Phase Space

This plugin creates a 2D phase space image for a user-given spatial and momentum coordinate.

External Dependencies

The plugin is available as soon as the libSplash and HDF5 libraries are compiled in.

.cfg file

Example for y-pz phase space for the electron species (.cfg file macro):

# Calculate a 2D phase space
# - momentum range in m_e c
TGB_ePSypz="--e_phaseSpace.period 10 --e_phaseSpace.filter all --e_phaseSpace.space y --e_phaseSpace.momentum pz --e_phaseSpace.min -1.0 --e_phaseSpace.max 1.0"

The distinct options are (assuming a species e for electrons):

Option

Usage Unit

--e_phaseSpace.period <N>

calculate each N steps

none

--e_phaseSpace.filter

Use filtered particles. Available filters are set up in particleFilters.param.

none

--e_phaseSpace.space <x/y/z>

spatial coordinate of the 2D phase space

none

--e_phaseSpace.momentum <px/py/pz>

momentum coordinate of the 2D phase space

none

--e_phaseSpace.min <ValL>

minimum of the momentum range

\(m_\mathrm{species} c\)

--e_phaseSpace.max <ValR>

maximum of the momentum range

\(m_\mathrm{species} c\)

Memory Complexity

Accelerator

locally, a counter matrix of the size local-cells of space direction times 1024 (for momentum bins) is permanently allocated.

Host

negligible.

Output

The 2D histograms are stored in .hdf5 files in the simOutput/phaseSpace/ directory. A file is created per species, phasespace selection and time step.

Values are given as charge density per phase space bin. In order to scale to a simpler charge of particles per \(\mathrm{d}r_i\) and \(\mathrm{d}p_i\) -bin multiply by the cell volume dV.

Analysis Tools

Data Reader

You can quickly load and interact with the data in Python with:

from picongpu.plugins.data import PhaseSpaceData
import numpy as np


ps_data = PhaseSpaceData('/home/axel/runs/lwfa_001')
# show available iterations
ps_data.get_iterations(ps="xpx", species="e", species_filter='all')

# show available simulation times
ps_data.get_times(ps="xpx", species="e", species_filter='all')

# load data for a given iteration
ps, meta = ps_data.get(ps='ypy', species='e', species_filter='all', iteration=2000)

# unit conversion from SI
mu = 1.e6  # meters to microns
e_mc_r = 1. / (9.109e-31 * 2.9979e8)  # electrons: kg * m / s to beta * gamma

Q_dr_dp = np.abs(ps) * meta.dV  # C s kg^-1 m^-2
extent = meta.extent * [mu, mu, e_mc_r, e_mc_r]  # spatial: microns, momentum: beta*gamma

# load data for a given time
ps, ps_meta = ps_data.get(ps="xpx", species="e", species_filter='all', time=1.3900e-14)

# load data for multiple iterations
ret = ps_data.get(ps="xpx", species="e", species_filter='all', iteration=[2000, 4000])

# data and metadata for iteration 2000
# (data is in same order as the value passed to the 'iteration' parameter)
ps, meta = ret[0]

Note that the spatial extent of the output over time might change when running a moving window simulation.

Matplotlib Visualizer

You can quickly plot the data in Python with:

from picongpu.plugins.plot_mpl import PhaseSpaceMPL
import matplotlib.pyplot as plt


# create a figure and axes
fig, ax = plt.subplots(1, 1)

# create the visualizer
ps_vis = PhaseSpaceMPL('path/to/run_dir', ax)

# plot
ps_vis.visualize(ps="xpx", iteration=200, species='e', species_filter='all')

plt.show()

# specifying simulation time is also possible (granted there is a matching iteration for that time)
ps_vis.visualize(ps="xpx", time=2.6410e-13, species='e', species_filter='all')

plt.show()

# plotting data for multiple simulations simultaneously also works:
ps_vis = PhaseSpaceMPL([
     ("sim1", "path/to/sim1"),
     ("sim2", "path/to/sim2"),
     ("sim3", "path/to/sim3")], ax)
ps_vis.visualize(ps="xpx", iteration=10000, species="e", species_filter='all')

plt.show()

The visualizer can also be used from the command line (for a single simulation only) by writing

python phase_space_visualizer.py

with the following command line options

Options

Value

-p

Path and filename to the run directory of a simulation.

-i

An iteration number

-s (optional, defaults to ‘e’)

Particle species abbreviation (e.g. ‘e’ for electrons)

-f (optional, defaults to ‘all’)

Species filter string

-m (optional, defaults to ‘ypy’)

Momentum string to specify the phase space

Jupyter Widget

If you want more interactive visualization, then start a jupyter notebook and make sure that ipywidgets and ìpympl are installed.

After starting the notebook server write the following

# this is required!
%matplotlib widget
import matplotlib.pyplot as plt
plt.ioff()

from IPython.display import display
from picongpu.plugins.jupyter_widgets import PhaseSpaceWidget

# provide the paths to the simulations you want to be able to choose from
# together with labels that will be used in the plot legends so you still know
# which data belongs to which simulation
w = PhaseSpaceWidget(run_dir_options=[
        ("scan1/sim4", scan1_sim4),
        ("scan1/sim5", scan1_sim5)])
display(w)

and then interact with the displayed widgets.

Out-of-Range Behavior

Particles that are not in the range of <ValL>/<ValR> get automatically mapped to the lowest/highest bin respectively. Take care about that when setting your range and during analysis of the results.

Known Limitations

  • only one range per selected space-momentum-pair possible right now (naming collisions)

  • charge deposition uses the counter shape for now (would need one more write to neighbors to evaluate it correctly according to the shape)

  • the user has to define the momentum range in advance

  • the resolution is fixed to 1024 bins in momentum and the number of cells in the selected spatial dimension

  • this plugin does not yet use openPMD markup.

References

The internal algorithm is explained in pull request #347 and in [Huebl2014].

Huebl2014

A. Huebl. Injection Control for Electrons in Laser-Driven Plasma Wakes on the Femtosecond Time Scale, chapter 3.2, Diploma Thesis at TU Dresden & Helmholtz-Zentrum Dresden - Rossendorf for the German Degree “Diplom-Physiker” (2014), https://doi.org/10.5281/zenodo.15924