*Index Terms*\--Cryostat, LHC interaction region, quadrupole,
superconducting magnet.

# INTRODUCTION

[There]{.smallcaps} are four interaction points (IP) in the LHC, two
high-luminosity and two low-luminosity IPs. Two magnet triplets will be
installed at each IP, one on each side, mirrored about the IP. Each
triplet consists of four superconducting quadrupole magnets in three
individual cryostats. The magnet assemblies are designated Q1, Q2a, Q2b,
and Q3. Q2a and Q2b are assembled in to a single cryostat designated Q2.
As part of their contributions to the LHC, Fermilab and KEK will each
provide half of the triplet quadrupoles. Fermilab will build all the Q2a
and Q2b magnet assemblies. KEK will build all of the Q1 and Q3 magnet
assemblies and ship them to Fermilab where all of the magnets will be
installed into their respective cryostats prior to shipping to CERN. The
cryostat consists of the vacuum vessel, structural support system,
insulation, thermal shield, and internal piping, that is, essentially
all of the complete magnet components except the magnet assembly itself.
The cryostat design for both Fermilab and KEK magnets has been developed
at Fermilab. This report documents that design development work. A
schematic layout of a complete triplet is shown in FIG. 1.

# CRYOSTAT DESIGN

As a rule, many of the constraints facing the design of cryostats for
superconducting magnets are at odds with one another. This design is no
different. The final assembly must be structurally strong to resist
loads imposed by shipping and handling and, in this case, overseas
shipment. The suspension must be stiff enough to resist long-term
degradation in alignment and yet must be thermally efficient to minimize
heat input to the 1.9 K helium system. The design shipping loads in the
vertical, lateral, and axial directions are 2 g, 1 g, and 4 g
respectively. These loads are those expected as input to the shipping
container and are not necessarily seen by the magnet assembly. A
combination of shock isolation systems and removable shipping restraints
is planned to minimize input to the mechanical structures in the magnets
themselves. Table 1 shows the current alignment criteria. The figures in
this table illustrate requirements for final alignment in the tunnel,
not necessarily for the magnet inside the cryostat. Finally, Table 2
contains the heat load estimates for a complete triplet. The estimates
reflected here assume the cold bore is passively cooled. Insertion of a
cold bore liner in all or part of the triplet would redistribute
estimates of the 1.9 K and 4.5 K heat loads.

The design of this cryostat is the first of its kind at Fermilab to make
exclusive use of 3-dimensional solid modeling and analysis software. The
original design development and the structural and thermal analyses were
done using a combination of SolidWorks and I-DEAS for modeling and ANSYS
for analysis. The final fabrication drawings were produced from the
I-DEAS solid models.

## Vacuum Vessel

The primary purpose of the vacuum vessel is to maintain the insulating
vacuum environment for the cold mass and other internal piping.
Additionally, it serves as a secondary containment in the event that an
internal pipe or bellows failure releases cryogens inside the vacuum
space. Finally, it provides the mechanical connection of the cold mass
and piping to the accelerator tunnel floor. Each of the different
quadrupoles, Q1 through Q3, requires a different vacuum vessel design,
differing mostly in length. The dominant features however are the same.
Each is constructed from rolled and welded sections of carbon steel pipe
connected at heavier-walled reinforcing sections. The reinforcing
sections are coincident with the internal supports and transfer the
weight and any mechanical loads to ground. The end flanges allow the
insulating vacuum bellows to be "parked" over the outermost section
while connecting the interconnect piping during installation in the
tunnel.

While ordinarily at room temperature, there are failure modes in which
the vacuum vessel might experience lower operating temperatures for
short periods. This could occur in the event of an internal failure that
spills cryogens inside the vessel. It can also occur during a loss of
insulating vacuum in which the vessel would be cooled by conduction from
the cold mass and internal piping. These conditions require a material
with higher fracture toughness at low temperature than normal
construction-grade carbon steel. CERN has developed a material
specification adopted by all collaborating laboratories that defines the
toughness in terms of a minimum Charpy impact strength at --50°C \[2\].
Several domestic and foreign materials commonly used in the gas pipeline
industry have been shown to meet this specification. Stainless steel and
aluminum were considered for fabrication of these vessels early in the
design process, but were dropped from consideration due to their high
cost. FIG. 2 shows a vacuum vessel assembly typical of those used in all
LHC triplet magnets.

## Structural Support System

The suspension system for this cryostat design represents a departure
from systems used in similar magnets over the course of the last several
years. SSC collider dipole cryostats, Tevatron low-beta quadrupole
cryostats, and LHC arc dipole and quadrupole cryostats all employ
composite support posts to support the weight of the cold assembly and
to resist loads experienced during shipping and handling \[3-5\]. Early
in the design phase of this project we elected to place the He II heat
exchanger outside the cold mass, but inside the vacuum vessel. This
precluded the use of a support post scheme like these other magnets
since it would have necessitated a vacuum vessel much larger in diameter
than could have been transported into the LHC tunnel. Rather, we opted
to use a composite "spider" to support the cold mass and internal piping
similar to structures used in the construction of cryogenic transfer
lines. These structures are very strong and stiff in the radial
direction and make good use of radial space. One of their weaknesses is
in resisting axial loads encountered during shipping, handling, and
cooldown. To better distribute these loads to all the support spiders
each pair of spiders has a set of axial tie-bars to tie them together.
The tie-bars are made from Invar tube to minimize axial forces imposed
on the supports during cooldown.

FIG. 3 illustrates a complete two-support suspension system including
the support spider axial tie-bars. Q1 and Q3 cryostats use two supports
and Q2 uses three. The cold mass is attached to the support at the
bearing blocks located at the horizontal plane. These are Garlock-DU®
sleeve bearings and allow the cold mass to slide axially during cooldown
to accommodate up to 14 mm of thermal contraction. The holes through the
composite are clearance holes for the internal piping. The band around
the outer perimeter is stainless steel and serves as an axial stiffener
and as the thermal intercept when attached to the 70 K shield. The
composite material is G-11CR.

## Multi-Layer Insulation System

Multi-layer insulation is installed on the outside of the 70 K thermal
shield, the cold mass, and around the internal helium pipes. The
insulation is made in blankets consisting of alternating layers of 6 µm
to 12 µm reflective Mylar and nylon spacer. The reflective aluminum
coating thickness is 300Å minimum on each side. The edges of the
blankets are terminated with Velcro strips to ensure a good seal and
easy, repeatable installation. The thermal shield uses two blankets of
15 layers each (15 layers of reflector, 14 layers of spacer) for a total
of 30 reflective layers. The cold mass and internal piping each have
single blankets of 10 reflective layers each. The expected performance
of the thermal shield blankets based on earlier testing at CERN is 1.2
W/m^2^ from 300 K to 70 K for all thirty layers \[6\]. Similarly, the
expected performance below 70 K is 50 to 100 mW/m^2^ for ten layers.
Aluminized Mylar is almost transparent to infrared radiation below 20 K
and so is nominally not effective on the cold mass or internal piping.
It is used as protection from residual gas conduction in the event of a
spoiled insulating vacuum and to reduce the transient heat load on the
cold mass and piping in the event of a catastrophic loss-of-vacuum
accident.

## Thermal Shield

A single thermal shield operating nominally at 70 K intercepts heat
radiating from room temperature at the inside surface of the vacuum
vessel. The temperature of the shield varies from about 60 K at the
beginning of the triplet to 75 K at the end. The shield consists of an
aluminum shell surrounding the entire cold assembly, attached at the
horizontal plane to two aluminum extrusions. The annular shell is
constructed in segments to minimize the effects of bowing due to
longitudinal temperature gradients during cooldown. Each extrusion has
an aluminum to stainless steel transition welded to the end to allow
welding to stainless steel interconnect bellows. FIG. 4 illustrates a
complete shield assembly. The holes through the shell allow the lugs on
the support spider shown in FIG. 3 to pass through for connection of the
suspension system to the vacuum vessel.

## Internal Piping

The internal piping consists of the He II heat exchanger, pumping line,
cooldown line, and 4.5 K intercept lines. The He II heat exchanger is a
corrugated copper tube inside a stainless steel shell. Helium in the
outer shell is common with the magnet volume. It operates at 1.9 K and
1.3 bar. The inside of the corrugated copper operates at 1.8 K and 16
mbar. A complete test of a full-scale mockup of this heat exchanger is
described in \[7\]. The cooldown line is used only during initial
filling of the magnet volume. The 4.5 K intercept lines will be used to
cool absorbers located at each magnet interconnect and potentially any
cold bore liner that might be installed at CERN. Pipe supports are
positioned at both ends of the cold mass and at one or two places along
the cold mass length. These supports position the ends of the pipes
accurately to facilitate magnet interconnections and stiffen the ends to
preclude interconnect bellows failure due to elastic instability caused
by high internal pipe pressures. Since the pipe supports have no direct
connection to anything other than the cold mass, there is no additional
heat load associated with any of the internal piping. FIG. 5 shows the
completed internal assembly and includes the cold mass, suspension
system, piping supports, and all the internal piping.

## Final Assembly

After the assembly is completed to the stage shown in FIG. 6, the
multi-layer insulation blankets are installed over the cold mass and
internal piping, the thermal shield and its insulation is installed, and
the entire assembly is rolled into the vacuum vessel. The lower support
blocks on each support spider contain a series of four cam rollers that
allow the assembly to roll on the inside surface of the vacuum vessel. A
close-up of this block and its associated rollers is shown in FIG. 6.

A rail system that is part of the final assembly tooling being
fabricated for the production magnets helps to effect the transition
from the assembly table to the vacuum vessel. A cable and winch system
is used to pull the assembly into the vessel on the rollers shown in
FIG. 6. Guides installed inside the vacuum vessel and assembly tooling
help to ensure the final alignment of the internal assembly with respect
to the vacuum vessel. Mechanical fasteners are inserted into the support
blocks from outside the vacuum vessel to anchor the internal assembly.
Fine tuning the rough alignment mentioned earlier takes place at this
stage through an adjustment mechanism built into the vacuum vessel
penetrations. Finally, Conflat gaskets and flanges are installed to
effect the final vacuum closure. FIG. 7 shows a model of the completed
first prototype assembly.

# SUMMARY

One prototype magnet and cryostat have been completed. The prototype is
half of a final Q2 assembly. The next complete assembly will be two cold
mass assemblies identical to the prototype, joined into a complete Q2 in
a full-length cryostat. Aside from small cosmetic changes to some of the
components and minor changes in some styles of mechanical fasteners
employed in the assembly the only significant change indicated by the
experience gained building the first prototype is in the design of the
MLI blankets on the internal piping. The prototype used a single blanket
around all the internal piping, including the cold mass. Production
magnets will use the separate cold mass and piping blankets referred to
earlier. In spite of the use of temporary tooling, the alignment of the
cold mass inside the vacuum vessel was very encouraging. The roll angle
of the cold mass with respect to gravity varied from 1.84 to 2.13 mrad,
with a maximum uncertainty of 0.03 mrad, over the course of three
thermal cycles \[8\]. Adjusting for the roll of the vacuum vessel of 1.3
mrad gives a maximum roll angle inside the vacuum vessel of 0.8 mrad.
The maximum warm-to-cold displacements were 77 laterally and 260
vertically. Opposite ends of the magnet did not move together so some
work remains to understand these measurements. It is important to note
that these roll and displacement values are measures of the alignment of
the cold mass with respect to the vacuum vessel. The final alignment
also relies on the precision of the installation of the vacuum vessel in
the tunnel.

Although it is difficult to quantify it seems clear that the extensive
use of solid modeling in all phases of this design have made the overall
assembly much more straightforward. The fit and finish of all phases of
the assembly are far superior to those in past projects. Rework of parts
at final assembly was almost non-existent and mounting on the Fermilab
test stand required no rework of piping to effect electrical or
cryogenic connections.

# ACKNOWLEDGMENTS

The authors would like to thank Arnie Knauf, Simmie Meredith, and Marsha
Schmidt for their help during the design and procurement of the first
prototype. We would also like to thank Jim Rife and his technical crew
of Damon Bice, Steve Gould, Paul Mayer, Don Nurczyk, Jamie Page, and
Lisa Ruiz for their help during fabrication of the first prototype.

# REFERENCES

1.  Mokhov, N., private communication, August 22, 2000.

2.  European Organization for Nuclear Research (CERN), "Technical
    Specification for the Supply of Vacuum Vessels for LHC Dipole
    Cryomagnets", LHC-QBQ-CI-0005, February 2000.

3.  Nicol, T.H., "Design Development for the 50MM Superconducting Super
    Collider Dipole Cryostat", in *Supercollider 3*, edited by John
    Nonte, Plenum Press, New York, 1991, pp. 1029-1036.

4.  Nicol, T.H., "TESLA Test Cell Cryostat Support Post Thermal and
    Structural Analysis", TESLA Report 94-01, Deutsches
    Elektronen-Synchrotron (DESY), Hamburg, 1994.

5.  European Organization for Nuclear Research (CERN), "The Large Hadron
    Collider Conceptual Design", edited by P. Lefevre and T. Pettersson,
    pp. 81-118.

6.  Darve, Ch., et al, "Thermal Performance Measurements on a 10-Meter
    Long Dipole Prototype Cryostat (Cryostat Thermal Model 2)", LHC
    Project Note 112, European Organization for Nuclear Research (CERN),
    Geneva, 1997.

7.  Darve, Ch., et al, "He II Heat Exchanger Test Unit for the LHC Inner
    Triplet", to be presented at CEC/ICMC 2001, Madison, July 16-20,
    2001.

8.  DiMarco, J., private communication, August 2001.

+-------------+-------------+-------------+-------------+-------------+
| TABLE 1     |             |             |             |             |
|             |             |             |             |             |
| Current IRQ |             |             |             |             |
| Alignment   |             |             |             |             |
| Table       |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| Magnet      | Transverse  | L           | Roll angle  | Pitch / Yaw |
|             | d           | ongitudinal |             |             |
|             | isplacement | d           |             |             |
|             |             | isplacement |             |             |
+-------------+-------------+-------------+-------------+-------------+
| Q1          | 300 m       | \~1 mm      | 200 rad     | 130 rad     |
+-------------+-------------+-------------+-------------+-------------+
| Q2          | 300 m       | \~1 mm      | 100 rad     | 130 rad     |
+-------------+-------------+-------------+-------------+-------------+
| Q3          | 300 m       | \~1 mm      | 100 rad     | 130 rad     |
+-------------+-------------+-------------+-------------+-------------+

+-------------+-------------+-------------+-------------+-------------+
| TABLE 2     |             |             |             |             |
|             |             |             |             |             |
| Inner       |             |             |             |             |
| Triplet     |             |             |             |             |
| Estimated   |             |             |             |             |
| Heat Loads  |             |             |             |             |
| (One        |             |             |             |             |
| Complete    |             |             |             |             |
| Triplet)    |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| Temperature | 50 to 75 K  | 4.5 K       | 1.9 K       | Notes       |
| level       |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| Static heat | 220         | 0           | 18          | 1,2         |
| loads (W)   |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| Dynamic     | 0           | 17          | 184         | 3           |
| heat loads  |             |             |             |             |
| (W)         |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| Total heat  | 220         | 17          | 202         |             |
| loads (W)   |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
|             |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| Notes       |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| 1\. Static  |             |             |             |             |
| heat load   |             |             |             |             |
| to outer    |             |             |             |             |
| shield: 140 |             |             |             |             |
| W           |             |             |             |             |
| conduction  |             |             |             |             |
| through     |             |             |             |             |
| supports +  |             |             |             |             |
| 80 W        |             |             |             |             |
| radiation   |             |             |             |             |
| and gas     |             |             |             |             |
| conduction. |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| 2\. Static  |             |             |             |             |
| heat load   |             |             |             |             |
| to 1.9 K:   |             |             |             |             |
| 12 W        |             |             |             |             |
| conduction  |             |             |             |             |
| through     |             |             |             |             |
| supports +  |             |             |             |             |
| 6 W         |             |             |             |             |
| radiation   |             |             |             |             |
| (assume     |             |             |             |             |
| emiss       |             |             |             |             |
| ivity=0.1). |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| 3\. Dynamic |             |             |             |             |
| heat loads  |             |             |             |             |
| based on    |             |             |             |             |
| nominal     |             |             |             |             |
| luminosity  |             |             |             |             |
| \[1\].      |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+

![](media/image1.wmf){width="3.442361111111111in"
height="1.0145833333333334in"}Fig. 1. Schematic layout of a typical LHC
interaction region triplet.

![](media/image2.png){width="3.446527777777778in"
height="1.5798611111111112in"}Fig. 2. Vacuum vessel assembly.

![](media/image3.png){width="3.4402777777777778in"
height="1.7722222222222221in"}

Fig. 3. Suspension system assembly.

![](media/image4.png){width="3.448611111111111in"
height="1.336111111111111in"}

Fig. 4. 70 K thermal shield assembly.

![](media/image5.png){width="3.4451388888888888in"
height="1.3659722222222221in"}

Fig. 5. Internal piping assembly.

![](media/image6.png){width="2.457638888888889in"
height="1.9916666666666667in"}

Fig. 6. Close-up of support block and rollers.

![](media/image7.png){width="3.4451388888888888in" height="1.5in"}

Fig. 7. Complete magnet assembly.