Overview of Possible LHC IR Upgrade Layouts

CARE HHH-2004 WorkshopCERN

8-11 November 2004J. Strait, N.V. Mokhov, T. Sen

Fermilab

bnl - fnal - lbnl - slacUS LHC Accelerator Research Program

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LHC Luminosity UpgradeWhy and When?

A luminosity upgrade of the LHC will be required by the middle of the next decade to keep the LHC physics program productive.

To raise the the LHCluminosity by x10,

from 1034 to 1035 cm-2s-1, will be very challenging. Must consider

several ways to achieve it.

Must start R&D now.

Must choose R&D directions judiciously.

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IR Upgrades – Opportunities and Challenges

A new IR is one straightforward way to raise the LHC luminosity:• Lower *.• Reduce effect of parasitic collisions.IR upgrade alone cannot achieve x10 increase in L.• At most x2~x3 seems possible from IR upgrade alone.• But IR must deal with higher beam current and with x10

increase in power from collision debris.Principle technical challenges of IR design:• Field quality and alignment . . . max is in IR magnets.• Energy deposition . . . 9 kW/beam from luminosity at 1035 cm-2s-1.

– Local peak power density => quench stability.– Total power into cryogenic system.– Radiation damage and activation.

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Baseline LHC IR

Baseline LHC IR => “Quadrupoles First.”• Quads as close as possible to IP => minimize max.• Quads are “inefficient” at sweeping charged particles:

=> “modest” peak power deposition. • But, beams share common channel:

=> many parasitic collisions.=> correction system acts on both beam simultaneously.

IP

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New IRs:“Straightforward” Designs

Quads 1st Dipoles 1st

J. Strait, et al., Towards a New LHC Interaction Region Design for a Luminosity Upgrade, PAC 2003.

Copy baseline IR with larger bore quads.

Fewer long-range collisions, but larger max.

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New IRs:Alternate Designs

Twin Dipole 1st Twin Quads 1stQuads between

Fewer long-range collisions,

intermediate max.,complex twin-aperture

quads and dipoles.

Very large crossing angle layouts.

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Preliminary IR Design Studies

Quad 1st

Dipoles 1st

Quad between

Twin D 1st

Twin Q 1st

cross (mrad) 0.30 0.53 0.42 0.49 7.5 7.8

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Energy Deposition – Quads First

Energy deposition and radiation are major issues for new IRs.• In quad-first IR, Edep increases with L and decreases with quad

aperture.– max > 4 mW/g, (P/L)max > 120 W/m, Ptriplet >1.6 kW

at L = 1035 cm-2 s -1.– Radiation lifetime for G11CR < 6 months at hottest spots.

T. Sen, et al., Beam Physics Issues for a Possible 2nd Generation LHC IR, EPAC 2002.

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Absorbers to Protect Triplet Quadrupoles

Front absorber (TAS) to limit flux hitting quads.

Internal absorbers to spread showers => limit peak power

density.

T. Sen, et al., Second Generation High Gradient Quadrupoles for the LHC IRs, PAC 2001.

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Cryogenic System Challenges

A.V. Zlobin, et al., Large-Aperture Nb3Sn Quadrupoles for 2nd Generation LHC IRs, EPAC 2002.

Many large cooling channels required to remove heat, even with super-fluid He.

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Energy Deposition – Dipoles First

• Problem is even more severe for dipole-first IR.– max on mid-plane ~ 50 mW/g; Pdipole ~3.5 kW

for L = 1035 cm-2 s -1.– “Exotic” magnet designs required, whose feasibility is not

known.

N.V. Mokhov, et al., Energy Dep.Limits in a Separation Dipole in Front of the LHC High-L Inner Triplet, PAC 2003.

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TAS Most charged particles entering dipole are swept into the magnet.

TAN

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Open Mid-Plane Dipole

Open mid-plane => showers originate outside the coils; peak power density in coils is reasonable.

Tungsten rods at LN temperature absorb significant radiation.

But, can this magnet be made to work??

R. Gupta and N.V. Mokhov, LARP Collaboration Meeting, Napa, CA, Oct 2004.

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IR Upgrade Questions and Issues

IR design concepts shown reduce * by x2 – x5 w.r.t. baseline design.

But…• Larger crossing and larger beam divergence limit the increase in L.

– Shorten bunches with more RF? (Expensive even for x2 reduction.)

– Crab crossing? (Difficult to provide enough crab cavity voltage. Any imperfections in crab system will blow up xy .)

– Increase bunch current? (Other factors may limit beam current below what it needed.)

• Factors limiting luminosity won’t be fully understood without LHC running experience.

• Other developments may influence design choice. (e.g. active beam-beam compensation; requirements by the experiments….)

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Other Beam Physics Questions

• Are the (very) large crossing angle schemes (twin-aperture dipole or quad first) in any way feasible?

• Can dispersion suppressors be designed for the non-parallel axis quadrupole cases?

• Can triplet errors be adequately corrected given the very large -functions?

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Magnet R&D Challenges

• All designs put a premium on achieving very high field:– Maximizes quadrupole aperture for a given gradient.– Separates the beams quickly in the dipole first IR

=> bring quads as close as possible to the IP.– Push Bop 8 T -> 13~15 T in dipoles or at pole of quad =>

Nb3Sn.• All designs put a premium on large apertures:

– Increasing max decreases * => quad aperture up to 110 mm?– Large beam offset at non-IP end of first dipole.

=> Dipole horizontal aperture >130 mm.• Energy deposition:

quench stability, cooling, radiation hard materials.• Nb3Sn is favored for maximum field and temperature margin, but

considerable R&D is required to master this technology.

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NbTi Magnets for IR Upgrades?

• Quad-first IR with Nb3Sn quads of 110 mm aperture and 6m length can achieve * = 16 cm.

• To achieve same * with NbTi (=> lower pole-tip field) requires aperture of 120~130 mm and length of 8~9 m.=> ~30% increase in max; 15~20% more parasitic collisions.

But• Current NbTi technology is not sufficiently radiation hard.• Smaller temperature margin => more sensitive to beam

heating.And dipole-first IR requires highest possible field:• Separate beams quickly.• Bring quads as close as possible to the IP.

=> Probably not practical without higher performance of Nb3Sn. See also F.Ruggiero, et al., Performance Limits and IR Design of a Possible LHC Luminosity Upgrade Based on NbTi SC Magnet Technology, EPAC 2004.

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Summary

• “Simple” IR upgrades – quad-first or dipoles-first – using Nb3Sn have the potential to reduce * by x2~x3.

• “Exotic” IR upgrades – “quads between” and large crossing angle layouts – might reduce * by x2.5~x5.

• Energy deposition and radiation hardness are major challenges for L = 1035 cm-2 s -1, especially for the dipole-first case.

• Nb3Sn technology offers greater upgrade potential than NbTi, but considerable R&D is required.

• The challenge of increasing LHC luminosity towards 1035 cm-2 s -1 is considerable, and many options need to be pursued now to ensure success.