Data Analysis

Beam Monitor

ImpactX provides a zero-sized beam monitor element that can be placed in lattices to output the particle beam at multiple positions in a lattice. Output is written in the standardized, open particle-mesh data schema (openPMD) and is compatible with many codes and data analysis frameworks.

For data analysis of openPMD data, see examples of many supported tools, Python libraries and frameworks. Exporting data to ASCII is possible, too.

See also WarpX’ documentation on openPMD.

Additional Beam Attributes

We add the following additional attributes on the openPMD beam species at the monitor position.

Reference particle:

  • beta_ref reference particle normalized velocity \(\beta = v/c\)

  • gamma_ref reference particle Lorentz factor \(\gamma = 1/\sqrt{1-\beta^2}\)

  • s_ref integrated orbit path length, in meters

  • x_ref horizontal position x, in meters

  • y_ref vertical position y, in meters

  • z_ref longitudinal position z, in meters

  • t_ref clock time * c in meters

  • px_ref momentum in x, normalized to mass*c, \(p_x = \gamma \beta_x\)

  • py_ref momentum in y, normalized to mass*c, \(p_y = \gamma \beta_y\)

  • pz_ref momentum in z, normalized to mass*c, \(p_z = \gamma \beta_z\)

  • pt_ref energy, normalized by rest energy, \(p_t = -\gamma\)

  • mass reference rest mass, in kg

  • charge reference charge, in C

Example to print the integrated orbit path length s at each beam monitor position:

import openpmd_api as io

series = io.Series("diags/openPMD/monitor.h5", io.Access.read_only)

for k_i, i in series.iterations.items():
    beam = i.particles["beam"]
    s_ref = beam.get_attribute("s_ref")
    print(f"step {k_i:>3}: s_ref={s_ref}")

Reduced Beam Characteristics

ImpactX calculates reduced beam characteristics like averaged positions, momenta, beam emittances and Courant-Snyder (Twiss) parameters during runtime. These quantities are calculated before, after, and during each step of the simulation. If diag.slice_step_diagnostics is enabled, they will also be calculated during each slice of each beamline element.

The code writes out the values in an ASCII file prefixed reduced_beam_characteristics containing the follow columns:

  • step

    Iteration within the simulation

  • s, ref_beta_gamma

    Reference particle coordinate s (unit: meter) and relativistic momentum normalized by the particle mass and the speed of light (unit: dimensionless)

  • x_mean/min/max, y_mean/min/max, t_mean/min/max

    Average / minimum / maximum beam particle position in the dimensions of x, y (transverse coordinates, unit: meter), and t (normalized time difference \(ct\), unit: meter)

  • sig_x, sig_y, sig_t

    RMS of the average beam particle positions (unit: meter)

  • px_mean/min/max, py_mean/min/max, pt_mean/min/max

    Average / minimum / maximum beam momenta normalized by reference particle momentum (unit: dimensionless, radians for transverse momenta)

  • sig_px, sig_py, sig_pt

    RMS of the average beam momenta (energy difference for pt) (unit: dimensionless)

  • emittance_x, emittance_y, emittance_t

    Normalized beam emittance (unit: meter)

  • alpha_x, alpha_y, alpha_t

    Courant-Snyder (Twiss) alpha (unit: dimensionless)

  • beta_x, beta_y, beta_t

    Courant-Snyder (Twiss) beta (unit: meter)

  • charge

    Cumulated beam charge (unit: Coulomb)

Interactive Analysis

When steering ImpactX from Python, one can at any point visualize the beam phase space with:

import matplotlib.pyplot as plt

from impactx import ImpactX, RefPart, distribution, elements

sim = ImpactX()

# ... setup and simulate ...

pc = sim.particle_container()

fig = pc.plot_phasespace()

# note: figure data available on MPI rank zero
if fig is not None:
In situ visualization of the beam phase space projections.

Fig. 12 In situ visualization of the beam phase space projections.