1. Study on Thermal Deposition in the Interaction Region Magnets
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2. Outline Chronology (where this study comes from)
Method of calculations
Cases and results
2005/06 reprise of the problem
Work done this year
Next
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3. Chronology
Study of a new Nb3Sn design for the LHC insertion quad started in 1995, suggested by the Milan group (L.Rossi)
Ended in 1998
(1999 final report)
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4. Purpose of the Study was:
Use of the Nb3Sn technology instead of the "traditional" NbTi, for the construction of the focussing quadrupoles in the interaction region of LHC in order to have :
Higher focussing gradient (higher luminosity), in the same magnet aperture or
The same focussing gradient in a larger aperture, (easier beam dynamics, especially at the injection).
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5. Calculation Procedure
DTUJET
Tracking of the secondary particles
(ad hoc code)
Evaluation of the power distribution in the various elements (FLUKA)
1300 p-p
7 TeV events
Thermal Analysis
(2D ANSYS)
This coupling with the thermal analysis was the key and innovative part of this study
The thermal analysis is a very important issue in this problem, because a good cooling system in the magnet allow to operate the magnets in very high radiation environment with a good safety margin.
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6. “Old” Cases Studied Version LHC 6.5 with G = 203 T/m and Beam Screens
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7. Modelization and Hypothesis
Detailed description of the magnets with the coil layers, insulation, wedges and yokes
TRACKING
Beam pipe thickness 1.5 mm
Detector peak field 2 T
Detector radius 1.1 m
Detector length 5.3 m
Crossing angle 200 mrad
Cross section
(inel.+ single diff. event) 80 mbarn
Luminosity cm-2s-1
FLUKA
Cut off for Hadrons MeV
Cut off for electrons/positron 1.5 MeV
Cut off for photons MeV
Cut off for neutrons eV
THERMAL ANALYSIS
MODEL D - Steady State
Thermal Conductivity Computed from cable-insulation stack
Insulation Non isotropic G10
Heat flux Purely longitudinal
Heat transfer coefficient W/m2K at 1.9 K
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8. Total Power into the Quads and the Inner Conductor Layer
Note the negative effect of the uniform cylindrical adsorbers (startup of the cascade from particles that would freely travel, deposing their energy in the following, less critical quads).
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9. Summary of the “old” cases
The statistical error has been evaluated from different runs with independent random seeds
1% for the region binning
3% medium binning (Bin Volume = 0.5x0.5x50 = 12.5 cm3)
4% small binning ( " " = 0.25x0.25x50 = 3.1 cm3)
7% very small binning ( " " = 0.25x0.25x10 = 0.63 cm3)
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10. Indications from the”old” cases I
Negative effect of the cylindrical absorbers
The case with higher gradient (G=300 T/m) is slightly worst than the reference case (G=235 T/m)
Case with lager aperture (F=85 mm) is better for the deposed power but it has a small increment in the peak power.
From the thermal analysis the power released by the radiation does not endanger the operating point of the magnets and guarantee a good stability margin, as a matter of fact the maximum temperature is in the median plane, where the magnetic field is lower enough to guarantee a safe combination of the two parameters.
Conversely in the higher field regions the temperature increment is not too high.
Larger stability margin can be achieved by using the aluminum collars instead
of the stainless steel ones (tradeoff with the mechanical properties).
Nb3Sn is a good solution for the second generation low-b quads, either to increase the focussing gradient, or to get the same focussing performance in a larger aperture.
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11. Indications from the”old” cases II
In the “nominal (academic) case” or LHC version 6.5 with beam screens:
quad aperture; 70 mm ;
quad gradient; 235/203 T/m;
trim quad between Q1 and Q2a; 82 T/m
The most loaded quad is the second or the last one (Q2a,Q3), absorbing some W with a peak power of about 7-10 mW/cm3
(depending on the bin volume)
With the actual luminosity value L=1034 cm2s-1
Indication of a positive effect by opening the aperture, this lead to consider large (100 mm) quad aperture for the “new” studies
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12. New Studies Why moving towards IP ?
What the energy/power deposed in the magnets if moving closer to IP
Why moving towards IP ?
Approaching the triplet to the IP opens the possibility of an increased focusing and hence of a significantly larger luminosity (J.-P. Koutchouk)
Conversely the energy deposition in the magnets becomes a critical point, because of the different secondary beam dynamics and the increased power
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13. (inel.+ single diff. event)
Hypothesis
TRACKING
Beam pipe aperture mm (TAS) mm (Beam screens after the TAS are taken into account)
Detector Solenoid Field with its fringing field (theoretical)
Detector peak field 2 T (ATLAS)
“ radius 1.1 m
“ length 5.3 m
Hard edge approx. for the quadrupole field
Cross section = 80 mbarn
(inel.+ single diff. event)
FLUKA
Quad aperture mm
Accurate definition of the quadrupole structure (current, insulation, wedges, collars, yokes)
Magnetic field in the quad material (ROXIE)
Cut off for Hadrons MeV
Cut off for electrons/positron 1.5 MeV
Cut off for photons MeV
Cut off for neutrons eV
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14. Geometry and materials
G10 Insulation (0.2, 0.7, 0.5 mm)
S.S. Beam pipe (1.75 mm)
S.S. Beam screen with cooling tubes
Nb3Sn Current shells (15 mm, 60°)
S.S. Pole wedges
S.S. Collar (20 mm)
Fe Yoke (18 cm)
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15. Cases studied Distance IP – Q1 = 23 m ; L = 8.64 1034 cm2s-1
Quad Gradient = 193 T/m ;qcr = 512 mrad
Distance IP – Q1 = 19 m ; L = cm2s-1
Quad Gradient = 204 T/m ;qcr = 514 mrad
Distance IP – Q1 = 16 m ; L = cm2s-1
Quad Gradient = 208 T/m ;qcr = 507 mrad
Distance IP – Q1 = 13 m ; L = cm2s-1
Quad Gradient = 213 T/m ;qcr = 500 mrad
(As from J.-P. K. computations)
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16. Results I (mean values)
The study is a parametric one, so it is should be used to check the effects of the insertion moving.
Cut-offs values and biasing option in order to have “reasonable” CPU Time
(~ 60 hours on Intel Centrino 2 GHz for each configuration)
More accurate absolute values can be obtained with different calculation option and higher CPU times.
Pcharged ~ 60%
Pneutral ~ 40%
PTAS~ 1 kW
15 % Hadron
73 % em showers
The power is increased of about one order of magnitude, respect to the actual case, because of the corresponding increase of the Luminosity
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17. Results II (mean values)
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18. Results III (peak values)
The maximum peak power deposition occurs in the second quadrupole (Q2a)
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19. Results IV (peak values)
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20. Results V (peak values)
The asymmetric distribution is due to the crossing angle (in these cases it is higher than the actual LHC parameter of 200 mrad)
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21. What indications ?
The power deposed in the TAS is almost constant for all the cases
The total power deposed into the quads increases, approaching the IP
The power deposition in Q1 and in its first shells increase almost linearly, approaching the IP
The power deposition in Q3 increase too, but the behaviour is less “linear”
The “hottest” quads is Q2a for IP-Q1 = 23 m while in the other cases the hottest one is Q3
The peak power is almost constant for all the situations examined
The highest peak power deposition is in Q2a and it is almost constant for all the cases studied
The power distribution in the last quad is more spreaded
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22. What’s Next ?
Complete this study with a thermal analysis of the quad behaviour
Get values as accurate as possible by “playing” with the cut-off and biasing parameters
Update the event generator from DTUJET to DPMJET (not big changes expected)
Carefully evaluate the actual LHC insertion (LHC 6.5 layout) with :
Very detailed description of the layout with NbTi coils (valves, flanges etc..)
Take into account the tile calorimeter and the engineering of the detector
Effect of the detector field (2 Tesla ATLAS, IP1; Tesla CMS, IP5)
Compare (if some difference is evident) between NbTi and Nb3Sn (academic/FLUKA exercise?)
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23. New Options/Solutions
Dipole first
Small Dipole first
Light quads
The power in the TAS is almost constant for all the positions
Every solution or possibility can/should be checked
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24. Thanks G.Battistoni(INFN), A. Ferrari and the CERN FLUKA Group
J.-P. Koutchouk (parameters for the study)
C. Vollinger (ROXIE quad field map)
C.Hoa, E. Wildner, G. Sterbini for the fruitful discussions and help in developing the study
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