[^1]

Abstract\-- **Accelerator magnet technology is currently dominated by
the use of NbTi superconductor. New and more demanding applications for
superconducting accelerator magnets require the use of alternative
materials. Several programs in the US are taking advantage of recent
improvements in Nb~3~Sn to develop high field magnets for new
applications. Highlights and challenges of the US R&D program are
presented along with the status of conductor development. In addition, a
new R&D focus, the US LHC Accelerator Research Program, will be
discussed.**

*Index Terms*---Accelerator magnets, superconducting magnets

# Introduction

[Performance]{.smallcaps} requirements of modern accelerators continue
to press the limits of magnet technology. Parameters such as field or
gradient and large beam-induced heat loads require the use of materials
well beyond the capabilities of NbTi, that is the current basis of
accelerator magnet technology. The magnets for the Large Hadron Collider
(LHC) \[1\], soon to be commissioned at CERN, represent the ultimate
application of NbTi. For applications requiring fields above 10 T, the
focus of accelerator magnet programs in the US is based on Nb~3~Sn, one
of the A15 compounds. The preference of Nb~3~Sn is based on a
combination of performance and commercial availability. The material
itself is over 4 decades old, however, the mechanical properties
(brittleness and strain sensitivity) have kept it from active
consideration for use in accelerator magnets until recently. While there
are still many challenges to face, much progress has been made. Nb~3~Sn
has been a cornerstone of the magnet program at Lawrence Berkeley
National Laboratory (LBNL) for over a decade. Other US magnet programs,
at Fermi National Accelerator Laboratory (FNAL), Texas A&M University
(TAMU) and Brookhaven National Laboratory (BNL) have more recently
joined the effort. These programs are being given additional impetus by
the newly launched US LHC Accelerator Research Program, a large fraction
of which is devoted to developing technology for future upgrades of the
LHC. For this application, Nb~3~Sn is the only possible choice.

# Conductor

It is well understood and appreciated by superconducting magnet builders
that conductor properties ultimately determine the performance limits.
Due to the increasing interest in Nb~3~Sn for accelerator magnet
applications, more effort has been put into conductor development.

## US DOE Conductor Development Program

Recognizing the need to further the development of Nb~3~Sn for future
accelerator magnet applications, the High Energy Physics division of the
US Department of Energy launched a Conductor Development Program (CDP)
in 1999. The goal is to provide cost-effective, high-performance
superconductor for the high-field magnets required for the next
generation of high-energy physics colliders, as well as upgrades to
existing colliders at Fermilab and CERN. Table I lists the target
parameters.

[TABLE I.]{.smallcaps}

[Target Parameters for the US Conductor Development Program]{.smallcaps}

  ----------------------------------- -----------------------------------
  J~c~ (non-copper, 12 T, 4.2 K)      3,000 A/mm^2^

  J~e~ (12 T, 4.2 K)                  1,000 A/mm2

  Effective filament diameter         \< 40 microns

  Process unit size                   100 kg, average piece length \> 10
                                      km with diameters 0.3 mm -- 1.0 mm

  Wire cost                           \< \$1.50/kA-m (12 T, 4.2 K)

  Heat treatment time                 Max 400 hrs; 50 hrs wind and react
  ----------------------------------- -----------------------------------

The program, managed by LBNL and a committee consisting of
representatives of US National Laboratories and Universities has made
excellent progress in a very short time. The modestly funded program
increased J~c~ by 50% in the first two years, reaching the goal of 3,000
A/mm^2^. This conductor, manufactured by Oxford Superconducting
Technology, uses the Restacked Rod Process (RRP). In this process, the
raw Niobium is in the form of rods that are less expensive than expanded
metal sheets used for Modified Jelly Roll (MJR) and is thus a better
candidate for scale-up and cost reduction. Initial results for the RRP
process, extrapolated to 12 T due to current testing limitations, were
reported in \[2\]. Later testing at LBNL with improved sample mounting
and testing procedures \[3\] gave a result of 3,016 A/mm^2^ at a field
(background + self-field) of 12.38 T. It is universally recognized that
improvements in conductor characterization are necessary to support the
Conductor Development Program. The programs at BNL \[4\] and FNAL \[5\]
are also developing high current strand measurement techniques. These
groups and others, from The University of Wisconsin and Ohio State
University are actively engaged in investigating stability,
magnetization, intra strand resistance measurements, strain sensitivity
and other parameters related to magnet performance \[6,7\].

A paper on high field magnets must at least mention High Temperature
Superconductor (HTS). Though there are several critical issues
(irreversible strain degradation, filament diameter) that have not been
sufficiently investigated, some progress has been made. The J~c~ of
Bi-2212 continues to improve but J~e~ is still low compared to Nb~3~Sn
strands, Fig. 1. However, for fields above 17 - 18 T, there is no other
choice. For this reason, programs at LBNL in HTS cable development \[8\]
and the Wind and React program at BNL \[9\], continue at some level.

![](media/image1.wmf)

Fig.1. Comparison of J~e~ for Bi-2212 and Nb~3~Sn

# Magnet Issues and Myths

Nb~3~Sn has many virtues related to high field magnets, but there are
some issues associated with it. Notably, like all the A15 materials, it
is brittle and strain sensitive. This impacts support structure design
and fabrication techniques, and generates concern about thermally
induced stress effects. Recent work by the U. S. program, has shown that
Nb~3~Sn is more robust than originally believed, particularly as applied
to accelerator magnets. Field quality is a perpetual issue and there was
some initial concern that Nb~3~Sn magnets might present difficulty in
achieving good field quality, but the D20 results at LBNL \[10\] and
preliminary studies by Fermilab \[11\] indicate that geometric harmonics
should be comparable to NbTi magnets. For Nb~3~Sn however, high J~c~
comes at the price of large filaments; combined, this leads to large
magnetization effects. Several groups have addressed this issue,
although the effects will never be on the order of those seen with NbTi.
High fields imply large stored energy and a concern for quench
protection issues.

# US Magnet Programs

The US magnet programs represent a diverse and complementary approach to
developing technology for high field magnets. They are primarily focused
on Nb~3~Sn, but, as mentioned above, there is some HTS work as well. The
US magnet programs combine to address the critical issues associated
with the application of Nb~3~Sn. A brief description of each of the
programs along with selected highlights is given below.

## Brookhaven National Laboratory

The BNL program has been involved with Nb~3~Sn for many years. Their
current high field magnet program is centered on the react and wind
approach. Techniques for winding the brittle cable are studied using
"10-turn coils", Fig. 2. Results so far have been mixed. Early tests
using ITER strand performed very well. Subsequent tests using the newer,
high performance strand have shown significant degradation.

![](media/image2.jpeg){width="3.5875in" height="2.4819444444444443in"}

Fig. 2. BNL 10-Turn Coil

Methods to reduce the degradation, such as decreasing the strain by
using smaller diameter strand and/or increasing the bending radius are
being investigated \[12\]. BNL's method of conductor evaluation has been
expanded to HTS Rutherford cable and given the challenge, has produced
encouraging results \[12\]. For the near future they are planning the
fabrication of 12 T background coils for high field insert tests.

## Fermi National Accelerator Laboratory

The magnet program at FNAL is focused on the development of 10 -- 12 T
accelerator dipoles for future Hadron Colliders. The diverse program
includes R&D approaches based on cos-theta and block coils using both
wind and react and react and wind techniques. The program has produced
three cos-theta dipole models. Although quench performance was limited,
the magnets have contributed to a better understanding of field quality
and control of magnetization effects \[13, 14\]. The quench performance
of the cos-theta design is being studied using a magnetic "mirror"
configuration. The react and wind program has produced three racetrack
dipoles in the common coil configuration. Structural changes have been
implemented on the third model in an attempt to improve the quench
performance \[15\]. The last two model coils reached 75 and 78%,
respectively, of the calculated short sample limit. The group has also
built a react and wind common coil magnet \[16\], addressing field
quality issues ( Fig. 3).

![](media/image3.wmf){width="1.4625in" height="1.3819444444444444in"}

Fig. 3. FNAL React and Wind Common Coil Magnet

This magnet has a maximum field of 11 T with a 40 mm clear aperture.
Test results for this magnet will be presented at this conference
\[17\].

## Lawrence Berkeley National Laboratory

The LBNL superconducting magnet program is directed towards advancing
all aspects of the technological infrastructure for high field magnet
development relevant to possible future accelerators. The program takes
three parallel paths, each with its own series of magnets. The Racetrack
Dipole (RD-series) is used to study collider magnet options in the
common-coil configuration. With the subscale model (SM-series) we
investigate new conductors and develop new technology. Finally, the
Horizontal Dipole (HD-series) employs the simplest geometries to achieve
high fields. In the past few years the program passed a number of key
milestones in both magnet fabrication and materials development work.
Recent efforts culminated in the successful test of HD-1, a
record-breaking dipole that reached a field of 16 T \[18, 19\]. Table 2
is a list of prototype magnets and coil tests that have been used to
study issues related to high field magnets, such as support structure
mechanics, field quality, strain effects, and fabrication techniques.

[Table II.]{.smallcaps}

[LBNL Nb~3~Sn Prototypes]{.smallcaps}

  ---------------- -------- -------- -------------- -----------------------
  Magnet/Coil      Year     Field    Type           Design Features

  D20              1997     13.5     Cos-theta      50 mm clear bore, Field
                                                    Quality

  RD-2             1998     6        Common Coil    New coil configuration

  RT-1             1999     12       Common Coil    High field coil test

  RD-3b            2001     14.5     Common Coil    New structure, high
                                                    stress

  RD-3c            2002     10       Common Coil    35 mm bore, field
                                                    quality fetures

  HD-1             2003     16       Block          New "HD" configuration
  ---------------- -------- -------- -------------- -----------------------

The model magnet program is supported by conductor studies and work on
sub-sized mechanical models. This program is now well underway, with
five tests completed and several more planned for this year. Sub-sized
models will be used to test support structure designs, new coil
geometries and cable designs. The program has recently been used to
investigate thermal stress effects relevant to long coil performance
\[20\]. If new materials such as Nb~3~Al, MgB~2~ or Bi-2212 become
available in sufficient quantity and with good properties, coils will be
fabricated and tested in the sub-sized models.

## Texas A&M University

The Texas A&M program is based on high field, high current density, wind
and react coils using internal structures to limit coil stress \[21\].
The program was launched three years ago with the successful testing of
TAMU1, a NbTi model in which a first test was done using many of the
construction details of the high-field design. During this last year
they built TAMU2, in which one of the three coil modules of a 12 T
design (Fig. 4) is housed in the flux return structure designed for the
whole dipole. This first Nb~3~Sn model was built using ITER conductor,
which has a lower current density than high-performance conductor of the
final version but will enable them to evaluate reaction and impregnation
procedures. TAMU2 is scheduled for testing at LBNL in fall 2003. Pending
the results, they will continue the program with TAMU3, an identical
single-module dipole using high-performance superconductor; then TAMU4,
containing the double-pancake coil that forms the center section of the
12 T dipole; and ultimately TAMU5, the full implementation of the
design, culminating in 12 T without stress concentration, minimal
deflection at shear interfaces and a uniform preload, maintained through
cool-down.. One of the magnetic features of the design is an inter-layer
ferric plate, intended to significantly reduce snap-back.

![](media/image4.wmf){width="2.573611111111111in"
height="1.9215277777777777in"}

Fig. 4. Texas A&M 12 T design

# LHC Accelerator Research Program

Still several years from commissioning, there are already discussions on
possible upgrades for the Large Hadron Collider (LHC) \[22\]. The
current LHC relies on pushing NbTi technology to the ultimate limit,
revealing the magnet systems as the limiting components for improved
machine performance and that any enhancements will require the use of
new materials.

The U.S. LHC Accelerator Research Program (LARP) is a follow-on activity
to the U.S. LHC Accelerator Construction Project, a collaboration of
LBNL, BNL and FNAL \[23\]. Participation in this program will build on
the previous investment in the construction project and ensure continued
development of domestic accelerator science and technology, keeping the
U.S. accelerator scientists at the forefront of the field. The
multi-year program includes participation in commissioning the
accelerator and U.S.-provided components, design and construction of
state-of-the-art beam instrumentation, accelerator physics studies, and
design and technology development required for an upgrade of the
interaction region magnet systems to increase luminosity. The program
was recommended for funding at a U.S. Department of Energy review
earlier this year.

## LARP Magnet R&D Program

The challenging requirements, along with recent progress, has led to the
choice of Nb~3~Sn as the conductor material for the proposed LHC
upgrades. The LARP will be an important application of Nb~3~Sn
technology, helping to drive it well beyond state-of-the-art,
significantly enhancing U.S. options in accelerator technology and
providing opportunities for new applications.

Even the simplest upgrade configurations, in terms of magnet
requirements, exceed the performance characteristics of NbTi and, in
fact, push the limits of Nb~3~Sn. The principal goal of the magnet
program is to produce designs for Interaction Region (IR) upgrade
quadrupoles and dipoles that utilize the full potential of the highest
performance superconducting materials. The program will extend and
quantify the limits on key performance parameters, providing accelerator
physicists with IR options that most efficiently address the beam
dynamics issues that limit machine operation.

Replacement of the existing inner triplets is a key step toward higher
luminosity. There are two fundamental inner triplet design options
\[24\]: a large-aperture, single-bore inner triplet followed by beam
separation dipoles or double-bore inner triplet with separation dipoles
first. The present inner triplet design is based on the first option.
This approach and the magnet requirements are well understood. The
second one needs a new layout of inner triplet optics with twin,
large-aperture high gradient quadrupoles and correctors and very high
field separation dipoles. This approach requires more study in terms of
accelerator physics and magnet design.

Some preliminary work on a variety of quadrupole designs has been done
by LBNL, FNAL and BNL \[25-27\]. Study of a reference design indicates
that a 90 mm Nb~3~Sn quadrupole with an operating gradient of 205 T/m,
that would allow a factor of 2 decrease in β\*, is feasible with
presently available material. With anticipated advances in Nb~3~Sn
performance and more aggressive magnet designs, even larger apertures
are likely to be possible, permitting an even larger decrease in β\* and
corresponding increase in luminosity. Several innovative options exist
that could meet the operation requirements and will be studied as part
of the program. These magnets can operate with energy deposition levels
a factor of 10 (5) higher at 1.9 K (4.5 K) than the present NbTi-based
system. Every design proposal requires pushing the limits of Nb~3~Sn
technology in several areas.

Recent progress in the development of Nb~3~Sn has encouraged the
prospects for use in accelerator magnets but the application for the LHC
upgrade adds additional issues to the already formidable list. The
magnets will operate in a high radiation environment, subject to
unprecedented beam induced heat loads, that will require development of
radiation hard materials for coil construction and understanding the
heat transfer characteristics of composite coils. The design process
will necessitate working closely with accelerator physicists to
understand trade-offs between magnet performance limits and IR upgrade
options.

A program to address the technological issues associated with high
gradient, large aperture quadrupoles and high field dipoles, is
necessary to realize the potential of this new regime and provide a
variety of options.

After 4 -- 5 years running at maximum luminosity, the LHC will need a
major upgrade to the machine and detectors in order to realize the full
physics potential. This implies that the magnet R&D must be completed
around 2102 for installation of components in 2013 -- 2014.

The program is comprised of two major phases; a beginning phase of
technology development, focused on issues driven by the requirements of
possible LHC upgrades, lasting 4 -- 5 years, and a second, overlapping
phase that takes that technology base and directs it to the chosen
upgrade application. The program culminates in production-ready
full-length prototypes.

The early stages of the program are dictated by slow turn-on of the
funding profile (Fig. 5) and will rely initially on support from the
base programs.

![](media/image5.wmf)

Fig. 5. LARP magnet program funding profile

The model is based on a well-coordinated, joint effort with national and
international collaborators that is fully leveraged on the substantial
existing infrastructure and intellectual resources of existing programs.
General magnet issues will continue to be investigated through the
existing base programs. The LARP will concentrate on issues specific to
the needs and time-frame required for application to LHC upgrades.

Initially, the program will focus on integrating the three U. S.
national laboratories and incorporating their technological strengths
into the overall program. The LARP offers an excellent opportunity to
strengthen the collaborative structure of not only the U.S. programs but
internationally as well. The LARP can be used to extend and strengthen
the environment developed in the framework of the current US/LHC
Accelerator project. One of the beneficial consequences of the program
will be the formation of a collaborative structure that makes efficient
use of existing infrastructure in the labs and that will continue beyond
the duration of the LARP.

### FY04 Program

The FY04 LARP R&D plan was discussed at a recent collaboration meeting
\[28\]. With a limited timeframe and a delayed funding profile, the
emphasis of the meeting was to identify the most critical issues in
magnet technology and accelerator physics and approach the task of
narrowing the possible IR options. The R&D effort is broken down into
three categories:

#### Technology Development

Technology development includes generic issues involving everything from
investigations into new materials and fabrication techniques to complex
model magnets for studying support structures and field quality. The
FY04 program will study two quadrupole geometries. The bladder and key
support structure developed by LBNL for dipoles \[29, 30\] will be
applied to a cos-theta quadrupole. A racetrack geometry quadrupole
(Figure 6), using existing coils from the LBNL sub-scale coil program
will be built and tested. The racetrack quadrupole has the virtue that
the conductor is removed from the highest radiation region of the
aperture. The trade-off is that the design is less efficient.

![](media/image6.png){width="2.2708333333333335in"
height="1.8180555555555555in"}

Fig. 6. Racetrack quadrupole geometry

#### Quadrupoles

One of the currently favored IR designs requires quadrupoles with
closely spaced, non-parallel bores. The FY04 plan calls for a
continuation of the recent work performed at FNAL to determine if this
is a viable option.

#### Dipoles

The separation dipoles represent one of the greatest challenges for
magnet designers. High field is a premium for any IR design option but
one favored scenario places the dipole next to the interaction point, in
an extreme radiation environment. FNAL \[31\] and BNL \[32\] have looked
at a cos-theta and block geometry respectively. The original cos-theta
design suffered unacceptable heat loads at the mid-plane. A modified
design with the conductor removed at the mid-plane (Fig. 7) was created.
The heat loads were significantly reduced but several other drawbacks
were discovered; excessive stress at the mid-plane, large stored energy
and large power dissipation in the cryogenic system. The open mid-plane
design, with an aspect ratio more appropriate for the beam and no
conductor or material on the mid-plane, looks much better in terms of
reducing the heat load, Fig. 8. However, the challenge is to provide
adequate support for this coil geometry. Therefore two issues will be
studied this year; a mechanical design for the open mid-plane geometry
and heat transfer analysis and studies using the LBNL sub-scale coils.

![](media/image7.wmf){width="2.6847222222222222in"
height="2.029166666666667in"}

Fig. 7. FNAL dipole design with reduced heat load

![](media/image8.wmf){width="2.879166666666667in"
height="2.9972222222222222in"}

Fig. 8. BNL open mid-plane dipole design

# Summary

The U.S. high field magnet program is making excellent progress. There
have been some significant successes but there is still much work to be
done in order to demonstrate the viability of magnet technology that
reaches beyond the limits of NbTi. The LARP is an excellent opportunity
to extend the expertise in high-field magnets, develop a strong
technological base for future projects and add vitality and diversity to
the overall US program. In addition to the technical aspects of the
program, it will also serve to develop and strengthen collaborative ties
between the US programs and international colleagues, laying a strong
foundation for future endeavors.

# Acknowledgment {#acknowledgment .Reference-Head}

The author would like to thank S. Caspi, R. Gupta , V. Kashikhin, A.
Lietzke, A.D. McInturff, G.L. Sabbi, R. Scanlan, and A. Zlobin, for
useful discussions and input for this paper.

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[^1]: ^^Manuscript received October 21, 2003. This work was supported
    by the Director, Office of Basic Energy Sciences, of the U.S.
    Department of Energy under Contract No. DE-AC03-76SF00098.

    S. A. Gourlay is with Lawrence Berkeley National Laboratory,
    Berkeley, CA 94720 USA (telephone: 510-486-7156, e-mail:
    sagourlay@lbl.gov).