# Chapter 2. Overview and Summary

## 2.1 General Description

The staged approach to the VLHC described in this feasibility study
requires placing two collider rings in a common tunnel. This requirement
imposes constraints on the lattices of the rings as well as on the civil
construction parameters. The injector to the collider is the existing
Fermilab accelerator complex, hence the collider must pass through or
close to Fermilab.

The plan view of the 233 km VLHC collider is shown in Fig. 2.1, which
shows the service buildings that contain cryogenic refrigerators and
other utilities. Stage 1 has six service areas (one on the Fermilab
site) for cryogenics and other operational requirements. Stage 2
requires six additional service sites.

The ring is almost circular, comprising two great arcs of 35 km average
radius connected by two clusters, one at Fermilab, one exactly opposite,
each six kilometers long and each containing straight sections and
dispersion suppressor arcs. The straight sections are needed for various
functions necessary for operation, such as injection, extraction,
collisions and beam cleaning.

![](media/image1.wmf)

Fig. 2.1 The 233 km VLHC Ring

A magnified view of the VLHC Fermilab cluster is shown in Fig. 2.2 with
one scale extremely expanded relative to the other. Grouped below or
nearly below the existing Fermilab site are the underground injection
and extraction beam lines, the beam absorber, and the RF acceleration
stations. In the on-site straight sections are two detectors, separated
sufficiently

![](media/image2.png)

Fig. 2.2 VLHC cluster at Fermilab, showing the various functions.\
Note the differences in E-W and N-S scales.

in distance and angle to avoid muon background from one detector hitting
the other. The cluster opposite the Fermilab site is mostly passive,
although a second campus could be developed there. The machine functions
at the far cluster are beam scraping and, eventually, the systems for
beam transfer from Stage 1 to Stage 2.

Fig 2.3 A cutaway view of the underground geology at Fermilab with the
VLHC tunnel in its approximate location. The vertical scale is expanded
relative to the horizontal scales.

Several different injection layouts from the Tevatron are possible and
are discussed in Chapter 4. To take advantage of the best geology in the
area, the VLHC tunnel is approximately 120 m below the surface at
Fermilab, much deeper than the Tevatron, as shown in Fig. 2.3. The
lengths of the injection lines are determined by the desire to descend
gradually to that depth because these ramps, and a similar one on the
other side of the VLHC ring are used during the construction phase for
the installation of magnets and other technical components.

A cross section view of the tunnel at its minimum width of 3.6 m and
minimum floor width of 3.0 m is shown in Fig. 2.4 with the Stage 1
collider installed. Notable are the small Stage 1 combined-function
magnet installed on stands on the floor of the tunnel, and the small
amount of necessary infrastructure. For example, there is only one small
cable tray, because all of the correction elements are powered from
local supplies installed in wall penetrations, and all of the
instrumentation and controls are local, with only fast communication to
the rest of the world. This design eliminates almost all long cables
except for some bundles of optical fiber.

![](media/image3.png){width="4.125in" height="3.115972222222222in"}

Fig. 2.4 A cross section of the VLHC tunnel with the Stage 1 magnet in
place.

Sections of the tunnel used for services will have to be larger than the
nominal minimum of 3.6 m. The long straight sections for injection and
extraction and experimental areas, the tunnel below the major service
buildings, and anywhere where two tunnels join will be among them. In
addition, there is an adit to the tunnel every 10 km that contains
cryogenic valve boxes and transformers and bulk power supplies for DC
power distribution in the tunnel. There are additional personnel exits
every 3.3 km between the 10 km points. Tunnel locations under the
service areas and the future locations of the Stage 2 service areas will
have adits sized for Stage 2 during the initial construction.

## 2.2 Geology

The terrain in northern Illinois is excellent for building large
accelerators. The surface is relatively flat, and there are thick layers
of self-supporting and competent rock underground, as can be seen in
Fig. 2.3. The geology is not perfect, however. The best rock, the
Galena-Platteville dolomite, is deep underground and is tilted down to
the east. There is an inactive fault west and south of Fermilab with a
large mismatch of geological properties on either side. In order to
sample all of the features that are in the area, and to get an idea of
the technical, environmental and financial issues involved in tunneling
through various media, we have included three tunnel orientations
discussed in detail in Chapter 7.

-   A south ring with a 0.08% incline and a depth at Fermilab of 235
    feet. This orientation requires tunneling through the Sandwich fault
    and is relatively shallow.

-   A north ring, with no incline and a depth at Fermilab of 330 feet.
    This orientation samples numerous types of underground media.

-   A north ring with a 0.20% incline and depth at Fermilab of 501 feet.
    This ring is inclined to stay almost exclusively in the
    Galena-Platteville dolomite. It is very deep in some locations.

The tunnel in the dolomite will not be lined but may have to be grouted
and sealed in places. Some of the tunnel will have to be lined in order
support poorer quality material and to avoid excessive inflow of water,
which we have limited to no more than 50 gallons per minute per mile of
tunnel. Large underground cisterns are constructed as part of the tunnel
near the six major service areas, and pumps with emergency backup empty
them. This is a potential environmental issue and the possibility of
lining the entire tunnel to reduce the water inflow is being studied.

## 2.3 Surface Features

The most prominent off-site surface feature of the VLHC will be the
service buildings required for refrigeration and other utilities for the
collider. Stage 1 needs only six service building, roughly 40 km apart.
A typical Stage 1 service area is shown in Fig. 2.5. One of these is on
the Fermilab site and is larger than the others to accommodate
additional utilities and heat load required by the experimental areas.

Fig. 2.5 A typical service area for the Stage 1 VLHC.

The Stage 1 service areas are modest, each requiring about ?? acres of
space, including heat rejection systems and 5 MW of installed power.
Stage 2 will require six additional sites approximately halfway between
the Stage 1 service areas, and an upgrade of all 12 sites to ?? acres
each, each with approximately 20 MW of installed power. The heat
rejection capability for Stage 2 compressors is about 8 MW per service
area. Minor areas are present at approximately three kilometer
intervals. They will have small buildings to cover and interlock the
personnel exits and contain backup (generator) power for the elevators
and lights. Every 10 km these buildings will be slightly larger because
of additional underground utilities and power requirements.

The two detector caverns and associated service buildings are on the
Fermilab site. These caverns are sized according to the SSC design and
are ? m by ? m by ? m high. It will be convenient but not necessarily
possible to construct them sufficiently deep that their roofs have a
thick enough dolomite cover that they will be self-supporting over that
large span. The two areas have bending between them so that the
background, particularly muons generated in one interaction region will
not appear in the other. Experiments similar to those designed for SSC
or being built for LHC are thought to be adequate for the 40 TeV
collisions of the Stage 1 VLHC.

## 2.8 The Lattice

While the Stage 2 collider is far in the future, one needs to anticipate
its requirements as best one can before laying out the components of the
first machine, since they ultimately must reside in same tunnel. This
entails anticipating the geometric, space and beam optics requirements
for future technical equipment -- much of which may not yet be known.
This creates the need for some compromises. For example, while the size
of the arcs is determined primarily by the low magnetic field of the
Stage 1 collider, the lengths of the straight sections and the
interaction regions are determined by the high energy (magnetic
rigidity) of the Stage 2 beam. Another complication of the geometry
comes about at the transition regions between bending and non-bending
portions of the accelerators. Special optical modules -- dispersion
suppressors -- are used to bring the orbits of off-momentum particles to
coincide with each other in the long straight sections. Since the Stage
1 and Stage 2 designs have different focusing characteristics, the
dispersion suppressors must be designed simultaneously to ensure that
the Stage 1 and Stage 2 orbits line up appropriately when entering the
long straight sections for the interaction regions and utility regions.

Table 2.2 Major Lattice Parameters for Stage 2

To allow for lattice and component designs to progress simultaneously
for both the Stage 1 and Stage 2 colliders, a general layout composed of
basic modules was developed. Each collider is made up of two major arcs
that connect two clusters of straight sections. The clusters contain
space for two nearby interaction regions and two utility regions that
are used for injection and extraction, accelerating cavities, and other
necessary functions. Short bending regions are located between each
interaction region and utility region to allow collision debris and muon
vectors to miss the downstream detector and utility enclosures. At the
outset, only one cluster region -- the one located at the Fermilab site
-- will be equipped with full interaction region and utility region
optics. The other cluster region will have the same straight section
lengths, but optically will consist of simple FODO-type modules, with
the exception of one area with large dispersion used for beam scraping.
Since the Stage 1 magnet has side-by-side apertures, each cluster
contains one insertion in which the beams are crossed without colliding,
to guarantee equal path lengths for the two beams. The exact length of
the standard cell is determined by the chosen bunch spacing. To ensure
that bunches will collide at any detector in the design, regardless of
what modules exist between interaction points, the ring module lengths
are designed in units of the bunch spacing.

## 2.5 Stage 1 Technical Components

### 2.5.1 Magnets

The major technical component of the VLHC is, of course, the main arc
magnet. The Stage 1 magnet, shown in perspective in Fig 2.6 is a
superferric gradient magnet of simple and elegant design. It is not only
less costly per Tesla-meter than the typical high-field superconducting
magnet, but also requires much simpler and less costly services, such as
cryogenics, power supplies, quench protection, and so forth. Since it is
a gradient magnet, quadrupoles are not required in the arcs. Its long
length, 65 m, presents some challenges in handling and transportation,
but reduces the number that need to be fabricated and tested and greatly
decreases the overall cost of production and installation.

![](media/image4.png)

Fig, 2.6 The superferric "Transmission-line" gradient magnet used in the
arcs for the Stage 1 VLHC

The magnet and its associated systems are discussed in detail in Chapter
5. The magnet is energized by passing 100 kA through the superconducting
cable in the center of the steel yoke. The magnetic field in the gaps on
either side are in opposite directions for the oppositely directed beams
of protons. The current returns in a similar cable in the helium
supply/return line in the cryogenic service pipe below the magnet. The
separation of the two cables generates a small dipole fringe field in
the tunnel. The gaps are narrow to keep the magnet and the excitation
current as small as possible. With a two centimeter gap on either side
the magnet will reach 2 T at 100 kA. The small beam pipe creates some
stability issues for the beam, and although standard methods seem to be
able to keep them within bounds, it is a subject for further study and
R&D. Since the steel yoke and the beam pipes of this magnet are warm,
the cold mass of this design is very low, so cool-down times and
inventory are impressively small. In addition, the electromagnetic force
on the conductor is small, permitting a support structure with very low
heat leak, hence, low operating power. Parameters for the main arc
dipole and dispersion suppressor dipole are listed in Table 5.2.

Every half-cell, 135 m, the conductor in the center of the magnet is
bent down to be close to the return conductor. This is a complication in
the cryostat, but creates a room-temperature gap that is used for the
various correction magnets: a closed-orbit dipole, a tune quadrupole and
a sextupole. Other correctors could be added as needed. All of the
correctors are conventional iron and copper magnets powered by local
DC-to-DC converters. There are also the usual collection of special
magnets in the utility straight sections. These comprise
Lambertson-style septa for the emergency beam-extraction system, which
operate in series with the main excitation bus so they are always at the
correct bend strength; injection and extraction kicker magnets, also in
the utility straight sections; conventional quadrupoles that carry the
FODO lattice across the utility straight sections; and strong, single
aperture quadrupoles and bending magnets to focus the beams and bring
them into collision at the two interaction regions.

The interaction region quadrupoles are typical of the magnets that will
be developed for LHC second-generation IRs. They operate at about 300
T/m gradient and have a larger aperture than the quadrupoles now being
built by Fermilab for the LHC, but otherwise look very similar. The
improved performance is gained by using Nb~3~Sn as the conductor.

### 2.5.2 Other Stage 1 Technical Components

#### 2.5.2.1 Cryogenics

The cryogenic system of the Stage 1 collider is distributed over a wide
area, but is about the same total refrigeration power as for the
Tevatron --- 30 MW installed, including 50 % overcapacity. The
refrigerator cold box is split, so that the cold stages of the heat
exchangers are at tunnel elevation. This reduces the helium head
pressure. There are two upstream and two downstream helium loops at each
refrigerator. One loop in each direction passes through the transmission
line and returns after 10 km. The other loop is bypassed for the first
10 km, and then is switched into the transmission line. This design
reduces the size of the transmission line cryostat. There is no liquid
nitrogen in the tunnel. The shield for the transmission line and for the
cryogenic service pipe is cooled with 40 K helium.

#### 2.5.2.1Tunnel-Resident Systems

Because of the size of the ring, we have taken special precautions to
maximize reliability and minimize maintenance in the long arcs. There is
no LCW system in the arcs, since the corrector magnets are air cooled.
Only the repetitive instrumentation and corrector power supplies at each
corrector location is required. These electronics are located in a
radiation-shielded modules buried in holes-in-the-wall at each quad
location. Average power dissipation is approximately 15 W per meter of
tunnel. All cables for instrumentation and corrector magnets are
pre-assembled on the magnet and factory tested before installation. At
alternate half-cells (270m), cryogenic thermometry and a valve
controllers are used to regulate the shield flow. Every 10km there is a
walk-in alcove that contains conventional electronics racks and provides
network connections, tunnel safety systems, bulk DC power for the
instrumentation modules, and cryogenic instrumentation for the cooldown
valve box.

#### 2.5.2.x System

ARE THERE ANY OTHER SYSTEMS THAT SHOULD BE DESCRIBED HERE? I can imagine
describing only the main magnets and leaving other stuff out. What about
you?

## 2.6 Construction and Installation Schedule

The construction of the tunnel is the major cost and schedule issue of
the VLHC. It is our goal that the entire construction, installation and
commissioning sequence be completed in 10 years. This will require a
major logistical effort, since the installation of infrastructure and
magnets must be done in one part of the ring while construction is
proceeding in another part of the ring. Because the magnets are so long,
they must be installed using ramps, not drops. It is presently planned
to have installation ramps in two locations, enabling installation in
four areas of the tunnel simultaneously. Although infrastructure can be
installed in any area in which we have occupancy, magnets can only be
installed in areas that are continuous with at least one ramp. To be
able to commission the installed components immediately after they are
installed, the surface facilities construction must also be coordinated
with the tunnel construction.

Because the VLHC is being built at Fermilab, there is the organizational
structure to move quickly to plan the construction and installation, and
to establish construction activities. Even so, we believe that it will
take about one year for the first tunneling to begin at the Fermilab
cluster and that the first section in which we can sensibly install will
be available at the start of year four. The tunneling and service
contracts will follow in sequence, with the last sector completed for
installation at the start of year eight. Hence, the complete tunnel
construction period is thought to take about six years, and the
installation period another six years, overlapped so that the total is
ten years, as shown in Fig. 2.7.

Some of the construction, such as the collision halls, will take
considerably longer. That will effect some specialized systems, although
they are all small compared to the arc magnet installation, except for
the detectors. A significant effort will be made to finish the collision
halls by the end of year five. There are about 600 magnets per sector,
so magnet installation will take about 2.5 years per sector at the rate
of one magnet per day. Hence, the peak production rate and total
installation rate in all sectors will be about four magnets per day. It
is assumed that once a sector adjacent to Fermilab is commissioned, beam
will be injected. This will cause some delay in the installation
process.

Fig, 2.7 An overview of the schedule for construction, installation of
infrastructure and magnets, and commissioning.

## 2.7 Operations

The Stage 1 collider can be filled in about one hour from the Tevatron.
Because of the cryogenic load of the rapid ramping cycle (50 s.), the
Tevatron can injects at no more than 900 GeV. The acceleration time to
20 TeV per beam is about 20 minutes, which could be shortened at the
expense of more RF. The long luminosity lifetime and long fill time
indicate that it will not be a significant advantage to decrease the
acceleration time.

The relatively low energy at injection, the small beam-tube aperture,
the large-circumference ring and the long fill time lead to some concern
about beam instabilities at injection. Although it appears that active
feedback systems can handle these instabilities, at this time they
appear to be the most serious accelerator physics issue for Stage 1
VLHC. If feedback is inadequate, there are other methods to minimize
this issue. For example, rebunching the beam in the Tevatron at 200 MHz
will reduce the peak bunch current by a factor of four. After the VLHC
is filled, the beam can be accelerated to about 3 TeV and rebunched at
53 MHz, where the instabilities are no longer an issue. Beam stability
should be one of the major R&D efforts for the VLHC.

Because of the size of the ring, emergency repairs will be time
consuming. This will put a premium on reliability, redundancy and
regular maintenance. For this reason fully redundant capability is
provided for all mission-critical subsystems in the arcs. Power
distribution is provided to each module by two independent loop feeders
with auto-resetting circuit breakers and remote disconnects. Two
independent power supplies are provided for each corrector magnet and
control function, summed by a diode. Redundant network connections to
each module are provided. The critical function of beam loss monitoring
is provided by redundant sensors read out from alternate quadrupoles
modules, so that even the death of a handful of control modules will not
require immediate maintenance. In order to keep such a system reliable,
maintenance periods every two weeks will be required to repair the many
"half power supplies" that have failed.

## 2.8 Stage 2 Issues

Round vs. flat

Synchrotron radiation

Energy reach

Power in the IR

Constant-power ramping

All to come later

I can't take it anymore

## 2.9 ES&H Issues

I see one interesting esh issue besides the worst case beam accident and
ground water, and that is the fringe field of the Stage 1 magnet. What
happens to the magnet is soemm schmuck leaves a heavy magnetic tool in
the tunnel. Does it accelerate to the magnet and smash the cryostat? Old
E-98 had a close call once with a gas rack and the fringe field of the
cyclotron. Do all tools going into the tunnel have to be bronze like in
the old cyclotron days?