### 5.1.4 Interaction Regions 

VLHC has provisions for two large colliding experiments located on the
present Fermilab site. The beam interaction points will be located
symmetrically from the two non-interaction beam crossing points. The
closest crossing point is13 half cells or 1700 meters from the
interaction points.

A system of magnets on each side of the interaction regions is required
to bring the beams into collision. With these magnets comes additional
requirements for the accelerator including , power supplies, beam
absorbers to protect the magnets and prevent activation of the tunnel,
and an alignment system for the magnetic elements. For the
superconducting magnets there is quench protection system, cryogenics,
and interconnects for room temperature to cold transitions of
instrumentation and powering.

![](media/image1.wmf){width="4.9in" height="2.54375in"}Figure 1 shows a
schematic of the interaction regions magnetic elements. Each interaction
region spans a space of 6 half cells, from 20 meter to approximately 810
meters from the interaction points. The interaction regions consist of
four high gradient "inner triplet" quadrupoles, two separation dipole
elements, and an outer system of lower gradient quadrupoles. There will
also be orbit and local field corrector magnets. The separation dipoles,
"outer" quadrupoles and correctors are used in other parts of the
accelerator and are thus described in section 5.1.5. The high gradient
inner triplet quadrupole is described below.

Figure 1. Principal magnetic elements for the VLHC-1 interaction region.

#### 5.1.4.1 Low-beta IR quadrupoles

These magnets form the final focus at the two collision points. The
required gradient is 300 T/m with a length of 10 meters and a single
aperture of \~80 mm. The field quality must be very good because the
beams could be separated by as much as 12.5 mm. due to the crossing
angle.

A design developed by INFN-LASA for a possible upgrade of the LHC low-β
quadrupoles, with an aperture of 85 mm \[1\], can be used as a starting
point to meet these requirements. The basic design as shown in figure 1
has two Nb~3~Sn layers in a cos (2Θ) geometry, wound using the cable
described in Table I. The cable has a target critical current density of
3000 A/mm2 @ 4.2 K, 12 T \[2\]. Internal splices can be avoided using
the double pancake technique. The main characteristics of the magnetic
design are reported in Table II. The field quality of the basic design
(aperture = 85 mm) gives acceptably low values for the allowed
harmonics. The value of the b6, b10 and b14 multipoles, normalized to
the gradient and using a 10 mm reference radius are respectively 1.5
e-6, 1.6 e-8 and lower than 1.0 e-9. The relative field error is lower
than 1.1 e-5, at 12.5 mm from the center on the midplane. Iron
saturation effect has to be studied.

Table I: Conductor characteristics

  ------------------------------------------ ----------------------------
  Superconductor                             Nb~3~Sn

  Critical current non Cu (@ 4.2K 12T)       3000 A/mm^2^

  Strand diameter                            0.825 mm

  α = Cu/nonCu                               1.3

  Number of strands                          36

  Cable dimension                            1.34 ÷ 1.60 x 15.0 mm^2^

  Effective filament diameter                \< 40 μm
  ------------------------------------------ ----------------------------

![](media/image2.wmf){width="3.3in" height="2.0416666666666665in"}

Figure 2 Basic design cross section with field quality at maximum
gradient

Table II: Magnetic design characteristics at short sample limit

  -------------------------------------- --------------------------------
  Aperture                               85 mm

  Gradient                               300 T/m

  Current                                20218 A

  Self inductance                        4 mH/m

  Stored energy                          820 KJ/m

  Horizontal Magnetic force (per octant) 2.78 MN/m

  Vertical Magnetic force (per octant)   3.43 MN/m

  Number of turns                        10 + 4 + 19

  Collars thickness                      20 mm

  Iron inner radius                      93.5 mm
  -------------------------------------- --------------------------------

The quench protection has been studied at an operating current
corresponding to 93% of the short sample limit. Simulations were
performed using a version of the QLASA code \[3\] modified at Fermilab
\[4\] for the study of FNAL Nb~3~Sn high field magnets. These studies
show that full coil coverage by heaters on both layers is required, and
the delay time between the spontaneous quench start and the beginning of
the heater-induced quench must be very short. This solution has an
impact on magnet technology, because if the insulation of the heaters
between the inner and outer layers cannot withstand the conductor heat
treatment, the double pancake technique must be abandoned. Furthermore,
protection redundancy will not be possible (or at least very
complicated) and fault scenarios have to be addressed. The short time
delay necessary to avoid excessive hot spot temperature and the lack of
protection redundancy should be the object of an appropriate R&D.

A possible solution for the mechanical design is the one proposed for
the upgraded version of the LHC IR low-beta quadrupoles presented in
\[5\]. It uses 20 mm thick stainless steel collars, which can provide
part of the required prestress and magnetic force containment. The rest
of prestress and support are provided by a thick skin. The yoke is only
in contact with the collars close to the midplanes, in order to transfer
the stress from the skin to the coils where it is most necessary. The
effectiveness of this design for the higher forces developed in the VLHC
IR quadrupoles is still under investigation. A finite element analysis
of coils and collars, simulating the rest of the mechanical structure by
use of an external load, showed that the maximum stress in the coil can
be kept below 160 MPa. The design of yoke, skin and contact regions must
still be optimized to provide the required support on the coils.

In summary, it can be concluded that a two layer design using Nb~3~Sn at
4.2 K is a good candidate to meet the requirements for these magnets, if
the critical current target is reached. Some design optimization will be
required for the quench protection and mechanical design.

#### 5.1.4.2 Interaction Region Subsystems

Most of the 800 meters of the interaction region will be in the standard
VLHC tunnel. Since these magnets elements need to be "tunable" they
require a special gallery to handle the power supply and electronics of
these magnets. There will likely be a global alignment system that
connects the experiments to the final focussing elements. Finally, the
tunnel will need to be wider near the IP to accommodate the extra
shielding and vacuum connections and instrumentation associated with the
collision detection and IP interconnect.

Four independent power systems are required for each side of an
interaction region: one for the inner triplet, one for each separation
dipole and one for the outer quadrupoles. Additional quadruples in the
IR region will likely be powered in series with other straight section
quadrupoles in the accelerator and are not include here. The high
gradient quadruples require a 25 kA, 10 Volt supply, the conventional
separation dipoles require 1 kA, 100 Volt supplies while the
conventional outer quadrupoles need a 7kA, 10 volt supply. Correctors
will be powered independently with supplies comparable to the outer
quadrupole conventional magnets.

Superconducting circuits will rely on quench heaters for protection.
With the relatively small amount of stored energy (50 MJ) and small
number of magnets/circuit (4 or less) it is not clear whether the
complications of an extraction circuit and bypass diodes are worth the
effort. Thus, in the event of a detected spontaneous quench, the circuit
power converter will be phased off, all quench protection heaters on all
the magnets in a circuit will be energized.

As stated, inner triplet quadruples will likely need protection levels
comparable or greater than a similarly sized LHC IR quadruple magnet.
Thus 4-8 quench heater circuits plus the requisite electronics are
required for each magnet. The total space for power supplies and
electronics required for each quench heater circuit is approximately 2
6U VME crates. Thus a space of approximately 70 6U crates are required
for each side/IR.

LCW and electrical power will be required for the power supplies and
conventional magnet. The LCW system will be required to deliver several
hundred liters/minute per IR

Superconducting magnets rely on 4.5 K helium. The estimated dynamic heat
load/side/IP will be approximately 600 Watts or 2.4 kW for both
interaction regions \[6\]. Local heating in the quadrupoles could be
quite large due to the strong focusing elements (G.dl \~ 3000 T/m \*m).
This could be a non-negligible load on the Phase I cryogenic
requirements.

This heat load, if taken directly by the 4.5 K helium will require
approximately ½ of a VLHC-1 power plant. Thus the cryogenic plant
located on site would have to be upsized. Since the IR temperature and
pressures profiles and requirements will differ from the transmission
lines, there will likely be a need to be a separate transfer line from
the cryogenic system. The helium requirements might be reduced by
carrying away some of the heat load by actively cooled absorbers in the
80 mm beam tubes.

A distribution box will be required in order to make a connection
between the room temperature and cryogenic electrical elements, as well
as supply the cryogens to the magnetic elements. Elements include
voltage taps, strip heater leads, thermometry and other cryogenic
instrumentation. 25-30 kA HTc leads would be used to supply the
excitation currents.

A particle absorber system will be required to protect the IR magnets
from the excessive radiation from particle produced from the interaction
region. Using the LHC as a guide, the system will require a hadron and
neutral particle absorber. The primary hadron absorber will be located
on the IP side of the inner triplets. The size of the absorber will be
comparable to the LHC absorber but will have significantly larger heat
loads, estimated to be 2 kW per IR \[7\]. Thus an actively cooled
absorber will likely be necessary. The neutral absorber, located between
the separation dipoles would be necessary to protect the outer
quadrupoles, possibly the transmission line magnets and the walls of the
tunnel from activation. It, too, will likely need to be actively cooled.
If water cooled the requirements would be modest and would be parasitic
to the magnet and power supply LCW system.

Alignment tolerances likely to be comparable to those of the LHC
Interaction region. Transverse alignment tolerances will thus need to be
on the order of 100-300 microns over both sides of the IP. Thus an
alignment system will be required to tying together the two sides of the
IR and the detector.

#### References {#references .Reference-heading}

\[1\] G. Ambrosio et al. *"Preliminary Proposal of a Nb3Sn Quadruple
Model for the Low-beta Insertions of the LHC"* INFN Report,
INFN/TC-95/25, Sept. 13, 1995.

\[2\] R. Scanlan, *"Conductor Development Program"* Low Temperature
Superconducting Workshop, Santa Rosa (CA), Nov. 8-10, 2000.

\[3\] M. Canali, L. Rossi, *"DYNQUE: a computer code for quench
simulation in adiabatic multicoil superconducting solenoids"*, INFN
Report, INFN/TC-93/06, 1993

\[4\] L. Imbasciati, G. Ambrosio, P. Bauer, V. Kashikhin, A. Zlobin,
*"Quench Protection of the Common Coil Racetrack (CCRT-1)-Results of
Adiabatic Model Calculations",* TD-00-056, FNAL (Sept. 2000)

\[5\] G. Ambrosio, G. Bellomo and L. Rossi, *"A 300 T/m Nb3Sn Quadrupole
for the Low-beta Insertions of the LHC"*, Proc. of EPAC96, Sitges
(Barcelona), p. 2290.

\[6\] Nikoli Mokhov, private communication

\[7\] Bill Turner, private communication