization capabilities (i.e. visualizing the data directly from the simulation, without dumping data files to disk).

• In situ Visualization with SENSEI
• In situ Visualization with Ascent

If you like the 3D rendering of laser wakefield acceleration on the WarpX documentation frontpage (which is also the avatar of the ECP-WarpX organization), you can find the serial analysis script video_yt.py as well as a parallel analysis script video_yt.py used to make a similar rendering for a beam-driven wakefield simulation, running parallel.

Warning: currently, quantities in the output file for iteration n are not all defined at the same physical time due to the staggering in time in WarpX. The table below provides the physical time at which each quantity in the output file is written, in units of time step, for time step n. “E part” means the electric field at each particle position.

quantity	staggering
E	n	n
B	n	n
j	n-1/2	n-1/2
rho	n	n
position	n	n
momentum	n-1/2	n-1/2
E part	n	n
B part	n	n