1. Q1 IR Septum Quadrupole for LHeC
LHeC IR Magnet Design Issues
Brett Parker, BNL/SMD
Introduce the eRHIC IR Sweet Spot Magnet Concept.
Review LHeC IR Quadrupole Challenges (Mirror Style Q1).
Revisit IR Layout with New Q1.
Few Observations re. Masks to Protect Central IR Region from Synrad Albedo Backscatter.
Q1 IR Septum Quadrupole for LHeC

2. eRHIC IR Machine Detector Interface Summary
16 mrad hadron dogleg has spectrometer function for forward going particles
10 mrad additional electron bending
Crab Cavities
Yellow boxes outline the cold beam tube apertures.
Warm Quad
Crab Cavities
Cryostat
Cryostat
Synrad
4.5m Region
Cryostat
Cryostat
Cryostat IP
ZDC
Cryostat
Hadrons
Electrons
Crab Cavities
Warm Quad
10 mrad total crossing angle
Crab Cavities
16 mrad hadron dogleg
The hadron beta functions are symmetric but the bending is antisymmetric.
Electron bending comes after the IP in order to avoid synrad background.

3. eRHIC IR Forward Region Layout Details
Dp/p = 20% Po
3 mrad
Roman
Pots
Forward
Detector
Dp/p = 50% Po
Forward acceptance needs for LHeC are?
Q2, has HERA style “butterfly shape chamber” B1 Q1 Q0
4 mrad neutron cone
ZDC
Main Beam 3 mrad
Q2 Front
Measure as much as possible before the next cold accelerator region.
Note that the coordinate system in these figures is aligned with the center of the 4 mrad neutron cone (neutral vector) indicated by the hatched region on the plot.

4. eRHIC IR Q1 with Sweet Spot Barely Outside Main Coil
Q1, Version Optics, Full Model
Q1 Version 2.5 Parameters
Magnetic Length = m
Main Gradient = T/m
e-Beam < Few Gauss
Note that the Q1 sweet spot region comes much closer to the main coil than the previous generic solution.
The Q1 outer coils contribute »36% of the total gradient and the peak field reaches 6.2 T on the inner main coil
Field quality is close to 1 x 10-4 at Rref = 20 mm (i.e. about 1 Unit).
Field inside elongated shielded region is less than 1 gauss in the main body.
These Q1 parameters would be very hard to achieve by any other means.
Coil ends do not preserve the body cross section, so we must carefully optimize end turns. Combining magnetic and superconducting materials, “magnetic cloaking,” may be helpful.

5. LHeC (CDR) 60 GeV x 7 TeV Interaction Region
Q1 B=0 Surface
Mirror quad has limited aperture vs. gradient.
High field mirror quad has large leakage field.
Bsep=0.3T
Controlling synrad back scatter albedo is critical, but the anti-backscatter protection masks need distance to be effective.

6. Magnet Septum Quadrupole Experience
HERA-II Septum Quadrupole
Q1 LHeC CDR Options
Useful horizontal aperture is small & beam steering losses even more.
For given aperture & peak field, gradient is half that of a standard quadrupole.
Parker et.al.
With high gradient, mirror plate saturates and natural field in the cutout region is then larger than that in the beam separation dipole!
Off-axis Beam Spot
Hera-II had warm magnets with split coils; LHeC cold magnets need complete protection (H2O cooling?).
LHeC Q1 Septum Quadrupole
Magnetic Septum Quadrupoles (MSQ), whether HERA-II (warm) or the LHeC (cold) version, focused beam passes off-axis and is deflected. MSQ aperture is surprisingly limited. Maybe eRHIC style IR sweet spot coil configurations could be useful?

7. First Attempt Sweet Spot Coil Configuration for LHeC Q1
Weak combined function outer coil minimizes average field over the sweet spot region.
Field center shift of -5.5 mm gives more space for synrad absorber inside the main quad aperture.
High gradient inner quadrupole coil
3-layer shell leaves small residual field

8. First Attempt Sweet Spot Coil Configuration for LHeC Q1
LHeC Q1 3D-Model
Boundary for warm insert through the Q1 cold mass
It is quite challenging to make such a wide region with B-field shielded.
But with this concept it is possible to put the shield in a warm insert through the whole length of Q1.
Thus we can absorb much of the synrad power in a warm region.

9. Ok so do we make best use of this new option for Q1?
Separation Dipole has 18. m active length (e.g. +/- 9 m from IP same as CDR)
Septum Bend Angle = +/- 9.8 mrad (+/ mrad in CDR)
Separation Dipole Field = T (0.300 T in CDR)
Critical Energy = 511. keV (718. in CDR and 110 for HERA-II)
Separation Dipole Synrad 6.6 mA = 25 kW ( mA in CDR)
Synrad Outer Edge w.r.t. e-Beam = 173. mm (242. mm with CDR bend angle)
With this geometry about 56% (e.g. 14. kW) hits the central synrad absorber.

10. Where is the rest of the 18 m of separation dipole?
Solenoid + Large Dipole Coils
The “central region” of the detector has a compact solenoid with an integrated large diameter dipole coil. But this provides less than half of the total separation dipole bend. IP IP
Small Diameter Warm Dipole
The rest of the separation bend is made up from compact warm dipoles on both sides of the detector. In consultation with Peter Kostka I learned that 400. mm diameter is space budgeted for these warm dipoles.
It turns out that indeed for 0.3 T and below, such a dipole can be accommodated with a low average current density.
A simple design would be to use solid Cu conductors with a simple integrated water cooled chill plate (due to confined space).
Synrad Spread
Y (mm)
e-Beam
X (mm)

11. Principles for Locating the Anti-Backscatter Masks
If even a quite small fraction of the backscattered synrad albedo hits sensitive central regions of the detector, it can cause unacceptable background and detector trips (HERA-II experience).
Masks placed between the closest primary absorbers and a sensitive region can protect against this but they must stay back from the primary beams.
Having the separation bend stop too close to the synrad absorber means that there is then no good place for these masks; we need some extra distance (as shown in the new layout) for these masks to be effective.
Background Reduction Mask Scheme Layout

12. Locating the Anti-Backscatter Masks for LHeC IR
Even with smaller bend angle and 15 m L*, we need a bit larger cutout region.
9 m of Q1
9 m of Q1
For Mask-1 to be effective we might need more space here.
With this new, improved beam separation scheme, it almost looks possible to protect the central 6 m beam pipe region from the backscatter albedo off the central synrad absorber (note LHeC CDR has only 1 m between the end of the separation dipole and the magnetic edge of Q1). But because in this design Q1 still does not pass the outer edge of the full synrad band, it is impossible to protect against albedo backscatter from this outer edge... this is an important lesson. Note that in the LHeC CDR the synrad band was much wider!

13. LHeC IR Magnet Design Issues: Summary and Lessons Learned
With a new Q1 design, a smaller beam separation angle and larger L*, it is now “almost possible” to protect the central detector region from synrad albedo backscatter and yet have reduced fields in a cutout region (for e-beam & non-interacting p-beam).
But to avoid backscatter albedo coming back at large angles, Q1 must be redesigned with a larger cutout region in order to pass the outer edge of the synrad band even further from the IP.
The warm sections of the separator dipole seem compatible with the available budgeted space.
Additional work is needed to integrate the rest of the IR magnets and for this the quick, semi-automated placement of non-standard apertures in 3D space would be most useful (and avoid mistakes).

14. Backup Slides
Proceed further only if you need encouragement....

15. Alternate Q1 Approach for the Same Shielded Region
The sweet spot concept works best, e.g. we can work very close to an inner coil structure, if the shielded region is not overly wide.
May have to go back to brute force yoke cutout shielded regions!
Small residual field
Putting a second shell inside the yoke cutout does help to reduce the residual field but unfortunately there is probably not enough space to have a warm shell as before.

16. Alternate Q1 Approach for the Same Shielded Region
Alternate Q1 Design, 3D-Model
Making the cutout region even larger without breaking quadrupole symmetry will drive use of very, very large cold yokes. Maybe we (sigh) just accept breaking symmetry and correct the main aperture field quality and reduce the cutout field by some clever arrangement and optimization of ancillary coils.