Proposal for modification of the tolerances at WP08, summary BJ, 2nd Dec. 2003
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Motivation
At the request of S. Fartoukh, in view of urgent dipole assignment in the tunnel to be
made by the MEB, the tolerances between WP08 and the position in the tunnel after one
year were re-evaluated by Stephane, J.P. Quesnel , D. Missiaen and BJ. We took into
account more consistently the dynamic deformation related to transportation also the
recently updated level arm effect of the survey/tunnel misalignment (X/Z differentiation,
more precise values)
This proposal must be endorsed by the WGA, but will be used temporarily by the MEB
as is. A confirmation of the content by the MEB is required.
This document applies only for aperture tolerances.
Data and method
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We considered the following components, which apply beyond WP08 to the body and to
the extremities of the dipole, all expressed as r.m.s. values and in mm. The value EX
below is computed as EX x H / D, with H=0.95 the beam height and D=0.53 the
transverse distance between the two feet which define the horizontality of the assembly.
The data are extracted from the best guess table of contributions to geometrical errors
prepared by the WGA, see
http://lhc-proj-wga-wgr.web.cern.ch/lhc-proj-wga-wgr/default.htm
Measurement error between GA and targets
Cold feet deformation
Cryostat ovalisation
Dynamic deformation after transport
Vertical displacement unit
Excursion of the MB, vertical (level arm 1.2)
Excursion of the MB, horizontal (no lever arm)
A
B
C
DZ
DX
E
EZ
EX
0.07
0.1
0.05
0.1
0.2
0.2
0.24
0.36
Quadratic sums
Vertical
A,B,C,DZ,EZ
Horizontal
A,B,C,DX,EX
TZ
TX
0.29
0.43
The value EX is a pessimistic upper limit, we therefore round TX down. Such that
finally, the contributions to tolerances beyond WP08 are
TZ = 0.3 mm and TX = 0.4 mm.
Overall tolerance in the tunnel (always for aperture): a racetrack with parameters
g_tunnel = 1.1 and r_tunnel = 1.65 for MB’s near a MQ and g_tunnel = 1.1 and
r_tunnel = 4 for mid-cell locations.
Results
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We therefore deduce new tolerances at WP08 with
Z_W08 = r_tunnel – 3 TZ and X_W08 = r_tunnel + g_tunnel – 3 TX, with the result
Near MQ
Mid-cell
Z_W08
X_W08
Z_W08
Z_W08
0.75
1.55
4.0
5.1
so,
r08 = 0.75
g08 = 0.8
so,
r08 = 3.1
g08 = 0.8
In the dispersion suppressors, where the aperture constraints are more stringent at a few
locations ( 4 x 16 = 64 MB’s ), we must use golden dipoles with a rectangular tolerance
of Z_WP08_ds = 0.5 and X_WP08_ds = 0.8.
ITP20
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By coherence with the new WP08 tolerance, we must also redefine the tolerance at
ITP20. In order to be installed near a MQ, the tolerances at ITP must be (rectangular, for
complying with standardized industrial tolerances ) :
Z_ITP20
X_ITP20
0.85
1.3
In the best possible world, the condition Z_I20 < Z_W08 should be satisfied. This is not
the case here, because of the intrinsic vertical deformation due to gravitation between the
three feet. But, the aperture tolerance is severe on the extremity of the MB which is
adjacent to the MQ. This can be used in the process of assignment of the location of the
MB (choosing upstream or downstream locations depending on which extremity is the
best). This complicates the task of the MEB, but there is presently no other easier
solution.
Interconnections
The tolerances for the offset at the interconnections depend on local differential
displacements, for which more work is needed. The tolerance at WP08 will be evaluated
later.