1. General Considerations for the Upgrade of the LHC Insertion Magnets
The subject and title of this talk is quite unusual for a workshop on advanced magnet designs. In fact, it is a long time now that Nb-Ti as magnet technology is not present in Conference sessions devoted to the front line of magnet R&D.
So, why this talk, and why in this opening session?
Well, the questions should probably be directed to the workshop organizers. They did not give me a direction but I hope that the intention was that we should evaluate the possibilities of the Nb-Ti technology one more time before we definitely cross the roads into new territories.
I dutifully accepted the challenge to present this aspect of magnet technology. I am not presenting this talk because I particularly believe in the scientific interest of Nb-Ti technonolgy. No, quite to the contrary, like many of my colleagues I consider that the capital R in the R&D in the domain of magnet science is in using the materials that are inherently much more performing than Nb—Ti.
My approach is therefore bottoms-up, looking ahead from the existing situation. I will bring forward a certain number of arguments why we at CERN (and not the magnet community at large) should not exclude using Nb-Ti for mid-term improvements of the LHC. My conclusion to the question in the title is a qualified “yes”, there are still some possibilities for improving certain magnets in the LHC by using state-of-the-art Nb-Ti technology. Some of these improvements could be considered at the level required for the Phase I of LHC upgrade.
R. Ostojic
CERN, AT Department

2. LHC Insertion Magnets 154 superconducting magnets:
Final focus
Dispersion suppressor
Matching section
Separation dipoles
154 superconducting magnets:
102 quadrupoles cooled at 1.9 K, with gradients of 200 T/m
52 dipoles and quadrupoles cooled at 4.5 K, with fields of 4 T and gradients of 160 T/m

3. LHC Magnet Classes MB – class (MB, MQ, MQM)
(8.5 T, Nb-Ti cable at 1.9 K; m-channel polyimide insulation)
1b. MQX- class (MQXA, MQXB)
(8.5 T; Nb-Ti cable at 1.9 K; closed-channel polyimide insulation)
2. MQY- class (MQM, MQY)
(5 T; Nb-Ti cable at 4.5 K; m-channel polyimide insulation)
3. RHIC – class (D1, D2, D3, D4)
(4 T; Nb-Ti cable at 4.5 K; closed-channel polyimide insulation)
4. MQTL – class (MQTL, MCBX and all correctors)
(3 T; Nb-Ti wire at 4.5 K; impregnated coil)
5. Normal conducting magnets (MBW, MBWX, MQW)
(1.4 T; normal conducting; impregnated coil)

4. Upgrade of the Matching Sections and Separation Dipoles
The present matching quadrupoles are state-of-the-art Nb-Ti quadrupoles which operate at 4.5 K.
The upgrade of the matching sections should in the first place focus on modifying the cooling scheme and operating the magnets at 1.9 K.
In case larger apertures are required, new magnets could be built as extensions of existing designs.
The 4 T-class separation dipoles should be replaced with higher field magnets cooled at 1.9 K.
The MQTL-class should be replaced by magnets more resistant to high radiation environment.

5. The LHC low-b triplet Q3 Q2 Q1 TASB DFBX MQXA MQXB MQXB MQXA MCSOX a3
6.37
5.5
5.5
6.37
2.985
1.0
2.715
MCSOX a3 a4 b4
MCBXA
MCBXH/V b3 b6
MQSX
MCBX
MCBXH/V
MCBX
MCBXH/V

6. LHC low-b triplets

7. Limits of the present LHC triplets
Aperture
70 mm coil
63 mm beam tube
60 mm beam screen  b* = 0.55 m
Gradient
215 T/m  operational 205 T/m
Field quality
Excellent, no need for correctors down to b* ~ 0.6 m
Peak power density
12 mW/cm3  L =
Total cooling power
420 W at 1.9 K  L =

8. Aperture issue
The coil aperture was the most revisited magnet parameter of the low-b quadrupoles.
Aperture of 70 mm defined in the “Yellow Book” (1995, nominal b*= 0.50 m, ultimate 0.25 m).
Subsequent studies showed a need for increasing the crossing angle by a factor of two.
e-cloud instability  introduction of beam screens.
Upgrade target remains a b* of 0.25 m (irrespective of magnet technology).
Luminosity increase by a factor ~1.5.
Higher luminosity implies substantially greater load on the cryogenic system.
feedback to the choice of aperture and magnet design.

9. Enabling operation of the LHC with minimal disruption
Maintenance and repair of insertion magnets:
Large number of magnets of different type means limited number of spare magnets ready for exchange.
A facility is planned at CERN for repair/rebuild of matching section quadrupoles.
Particular problem: low-beta quadrupoles and separation dipoles
Only one spare of each type (best magnets already in the LHC).
As of 2006, there will be no operating facility for repair and testing of these magnets.

10. Quadrupole-first layouts
Optimize the aperture and length of the quadrupoles according to their position in the triplet.
Use of aperture:
Increase the aperture to reduce heat loads (peak and total)
Profit from better field quality to reduce the number of correctors and introduce stronger orbit correctors
Decrease b* to complement other ways of increasing luminosity.
Reduce the number of correctors to free space for stronger dipole correctors which will be needed in the new configuration

11. Large aperture quadrupoles using existing LHC cables

12. Large aperture quadrupoles
Operating current at 80% of conductor limit
As the quadrupole aperture increases, the operating gradient decreases by 20 T/m for every 10mm of coil aperture.
To get a GL similar to the present triplet, quadrupole lengths need to be increased by 20-30%.
The Nb-Ti technology proven for quadrupoles up to 12 m long.

13. R&D directions for Nb-Ti quads
Technology and manufacturing issues are well mastered.
Relatively easy extension of main magnet parameters (aperture and length) without extensive R&D.
Focus R&D on magnet “transparency”:
Cable and coil insulation
Thermal design of the collaring and yoking structures
Coupling to the heat exchanger
C. Meuris et al, 1999
LHC dipoles
FRESCA, 10 T, 88 mm
D. Leroy et al., 1999

14. Summary
LHC contains several generations of Nb-Ti magnets. Extensive experience exists in building magnets of different aperture and length. Upgrading the magnets to a higher class should be considered as a first option.
Nb-Ti (1.9K) technology is a clear choice for upgrading the large number of magnets in the LHC insertions (dipoles and quadrupoles) of the 4 T class.
The availability of spare low-b triplets and separation dipoles is a serious concern. Any proposal for the upgrade must take this issue into account and provide an appropriate solution.
The shortest route for providing new magnets in a time frame compatible with LHC luminosity runs is to use Nb-Ti technology.
Nb-Ti (1.9K) technology has reached its limits for large series production with the LHC main dipoles; improvements for small series are still possible.

15. Comment
It is generally accepted that a new generation of magnets (Nb3Sn, HTS,…) will be required for the next hadron collider. CERN should take part in a wider effort to develop and demonstrate the feasibility of the new technology.
In the interest of LHC operation, we must have an alternative; Nb-Ti technology can offer an appropriate intermediate solution.
The pitfalls in building Nb-Ti magnets should not be underestimated. There is a need to start design studies and development before LHC construction teams move on to other projects.
Initial experience from operating the LHC with beam is crucial for refining magnet parameters and making sure there are no “unknown unknowns”.
We should support US-LARP. However, LHC is too serious an investment that we can rely only on this programme (and the whims of the funding agency) for its future. We must have an alternative, and it is fortunate that Nb-Ti can offer such an intermediate solution.