# . R&D Programs and Related Studies

The purposes of Research and Development programs and engineering
studies are to reduce technical risk and cost, and to improve
performance. The following sections briefly discuss what we see now as
the most important R&D that should be vigorously pursued for both
Stage‑1 and Stage-2. There is a distinct difference in the nature of the
R&D between Stage-1 and Stage-2; the work on the low-energy collider
emphasizes systems issues, engineering and cost reduction, while the R&D
for the high-energy machine is strongly slanted toward the development
of components. This difference is appropriate, because the components of
the Stage-1 collider are relatively straightforward, whereas the Stage-2
machine requires cutting-edge components, particularly magnets.

## 10.1 R&D for the Stage-1 VLHC {#rd-for-the-stage-1-vlhc .unnumbered}

Because of the simplicity of the magnet and accelerator systems of the
Stage-1 collider, there are only a few risk-associated R&D studies
needed. Most of the R&D effort is devoted to reducing costs.

### 10.1.1 Tunneling R&D and Engineering {#tunneling-rd-and-engineering .unnumbered}

The major cost driver in the Stage-1 collider is tunneling and other
underground construction. Large investments in underground construction
R&D and engineering studies are easily justified because even small
improvements in efficiency will save considerable cost and avoid
significant financial risks. Furthermore, the R&D results would be
useful to a broad range of business and government far beyond
high-energy physics. It might be possible to find funding from sources
outside of HEP to support this R&D. The R&D is of two types:
improvements in the current tunnel boring machine and conveyor belt muck
removal systems, and trying new and innovative methods that might, if
successful, reduce cost and improve safety.

Typically, forty percent or more of tunneling costs are in labor. Hence,
development of automated or semi-automated procedures looks very
promising. Such procedures could also result in improved safety, since
fewer workers would be underground and at risk of injury. This is not
fancy technology that will be difficult to develop and sell to the
conservative tunneling industry, but relatively straightforward
application of existing techniques that are widely used in other
industries.

From a study carried out for Fermilab \[[^1]\] cost drivers in
conventional tunneling in Fermilab region geology are well understood.
Improvement in tunnel boring machine (TBM) utilization can be achieved
by fully instrumenting the TBM and trailing gear with pressure sensors,
flow meters in the hydraulic and water systems, vibration sensors placed
on all critical devices, and TV cameras positioned to watch all moving
components. The TBM can be remotely operated from any place on the
construction site. This dramatically reduces labor costs and improves
job safety. Maintenance can be done as scheduled. Breakdowns aren\'t as
costly, because you don\'t have large crews standing around waiting for
a repair to be completed. The cost per meter of tunnel can be
significantly reduced \[[^2]\]. A Fermilab group has formed an informal
collaboration with the Kenny Construction Company to instrument a TBM
and gather its data in a TARP Tunnel under construction \[[^3]\].

There are also many possibilities for the development of better tools
and machinery to make tunneling faster, safer and cheaper. In a
systematic R&D program aimed at reducing tunnel costs, new approaches to
tunneling can be explored. A visionary goal is to have no people
underground except during maintenance. The mining industry is moving
toward totally robotic systems but this technological evolution has not
yet reached the construction industry. A modified TBM with corner
cutters \[[^4], [^5]\] will produce a flat floor and allow use of a
diesel-fume free battery operated, rubber tired autonomous muck removal
and transportation system.

Close collaboration between accelerator designers and tunnel engineers
can also cut costs. The many additions, side caverns and adits that an
accelerator requires drive up the cost of underground construction.
Careful engineering of the accelerator in conjunction with the tunnel
may be able to eliminate some of these and significantly reduce costs.
This study will involve a multi-disciplinary team of accelerator
scientists, engineers and underground construction experts.

### 10.1.2 Vacuum System {#vacuum-system .unnumbered}

The vacuum system for the Stage-1 collider is a surprising cost driver.
Development and studies of different methods to reduce its cost are
worthwhile. These could include simpler and less costly ion pumps and
improved beam-tube materials and fabrication techniques. There are a
number of possibilities. There may be practical non-evaporable getter
material (NEG) that will pump semi-inert gasses such as methane. It
would also be worthwhile to investigate the possibility that NEG strips
exposed to synchrotron radiation and/or secondary electrons can pump
methane. If so, this would eliminate the need for ion pumps. Another
fruitful area of investigation is thin film NEG coatings which are under
development at CERN. At a minimum these could be useful in special
regions of the machine where conventional pumping solutions are
difficult or impossible. The possibility that these coatings could
replace the NEG strips, and possibly eliminate the need for ion pumps
when exposed to synchrotron radiation, should also be investigated.
Photodesorption coefficients of thin film NEG coated beam tubes and the
pumping speed of these coatings in the presence of synchrotron radiation
need to be documented.

The possibility of using the already present cryogenics of the magnet
transmission line for a beam tube cryopump was recognized at the
beginning of the study but it was felt that the cost of the cold to warm
transitions and thermal shields would exceed the cost of the system that
was analyzed. Perhaps this question should be reexamined. Unless the
cryopump temperature is equal to or less than \~3 K, cryosorber will
need to be added to pump hydrogen beyond one monolayer.

### 10.1.3 Beam Stability {#beam-stability .unnumbered}

There are a number of potential beam stability issues in the Stage-1
collider. As discussed in Chapter 3, these have been solved in the
design study by rebunching the beam in the Tevatron before injection
into the collider, using a uniform filling sequence, and by using an
audio frequency tune-equalization quadrupole in the Stage-1 collider.
None of these steps seem difficult or expensive, but they do need to be
studied, both on paper and by experiments on existing machines. In
addition, there are some other promising possibilities, particularly
TMCI feedback systems, that are even less costly and also deserve study.

Studies at Fermilab that might lead to reduced transverse beam emittance
would allow lower current in the Stage-1 collider and have many good
effects, including reducing the possibility of beam instabilities.

### 10.1.4 Magnetic Field Quality {#magnetic-field-quality .unnumbered}

Superferric magnets are unlikely to have many field-quality problems
except near saturation. Nevertheless, we have not yet made a magnet with
good field quality using the correct cross section for the design-study
lattice. There is a contract with the D.V. Efremov Scientific Research
Institute (St. Petersburg, Russia) to make two 6-meter long magnets with
accurate laminations. It might be useful at the same time to build and
test cross sections with some fast-turnaround fabrication method, such
as wire EDM machining. A study of the sensitivity of the magnetic field
to steel chemistry and production tolerances would also be informative
and might lead to cost-savings. Many of these measurements have been
carried out on the magnet test bench shown in Figure 10.1. In this
apparatus, 10 turns of water-cooled conductor are used to emulate the
100 kA single-turn conductor in the Transmission Line magnet. It is a
simple matter to make yokes of any desired pole shape and size, clamp
them around the conductor, and make accurate measurements.

![](media/image1.jpeg){width="5.30625in" height="4.229861111111111in"}

Figure 10.1. Apparatus for magnetic field quality measurements.

### 10.1.5 High-Gradient IR Quadrupoles {#high-gradient-ir-quadrupoles .unnumbered}

The interaction regions of the VLHC require high-gradient quadrupoles
that seem feasible but are beyond the present state-of-the-art. These
magnets need to be developed, and since such developments have
traditionally taken many years, the R&D should start now. The target
gradient for these magnets, 300 T/m, requires Nb~3~Sn of improved
performance, and hence, a superconducting materials R&D program is also
required in parallel. The U.S. LHC collaboration has proposed developing
quadrupoles of similar performance for a luminosity upgrade of the LHC.
If that effort is funded, it may accomplish the necessary R&D for the
VLHC as well.

Fermilab is particularly well-positioned to lead the high-gradient
quadrupole R&D effort. In recent years, we have built the infrastructure
necessary to do this work---reaction ovens, a strand-testing facility, a
compact cable-making machine, winding and curing tooling for model and
full-sized magnets, and a vertical dewar test facility (Figure 10.2)
that will soon have 30 kA capability. In addition, if wide cables are
required, LBNL is the world leader in cable-making R&D, and has a
facility to make cables containing up to 60 strands. This infrastructure
at the two labs allows the flexibility of design and fast turn-around
necessary for a successful R&D program involving such challenging
magnets.

![](media/image2.png){width="4.430555555555555in"
height="4.072916666666667in"}

Figure 10.2. The Vertical Magnet Test Facility with a model LHC IR
quadrupole\
ready to be inserted for testing.

### 10.1.6 Magnet Production, Handling and Operation {#magnet-production-handling-and-operation .unnumbered}

The very long magnets discussed in the design study raise a number of
issues that are certainly soluble but need both detailed engineering and
some R&D. These include the welding of the half-cores from the inside to
permit an open-gap magnet; design of the cryogenic pipes and support
system for large thermal contractions; beam pipes and transmission lines
with short or non-existent bellows; the use of thicker laminations,
extruded steel yokes, and less costly low-carbon steel to reduce the
cost of the steel yokes; and engineering and prototyping of handling,
transportation and installation needs. The R&D plan must contain the
fabrication and operation of long magnets in a "string test" that will
prototype all of the issues involving the production, handling,
installation and operation of the transmission-line magnets.

### 10.1.7 Cryogenic System {#cryogenic-system .unnumbered}

Figure 10.3 shows the apparatus used for developing the transmission
line. The large magnet is used as the primary of a transformer to induce
100 kA in the single-turn superconducting transmission line. During the
past 12 months several different transmission-line configurations were
successfully tested in the current loop to 100 kA and beyond.

![](media/image3.wmf){width="4.916666666666667in"
height="3.5479166666666666in"}

Figure 10.3. The test apparatus used at MW-9 for developing the
transmission line.

The transmission-line magnet can, in principle, have a very small heat
load, which leads to small cryogenic plants spaced far apart, and low
operating costs. Several concepts exist for further optimization of this
heat leak. With the plants spaced 40 km apart, there is the possibility
of flow instabilities if there are regions of high relative heat load
due to assembly errors or bad vacuum. These issues can only be
understood and resolved by the combined use of computer simulation,
bench-top laboratory experiments and large-scale demonstrations, using
components fabricated in a realistic production environment. The results
of these efforts will be fed back into the production and quality
assurance methods, and will influence the designs of the instrumentation
and controls systems, including software for stable control and recovery
during upset conditions. The test string planned for operation at the
MS-6 location at Fermilab will answer many of these questions. It will
have magnets and transmission line mock-ups connected to a
well-instrumented refrigerator close by. It is expected to be fully
operational in mid-2002, although earlier tests will be done using
helium from dewars.

Using turbo compressors could increase the efficiency of the cryogenic
plant. In order to use turbo compressors, helium must be mixed with high
molecular-weight gasses at the inlet of the machine and effectively
separated at the outlet. It is necessary to develop a reliable
separation/purification system to keep contamination of the helium below
the required level.

As mentioned above, one of the issues of operating with supercritical
helium is control of instabilities. One way to avoid these problems is
to operate at higher temperature, in gaseous helium. It seems possible
to add superconductor to the transmission line, which will have a small
cost impact, permitting the transmission line to operate at a higher
temperature. This would make control easier and the refrigerator more
efficient, and may lead to a net cost reduction.

### 10.1.8 Public Acceptance and Outreach {#public-acceptance-and-outreach .unnumbered}

One of the major hurdles that must be overcome before any big machine
can be built is convincing the public that building and operating the
collider under their homes and neighborhoods is both safe and
environmentally sound. As a community, we have not been very adept at
this activity in the past, but we will need to be in the future. A
program of understanding the attitudes and real concerns of our
neighbors and interacting with them in constructive ways must be
started. In addition, understanding what other labs, such as CERN and
DESY have done to permit them to construct off their sites would be
useful.

Even though we believe the VLHC to be the least expensive way to advance
the energy frontier, it is not cheap. Getting the public and political
support necessary to be funded will be a very challenging task. It must
begin by educating and informing American citizens and their leaders of
the importance of this research to their culture, civilization and
well-being. As a community, we need to lead in this effort and not leave
it up to the funding agencies. Again, understanding what our citizens
and leaders view as important and how we might fit in among those
perceptions is the first step.

### 10.1.9 Other Engineering Studies {#other-engineering-studies .unnumbered}

There are numerous issues for the construction and operation of the VLHC
that can be solved by engineering studies that will not involve much
R&D. In many cases, these are as important and challenging as the R&D
itself. Among them are studies of the integration of construction and
installation to reduce the total construction period. This is a complex
organizational problem that could be studied through computer
simulation. Other computer simulation studies might include construction
and operational safety, which could inform us about the number of
necessary egress points, the design of tunnel vehicles, and even safety
and administrative practices.

Studies and simulations of beam accidents and their effect on the
environment are absolutely necessary (see Section 5.3.6), as are studies
of the environmental impacts of the actual construction of the
underground and surface facilities. These studies must include the issue
of groundwater handling and radiological contamination; for a detailed
discussion, see Section 7.8.

The cost analysis presented in this study is very preliminary. It must
be improved by continued iteration and deeper detail. In addition, there
are other staging options and designs beside the one chosen for this
study. It is important that some effort be invested early in the program
to understand what the VLHC might look like and cost if those other
designs were used. Exercises of this type must go hand-in-hand with
discussions of high-energy physics goals and the politics of
international collaboration and planning.

## 10.2 R&D for the Stage-2 VLHC {#rd-for-the-stage-2-vlhc .unnumbered}

### 10.2.1 High Field Magnet R&D {#high-field-magnet-rd .unnumbered}

The Stage-2 magnet system configuration with the vertical bore
arrangement adopted in this study dictated the configuration of the
superconducting magnets described in Section 6.1. Arc dipole magnets are
based on the common coil design and react-and-wind fabrication
technique. This is regarded at this time as the most innovative and cost
effective approach, although it requires significant efforts to prove it
experimentally. Extensive R&D efforts in this direction are in progress
at Fermilab, LBNL and BNL.

![](media/image4.png)

Figure 10.4. Cos-theta cold-warm yoke dipole with vertical bore
arrangement. The yoke cold part is shown in the picture, the 15 mm thick
yoke warm part with ID=680 mm is not shown.

![](media/image5.png)

Figure 10.5. Cos-theta warm yoke dipole with minimized size and weight.

There are also other magnet design approaches based on the traditional
cos-theta (shell-type) coil geometry which allows both horizontal and
vertical bore arrangements as shown in and . These magnets are being
developed at Fermilab for the VLHC and meet the Stage-2 VLHC
requirements, including operating field range, field quality, critical
current and critical temperature margin, quench protection, etc.
\[[^6]\]. The designs and parameters of arc quadrupole magnets that
match these dipole magnets are described in \[[^7], [^8]\].

Due to the small bending radii in the cos-theta type coils, we are
forced to use the wind-and-react technique in order to avoid a large
degradation of the cable critical current during coil winding. For the
same coil bore diameter, the cos-theta coil configurations allow a
reduction of the coil cross section area by about 20 percent with
respect to the block-type coil. These coils can be placed much closer to
each other, especially in the horizontal bore arrangement, reducing the
iron yoke size and the total magnet size, weight and cost. The most
straightforward design for the warm yoke \[[^9]\] is shown in Figure
10.5.

The model magnet R&D programs carried out in several U.S. national labs
and universities have to address the main question: which design
approach and fabrication techniques are the best for Stage-2 VLHC? The
Stage-2 magnet R&D program will consist of three traditional phases:
Phase 1 - short model R&D; Phase 2 - full-scale prototype fabrication
and tests; and Phase 3 - magnet string (half-cell) test. It will require
funding at the level of \$5 million-\$10 million per year for five to 10
years to study different magnet designs and technologies, to select the
best arc-magnet design, to fabricate and test a series of full-scale
prototypes, to build and test a magnet system half-cell to demonstrate
the feasibility of the Stage-2 magnet system. Results of this program
will provide the basis for the cost estimate and schedule of the Stage-2
VLHC.

### 10.2.2 Strand and Cable R&D {#strand-and-cable-rd .unnumbered}

To build the high field SC magnets with the required Stage-2 nominal
field and gradient and reliable operational margins, the superconducting
strands must provide certain parameters \[[^10]\] discussed in Section
6.1 and summarized in Table 10.1.

Table 10.1. Strand target parameters for the Stage-2 high field magnets.

  ---------------------------------------------- ------------------------
  **Parameter**                                  **Value**

  Strand diameter                                0.300-1.000 mm

  Critical current density J~c~ (4.2 K, 12 T)    \>3000 A/mm^2^

  Effective filament diameter d~eff~             \<40 μm

  Cu stabilizer                                  \>50%

  Residual resistivity ratio (RRR)               \>100
  ---------------------------------------------- ------------------------

Nb~3~Sn is currently the most likely superconducting material for the
Stage-2 high-field Superconducting magnets, because of its properties
and commercial availability. At this time there are three technologies
that may reach the desired technical goals: Internal Tin (IT), Modified
Jelly Roll (MJR), and Powder-in-Tube (PIT). In the second half of 2000,
a Superconductor R&D National Program for HEP applications was started
by the DOE \[[^11]\]. This program is focussed on reaching the conductor
target parameters presented in Table 10.1 and on reducing Nb~3~Sn strand
cost. This program has proved remarkably successful. In only a few
months of R&D, U.S. companies IGC and OST improved the critical current
density of Nb~3~Sn strands by 30%. Figure 10.6 shows the progress in
time of Nb~3~Sn strand critical current density since 1984. A trendline
extrapolation suggests that reaching the goal of 3000 A/mm^2^ is
possible by the 2005.

The critical current density of Nb~3~Sn strands is controlled by a few
parameters, such as the volumetric fraction of the Nb~3~Sn phase that
can be packed in the non-copper part of a strand, the strand heat
treatment procedure, and the flux-pinning mechanisms. All of these
parameters are now under investigation and optimization \[[^12], [^13],
[^14]\]\[Error: Reference source not found-Error: Reference source not
found\] in order to reach the required J~c~.

A critical parameter responsible for the persistent current effect in
magnets and strand stability is the strand magnetization determined by
J~c~**^.^**d~eff~, where d~eff~ is an effective filament diameter. For
the Nb~3~Sn strands produced using the most cost effective technologies
such as IT or MJR, d~eff~ is quite large about 100-120 μm, and only PIT
provides a small d~eff~, on the level of 50 μm, which significantly
decreases the magnetization effects, but results in increased strand
cost \[[^15]\]. An optimization of the strand design and technology
aimed at reducing d~eff~ in the high J~c~ Nb~3~Sn strands is an
important next step of the conductor R&D program.

![](media/image6.wmf)

Figure 10.6. Improvement in J~c~ (4.2 K, 12 T) of Nb~3~Sn since 1984.

It is necessary during magnet and cable fabrication and operation to
reduce the critical current degradation relative to that of the original
virgin strands due to strain as well as to control the interstrand
resistance responsible for the thermal and electromagnetic coupling
between the strands. In the wind-and-react technique, both strand
plastic deformation during cabling (before reaction), and cable
compression in the coil during magnet fabrication and operation (after
reaction, due to coil pre-compression and Lorenz force) contribute to
I~c~ degradation and interstrand resistive coupling. In the
react-and-wind method, there is also I~c~ degradation due to the bending
strain introduced during winding. The cable R&D program goals are
summarized in Table 10.2. Cable studies performed at Fermilab and LBNL
show significant progress in developing SC cable for accelerator magnets
during last 1-2 years. The results obtained prove that cable R&D goals
will be certainly achieved \[[^16], [^17], [^18], [^19]\] \[Error:
Reference source not found-Error: Reference source not found\].

As was shown in section 6.1.8, the cost of Nb~3~Sn strands is one of the
major cost drivers of the Stage-2 magnet system. NbTi strands for
accelerator magnets, thanks to previous HEP projects and the MRI magnet
business, can be purchased in large quantities for about \$100/kg. The
small quantity price (e.g., 2 tons purchased for the U.S. LHC program)
is as much as 50% higher -- about \$150/kg. It is apparent that lower
prices in the future for this conductor is unlikely to be realized,
since most of the cost savings and property improvements available for
NbTi have already been realized. The status of Nb~3~Sn is in stark
contrast to NbTi. The quantities being purchased annually are small at
present (about 0.5 ton for accelerator magnets, two tons for MRI
magnets, and two tons for fusion energy magnets). This factor of 100
lower volume means that the economies of scale available for NbTi do not
exist. Consequently, at this time, Nb~3~Sn conductor costs about
\$750/kg.

Table 10.2. Cable R&D goals.

  ----------------------------------------- -----------------------------
  **Parameter**                             **Value**

  Number of strands                         28-60

  Packing factor                            87-90%

  Cabling degradation                       \< 10 %

  Bending degradation                       \< 10 %

  Compression degradation                   \< 10 %

  Strand crossover resistance               \>10 μOhm
  ----------------------------------------- -----------------------------

The goal of the HEP Conductor Development Program is to demonstrate that
Nb~3~Sn costs can be reduced to the \$150/kg range when the production
processes are scaled-up to those used for NbTi. In order to demonstrate
this, the key is to scale up the unit process quantities, i.e. billet
sizes, to the NbTi equivalent, and also to procure the raw materials in
quantities that will show economies of scale. The necessary
technological steps and the costs to do this, as well as the overall
process costs are discussed in reference \[[^20]\]. These scale-up
efforts will require about \$1 million per year for three years for each
process that is selected for scale-up. However, the cost savings
projected for a VLHC-size project will return this investment many times
over, and may be the critical factor determining the feasibility of such
a project.

### 10.2.3 IR Magnets for Flat Beam Optics {#ir-magnets-for-flat-beam-optics .unnumbered}

The parameters of various IR magnets required for the flat beam optics
are given in Table 6-6. The layout is shown in Figure 3.10. The
following are the major design considerations of these IR magnets:

-   Small aperture (especially in quadrupoles for generating high
    gradients)

-   Brittle superconductors that must be used for generating high
    field/gradient

-   Large Lorentz forces (associated with high fields)

-   Small separation between the two apertures (associated with the
    doublet optics)

Given the importance of these few magnets, state-of-the-art
superconductors are used in the design. The ends of conventional
cos-theta designs put a practical limit on the minimum aperture,
particularly in quadrupole magnets made with brittle material. To
overcome this and other limitations, Stage-2 VLHC IR doublets are based
on non-traditional magnet designs with racetrack coils. These are
conductor friendly designs with large bend radii and are suitable for
containing large Lorentz forces.

The minimum separation between the two apertures in first quadrupole
from the interaction point determines the layout of the entire
interaction region and the maximum β-function for the given optics,
since it sets the distance of that quadrupole from the IP. In addition,
it also establishes the maximum pole-tip field of this and other
magnets. In conventional 2-in-1 designs, the minimum separation is
determined by the conductor width required for generating field gradient
and the support structure required for containing large Lorentz forces.
In the proposed design the amount of conductor between the two apertures
is much smaller than on any other side and no support structure is
required between the two apertures. This brings a large reduction in
spacing (by about a factor of five) between the two apertures. The cross
section of the proposed design is shown in Figure 10.7. In order to
facilitate large bend radii, the return path of all turns is further
away from the aperture. Field contours and field lines in the aperture
of this magnet are shown on the right in Figure 10.7. The design is
based on react-and-wind Nb~3~Sn superconductor with a current density in
the coil of 2500 A/mm^2^ at 12 T. The magnets based on these design
principles use a much larger amount of conductor than in a conventional
design. However, the cost of conductor is not a major issue in designing
a few critical high performance magnets.

![](media/image7.jpeg){width="2.4944444444444445in"
height="2.040277777777778in"}
![](media/image8.jpeg){width="1.5013888888888889in"
height="2.0618055555555554in"}

Figure 10.7. A conceptual design of 2-in-1 Q1A (left) and flux
distribution in the coil region of the magnet (right). The design
minimizes the spacing between two apertures.

This design also introduces a strong coupling and cross talk between the
two apertures. The superimposition of a dipole field on the quadrupole
coils increases the peak field in the conductor and reduces the maximum
achievable gradient. The maximum gradient in Q1A (minimum separation)
is, therefore, 400 T/m as compared to 600 T/m in Q1B, Q2A and Q2B where
this effect is much smaller. The goal is to minimize the cross talk
induced harmonics with the exception of the dipole field. By symmetry,
the normal even harmonics and skew odd harmonics are theoretically zero.
The target field harmonics will be obtained either by design or by
correction. As the design evolves, the harmonics are expected to be
reduced. All four quadrupoles will have a different cross section.

The VLHC IR region uses three types of dipole magnets. The design of the
insertion region dipoles is also based on racetrack coils. D1A, D1B (one
each on each side) are single aperture dipoles and D2 (two on each side)
is a 2-in-1 dipole. The operating field in the smaller aperture (25 mm)
dipole D1A is 16 T (quench field \~18 T) and it uses High Temperature
Superconductor (BSCCO-2212) in a hybrid design. The operating field in
the larger aperture (50 mm) dipole D1B is reduced to 12 T to reduce the
Lorentz forces in larger aperture. This field can be entirely obtained
by Nb~3~Sn superconductor.

### 10.2.4 Cryogenic-Related R&D for a High Field VLHC  {#cryogenic-related-rd-for-a-high-field-vlhc .unnumbered}

The large total cryogenic system power, the magnet string lengths, total
cold mass, and total helium inventory for a Stage-2 cryogenic system go
well beyond that of previous systems. Scaling up standard cryogenic
methods and systems to the size of a VLHC results in some new problems
which call for R&D. The following is a list of possible areas for R&D in
cryogenics for a high field VLHC.

1.  It would be very desirable to reduce the installed cryogenic system
    power to significantly less than the presently estimated 105 MW. The
    heat load with the biggest potential for reduction by some
    innovative method is the synchrotron radiation load at 100 K.
    Developing methods for removing a fraction of the synchrotron
    radiation at room temperature could permit a dramatic reduction in
    cryogenic system size (both power and inventory) for a high-field
    VLHC.

2.  Static heat load (thermal radiation and conduction) reduction might
    be possible. One should investigate the possible utilization of
    techniques developed for MRI or by NASA (e.g., trace cooling of
    supports and/or trace cooling of an inner shield). It could also be
    beneficial to fund a development effort toward better multilayer
    insulation (MLI) and MLI installation.

3.  Improve cryogenic plant efficiency. Although there is no
    thermodynamic advantage in a heavier gas as a working fluid in a
    cycle, there might be some advantage of a higher molecular weight
    refrigerant (nitrogen or neon) in permitting the use of very
    large-scale air separation compressors and expanders. Hans Quack
    (cryogenics professor at University of Dresden, Germany) states
    \[[^21]\]: "At a paper, which I am going to present at the CEC in
    Madison, I am going to propose a new system with a mixture of helium
    and neon as refrigerant. Such a refrigerant is nearly as efficient
    as helium, it allows power recovery of the turbines and a turbo
    compressor as main compressor and one can go down to 27 K."

4.  Improve helium screw compressor efficiency. This R&D item was listed
    at the Bloomington workshop in 1994 and the Jefferson Lab workshop
    in 1999. Quoting from the 1999 report \[[^22]\]: "Screw compressor
    efficiency (FY94 recommendation): This one component accounts for
    more than half of total inefficiency of a refrigeration system. We
    use screw compressors rather than the more efficient reciprocating
    compressors because of their very high reliability. The root problem
    is that the He market is so small compared to Freon that it is
    difficult to get the vendor's R&D attention."

5.  Another R&D item from the 1999 Jefferson Lab workshop which is also
    worth listing is a study of flow instabilities in long systems.
    Density wave instabilities could cause serious trouble for a VLHC,
    and a thorough understanding of this phenomenon would be required to
    ensure good design.

### 10.2.5 Synchrotron Radiation and Vacuum {#synchrotron-radiation-and-vacuum .unnumbered}

Almost twenty years ago, when people started studying very high energy
colliders like the SSC, the LHC and the Eloisotron, it was immediately
recognized that the effect of synchrotron radiation on the beam tube
vacuum and the cryogenic system would be a major issue and R&D challenge
\[[^23] [^24] [^25] --[^26]\]. Numerous experiments and calculations
have been done since that time, and they still remain major issues for
the design and operation of high energy hadron colliders.

The cryogenic issues fall into two categories: removal of heat from the
beam-tube liner, and the total heat load. These are complex but well
understood cryogenic issues involving cryogen flow, thermal
conductivity, pressure drop, and other parameters. So many parameters,
in fact, that the solutions must be experimentally verified in addition
to being simulated on computers. We anticipate, however, that these
simulations of this complex system will be very useful in converging on
a proper design.

The vacuum system is more difficult to understand, and must be
experimentally verified. The usual solution proposed, but which will be
tested first in the LHC approximately six years from now, is to have a
beam-tube liner, shown in Figure 10.8, which intercepts the synchrotron
radiation at elevated temperature (to take advantage of improved Carnot
efficiency), and which has small perforations that permit gas to escape
and be trapped, cryopumped and shadowed between the liner and the beam
tube. This system must be tested at synchrotron light sources, although
a lot will be learned during the early operation of the LHC. Among the
issues are pumping speed, desorption coefficients and clean-up time.

![](media/image9.png)

Figure 10.8. A model of the VLHC beam-tube liner.

A photon stop has been proposed as a possibility to intercept the
intense synchrotron radiation in the Stage-2 VLHC \[[^27]\]. The photon
stop, shown in Figure 10.9, is a piece of metal or semiconductor that
protrudes into the beam pipe between magnets, and intercepts all or most
of the synchrotron radiation before it hits the beam-tube liner. The
advantage of the photon stop is that it may be able to be operated at
temperatures close to 300 K, thus saving significant amounts of
cryogenic power, possibly more than 30 percent. A first pass engineering
design of such a device has been completed \[[^28]\]. The results of
numerical impedance calculations \[[^29]\] give additional support to
the viability of the approach.

![](media/image10.wmf){width="3.683333333333333in"
height="2.232638888888889in"}

Figure 10.9. A model of a proposed VLHC photon stop, to be placed
between magnets to absorb synchrotron radiation at 300 K.

An Argonne National Laboratory-Fermilab collaboration proposes a
multi-step R&D plan towards the realization of such a device \[[^30]\].
In the first step, a photon stop prototype would undergo tests in the
Advanced Photon Source (APS) photon beam in a device that could be
operated wholly at room temperature. Simultaneously, cryogenic tests of
prototypes could take place at Fermilab. A photon beam is not required
for such tests. Full-blown cryogenic and vacuum test series of the
photon-stop in a VLHC-like setting is proposed as the final stage. An
important part of the R&D is to understand the effect of the photon
stops on the beam stability. This can be accomplished with a combination
of sophisticated calculations and bench or beam measurements.

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

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[^2]: \[?\] The Robbins Company, \"Tunnel Cost Reduction Study,\" July
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[^3]: \[?\] Joseph Lach, \"Instrumentation of a 17ft Diameter Tunnel
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[^4]: \[?\] G.W. Foster, \"An Unmanned, Battery-Operated TBM Tunneling
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[^18]: \[?\] P. Bauer et al., "Fabrication and Testing of
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