mechanical constraints on the beampipe diameter
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Mechanical Constraints on the Beampipe Diameter. Ray Veness / TE - VSC. Introduction. Assuming The ATLAS, CMS (and ALICE?) would like to reduce the diameter of the central ~±3.5m of beampipe This new radius should be compatible with LHC Phase I upgrade - PowerPoint PPT PresentationTRANSCRIPT
Mechanical Constraints on the Beampipe Diameter
Ray Veness / TE - VSC
Mechanical constraints - R.VenessLEB 5 III 09
Introductiono Assuming
o The ATLAS, CMS (and ALICE?) would like to reduce the diameter of the central ~±3.5m of beampipe
o This new radius should be compatible with LHC Phase I upgrade
o There are too many uncertainties to specify a geometry for Phase II upgrade
o Then… (contents)o How did we reach the current beampipe radius?o What to the existing designs look like?o What have we learned since the beampipe was designedo What are possible ‘show stoppers’ on the way?o How long will it take to construct?
Mechanical constraints - R.VenessLEB 5 III 09
How the Start-up Beampipe was Specified
o LHC Technical Committee 4/2/97“… the LHCTC agreed there must be no question of the beampipe
in an experiment introducing a potential performance limitation, in all cases adequate aperture and vacuum stability at maximum machine performance must be ensured.”
o Beampipe was specified for ‘ultimate LHC’ in terms of:o Aperture
o beam stay clear – for the tracker region, this was most critical at INJECTIONo Tolerances to ensure beampipe does not enter beam stay-clear
o dynamic vacuumo Beam lifetime, quench of triplet, beam-induced instabilities (ion-induced
desorption, electron cloud etc)o Impedance
o Transverse wall instabilities, trapped modes, resistive wall impedanceo Background in the experiment – not a machine requirement
o depends on machine layout, pressure in the whole insertion and requirement from the experiment – not linked the central diameter to the first order
Mechanical constraints - R.VenessLEB 5 III 09
Beampipe Diameter for LHC Baseline (1997 values)
o Beam stay-clear 14mmo Composed of beam size, beam separation, closed orbit and
crossing angle componentso within the tracker region (±5 m) at injectiono In forward regions, this may be higher for collision optics
o Survey Precision ~ 2.6mmo See presentation from survey
o Mechanical construction ~ 2.6mmo Tolerances on straightness, circularity, wall thickness, sag under
self-weight and construction of survey targetso Instabilities ~ 9.8mm
o Stability of the cavern, detector movements due to electro-magnetic forces and thermal expansion
Mechanical constraints - R.VenessLEB 5 III 09
Baseline beampipe – the designso Geometrical
o ALICE, ATLAS and CMS beryllium pipes have the same basic geometry
o 58mm inner diameter, 0.8mm wall thicknesso This greatly simplified the development cost, tooling costs and spares inventory
o With some differenceso Beryllium lengths vary from 4 to 7 mo Transitions from beryllium to AA2219 in ATLAS and stainless steel in ALICE and CMS
o Vacuumo Fully NEG (non-evaporable getter) coated and in-situ baked surface –
low SEY, PDY, thermal outgassingo Gives a high distributed pumping speed for most gases seen in a UHV system
o Sputter-ion pumps at outside central region for non-gettered gasseso Requires a certain beampipe diameter to provide conductance for non-NEG pumped
gaseso Electrical
o Made from beryllium – low electrical resistivity, o no significant heating of chamber, no problems of transverse wall instabilities
o No sharp transitions in geometry (no trapped modes etc)
Mechanical constraints - R.VenessLEB 5 III 09
ATLAS Central beampipe during integration on the pixel assembly bench
LEB 5 III 09 Mechanical constraints - R.Veness
Central beampipe during in CMS
Mechanical constraints - R.VenessLEB 5 III 09
What have we learned since 1997?o Beam stay-clear:
o Waiting for input from machine phase I upgrade studieso Survey precision
o See talk from surveyo Mechanical tolerances
o Construction: Straightness, circularity, wall thicknesso Better than expected
o Sag under self-weighto Depends on number of supports, and supported weighto ATLAS would like to suppress one of 4 supports, and support the detector from the pipeo Will be larger than the baseline pipe in ATLAS
o Precision of survey target constructiono As expected
o Instabilitieso Cavern stability – see surveyo Electro-magnetic and thermal movements
o Survey input?o Will be know before we have feedback from operation with beam giving tracks?
Mechanical constraints - R.VenessLEB 5 III 09
Other Issues for a Reduction in Radius
o Resistive wall beam heatingo Calculated heat loss in 58mm ID beryllium chamber is 8.6 W/m for ultimate
(0.9 A @ 7 TeV/c) machine [1]o Resistive wall impedance scales with 1/r3 (??) so decreasing the beampipe
radius from 29 mm to 25 mm would increase heat load by 56%o Dynamic Vacuum
o Reducing the diameter could lead to problemso Synchrotron radiation, pumping of non-NEG gases such as methaneo However, engineering solutions outside the pixel should be possible, eg, SR
masks,o Electron cloud effects, depending on bunch spacing
o Other constraints linked to approaching the beamo Collimation, machine protection, background
o These factors will be estimated once a provisional beampipe geometry is selected
[1] L.Vos, Private communication
Mechanical constraints - R.VenessLEB 5 III 09
Contract timescale for Baseline beryllium pipes
2001 2002 2003 2004 2005 2006 2007
Market survey placed
Finance committee approval for contract placement
Call for tenders
Contract placed
ATLAS chamber delivered to CERN
Chamber assembled ready for installation
Installation in ATLAS starts
Total project duration: 6 years, 5 months
Engineering design agreed
CMS chamber delivered to CERN
Mechanical constraints - R.VenessLEB 5 III 09
Summaryo Beampipe radius
o The mechanical tolerances were not a major factor in the composition of the beampipe radius
o Considering the uncertainties remaining on the supporting systems and loads that the beampipe will have to support, it appears unwise to reduce them
o Instabilitieso This was the largest factor in the definition of the diameter after
apertureo ‘Known unknowns’ such as stability of the cavern should be better
understoodo ‘Unknown unknowns’… will we know more before operation with
beam?o Other parameters
o As soon as we have a provisional geometry for the beampipe, we must feed this back to the relevant experts for analysis