Generation of beam distributions

The following examples are tests of beam initialization for distributions of various types.

Note

Please consult Beam Distribution Input for our conventions on beam distribution input. We currently offer two different methods:

  1. Phase space ellipse axis intersections (ImpactX native)

  2. Courant-Snyder (Twiss) parameters

For illustration purposes, some of the following examples use Twiss initialization while some use the ImpactX native initialization. Both are equivalent.

In each example, we use a 2 GeV electron beam with initial unnormalized rms emittance of 2 nm.

The matched Twiss parameters are the same as those used in the FODO example:

  • \(\beta_\mathrm{x} = 2.82161941\) m

  • \(\alpha_\mathrm{x} = -1.59050035\)

  • \(\beta_\mathrm{y} = 2.82161941\) m

  • \(\alpha_\mathrm{y} = 1.59050035\)

The second moments of the particle distribution after the FODO cell should coincide with the second moments of the particle distribution before the FODO cell, to within the level expected due to noise due to statistical sampling.

In this test, the initial and final values of \(\sigma_x\), \(\sigma_y\), \(\sigma_t\), \(\epsilon_x\), \(\epsilon_y\), and \(\epsilon_t\) must agree with nominal values.

A 6d Gaussian distribution from Twiss functions

A Gaussian distribution in all 6 phase space variables, initialized using Courant-Snyder (Twiss) functions.

In this test, the initial and final values of \(\sigma_x\), \(\sigma_y\), \(\sigma_t\), \(\epsilon_x\), \(\epsilon_y\), and \(\epsilon_t\) must agree with nominal values.

Run

This example can be run either as:

  • Python script: python3 run_gaussian_twiss.py or

  • ImpactX executable using an input file: impactx input_gaussian_twiss.in

For MPI-parallel runs, prefix these lines with mpiexec -n 4 ... or srun -n 4 ..., depending on the system.

Listing 180 You can copy this file from examples/distgen/run_gaussian_twiss.py.
#!/usr/bin/env python3
#
# Copyright 2022-2024 ImpactX contributors
# Authors: Marco Garten, Axel Huebl, CHad Mitchell
# License: BSD-3-Clause-LBNL
#
# -*- coding: utf-8 -*-

from impactx import ImpactX, distribution, elements, twiss

sim = ImpactX()

# set numerical parameters and IO control
sim.space_charge = False
# sim.diagnostics = False  # benchmarking
sim.slice_step_diagnostics = True

# domain decomposition & space charge mesh
sim.init_grids()

# load a 2 GeV electron beam with an initial
# unnormalized rms emittance of 2 nm
kin_energy_MeV = 2.0e3  # reference energy
bunch_charge_C = 1.0e-9  # used with space charge
npart = 10000  # number of macro particles

#   reference particle
ref = sim.particle_container().ref_particle()
ref.set_charge_qe(-1.0).set_mass_MeV(0.510998950).set_kin_energy_MeV(kin_energy_MeV)

#   particle bunch
distr = distribution.Gaussian(
    **twiss(
        beta_x=2.8216194100262637,
        beta_y=2.8216194100262637,
        beta_t=0.5,
        emitt_x=2e-09,
        emitt_y=2e-09,
        emitt_t=2e-06,
        alpha_x=-1.5905003499999992,
        alpha_y=1.5905003499999992,
        alpha_t=0.0,
    )
)
sim.add_particles(bunch_charge_C, distr, npart)

# add beam diagnostics
monitor = elements.BeamMonitor("monitor", backend="h5")

# design the accelerator lattice)
ns = 25  # number of slices per ds in the element
fodo = [
    monitor,
    elements.Drift(name="d1", ds=0.25, nslice=ns),
    monitor,
    elements.Quad(name="q1", ds=1.0, k=1.0, nslice=ns),
    monitor,
    elements.Drift(name="d2", ds=0.5, nslice=ns),
    monitor,
    elements.Quad(name="q2", ds=1.0, k=-1.0, nslice=ns),
    monitor,
    elements.Drift(name="d3", ds=0.25, nslice=ns),
    monitor,
]
# assign a fodo segment
sim.lattice.extend(fodo)

# run simulation
sim.track_particles()

# clean shutdown
sim.finalize()
Listing 181 You can copy this file from examples/distgen/input_gaussian_twiss.in.
###############################################################################
# Particle Beam(s)
###############################################################################
beam.npart = 10000
beam.units = static
beam.kin_energy = 2.0e3
beam.charge = 1.0e-9
beam.particle = electron
beam.distribution = gaussian_from_twiss
beam.alphaX = -1.5905003499999992
beam.alphaY = 1.5905003499999992
beam.alphaT = 0.0
beam.betaX = 2.8216194100262637
beam.betaY = beam.betaX
beam.betaT = 0.5
beam.emittX = 2e-09
beam.emittY = beam.emittX
beam.emittT = 2e-06


###############################################################################
# Beamline: lattice elements and segments
###############################################################################
lattice.elements = monitor drift1 quad1 drift2 quad2 drift3 monitor

monitor.type = beam_monitor
monitor.backend = h5

drift1.type = drift
drift1.ds = 0.25

quad1.type = quad
quad1.ds = 1.0
quad1.k = 1.0

drift2.type = drift
drift2.ds = 0.5

quad2.type = quad
quad2.ds = 1.0
quad2.k = -1.0

drift3.type = drift
drift3.ds = 0.25


###############################################################################
# Algorithms
###############################################################################
algo.space_charge = false

Analyze

We run the following script to analyze correctness:

Script analysis_gaussian.py
Listing 182 You can copy this file from examples/distgen/analysis_gaussian.py.
#!/usr/bin/env python3
#
# Copyright 2022-2023 ImpactX contributors
# Authors: Axel Huebl, Chad Mitchell
# License: BSD-3-Clause-LBNL
#

import numpy as np
import openpmd_api as io
from scipy.stats import moment


def get_moments(beam):
    """Calculate standard deviations of beam position & momenta
    and emittance values

    Returns
    -------
    sigx, sigy, sigt, emittance_x, emittance_y, emittance_t
    """
    sigx = moment(beam["position_x"], moment=2) ** 0.5  # variance -> std dev.
    sigpx = moment(beam["momentum_x"], moment=2) ** 0.5
    sigy = moment(beam["position_y"], moment=2) ** 0.5
    sigpy = moment(beam["momentum_y"], moment=2) ** 0.5
    sigt = moment(beam["position_t"], moment=2) ** 0.5
    sigpt = moment(beam["momentum_t"], moment=2) ** 0.5

    epstrms = beam.cov(ddof=0)
    emittance_x = (sigx**2 * sigpx**2 - epstrms["position_x"]["momentum_x"] ** 2) ** 0.5
    emittance_y = (sigy**2 * sigpy**2 - epstrms["position_y"]["momentum_y"] ** 2) ** 0.5
    emittance_t = (sigt**2 * sigpt**2 - epstrms["position_t"]["momentum_t"] ** 2) ** 0.5

    return (sigx, sigy, sigt, emittance_x, emittance_y, emittance_t)


# initial/final beam
series = io.Series("diags/openPMD/monitor.h5", io.Access.read_only)
last_step = list(series.iterations)[-1]
initial = series.iterations[1].particles["beam"].to_df()
final = series.iterations[last_step].particles["beam"].to_df()

# compare number of particles
num_particles = 10000
assert num_particles == len(initial)
assert num_particles == len(final)

print("Initial Beam:")
sigx, sigy, sigt, emittance_x, emittance_y, emittance_t = get_moments(initial)
print(f"  sigx={sigx:e} sigy={sigy:e} sigt={sigt:e}")
print(
    f"  emittance_x={emittance_x:e} emittance_y={emittance_y:e} emittance_t={emittance_t:e}"
)

atol = 0.0  # ignored
rtol = 2.3 * num_particles**-0.5  # from random sampling of a smooth distribution
print(f"  rtol={rtol} (ignored: atol~={atol})")

assert np.allclose(
    [sigx, sigy, sigt, emittance_x, emittance_y, emittance_t],
    [
        7.5451170454175073e-005,
        7.5441588239210947e-005,
        9.9775878164077539e-004,
        1.9959540393751392e-009,
        2.0175015289132990e-009,
        2.0013820193294972e-006,
    ],
    rtol=rtol,
    atol=atol,
)


print("")
print("Final Beam:")
sigx, sigy, sigt, emittance_x, emittance_y, emittance_t = get_moments(final)
print(f"  sigx={sigx:e} sigy={sigy:e} sigt={sigt:e}")
print(
    f"  emittance_x={emittance_x:e} emittance_y={emittance_y:e} emittance_t={emittance_t:e}"
)

atol = 0.0  # ignored
rtol = 2.3 * num_particles**-0.5  # from random sampling of a smooth distribution
print(f"  rtol={rtol} (ignored: atol~={atol})")

assert np.allclose(
    [sigx, sigy, sigt, emittance_x, emittance_y, emittance_t],
    [
        7.4790118496224206e-005,
        7.5357525169680140e-005,
        9.9775879288128088e-004,
        1.9959539836392703e-009,
        2.0175014668882125e-009,
        2.0013820380883801e-006,
    ],
    rtol=rtol,
    atol=atol,
)

A K-V distribution initialized from Twiss functions

This example is initialized using Courant-Snyder (Twiss) functions.

In this test, the initial and final values of \(\sigma_x\), \(\sigma_y\), \(\sigma_t\), \(\epsilon_x\), \(\epsilon_y\), and \(\epsilon_t\) must agree with nominal values.

Run

This example can be run either as:

  • Python script: python3 run_kvdist_twiss.py or

  • ImpactX executable using an input file: impactx input_kvdist_twiss.in

For MPI-parallel runs, prefix these lines with mpiexec -n 4 ... or srun -n 4 ..., depending on the system.

Listing 183 You can copy this file from examples/distgen/run_kvdist_twiss.py.
#!/usr/bin/env python3
#
# Copyright 2022-2024 ImpactX contributors
# Authors: Marco Garten, Axel Huebl, CHad Mitchell
# License: BSD-3-Clause-LBNL
#
# -*- coding: utf-8 -*-

from impactx import ImpactX, distribution, elements, twiss

sim = ImpactX()

# set numerical parameters and IO control
sim.space_charge = False
# sim.diagnostics = False  # benchmarking
sim.slice_step_diagnostics = True

# domain decomposition & space charge mesh
sim.init_grids()

# load a 2 GeV electron beam with an initial
# unnormalized rms emittance of 2 nm
kin_energy_MeV = 2.0e3  # reference energy
bunch_charge_C = 1.0e-9  # used with space charge
npart = 10000  # number of macro particles

#   reference particle
ref = sim.particle_container().ref_particle()
ref.set_charge_qe(-1.0).set_mass_MeV(0.510998950).set_kin_energy_MeV(kin_energy_MeV)

#   particle bunch
distr = distribution.KVdist(
    **twiss(
        beta_x=2.8216194100262637,
        beta_y=2.8216194100262637,
        beta_t=0.5,
        emitt_x=2e-09,
        emitt_y=2e-09,
        emitt_t=2e-06,
        alpha_x=-1.5905003499999992,
        alpha_y=1.5905003499999992,
        alpha_t=0.0,
    )
)
sim.add_particles(bunch_charge_C, distr, npart)

# add beam diagnostics
monitor = elements.BeamMonitor("monitor", backend="h5")

# design the accelerator lattice)
ns = 25  # number of slices per ds in the element
fodo = [
    monitor,
    elements.Drift(name="d1", ds=0.25, nslice=ns),
    monitor,
    elements.Quad(name="q1", ds=1.0, k=1.0, nslice=ns),
    monitor,
    elements.Drift(name="d2", ds=0.5, nslice=ns),
    monitor,
    elements.Quad(name="q2", ds=1.0, k=-1.0, nslice=ns),
    monitor,
    elements.Drift(name="d3", ds=0.25, nslice=ns),
    monitor,
]
# assign a fodo segment
sim.lattice.extend(fodo)

# run simulation
sim.track_particles()

# clean shutdown
sim.finalize()
Listing 184 You can copy this file from examples/distgen/input_kvdist_twiss.in.
###############################################################################
# Particle Beam(s)
###############################################################################
beam.npart = 10000
beam.units = static
beam.kin_energy = 2.0e3
beam.charge = 1.0e-9
beam.particle = electron
beam.distribution = kvdist_from_twiss
beam.alphaX = -1.5905003499999992
beam.alphaY = -beam.alphaX
beam.alphaT = 0.0
beam.betaX = 2.8216194100262637
beam.betaY = beam.betaX
beam.betaT = 0.5
beam.emittX = 2e-09
beam.emittY = beam.emittX
beam.emittT = 2e-06


###############################################################################
# Beamline: lattice elements and segments
###############################################################################
lattice.elements = monitor drift1 quad1 drift2 quad2 drift3 monitor

monitor.type = beam_monitor
monitor.backend = h5

drift1.type = drift
drift1.ds = 0.25

quad1.type = quad
quad1.ds = 1.0
quad1.k = 1.0

drift2.type = drift
drift2.ds = 0.5

quad2.type = quad
quad2.ds = 1.0
quad2.k = -1.0

drift3.type = drift
drift3.ds = 0.25


###############################################################################
# Algorithms
###############################################################################
algo.space_charge = false

Analyze

We run the following script to analyze correctness:

Script analysis_kvdist.py
Listing 185 You can copy this file from examples/distgen/analysis_kvdist.py.
#!/usr/bin/env python3
#
# Copyright 2022-2023 ImpactX contributors
# Authors: Axel Huebl, Chad Mitchell
# License: BSD-3-Clause-LBNL
#

import numpy as np
import openpmd_api as io
from scipy.stats import moment


def get_moments(beam):
    """Calculate standard deviations of beam position & momenta
    and emittance values

    Returns
    -------
    sigx, sigy, sigt, emittance_x, emittance_y, emittance_t
    """
    sigx = moment(beam["position_x"], moment=2) ** 0.5  # variance -> std dev.
    sigpx = moment(beam["momentum_x"], moment=2) ** 0.5
    sigy = moment(beam["position_y"], moment=2) ** 0.5
    sigpy = moment(beam["momentum_y"], moment=2) ** 0.5
    sigt = moment(beam["position_t"], moment=2) ** 0.5
    sigpt = moment(beam["momentum_t"], moment=2) ** 0.5

    epstrms = beam.cov(ddof=0)
    emittance_x = (sigx**2 * sigpx**2 - epstrms["position_x"]["momentum_x"] ** 2) ** 0.5
    emittance_y = (sigy**2 * sigpy**2 - epstrms["position_y"]["momentum_y"] ** 2) ** 0.5
    emittance_t = (sigt**2 * sigpt**2 - epstrms["position_t"]["momentum_t"] ** 2) ** 0.5

    return (sigx, sigy, sigt, emittance_x, emittance_y, emittance_t)


# initial/final beam
series = io.Series("diags/openPMD/monitor.h5", io.Access.read_only)
last_step = list(series.iterations)[-1]
initial = series.iterations[1].particles["beam"].to_df()
final = series.iterations[last_step].particles["beam"].to_df()

# compare number of particles
num_particles = 10000
assert num_particles == len(initial)
assert num_particles == len(final)

print("Initial Beam:")
sigx, sigy, sigt, emittance_x, emittance_y, emittance_t = get_moments(initial)
print(f"  sigx={sigx:e} sigy={sigy:e} sigt={sigt:e}")
print(
    f"  emittance_x={emittance_x:e} emittance_y={emittance_y:e} emittance_t={emittance_t:e}"
)

atol = 0.0  # a big number
rtol = 1.5 * num_particles**-0.5  # from random sampling of a smooth distribution
print(f"  rtol={rtol} (ignored: atol~={atol})")

assert np.allclose(
    [sigx, sigy, sigt, emittance_x, emittance_y, emittance_t],
    [
        7.5451170454175073e-005,
        7.5441588239210947e-005,
        9.9775878164077539e-004,
        1.9959540393751392e-009,
        2.0175015289132990e-009,
        2.0013820193294972e-006,
    ],
    rtol=rtol,
    atol=atol,
)


print("")
print("Final Beam:")
sigx, sigy, sigt, emittance_x, emittance_y, emittance_t = get_moments(final)
print(f"  sigx={sigx:e} sigy={sigy:e} sigt={sigt:e}")
print(
    f"  emittance_x={emittance_x:e} emittance_y={emittance_y:e} emittance_t={emittance_t:e}"
)

atol = 0.0  # a big number
rtol = 1.5 * num_particles**-0.5  # from random sampling of a smooth distribution
print(f"  rtol={rtol} (ignored: atol~={atol})")

assert np.allclose(
    [sigx, sigy, sigt, emittance_x, emittance_y, emittance_t],
    [
        7.4790118496224206e-005,
        7.5357525169680140e-005,
        9.9775879288128088e-004,
        1.9959539836392703e-009,
        2.0175014668882125e-009,
        2.0013820380883801e-006,
    ],
    rtol=rtol,
    atol=atol,
)

A 4D Kurth Distribution

A 4D Kurth distribution in the transverse phase space variables ( + a longitudinally uniform distribution in \(t\) + a Gaussian distribution in \(p_t\) ).

In this test, the initial and final values of \(\sigma_x\), \(\sigma_y\), \(\sigma_t\), \(\epsilon_x\), \(\epsilon_y\), and \(\epsilon_t\) must agree with nominal values.

Run

This example can be run either as:

  • Python script: python3 run_kurth4d.py or

  • ImpactX executable using an input file: impactx input_kurth4d.in

For MPI-parallel runs, prefix these lines with mpiexec -n 4 ... or srun -n 4 ..., depending on the system.

Listing 186 You can copy this file from examples/distgen/run_kurth4d.py.
#!/usr/bin/env python3
#
# Copyright 2022-2023 ImpactX contributors
# Authors: Axel Huebl, Chad Mitchell
# License: BSD-3-Clause-LBNL
#
# -*- coding: utf-8 -*-

from impactx import ImpactX, distribution, elements

sim = ImpactX()

# set numerical parameters and IO control
sim.space_charge = False
# sim.diagnostics = False  # benchmarking
sim.slice_step_diagnostics = True

# domain decomposition & space charge mesh
sim.init_grids()

# load a 2 GeV proton beam with an initial
# normalized transverse rms emittance of 1 um
kin_energy_MeV = 2.0e3  # reference energy
bunch_charge_C = 1.0e-9  # used with space charge
npart = 10000  # number of macro particles

#   reference particle
ref = sim.particle_container().ref_particle()
ref.set_charge_qe(1.0).set_mass_MeV(938.27208816).set_kin_energy_MeV(kin_energy_MeV)

#   particle bunch
distr = distribution.Kurth4D(
    lambdaX=1.0e-3,
    lambdaY=1.0e-3,
    lambdaT=1.0e-3,
    lambdaPx=1.0e-3,
    lambdaPy=1.0e-3,
    lambdaPt=2.0e-3,
    muxpx=-0.0,
    muypy=0.0,
    mutpt=0.0,
)

sim.add_particles(bunch_charge_C, distr, npart)

# add beam diagnostics
monitor = elements.BeamMonitor("monitor", backend="h5")

# design the accelerator lattice)
constf = [
    monitor,
    elements.ConstF(name="constf1", ds=2.0, kx=1.0, ky=1.0, kt=1.0e-4),
    monitor,
]

# assign a constf segment
sim.lattice.extend(constf)

# run simulation
sim.track_particles()

# clean shutdown
sim.finalize()
Listing 187 You can copy this file from examples/distgen/input_kurth4d.in.
###############################################################################
# Particle Beam(s)
###############################################################################
beam.npart = 10000
beam.units = static
beam.kin_energy = 2.0e3
beam.charge = 1.0e-9
beam.particle = proton
beam.distribution = kurth4d
beam.lambdaX = 1.0e-3
beam.lambdaY = beam.lambdaX
beam.lambdaT = 1.0e-3
beam.lambdaPx = 1.0e-3
beam.lambdaPy = beam.lambdaPx
beam.lambdaPt = 2.0e-3
beam.muxpx = 0.0
beam.muypy = 0.0
beam.mutpt = 0.0


###############################################################################
# Beamline: lattice elements and segments
###############################################################################
lattice.elements = monitor constf1 monitor

monitor.type = beam_monitor
monitor.backend = h5

constf1.type = constf
constf1.ds = 2.0
constf1.kx = 1.0
constf1.ky = 1.0
constf1.kt = 1.0e-4


###############################################################################
# Algorithms
###############################################################################
algo.space_charge = false

Analyze

We run the following script to analyze correctness:

Script analysis_kurth4d.py
Listing 188 You can copy this file from examples/distgen/analysis_kurth4d.py.
#!/usr/bin/env python3
#
# Copyright 2022-2023 ImpactX contributors
# Authors: Axel Huebl, Chad Mitchell
# License: BSD-3-Clause-LBNL
#

import numpy as np
import openpmd_api as io
from scipy.stats import moment


def get_moments(beam):
    """Calculate standard deviations of beam position & momenta
    and emittance values

    Returns
    -------
    sigx, sigy, sigt, emittance_x, emittance_y, emittance_t
    """
    sigx = moment(beam["position_x"], moment=2) ** 0.5  # variance -> std dev.
    sigpx = moment(beam["momentum_x"], moment=2) ** 0.5
    sigy = moment(beam["position_y"], moment=2) ** 0.5
    sigpy = moment(beam["momentum_y"], moment=2) ** 0.5
    sigt = moment(beam["position_t"], moment=2) ** 0.5
    sigpt = moment(beam["momentum_t"], moment=2) ** 0.5

    epstrms = beam.cov(ddof=0)
    emittance_x = (sigx**2 * sigpx**2 - epstrms["position_x"]["momentum_x"] ** 2) ** 0.5
    emittance_y = (sigy**2 * sigpy**2 - epstrms["position_y"]["momentum_y"] ** 2) ** 0.5
    emittance_t = (sigt**2 * sigpt**2 - epstrms["position_t"]["momentum_t"] ** 2) ** 0.5

    return (sigx, sigy, sigt, emittance_x, emittance_y, emittance_t)


# initial/final beam
series = io.Series("diags/openPMD/monitor.h5", io.Access.read_only)
last_step = list(series.iterations)[-1]
initial = series.iterations[1].particles["beam"].to_df()
final = series.iterations[last_step].particles["beam"].to_df()

# compare number of particles
num_particles = 10000
assert num_particles == len(initial)
assert num_particles == len(final)

print("Initial Beam:")
sigx, sigy, sigt, emittance_x, emittance_y, emittance_t = get_moments(initial)
print(f"  sigx={sigx:e} sigy={sigy:e} sigt={sigt:e}")
print(
    f"  emittance_x={emittance_x:e} emittance_y={emittance_y:e} emittance_t={emittance_t:e}"
)

atol = 0.0  # ignored
rtol = 1.3 * num_particles**-0.5  # from random sampling of a smooth distribution
print(f"  rtol={rtol} (ignored: atol~={atol})")

assert np.allclose(
    [sigx, sigy, sigt, emittance_x, emittance_y, emittance_t],
    [
        9.970922e-04,
        9.908808e-04,
        9.992460e-04,
        9.878659e-07,
        9.966353e-07,
        1.994764e-06,
    ],
    rtol=rtol,
    atol=atol,
)


print("")
print("Final Beam:")
sigx, sigy, sigt, emittance_x, emittance_y, emittance_t = get_moments(final)
print(f"  sigx={sigx:e} sigy={sigy:e} sigt={sigt:e}")
print(
    f"  emittance_x={emittance_x:e} emittance_y={emittance_y:e} emittance_t={emittance_t:e}"
)

atol = 0.0  # ignored
rtol = 1.3 * num_particles**-0.5  # from random sampling of a smooth distribution
print(f"  rtol={rtol} (ignored: atol~={atol})")

assert np.allclose(
    [sigx, sigy, sigt, emittance_x, emittance_y, emittance_t],
    [
        9.885251e-04,
        1.006606e-03,
        1.103184e-03,
        9.878658e-07,
        9.966353e-07,
        1.994764e-06,
    ],
    rtol=rtol,
    atol=atol,
)

A Semigaussian distribution

A 6D semigaussian distribution (uniform in position, Gaussian in momentum).

In this test, the initial and final values of \(\sigma_x\), \(\sigma_y\), \(\sigma_t\), \(\epsilon_x\), \(\epsilon_y\), and \(\epsilon_t\) must agree with nominal values.

Run

This example can be run either as:

  • Python script: python3 run_semigaussian.py or

  • ImpactX executable using an input file: impactx input_semigaussian.in

For MPI-parallel runs, prefix these lines with mpiexec -n 4 ... or srun -n 4 ..., depending on the system.

Listing 189 You can copy this file from examples/distgen/run_semigaussian.py.
#!/usr/bin/env python3
#
# Copyright 2022-2023 ImpactX contributors
# Authors: Axel Huebl, Chad Mitchell
# License: BSD-3-Clause-LBNL
#
# -*- coding: utf-8 -*-

from impactx import ImpactX, distribution, elements

sim = ImpactX()

# set numerical parameters and IO control
sim.space_charge = False
# sim.diagnostics = False  # benchmarking
sim.slice_step_diagnostics = True

# domain decomposition & space charge mesh
sim.init_grids()

# load a 2 GeV electron beam with an initial
# unnormalized rms emittance of 2 nm
kin_energy_MeV = 2.0e3  # reference energy
bunch_charge_C = 1.0e-9  # used with space charge
npart = 10000  # number of macro particles

#   reference particle
ref = sim.particle_container().ref_particle()
ref.set_charge_qe(-1.0).set_mass_MeV(0.510998950).set_kin_energy_MeV(kin_energy_MeV)

#   particle bunch
distr = distribution.Semigaussian(
    lambdaX=3.9984884770e-5,
    lambdaY=3.9984884770e-5,
    lambdaT=1.0e-3,
    lambdaPx=2.6623538760e-5,
    lambdaPy=2.6623538760e-5,
    lambdaPt=2.0e-3,
    muxpx=-0.846574929020762,
    muypy=0.846574929020762,
    mutpt=0.0,
)
sim.add_particles(bunch_charge_C, distr, npart)

# add beam diagnostics
monitor = elements.BeamMonitor("monitor", backend="h5")

# design the accelerator lattice)
ns = 25  # number of slices per ds in the element
fodo = [
    monitor,
    elements.Drift(name="d1", ds=0.25, nslice=ns),
    monitor,
    elements.Quad(name="q1", ds=1.0, k=1.0, nslice=ns),
    monitor,
    elements.Drift(name="d2", ds=0.5, nslice=ns),
    monitor,
    elements.Quad(name="q2", ds=1.0, k=-1.0, nslice=ns),
    monitor,
    elements.Drift(name="d3", ds=0.25, nslice=ns),
    monitor,
]
# assign a fodo segment
sim.lattice.extend(fodo)

# run simulation
sim.track_particles()

# clean shutdown
sim.finalize()
Listing 190 You can copy this file from examples/distgen/input_semigaussian.in.
###############################################################################
# Particle Beam(s)
###############################################################################
beam.npart = 10000
beam.units = static
beam.kin_energy = 2.0e3
beam.charge = 1.0e-9
beam.particle = electron
beam.distribution = gaussian
beam.lambdaX = 3.9984884770e-5
beam.lambdaY = beam.lambdaX
beam.lambdaT = 1.0e-3
beam.lambdaPx = 2.6623538760e-5
beam.lambdaPy = beam.lambdaPx
beam.lambdaPt = 2.0e-3
beam.muxpx = -0.846574929020762
beam.muypy = -beam.muxpx
beam.mutpt = 0.0


###############################################################################
# Beamline: lattice elements and segments
###############################################################################
lattice.elements = monitor drift1 quad1 drift2 quad2 drift3 monitor

monitor.type = beam_monitor
monitor.backend = h5

drift1.type = drift
drift1.ds = 0.25

quad1.type = quad
quad1.ds = 1.0
quad1.k = 1.0

drift2.type = drift
drift2.ds = 0.5

quad2.type = quad
quad2.ds = 1.0
quad2.k = -1.0

drift3.type = drift
drift3.ds = 0.25


###############################################################################
# Algorithms
###############################################################################
algo.space_charge = false

Analyze

We run the following script to analyze correctness:

Script analysis_semigaussian.py
Listing 191 You can copy this file from examples/distgen/analysis_semigaussian.py.
#!/usr/bin/env python3
#
# Copyright 2022-2023 ImpactX contributors
# Authors: Axel Huebl, Chad Mitchell
# License: BSD-3-Clause-LBNL
#

import numpy as np
import openpmd_api as io
from scipy.stats import moment


def get_moments(beam):
    """Calculate standard deviations of beam position & momenta
    and emittance values

    Returns
    -------
    sigx, sigy, sigt, emittance_x, emittance_y, emittance_t
    """
    sigx = moment(beam["position_x"], moment=2) ** 0.5  # variance -> std dev.
    sigpx = moment(beam["momentum_x"], moment=2) ** 0.5
    sigy = moment(beam["position_y"], moment=2) ** 0.5
    sigpy = moment(beam["momentum_y"], moment=2) ** 0.5
    sigt = moment(beam["position_t"], moment=2) ** 0.5
    sigpt = moment(beam["momentum_t"], moment=2) ** 0.5

    epstrms = beam.cov(ddof=0)
    emittance_x = (sigx**2 * sigpx**2 - epstrms["position_x"]["momentum_x"] ** 2) ** 0.5
    emittance_y = (sigy**2 * sigpy**2 - epstrms["position_y"]["momentum_y"] ** 2) ** 0.5
    emittance_t = (sigt**2 * sigpt**2 - epstrms["position_t"]["momentum_t"] ** 2) ** 0.5

    return (sigx, sigy, sigt, emittance_x, emittance_y, emittance_t)


# initial/final beam
series = io.Series("diags/openPMD/monitor.h5", io.Access.read_only)
last_step = list(series.iterations)[-1]
initial = series.iterations[1].particles["beam"].to_df()
final = series.iterations[last_step].particles["beam"].to_df()

# compare number of particles
num_particles = 10000
assert num_particles == len(initial)
assert num_particles == len(final)

print("Initial Beam:")
sigx, sigy, sigt, emittance_x, emittance_y, emittance_t = get_moments(initial)
print(f"  sigx={sigx:e} sigy={sigy:e} sigt={sigt:e}")
print(
    f"  emittance_x={emittance_x:e} emittance_y={emittance_y:e} emittance_t={emittance_t:e}"
)

atol = 0.0  # ignored
rtol = 2.3 * num_particles**-0.5  # from random sampling of a smooth distribution
print(f"  rtol={rtol} (ignored: atol~={atol})")

assert np.allclose(
    [sigx, sigy, sigt, emittance_x, emittance_y, emittance_t],
    [
        7.5451170454175073e-005,
        7.5441588239210947e-005,
        9.9775878164077539e-004,
        1.9959540393751392e-009,
        2.0175015289132990e-009,
        2.0013820193294972e-006,
    ],
    rtol=rtol,
    atol=atol,
)


print("")
print("Final Beam:")
sigx, sigy, sigt, emittance_x, emittance_y, emittance_t = get_moments(final)
print(f"  sigx={sigx:e} sigy={sigy:e} sigt={sigt:e}")
print(
    f"  emittance_x={emittance_x:e} emittance_y={emittance_y:e} emittance_t={emittance_t:e}"
)

atol = 0.0  # ignored
rtol = 2.3 * num_particles**-0.5  # from random sampling of a smooth distribution
print(f"  rtol={rtol} (ignored: atol~={atol})")

assert np.allclose(
    [sigx, sigy, sigt, emittance_x, emittance_y, emittance_t],
    [
        7.4790118496224206e-005,
        7.5357525169680140e-005,
        9.9775879288128088e-004,
        1.9959539836392703e-009,
        2.0175014668882125e-009,
        2.0013820380883801e-006,
    ],
    rtol=rtol,
    atol=atol,
)