Shadowgraphy

Computes a 2D image by time-integrating the Poynting-Vectors in a fixed plane in the simulation. This can be used to extract a laser from an simulation, which obtains the full laser-plasma interactions through the PIC code. If the probe laser propagates through plasma structures, the plasma structures lead to modulations in the probe laser’s intensity, resulting in a synthetic shadowgram of the plasma structures. The plugin performs the time-integration of the probe laser and the application of various masks on the probe pulse in Fourier space. Thus, one needs to manually add the probe pulse to the simulation with e.g. the incident field param files. Since the plugin currently only works in the xy-plane, the probe pulse should propagate in z direction. The integration plane for the plugin must lie within the simulation volume and the position of the field absorber should be considered when placing the plugin plane.

External Dependencies

The plugin is available as soon as the FFWT3 is compiled in.

Usage

Command line option

Description

--shadowgraphy.start

Step to start the plugin and with this the time integration.

--shadowgraphy.duration

Duration of shadowgraphy calculation in simulation time steps. The plugin will be called duration / params::tRes times. The final plugin call (and the file output) happens at the time defined in .period plus the duration.

--shadowgraphy.slicePoint

Specifies at what ratio of the total depth of the z dimension, the slice for the field extraction should be set. The value given should lie between 0.0 and 1.0.

--shadowgraphy.focusPos

Focus position of lens system relative to slicePoint. The focus position is given in SI units.

--shadowgraphy.fileName

Output file prefix for openPMD output.

--shadowgraphy.ext

Backend for openPMD output.

--shadowgraphy.fourierOutput

If enabled, the fields will also be stored on disk in in (x, y, \omega) Fourier space in an openPMD file.

Note

Currently the plugin only supports an integration slice in the xy-plane, which means that for probing setups the probe pulse should propagate in z direction. The moving window can’t be activated or deactivated during the plugin integration loop.

Reqired param file options

Description

tRes

Use the fields at each tRes’th time-step of simulation

xRes

Use each xRes’th field value in x direction

yRes

Use each yRes’th field value in y direction

omegaWfMin

Minimum non-zero value for abs(omega)

omegaWfMax

Maximum non-zero value for abs(omega)

masks::positionWf

Mask that is multiplied to E and B field when gathering the slice

masks::timeWf

Mask that is multiplied to E and B field during the time integration

masks::maskFourier

Mask that is multiplied to E and B field in Fourier domain

Output

Plot the first shadowgram that is stored in the simulation output directory simOutput.

import matplotlib.pyplot as plt
import numpy as np
import openpmd_api as opmd


def load_shadowgram(series):
   i = series.iterations[[i for i in series.iterations][0]]

   shadowgram_tmp = i.meshes["shadowgram"][opmd.Mesh_Record_Component.SCALAR].load_chunk()
   unit = i.meshes["shadowgram"].get_attribute("unitSI")
   series.flush()

   return shadowgram_tmp * unit


def load_meshgrids(series):
   i = series.iterations[[i for i in series.iterations][0]]

   xspace_tmp = i.meshes["Spatial positions"]["x"].load_chunk()
   xunit = i.meshes["Spatial positions"]["x"].get_attribute("unitSI")
   series.flush()
   xspace = xspace_tmp * xunit

   yspace_tmp = i.meshes["Spatial positions"]["y"].load_chunk()
   yunit = i.meshes["Spatial positions"]["y"].get_attribute("unitSI")
   series.flush()
   yspace = yspace_tmp * yunit

   return np.meshgrid(xspace, yspace)


path = "PATH/TO/simOutput"

series = opmd.Series(path + "/shadowgraphy_" + "%T." + "bp5", opmd.Access.read_only)
shadowgram = load_shadowgram(series)
xm, ym = load_meshgrids(series)
series.close()

fig, ax = plt.subplots(figsize=(10, 10))
ax.pcolormesh(xm, ym, shadowgram)
ax.set_aspect("equal")

Shadowgram Size and Moving Window

The size of the pixels is the size of the cells in the simulation divided by the resolution in the plugin CELL_WIDTH_SI * shadowgraphy::params::xRes and CELL_HEIGHT_SI * shadowgraphy::params::yRes. The shadowgram itself does not include cells that lie outside of the field absorber in both x and y direction. When the moving window is activated, the resulting shadowgram is smaller in moving window propagation direction y. The size difference is equal to the speed of light times the time it would take for light to propagate from the -z border of the simulation box to the plugin integration plane plus the integration duration. This prevents artifacts from the laser being cut off due to the moving window or the laser not fully being propagated through the plasma structures.

References