500
F. Bordry et al.
After this operation, the less critical components from the alignment point of
view are positioned with respect to the “smoothed” components.
During the exploitation of the accelerator, when making successive maintenance
surveys of these long and flexible figures, absolute comparisons would be a non
sense and the differences between trend curves, corresponding to each survey, must
be analytically eliminated. The state of the alignment is expressed by the statistical
dispersion observed around the mean trend curve in these successive comparisons.
In any case, and for successive measurements as well, the problem is the difference
between these distorted curves and the theoretical geometry. Each “image” of the
ideal line has the same likelihood of being “true”, within the envelope of errors,
but is nevertheless different. This difference has (globally) no physical meaning, but
local discrepancies or distortions may be the signal of a move, either for a single
element or for a group, depending on the deformations of the supporting structure
(floor and tunnel) due to geo-mechanical forces, and/or to some constraints along
the machine (vacuum, dissipated energy, etc.).
8.9.10 Permanent Monitoring and Remote Alignment of Low
Beta Quadrupoles
The low beta quadrupoles magnets ensure the final focus of the particle beam
before the collisions and for this purpose they are located in close vicinity to
the experimental detectors. Their positioning accuracy (1σ) is requested to be as
followed:
• 0.1 mm (1σ) for the positioning of one triplet of quadrupoles with respect to the
triplet located on the other side.
• A few microns of stability of the position of one quadrupole inside its triplet
These tight tolerances, the absence of a direct line of sight through the detectors
and the difficult environment (radiations, magnetic fields) have justified the permanent monitoring of the position of these critical components and the addition of
dedicated survey galleries (Fig. 8.102).
In the vertical direction, the permanent monitoring is realized by Hydrostatic
Leveling System (HLS). The principle is based on communicating vessels, the
sensors measuring the distances to a water level by means of capacitive technology
with a resolution below the micron. This high accuracy can only be obtained by
taking into account the temperature gradients, the difference of pressure, the tidal
effects and the influence of the deviation of the vertical. Both sides of the experiment
are linked by a network of pipes (Fig. 8.103—below) running along the cavern and
avoiding big changes in height which would generate temperature gradients.
In the horizontal plane, on each side of the experiment, a “short” (30 m to
45 m) stretched wire detected by biaxial sensor from Wire Positioning Systems
(WPS), also using the capacitive technology, measures the relative position of
F. Bordry et al.
After this operation, the less critical components from the alignment point of
view are positioned with respect to the “smoothed” components.
During the exploitation of the accelerator, when making successive maintenance
surveys of these long and flexible figures, absolute comparisons would be a non
sense and the differences between trend curves, corresponding to each survey, must
be analytically eliminated. The state of the alignment is expressed by the statistical
dispersion observed around the mean trend curve in these successive comparisons.
In any case, and for successive measurements as well, the problem is the difference
between these distorted curves and the theoretical geometry. Each “image” of the
ideal line has the same likelihood of being “true”, within the envelope of errors,
but is nevertheless different. This difference has (globally) no physical meaning, but
local discrepancies or distortions may be the signal of a move, either for a single
element or for a group, depending on the deformations of the supporting structure
(floor and tunnel) due to geo-mechanical forces, and/or to some constraints along
the machine (vacuum, dissipated energy, etc.).
8.9.10 Permanent Monitoring and Remote Alignment of Low
Beta Quadrupoles
The low beta quadrupoles magnets ensure the final focus of the particle beam
before the collisions and for this purpose they are located in close vicinity to
the experimental detectors. Their positioning accuracy (1σ) is requested to be as
followed:
• 0.1 mm (1σ) for the positioning of one triplet of quadrupoles with respect to the
triplet located on the other side.
• A few microns of stability of the position of one quadrupole inside its triplet
These tight tolerances, the absence of a direct line of sight through the detectors
and the difficult environment (radiations, magnetic fields) have justified the permanent monitoring of the position of these critical components and the addition of
dedicated survey galleries (Fig. 8.102).
In the vertical direction, the permanent monitoring is realized by Hydrostatic
Leveling System (HLS). The principle is based on communicating vessels, the
sensors measuring the distances to a water level by means of capacitive technology
with a resolution below the micron. This high accuracy can only be obtained by
taking into account the temperature gradients, the difference of pressure, the tidal
effects and the influence of the deviation of the vertical. Both sides of the experiment
are linked by a network of pipes (Fig. 8.103—below) running along the cavern and
avoiding big changes in height which would generate temperature gradients.
In the horizontal plane, on each side of the experiment, a “short” (30 m to
45 m) stretched wire detected by biaxial sensor from Wire Positioning Systems
(WPS), also using the capacitive technology, measures the relative position of