Considerations for required
TCT stroke in HL-LHC
R. Bruce, R. de Maria, S. Redaelli
R. Bruce, 2015.06.12
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Introduction
•
Present collimators have a maximum half gap of 30 mm, which is
given by the mechanical design
•
In HL-LHC: very large beta functions in the IRs and upgraded
larger triplet aperture
•
Question: What are the required openings in mm of the IR
collimators in HL-LHC, and is the present design of the IR
collimators sufficient?
R. Bruce, 2015.06.12
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HL-LHC collimator settings
•
Design settings for collimators and allowed aperture:
CERN-ACC-2014-0044
R. Bruce, 2015.06.12
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Numeric example for TCTs
•
nominal situation: TCT jaws opened symmetrically around
nominal orbit at 10.9 σ half gap (15 cm round optics HLv1.1)
•
Max. opening possible is around 14 σ – 2 σ larger than allowed
triplet aperture. So far so good…
R. Bruce, 2015.06.12
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Orbit shifts and better aperture
•
Assumed that TCT is perfectly centered and that orbit is nominal
– Allowing a 3 mm combined orbit offset and TCT misalignment: max
opening is 12.5 σ => slightly larger than allowed aperture
•
However, this assumes a possibly pessimistic aperture
– 12 σ allowed aperture is after the MADX tolerances of orbit, β-beat etc
have been applied. Ideal aperture is larger: about 14 σ
– Aperture could be close to ideal as for LHC – maybe open TCTs to increase
margin. For 14 σ opening and 3mm offset => Required stroke is 33 mm
R. Bruce, 2015.06.12
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Optics changes?
•
So far assumed nominal β-function for round optics
•
Most critical case for flat optics:
•
About 39 mm stroke required, if we assume 14 sigma aperture
and 3 mm offset
•
Super-round and super-flat: not considered at the moment.
Unclear what collimator setting can be used (aperture much
smaller than 12 σ)
R. Bruce, 2015.06.12
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Re-matched optics?
•
Request to increase β-functions at the crab cavities – possible to do without
significant change of β at triplet and triplet strength
•
Means increasing β at the TCTs – larger stroke needed for same setting in σ
•
Hard limit: aperture of D2. Extreme case: D2 at same aperture as triplet
– Implies a major change of the IR optics or worse than expected local optics
correction (today no direct measurement of Q4 β-function proven for low β*)
– Must have TCT5(6) to protect bottlenecks upstream of TCT4
– What about physical aperture of TAXN?
R. Bruce, 2015.06.12
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Increased β at TCTs?
R. Bruce, 2015.06.12
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TCT stroke with increased β
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Check increase factor of β in D2 that brings the D2 aperture to
the level of the triplet
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Apply the same increase factor at the TCTs
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Assuming we want to go with the TCT to the aperture, the max
stroke is 𝑋𝑚𝑎𝑥 = 𝑥 + 𝑛σ,𝑡𝑟𝑖𝑝𝑙 𝑓 β 𝑇𝐶𝑇 ε, where f is the increase
factor in β needed to make the D2 aperture go to the level of the
triplet:
𝐴𝑡𝑟𝑖𝑝𝑙𝑒𝑡 −|𝑥𝑡𝑟𝑖𝑝𝑙𝑒𝑡 |
β𝑡𝑟𝑖𝑝𝑙𝑒𝑡 ε
=
𝐴𝐷2 −|𝑥𝐷2 |
𝑓 β𝐷2 ε
– Most critical case: 𝑋𝑚𝑎𝑥 = 48 mm.
– Close to the physical D2 aperture (as expected if going in σ to the D2
aperture, since we have similar but larger orbit and β at D2 and TCT)
– Adding again 3mm for TCT orbit+misalignment => 51 mm needed
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Conclusions – for discussion
•
Studied the required opening in mm of the TCTs in HL-LHC
– Presented results using a 1D aperture calculation with MADX, however,
results confirmed within 1 mm using the MADX aperture model
•
For standard HL-LHC v1.1, the max required jaw setting could be
about 33 mm for round and 39 mm for flat optics
– Assuming a combined orbit and misalignment of 3 mm, and that the TCTs
could be moved out to the level of the ideal triplet aperture
•
Optics changes so the D2 gets same normalized aperture as
triplet => required TCT setting at D2 physical aperture + margin
– In total about 51mm for worst case
•
Conclusion: should investigate if the TCT stroke can be increased
– Presently have only 30 mm
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