Sextupole implementation

[jank324]

Description

Implements a sextuple magnet element.

Motivation and Context

Alternative potentially cleaner approach to #399. Would close #42. Closes #230. Related to #277.

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist

• I have updated the changelog accordingly (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have reformatted the code and checked that formatting passes (required).
• I have have fixed all issues found by flake8 (required).
• I have ensured that all pytest tests pass (required).
• I have run pytest on a machine with a CUDA GPU and made sure all tests pass (required).
• I have checked that the documentation builds (required).

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[Copilot]

Pull Request Overview

This PR implements a new Sextupole element for particle accelerator simulations, including corresponding tests and integration into the element registry.

• Introduces the Sextupole element with transfer map and tracking logic.
• Adds tests comparing Sextupole tracking against Ocelot and equivalence to a drift when k2 is zero.
• Updates element registration and changelog to include the new sextupole.

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/test_sextupole.py	Adds tests comparing Sextupole tracking to Ocelot and drift behavior.
tests/test_elements.py	Registers the Sextupole element with a default length value.
cheetah/track_methods.py	Introduces a new second-order transfer matrix via base_tmatrix.
cheetah/accelerator/sextupole.py	Implements the Sextupole element with transfer mapping and tracking.
cheetah/accelerator/drift.py	Updates device/dtype handling using verify_device_and_dtype.
cheetah/accelerator/init.py	Adds Sextupole to accelerator module exports.
cheetah/init.py	Exports Sextupole as part of the core element suite.
CHANGELOG.md	Documents the addition of the Sextupole element.

Comments suppressed due to low confidence (2)

cheetah/accelerator/sextupole.py:10

• [nitpick] The docstring for Sextupole still contains placeholder TODOs for parameters such as 'k2', 'misalignment', and 'tilt'. Consider updating these docstrings to clearly describe the expected units and purpose of these parameters.

class Sextupole(Element):

tests/test_sextupole.py:61

• While the tests verify the Sextupole against Ocelot and drift behavior, there is no test covering the misalignment functionality. Adding a test case with a non-zero misalignment would ensure the misalignment handling in the transfer_map is properly validated.

def test_sextupole_parameter_beam_particle_beam_agreement():

[Hespe]

Note for the future: We might want to look for a general solution for tests that should be applied to most if not all element classes. Maintaining these lists by hand like for the dtype and clone tests is prone to forgetting newly added elements as seen above.

[jank324]

Note for the future: We might want to look for a general solution for tests that should be applied to most if not all element classes. Maintaining these lists by hand like for the dtype and clone tests is prone to forgetting newly added elements as seen above.

Can you create an issue for that and link it here?

[Hespe]

Note for the future: We might want to look for a general solution for tests that should be applied to most if not all element classes. Maintaining these lists by hand like for the dtype and clone tests is prone to forgetting newly added elements as seen above.

Can you create an issue for that and link it here?

I have created #416

[jank324]

This PR is done, awaiting only review from @cr-xu, re-review from @Hespe and that the general GitHub runner trap: 6 issue is resolved such that we can actually run all the tests.

[cr-xu]

The PR looks mostly good to me, I didn't go through the T matrix line by line though. If I read correctly, this is using the mad calculation conventions? Maybe we should mention it in the comments.