# . Overview and Summary

## 2.1 General Description {#general-description .unnumbered}

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 Figure 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, beam
cleaning, and the physics detectors.

![](media/image1.wmf){width="4.082638888888889in"
height="4.377777777777778in"}

Figure 2.1. The 233-km VLHC ring.

A magnified view of the VLHC Fermilab cluster is shown in Figure 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.wmf){width="5.986805555555556in"
height="3.2159722222222222in"}

Figure 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 cleaning and, eventually, the systems for
beam transfer from Stage 1 to Stage 2.

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 (Figure 2.5), much deeper than the Tevatron, but about the same
as the LHC.

The lengths of the injection lines are determined by the desire to
descend gradually to that depth so that these ramps can be used during
the construction phase for the installation of magnets and other
technical components. A similar ramp is used for installation on the far
side of the VLHC ring.

The tunnel for this Design Study is a standard tunnel, similar in size
to the SSC and LHC tunnels, and made by standard construction
techniques. A cross section view of the tunnel at its minimum finished
diameter of 12 feet and minimum floor width of 10 feet is shown in
Figure 2.3 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. An electric trolley line
provides power for tunnel transportation and local work power. This
design eliminates almost all long cables except for ring-wide power
cables and some bundles of optical fiber.

The Stage-2 collider is shown in Figure 2.4. The Stage-2 magnets are
installed vertically above the Stage-1 magnets to preserve the ring
circumference. Additional cryogenic service piping, power and cable
trays are required.

![](media/image3.wmf){width="5.070138888888889in"
height="3.7895833333333333in"}

Figure 2.3. A cross section of the VLHC tunnel with the Stage-1 magnets
in place.

![](media/image4.wmf){width="4.986805555555556in"
height="3.713888888888889in"}

Figure 2.4. A cross section of the VLHC tunnel after the Stage-2 magnets
have been installed.

Sections of the tunnel used for services will have to be larger than the
nominal minimum of 12 feet. 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 5 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.

![](media/image5.jpeg){width="5.983333333333333in"
height="3.2118055555555554in"}

Figure 2.5. 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.

## 2.2 Geology {#geology .unnumbered}

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
Figure 2.5. The geology is not perfect, however. The best rock for
tunneling, 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,
and a disturbance from a large meteor strike near Des Plaines. 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 500 feet.
    This ring is inclined to stay 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 specified as no more than an average of 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.

![](media/image6.jpeg){width="6.3909722222222225in"
height="3.6631944444444446in"}

Figure 2.6. A typical service area for the Stage-1 VLHC. The layout of
trees, low-profile buildings, and cooling pond provides an aesthetic
10-acre site appealing to the local community.

## 2.3 Surface Features {#surface-features .unnumbered}

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 buildings, roughly 40 km apart.
A typical Stage-1 service area is shown in Figure 2.6. One of these is
on the Fermilab site and is larger than the others to accommodate
additional utilities and cryogenic power required by the experimental
areas.

The six Stage-1 service areas are modest, each requiring up to \~10
acres of space, including cooling ponds for 4 MW of installed power.
Stage 2 will require six additional sites approxi­mately halfway between
the Stage-1 service areas. In this upgrade, all 12 sites will require 40
acres each, with approximately 20 MW of installed power per site. The
land use requirements will depend on the mix of cooling ponds versus
cooling towers chosen for each service area. Land availability and
aesthetics are as important as the technical cost issues in these
choices. The heat rejection capability for Stage-2 compressors is about
8 MW per service area. Minor areas are present at approximately
five-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 100 m by 30 m by 45 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.

## 2.4 The Lattice {#the-lattice .unnumbered}

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 conservatism and some compromises. For
example, while the size of the arcs is determined by the energy and bend
fields of both colliders, 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.

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
collimation. 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. Details of
both colliders are shown in Table 1.1 and 1.2, and in Chapter 3.

## 2.5 Stage-1 Technical Components {#stage-1-technical-components .unnumbered}

### 2.5.1 Magnets {#magnets .unnumbered}

The major technical component of the VLHC is, of course, the main arc
magnet. The Stage-1 magnet, shown in perspective in Figure 2.7, 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/image7.wmf){width="6.000694444444444in"
height="4.864583333333333in"}

Figure 2.7. 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, which induces a magnetic field
that circulates in the yoke and through the gaps. The magnetic fields 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 90 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 is 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.

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 field-free region at room-temperature 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 air-cooled 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 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.

Table 2.1. Main parameters and count of magnets in the Stage-1 VLHC.

+--------------+-------+--------+--------+--------+------------------+
| **Magnet     | **B   | **     | **     | **     | **Notes**        |
| Type**       | ~nom~ | G~nom~ | L~mag~ | Number |                  |
|              | (T)** | (      | (m)**  | of     |                  |
|              |       | T/m)** |        | elem   |                  |
|              |       |        |        | ents** |                  |
+--------------+-------+--------+--------+--------+------------------+
| Gradient     | 1.97  | 9.73   | 65.75  | 3136   | Superferric      |
| dipole (arc) |       |        |        |        | transmission     |
|              |       |        |        |        | line             |
+--------------+-------+--------+--------+--------+------------------+
| Gradient     | 1.80  | 16.88  | 48.81  | 160    | Superferric      |
| dipole       |       |        |        |        | transmission     |
| (Disp.       |       |        |        |        | line             |
| Suppressor)  |       |        |        |        |                  |
+--------------+-------+--------+--------+--------+------------------+
| Special      | 1.95  |        | 25 --  | 52     | Separation,      |
| dipoles      |       |        | 35     |        | recombina­tion,   |
|              |       |        |        |        | and cross-over   |
+--------------+-------+--------+--------+--------+------------------+
| Straight     |       | 70     | 4.8 -- | 464    | Water-cooled     |
| sect quads   |       |        | 6.8    |        | copper & steel   |
+--------------+-------+--------+--------+--------+------------------+
| Low β        |       | 300    | 9.2 -- | 16     | Nb~3~Sn          |
| quadrupoles  |       |        | 10.9   |        | Conductor        |
+--------------+-------+--------+--------+--------+------------------+
| Correctors   |       |        |        |        | Air-cooled       |
|              |       |        |        |        | copper & steel.  |
+--------------+-------+--------+--------+--------+------------------+
| > Dipole     | 1.0   |        | 0.50   | 1648   |                  |
| > (horiz.)   |       |        |        |        |                  |
+--------------+-------+--------+--------+--------+------------------+
| > Dipole     | 1.0   |        | 0.50   | 1648   |                  |
| > (vert.)    |       |        |        |        |                  |
+--------------+-------+--------+--------+--------+------------------+
| > Quadrupole |       | 25     | 0.50   | 3296   |                  |
+--------------+-------+--------+--------+--------+------------------+
| > Sextupole  |       | 1750   | 0.80   | 3296   |                  |
|              |       | T/m^2^ |        |        |                  |
+--------------+-------+--------+--------+--------+------------------+

### 2.5.2 Other Stage-1 Technical Components {#other-stage-1-technical-components .unnumbered}

#### 2.5.2.1 Cryogenics {#cryogenics .unnumbered}

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 --- 20 MW installed, including 50 % overca­pacity. It is a very
simple system, with only piping and no pressure vessels in the tunnel.
Each refrigeration plant services 38 km of arc magnets. The refrigerator
cold box is split, so that the cold stages of the heat exchangers are at
tunnel elevation. This reduces the helium gravity 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 thermal shield for the transmission line and for the
cryogenic service pipe is cooled with 40 K helium.

#### 2.5.2.1 Tunnel-Resident Systems {#tunnel-resident-systems .unnumbered}

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 low-conductivity water system in the arcs, since the corrector
magnets are air-cooled. Only repetitive instrumentation and corrector
power supplies at each corrector location are required. These
electronics are located in radiation-shielded modules buried in cavities
(holes in the wall) at each quadrupole location. Average power
dissipation is approximately 15 W/m of tunnel. All cables for
instrumentation and corrector magnets are pre-assembled on the magnet
and factory tested before installation. At alternate half-cells (270 m),
cryogenic thermometry and valve controllers are used to regulate the
shield flow. Every 10 km 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 cool down valve box.

## 2.6 Stage-1 Construction and Installation Schedule {#stage-1-construction-and-installation-schedule .unnumbered}

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 Figure 2.8. It will take a significant effort to
finish the collision halls by the end of year five so that detector
installation can start. After magnet installation is completed in a
sector, that sector will be commissioned, that is, cooled down and
energized, even while installation and perhaps tunneling is continuing
in other sectors of the ring.

![](media/image8.wmf)

Figure 2.8. An overview of the schedule for construction, installation
of\
infrastructure and magnets, and commissioning.

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
total installation rate in all sectors will be about four magnets per
day. Since our plan has only two magnet-assembly factories, the magnet
production will need to start earlier than installation, and a
significant amount of magnet storage space will need to be constructed.
Other possibilities are to have more magnet-assembly areas or to extend
the construction period to 12 years. It is assumed that once a sector
adjacent to Fermilab is commissioned, the magnets will be energized and
beam may be injected. This will cause some delay in the installation
process.

## 2.7 Stage-1 Operations {#stage-1-operations .unnumbered}

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 inject 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.

Because of the size of the ring, emergency repairs will be time
consuming. This will put a premium on reliability, redundancy and
regular maintenance. 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. Redundant
network connections to each module are provided. The critical function
of monitoring beam loss is provided by redundant sensors read out from
alternate quadrupole modules, so that even the failure 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 "half-power supplies" that have failed during
that period. Because the helium refrigerators have expanders in the
tunnel, that part of the cold box is in an alcove far enough from the
tunnel to allow access while beam is on, which, in conjunction with
generous liquid helium storage, will lead to improved availability.

## 2.8 Stage-2 Installation {#stage-2-installation .unnumbered}

At some point in the operation of the Stage-1 VLHC it will be decided
that an energy upgrade is necessary. Production will begin on the
high-field dipoles, which although more difficult technically than the
superferric magnets of Stage 1 are more conventional in size and
appear­ance. Since Stage 2 will require more than 11,400 double-aperture
dipoles, 1700 double-aperture quadrupoles, and about 1700 spool pieces
with more than 10,000 correctors of different types, production will
require the participation of many industrial companies throughout the
world. The magnets can be transported by conventional means and
accumulated in industrial storage space so that a large inventory is
available at the start of the shutdown. Since the tunnel has a complete
infrastructure, magnets can be installed in many locations
simultaneously. It is anticipated that the peak installation rate will
be about 12 magnets per day, and that the conver­sion from Stage 1 to
Stage 2 will take less than seven years. The major impediment to this
fast-track schedule appears to be the ability of what is now a small
industrial base to make enough Nb~3~Sn wire and cable in the five-year
magnet production period. If the magnet production begins earlier and
more effort is put into installation, the conversion time could be made
shorter.

At the same time as magnet installation is proceeding, the tunnels must
be constructed for the Stage-1 bypass, the Stage-1 machine must be
rerouted through the bypasses with a few higher-field (about 4 T)
magnets, and two new or upgraded detectors must be assembled in the
existing collider halls. It seems reasonable that all of these
operations could be completed in less than the seven-year conversion
time.

## 2.9 Stage-2 Operation {#stage-2-operation .unnumbered}

Since the Stage-2 collider is the same size as the Stage-1 collider, it
can be filled in about 30 seconds by transferring multiple
Tevatron-length batches already stored in the Stage-1 ring, freight-car
style, into the Stage-2 ring. The ramp up to full energy of 87.5 TeV per
beam will take about 35 minutes, limited first by the RF accelerating
voltage and later by the amount of installed power for the magnet
supplies. The energy stored in the magnetic field is returned to the
power grid when the magnets are ramped down in preparation for the next
cycle. Hence, the minimum time from physics collisions to physics
collisions will be about 1.2 hours. We have chosen 10 TeV as the
injection energy since that seems completely adequate from the point of
view of beam instabilities and dynamic aperture, and because the lower
the injection energy, the less costly is the injection equipment, such
as kickers and septa.

From an accelerator physics viewpoint the most interesting feature of
the Stage-2 machine is that synchrotron radiation will lead to damping
of the beam emittance, and hence, a noticeable improvement in luminosity
during the store even as the beam is evaporating due to inelastic
collisions at the interaction points. This enhancement allows excellent
luminosity at reduced beam current, which has the effect of limiting the
stored energy in the beam and the compressor power needed to remove the
synchrotron radiation power in the cryogenic system. The reduced beam
current also reduces the optimum storage time, but at eight hours, that
is a small effect on the integrated luminosity. Another interesting
consequence of synchrotron radiation is that one can trade off
luminosity against operating energy: going up in energy, one does not
lose as fast as one might imagine, because the damping becomes more
effective. It appears that the Stage-2 VLHC could operate at 200 TeV and
2 × 10^34^, as shown in Table 1.2.

## 2.10 ES&H Issues {#esh-issues .unnumbered}

During construction, the major environmental issues will be disposal of
rock chips from the tunnel and the disturbance to neighborhoods during
the construction period. These are non-trivial issues, but they can be
handled with appropriate planning, communication and sensitiv­ity. It is
clear that a lot of thought will need to be put into organizing the
construction effort, particularly since most of the construction will be
staged off site in close proximity to private land.

During operation there are a few issues that are similar to those
occurring now in all accel­erators, but exacerbated by the higher
particle energy and total stored beam energy of these machines, and the
fact that the VLHC is so large in circumference. Most obvious among
these is the so-called "worst-case accident," which we believe to be the
loss of the entire beam at a single point in the arcs of the Stage-2
collider. We have made a preliminary study of this accident, and it
appears that it can be handled. It will scar a section of tunnel wall
and create a local radiation cleanup problem in the tunnel. It will also
irradiate some ground water with tritium and sodium 22, but not to a
level that creates a severe environmental or health hazard. The total
activation is equivalent to losing the Main Injector beams at a single
point for an eight-hour shift. A more plausible accident involves beam
damage from an accidental partial firing of the beam extraction kickers.
The potential beam damage in this case is confined to several
well-defined places which can be protected by special graphite
collimators to intercept the beams \[1\].

One interesting equipment hazard that is unique to the Stage-1 machine
is the fringe field caused by the large separation of the drive and
return currents in the magnets. When the mag­nets are energized to full
field, there is a region of about 30 cm radius around the magnets and
centered approximately at the bottom of the steel yoke, where steel
tools can be levitated and accelerated toward the magnets. If they hit
the magnets in particular spots they could cause localized but
significant damage. This is not a personnel safety issue because no one
can be in the tunnel when the magnets are energized, but precautions
need to be taken to avoid equipment damage. There will be no measurable
magnetic field at the surface for either machine.

## 2.11 Foci for Future Development {#foci-for-future-development .unnumbered}

In order to focus the R&D over the next few years, it is first necessary
to understand the techni­cal problems that might limit performance or
even make the operation of the collider impossible or unsafe; second, to
understand the issues connected with construction, fabrication and
instal­lation that could delay startup or create unwanted environmental
concerns; and third, to enu­merate those technical or scientific
developments that could, if successful, make the VLHC less costly or
lead to improved performance.

### 2.11.1 Performance Issues for Stage 1 {#performance-issues-for-stage-1 .unnumbered}

The relatively low energy at injection, the small beam-tube aperture and
the large-circumference ring have led to some concern about beam
instabilities and tune shifts at injection. Calculations to date and
experience on working machines show that a combination of active
feedback systems and natural damping mechanisms can handle the
instabilities.

During the course of this study we have identified and incorporated two
design changes to minimize these issues. Firstly, a bunch coalescing
scheme similar to the one used in Tevatron Collider operations has been
incorporated to avoid potential single-bunch instability problems at
injection. In this scheme, four low intensity 200 MHz bunches are
injected, accelerated, and then combined into single high-intensity 53
MHz bunch at collision energy where the instabili­ties are not an issue.

A second change was made to minimize the tune spread due to
time-dependent magnetic image currents in the steel pole tips. This
effect is minimized by loading the VLHC ring uni­formly around the
circumference as described in Chapters 3 and 5. Residual tune spread can
be eliminated, if necessary, with a pair of trim quadrupoles operating
at audio frequencies.

At this time these appear to be solutions for the most serious potential
accelerator physics issue for the Stage-1 VLHC. If further calculations
show this to be inadequate, the magnets will have to be redesigned with
larger gaps, requiring more steel and a higher drive current. Both of
these are cost issues but neither affect the feasibility of the machine.
Beam stability will be one of the major R&D efforts for the VLHC.

Other magnet issues are verifying the magnetic field quality for
significant quantities of the final magnet design, and systems issues
such as thermal load in the transmission line and interconnect design
and tests. The issues associated with producing and handling very long
magnets need to be simulated and then tested. The insertion quadrupoles
have challenges that appear able to be conquered, but the realization
needs to be demonstrated. They will require Nb~3~Sn technology. A
fallback using NbTi would entail approximately a 25% luminosity pen­alty.

Other issues that might seem problematic have been looked at and
although they all need further study, they do not at this time appear to
be performance limiting. Among these are ground motion degradation of
beam emittance or alignment, the alignment process itself, and the study
of potential supercritical flow instabilities in the cryogenic piping of
Stage 1.

### 2.11.2 Construction Issues for Stage 1 {#construction-issues-for-stage-1 .unnumbered}

The most obvious construction issues for Stage 1 are the long tunnel
running under land that is in private or public use, and the massive
amount of fabrication and installation. The tunnel construction was
mentioned above as an environmental issue, and in Chapter 1 as a public
acceptance issue. The planning, tooling and equipment required for
handling, assembling, testing, storing and installing and commissioning
the long magnets and the subsystems and infrastructure will be a
significant challenge. We have created a model of a ten-year
construc­tion period with two factories and two installation ramps. It
appears that this schedule may be hard to keep, and we may need either
more assembly factories and installation ramps, more installation
equipment, or a few years longer to complete the job. It may be more
cost-effective to have an additional factory, since it also opens up
more of the ring to timely installation.

### 2.11.3 R&D Aimed at Improvements and Cost Reduction for Stage 1 {#rd-aimed-at-improvements-and-cost-reduction-for-stage-1 .unnumbered}

Considering the cost drivers, there are two obvious possibilities for
cost savings in the con­struction of the Stage-1 VLHC: improved and less
costly tunneling and underground construc­tion techniques, and cheaper
magnets. Investing in R&D in these subjects holds promise for
significant reductions in the cost of the VLHC. The major component of
tunneling costs is labor, and automation of the process would not only
reduce the cost, but make tunneling safer. This R&D would be useful to
society at large, in addition to its specific application to the VLHC.
There are also many possible tunnel designs. Some may eliminate many of
the special underground additions, such as sump pits, egress and access
side-rooms, and so forth, all of which add greatly to the cost. These
issues need serious engineering that they have not yet received.

Our cost-driver analysis indicates that the superferric magnet is
between two and three times less costly per Tesla-meter than
conventional superconducting magnets. Automation of the production might
make them even more cost effective by reducing assembly and sub-assembly
labor. The single most expensive component is the yoke steel, so being
able to produce high-quality magnet steel assemblies inexpensively would
be a big money saver.

The vacuum system is surprisingly expensive, driven by the costs of the
standard ion pumps needed to reduce the partial pressure of non-reactive
gasses. Finding a simpler pump design or a getter that will pump methane
would drastically reduce the cost of the vacuum system.

### 2.11.4 Performance and Development Issues for Stage 2 {#performance-and-development-issues-for-stage-2 .unnumbered}

We chose 175 TeV collision energy as the operating energy of the
high-energy VLHC, but this study has shown that neither vacuum problems
nor local heat-removal issues will arise from synchrotron radiation
impinging on the beam-tube liner. The only limitations to operation up
to 200 TeV and 2 × 10^34^ cm^-2^s^-1^ luminosity appear to be the amount
of beam debris power in the IRs and the total compressor power needed to
remove the synchrotron radiation power. The first of these will need a
dedicated team to study the flow of particles and energy into the
detectors and into the magnets close by the interaction point. More
sophisticated algorithms to match finer grained and faster detectors
close to the IP may be developed and may make a big im­provement. The
development programs on more radiation-resistant electronics, such as
deep sub-micron silicon or diamond-based detectors need to be continued
and enhanced. Computer simulations of energy deposition and tests on
radiation damage need to be started to understand design and materials
issues that may influence the parameters of the interaction insertions
and the detectors.

As to the removal of synchrotron radiation power, it appears entirely
feasible to place a room-temperature "finger," or synchrotron radiation
mask between magnets that are about 14 meters long that will intercept
all or most of the synchrotron radiation, and that will not interfere
with the beam or create impedance problems. This can greatly reduce the
compressor power, and allow higher luminosity, higher energy, or both.
With such a device, collision energies greater than 200 TeV and
luminosities greater than 2 × 10^34^ cm^-2^s^-1^ are certainly possible.

A possible enhancement of performance could come from using the
synchrotron radiation damping to obtain flat beams in collision as is
done in all electron colliders. The advantages appear to be in doublet
optics in the interaction region, and smaller β~max~ in the quadrupoles.
The disadvantages have to do with extremely challenging magnets that
would have to operate in the environment of the interaction insertion.
This issue and the necessary magnets deserve some study.

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

\[1\] J. Donald Cossairt et al., "Environment, Safety & Health
Considerations for a New Accelerator Facility," FERMILAB-Conf-01/051-E,
April, 2001.