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Implement Strang splitting with space charge #920

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Implement Strang splitting with space charge #920

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a0017d2
Implement test for envelope tracking.
cemitch99 Mar 26, 2025
6eaf2e3
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Mar 26, 2025
17bfdf1
Ensure nslice is even.
cemitch99 Mar 26, 2025
2e09182
Remove modification of nslice
cemitch99 Mar 26, 2025
4ea732d
Redefine nslice = number of half-steps.
cemitch99 Mar 26, 2025
0dcea99
Fine-tune tolerances and targets.
cemitch99 Mar 27, 2025
d1c69a5
Replace `floor` by `round`.
cemitch99 Mar 28, 2025
dcae328
Merge pull request #1 from BLAST-ImpactX/development
cemitch99 Mar 28, 2025
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6 changes: 3 additions & 3 deletions examples/expanding_beam/analysis_expanding_envelope.py
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Original file line number Diff line number Diff line change
Expand Up @@ -104,9 +104,9 @@ def read_time_series(file_pattern):
assert np.allclose(
[sig_xf, sig_yf, sig_tf],
[
9.029112e-04,
9.029112e-04,
1.841402e-06,
8.94427191e-04,
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@cemitch99 cemitch99 Mar 27, 2025

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Here the targets were tuned to analytically-determined values, instead of being set to coincide with a low-resolution run.

8.94427191e-04,
1.824483738e-06,
],
rtol=rtol,
atol=atol,
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2 changes: 1 addition & 1 deletion examples/kurth/analysis_kurth_10nC_periodic_envelope.py
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Expand Up @@ -98,7 +98,7 @@ def read_time_series(file_pattern):
)

atol = 0.0 # ignored
rtol = 2.0e-3 # from random sampling of a smooth distribution
rtol = 3.5e-3 # from random sampling of a smooth distribution
print(f" rtol={rtol} (ignored: atol~={atol})")
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@cemitch99 cemitch99 Mar 27, 2025

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The original tolerance was too tight.


assert np.allclose(
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1 change: 1 addition & 0 deletions src/initialization/InitElement.cpp
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Expand Up @@ -66,6 +66,7 @@ namespace detail
int nslice = nslice_default;
pp_element.getWithParser("ds", ds);
pp_element.queryAddWithParser("nslice", nslice);
nslice = 2*nslice; // ensure nslice is even

AMREX_ALWAYS_ASSERT_WITH_MESSAGE(nslice > 0,
pp_element.getPrefix() + ".nslice must be > 0.");
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20 changes: 17 additions & 3 deletions src/tracking/envelope.cpp
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Expand Up @@ -137,7 +137,8 @@ namespace impactx
}, element_variant);

// sub-steps for space charge within the element
for (int slice_step = 0; slice_step < nslice; ++slice_step)
int nsteps = std::floor(nslice/2.0);
for (int slice_step = 0; slice_step < nsteps; ++slice_step)
{
BL_PROFILE("ImpactX::track_envelope::slice_step");
step++;
Expand All @@ -147,14 +148,27 @@ namespace impactx
<< " slice_step=" << slice_step << "\n";
}

std::visit([&ref, &cm](auto&& element)
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@ax3l ax3l Jun 25, 2025

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Is the reference particle pushed twice now? o.0

Happens again in line 176 below

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@cemitch99 cemitch99 Jun 27, 2025 •
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Clarified in person:

  • also includes envelope push
  • note the difference in weights (half step for push, full step for SC & CSR, another half step for push)

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@cemitch99 cemitch99 Jun 27, 2025 •
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We have to make sure that thin elements, which only go through this loop body once, are still only kicked once though.

{
// push reference particle in global coordinates
{
BL_PROFILE("impactx::Push::RefPart");
element(ref);
}

// push Covariance Matrix in external fields
element(cm, ref);

}, element_variant);

if (space_charge == SpaceChargeAlgo::True_2D)
{
// push Covariance Matrix in 2D space charge fields
envelope::spacecharge::space_charge2D_push(ref, cm, intensity, slice_ds);
envelope::spacecharge::space_charge2D_push(ref, cm, intensity, 2*slice_ds);
} else if (space_charge == SpaceChargeAlgo::True_3D)
{
// push Covariance Matrix in 3D space charge fields
envelope::spacecharge::space_charge3D_push(ref, cm, intensity, slice_ds);
envelope::spacecharge::space_charge3D_push(ref, cm, intensity, 2*slice_ds);
} else {
amrex::Print() << "Warning: Space charge is off by default." << "\n";
}
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