# . R&D Programs

## 10.2 Needed R&D, Stage 2 {#needed-rd-stage-2 .unnumbered}

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

The VLHC-2 magnet system configuration with vertical bore arrangement
adopted in this study dictated the configuration of the SC magnets
described in Section 6.1. Arc dipole magnets are based on the common
coil design and react-and-wind fabrication technique. This approach is
regarded at this time as most innovative and cost effective although it
requires significant efforts to prove it experimentally. Extensive R&D
works in this direction are carried out at Fermilab, LBNL and BNL. There
are also other magnet design approaches based on the traditional
cos-theta (shell-type) coil geometry which allows both horizontal and
vertical bore arrangement as it is shown in Figures 10-1 and 10-2. All
those magnets were developed at Fermilab for VLHC and meet the VLHC-2
requirements including the operation field range, field quality,
critical current and critical temperature margin, quench protection,
etc. \[1\]. The designs and parameters of arc quadrupole magnets that
match these dipole magnets are described in \[2,3\]. Due to small
bending radii in the cos-theta type coils they are forced to use the
wind-and-react technique in order to avoid a large degradation of the
cable critical current during coil winding. The cos-theta coil
configurations allow reducing the coil cross section area by \~20% with
respect to the block type coil with the same coil bore diameter. These
coils can be placed much closer to each other, especially in case of the
horizontal bore arrangement, reducing the iron yoke (and magnet as a
whole) size, weight and cost. It is most straightforward for the warm
yoke design \[4\] shown in Figure 10-2.

![](media/image1.png)

*Fig. 10-1. Cos-theta cold-warm yoke dipole with vertical bore
arrangement. The yoke cold part is shown on the picture, the 15 mm thick
yoke warm part with ID=680mm is not shown.*

![](media/image2.png)

*Fig. 10-2. Cos-theta warm yoke dipole with minimized size and weight.*

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 the VLHC-2. The VLHC-2
Magnet R&D program will consist of three traditional stages: stage 1 -
short model R&D, stage 2 - full-scale prototype fabrication and tests,
stage 3 - magnet string (hale-cell) test. It would require funding on
the level of 5-10 M\$ per year during 5-10 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 magnet system half-cell to demonstrate the feasibility of VLHC-2
magnet system. Based on the results of this program the input (base) for
the cost estimate and schedule of VLHC-2 will be provided.

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

To build the high field SC magnets with a required for VLHC-2 nominal
field/field gradient and reliable operation margins, the superconducting
strands must provide certain parameters discussed in Section 6.1 and
summarized in Table 10.1 \[5\].

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

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

  Strand diameter                                0.300-1.000 mm

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

  Effective filament diameter d~eff~             \<40 m

  Cu stabilizer                                  \>50%

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

Nb~3~Sn is currently the material foreseen for the development of VLHC-2
high field SC magnets thanks to its properties and commercial
availability. At this time there are three technologies that may reach
the above technical goals: Internal Tin (IT), Modified Jelly Roll, and
Powder-in-Tube (PIT). In the second half of 2000, a Superconductor R&D
National Program for HEP applications was started by DOE \[6\]. This
program was oriented on reaching the conductor target parameters
presented in Table 10.1 and optimization the 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 10-3 shows the progress in time of
Nb~3~Sn strand critical current density since 1984. A trendline
extrapolation suggests an achievement of the 3000 A/mm^2^ goal within
year 2004-2005.

![](media/image3.wmf)

*Fig. 10-3. Improvement in J~c~ (4.2 K, 12 T) of Nb~3~Sn since 1984 up
to date.*

The critical current density of Nb~3~Sn strands is controlled by a few
parameters, such as the volumetric fraction of Nb~3~Sn phase that can be
packed in the non-Cu part of a strand, strand heat treatment procedure,
flux pinning mechanisms. All these parameters are now under
investigation and optimization \[7-9\] in order to reach the required
Jc.

Another critical parameter responsible for the persistent current effect
in magnets and strand stability is the strand magnetization determined
by Jcd~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 \~100-120 m, and only PIT provides it
on the level of 50 m resulting in a significant increase of the strand
cost \[10\]. An optimization of the strand design and technology aming
on the reduction of d~eff~ in the high Jc Nb~3~Sn strands is an
important next step of the Conductor R&D program.

In magnet design, it is also necessary to reduce the critical current
degradation of the original virgin strands as well as to control the
interstrand resistance, responsible for the thermal and electromagnetic
coupling between the strands, during magnet fabrication and operation.
In case of 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 case of the react-and-wind method,
there is also to take into account the 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 of SC cable for accelerator
magnets during last 1-2 years. The results obtained prove that cable R&D
goals will be certainly achieved \[11-14\].

*Table 10.2.* Cable R&D goals.

  ----------------------------------------- -----------------------------
  **Parmeter**                              **Value**

  Number of strands                         28-60

  Packing factor                            87-90%

  Cabling degradation                       \< 10 %

  Bending degradation                       \< 10 %

  Compression degradation                   \< 10 %

  Strand crossover resistance               \>10 mcOhm
  ----------------------------------------- -----------------------------

As was shown in section 6.1.8, the cost of Nb~3~Sn strands is one of the
major cost drivers of the VLHC-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 considerably higher -- about \$150/kg. It is apparently the lowest
prices for this conductor since most of the cost savings and property
improvements available for NbTi have been realized at this point. The
status of Nb~3~Sn presents a stark contrast to NbTi. The quantities
being purchased are small at present (annually approximately 0.5 ton for
accelerator magnets, 2 tons for NMR magnets, and 2 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.

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 \[15\]. These scale-up efforts will
require about 1 M\$/year for three years for each process that is
selected for scale-up. However, the cost savings projected for a
LHC-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 Fig.3-25. The following
are the major design considerations of these IR magnets:

-   Small aperture (specially 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, the state of the art
superconductors are used in the design. The ends of conventional cosine
theta designs put a practical limit on the minimum aperture,
particularly in quadrupole magnets made with brittle material. To
overcome this and other limitations VLHC-2 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 Q1A determines the
layout of the entire IR region and the maximum beam size for the given
optics. 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
Fig.10-4. In order to facilitate large bend radii, returned path of all
turns is further away from the aperture. Field contours and field lines
in the aperture of this magnet are also shown in Fig.10-4. 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 much larger amount of conductor than that in a
conventional design. However, the cost of conductor is not a major issue
in designing a few critical high performance magnets.

![](media/image4.jpeg){width="2.4944444444444445in"
height="2.040277777777778in"}
![](media/image5.jpeg){width="1.5013888888888889in"
height="2.0618055555555554in"}

Fig.10-4: 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 a cross talk between
the two apertures. The superimposition of a dipole field on the
quadrupole coils increases the peak field on 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 are given in Table 3-34.
These values will be obtained either by design or by correction. Table
3-34 includes harmonics from both design and construction errors,
however, the harmonic errors are dominated by the design. These
harmonics are expected to come down as the design evolves. All four
quadrupoles will have a different cross section.

VLHC IR region uses three types of dipole magnets. The design of all
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 VLHC-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 some 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 probably the synchrotron radiation. Developing
    methods for removing some significant 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: "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: "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 again
    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.

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

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