Setup new far backwards geometry

[simonge]

Briefly, what does this PR introduce?

Makes the changes required to match the current EIC accelerator beamline setup in the Far Backward region. The vacuum pipe spanning the tagger drift volume has needed some significant revisions, simplifying the implementation but removing some functionality to allow the "antechamber" design.

Incorporates the volumes reviewed in #978, placed correctly.

Introduces a new script which automatically updates the xml description of the magnets, strengths and positions based on a provided twiss and survey file. (The current files have been added to the calibrations but could be removed as don't do anything other than act as an input to the script.)

Some extra tinkering of positions may be required. In particular the luminosity pipe along the z-axis doesn't extend into the B2AeR magnet. There is a small vacuum/air boundary between the tagger exit windows and the beampipe cylinder. These could be fixed in this PR or a future PR as they won't have the most significant impacts.

The Luminosity components will need their positions updated to match the new layout.

Not the cleanest PR.

What kind of change does this PR introduce?

• Bug fix (issue Update the FarBackward beamline, magnets and taggers #963)
• New feature (issue #__)
• Documentation update
• Other: __

Please check if this PR fulfills the following:

• Tests for the changes have been added
• Documentation has been added / updated
• Changes have been communicated to collaborators

Does this PR introduce breaking changes? What changes might users need to make to their code?

Hopefully nothing.

Does this PR change default behavior?

There is likely to be significant changes to the acceptance and reconstruction of the taggers.

[simonge]

Visual comparison between the CAD and simulation. Looks like definitely some positions to adjust still.

[simonge]

@nat93 Please can I check something quickly. In your CryostatMagnet geometry, does the beampipe have to fit inside the InnerSupport_Tube or is it the beampipe?

[nat93]

@nat93 Please can I check something quickly. In your CryostatMagnet geometry, does the beampipe have to fit inside the InnerSupport_Tube or is it the beampipe?

Hi @simonge , the support tube is a part of the magnet - superconducting coils are placed/attached on the support tube. The vacuum beam pipe is independent of the cryostat. So, the beam pipe has to fit inside the inner support tune. In other words: the outer radius of the beam pipe should be smaller than the inner radius of the inner support tube. Hope this helps. Please let me know if there are any inconsistencies in the current model.

[simonge]

Great, thanks @nat93 just thought I'd double check. Where have the values you've used come from, was this the magnet CAD drawings?

Presumably there will need to be some gap between the pipe and support tube, do you have an idea of what this would need to be?

[nat93]

Great, thanks @nat93 just thought I'd double check. Where have the values you've used come from, was this the magnet CAD drawings?

The corresponding STEP file of the cryostat can be found on SharePoint: EIC Public Sharing Docs → Documents → Experimental Program → ePIC → Engineering → STR-Files → IR6_CRYOSTAT_2200m_top_4-24-2025.stp [https://brookhavenlab.sharepoint.com/:u:/s/EICPublicSharingDocs/EdR44ODny1BEmMTTks61CCwBe8tJYMc0iWkPIYY_EjYEuw?e=2XLUEt]

The central beam pipe model follows the CAD drawing released by the EIC vacuum group: https://indico.bnl.gov/event/27631/#26-update-on-beampipe-modeling

Presumably there will need to be some gap between the pipe and support tube, do you have an idea of what this would need to be?

The beam pipe is changing frequently and there is no strict limit on the gap between. However, we plan to install additional high-Z material shielding around the beam pipe to protect the cryostat from SR. The shielding is about 3 mm thick. So, I would expect at least a 5-mm gap between the inner support tube and beam pipe.

[simonge]

Most of the issues and overlaps have been addressed.

There is currently still a discontinuity linking the luminosity line with the electron line coming out of B2AeR. This will need changes to BeamPipeChain, where I plan to create a separate PR which allows merging the a chain of the current volumes into boolean solids and also adding an optional branched pipe which will also need to be a boolean solid with the summed vacuum subtracted from the summed pipes.