Conductor cost/performance status report for Snowmass 2001

R.M.Scanlan

2-15-01

[NbTi]{.underline}

The MRI magnet business has provided a solid foundation for NbTi for the
past 10 years, and HEP has benefited from this in two ways. First, MRI
magnets use approximately 200 tons of conductor per year, and this
allows economies of large scale production to be realized. Second, the
MRI conductor business is very competitive, and this has resulted in a
relatively constant price for both raw materials and finished conductor.
For example, the cost of NbTi alloy and finished conductor remained
essentially flat between the SSC procurements in 1990 and the recent LHC
procurements in 1999-2000. As a result of these factors, NbTi wire for
accelerator quality magnets can be purchased in large quantities for
about \$100/kg or \$1.00/kA-m (7 T, 4.2K). Of course, the small quantity
price (e.g., 2 tons purchased for the U.S. LHC program) is considerably
higher\--about \$150/kg. However, the downside of this situation is that
most of the cost savings and property improvements available for NbTi
have been realized at this point.

[Nb3Sn]{.underline}

The status of Nb3Sn presents a stark contrast to NbTi. Nb3Sn
superconductor is the enabling technology for high field (\>10T)
accelerator magnets, NMR spectrometer magnets operating at 500 MHz and
higher, and for magnetic fusion energy applications. However the
quantities being purchased for these projects are small at present
(approximately 0.5 tons/yr for accelerator magnets; 2 tons/yr for NMR
magnets, and 2 tons/yr 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, Nb3Sn conductor costs about \$750/kg.
In terms of properties, significant progress has been made since the
mid-1990s when the ITER fusion program was the main consumer of Nb3Sn.
The critical current improvement is shown in Fig.1, and at present, a Jc
greater than 2400 A/mm2 (12 T, 4.2K) has been produced by two U.S.
manufacturers. Thus, on a \$/kA-m at the operating field, the present
day cost for small quantities of Nb3Sn superconductor is \$4.60/kA-m
(10T, 4.2K) and \$7.35/kA-m (12T, 4.2K). The goal of the HEP Conductor
Development Program is to demonstrate that Nb3Sn costs can be reduced to
the \$1.50/kA-m range when the production processes are scaled-up to
those used for NbTi. In order to demonstrate this, we believe that 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 demonstrate economies of scale. The necessary steps and the
costs to do this will be discussed, and the overall process costs will
be presented in a series of spreadsheet calculations.

Nb3Sn Conductor scale-up

The key factor in producing a cost effective NbTi superconductor has
been the hot extrusion of very large billets. The U.S. industry standard
at present is a 300 mm diameter billet yielding approx. 400 kg of
conductor. The HEP Conductor Development Program is funding the
development of two Nb3Sn processes that are compatible with the hot
extrusion of 300 mm diameter billets\--the hot extruded rod (HER)
process at Oxford Instruments-Superconductors (OI-S), and the internal
tin (IT) process at Intermagnetics General Corp.(IGC).

In the HER process, a first stage billet is prepared by drilling holes
in a Cu ingot, loading Nb rods, and extruding. A second stage billet is
then prepared by making hex rods from the first stage, drilling out the
central copper core, stacking the hexes in a copper can, and loading
powdered salt into the holes. During extrusion, the Cu/Cu surfaces are
diffusion-bonded together to produce a strong but ductile structure. The
salt is washed out and Sn alloy rods are inserted into the holes. From
this point, the rods are cold drawn on the same equipment as NbTi,
except that the intermediate heat treatments (typically 4 anneals at 375
C for 80 hrs each) given to NbTi to enhance Jc, are eliminated. Instead,
one heat treatment is performed at the end of the mechanical processing,
or in the final coil, at a temperature of 650 C for 120-200 hrs in order
to form the Nb3Sn. Thus, the fabrication process is analogous to that
used for NbTi, and the major pieces of capital equipment (extrusion
press, rod draw bench, fine wire drawing) are the same as used to
process NbTi. The extra steps in Nb3Sn fabrication are (1)drilling the
core hole and inserting the salt, (2)washing out the salt and inserting
a Sn rod, and (3) the final heat treatment to form Nb3Sn. However, the
elimination of the intermediate heat treatments largely compensates for
the these extra processing steps.

Similarly for the internal tin process, the first stage billet is
prepared by drilling holes in a Cu ingot and loading Nb rods.
Alternatively, Cu clad Nb can be extruded, drawn and hexed, loaded into
another extrusion can, and extruded. Then, a central hole is drilled
into the rods, a Sn alloy core is inserted, and the material is drawn.
In one process option, called the monoelement internal tin (MEIT), only
one Sn core is used, the wire is drawn to fine size, and then cabled
with additional stabilizing Cu. Another option is to restack the Sn
core/Nb filament rod into another Cu tube and then draw to a larger
diameter wire that does not require the cabling step. Spreadsheet
calculations are provided for both these options.

Raw materials

Next, let\'s look at raw materials. The copper components, i.e.
extrusion cans, nose, and tail pieces, are similar and the costs in
either case are relatively small. NbTi alloy rod costs \$100/kg. The Nb
raw material cost is strongly dependent on the starting material shape
and the quantity ordered. For example, the Nb expanded metal used for
the modified jelly roll (MJR) process costs over \$160/kg, whereas the
Nb rod cost from the same raw materials source is \$110/kg. However, Nb
ingot material obtained from another supplier, with fewer refining
steps, has been shown to be acceptable for fabrication into Nb rods. The
cost of Nb rods from this source is \$62/kg. Fine filament NbTi requires
the use of Nb foil as a barrier against Cu-Ti intermetallic formation,
which costs \$150/kg in large quantities. Nb3Sn requires a diffusion
barrier to prevent Sn contamination of the Cu stabilizer, and the
material of choice until recently was Ta. Historically, the price of Nb
has been very stable, while the price of Ta fluctuates widely, depending
on the business cycles in the electronics industry where it is used in
high quality capacitors. The recent high demand for handheld electronic
devices has driven up the price of Ta to historically high levels, over
\$1300/kg. Although only 5 volume % Nb or Ta is required as diffusion
barrier material, the very high price of Ta makes it a relatively
important factor in the raw materials cost at present. In certain cases,
Nb or NbTa alloys or perhaps V, can be substituted for Ta, and these
alternatives are being investigated. The option of an outer Nb sheet
plus a thin inner Ta sheet represents an cost-effective option, and is
used in the spreadsheet calculations.

The spreadsheet in Table 1 shows the cost for full scale wire production
costs for the internal tin process utilizing 300 mm diam extrusion
billets. Here, production scale is defined as 5 billets/month, 60
billets/yr, yielding about 25 tons/yr. For reference, the LHC requires
about 400 tons of NbTi (100 tons/yr for 4 years). This production rate
will allow a relatively attractive overhead rate (300%) and profit
margin (40%). When these values are used, the price of the Nb3Sn wire is
\$259/kg for the more expensive Nb raw material and \$215/kg for the
alternative Nb source. The corresponding prices \$/kA-m (for a 0.8 mm
diam wire with Ic =900 A at 12 T, 4.2K) are \$1.26 and \$1.05,
respectively. A similar calculation done for the MEIT process by
B.Zeitlin yields values of \$1.08 and \$0.88/kA-m, respectively. Thus,
we conclude from these projections that Nb3Sn can be produced in the
quantities required for a VLHC accelerator for the price target
established for the Conductor Development Program of \$1.00/kA-m.

What must be done to make these cost projections a reality? The
following are a series of tests that need to be done in a scale-up and
cost reduction program, for both HER and internal Sn processes:

1.  Fabrication of 300 mm diameter, 400 kg production billets. The issue
    here is the quality of wire produced. The present billet size is 200
    mm diameter, 100 kg. The cost of a 300mm diameter billet is approx.
    \$100 K.

2.  Fabrication of wire with long piece lengths. Long piece lengths
    without wire breaks are the key to controlling production costs. The
    piece length goal of the Conductor Development Program is 10,000
    m(40 kg of 0.8 mm diam wire), or about 10 pieces/billet.

3.  Demonstrate that the alternative source of less expensive Nb raw
    material produces wire that is reliable and reproducible.

4.  Billet designs must be optimized to yield the effective filament
    size target of 40 micrometers.

In addition, there are some specific R&D goals that must be met for each
fabrication process. It must be demonstrated that the HER composition
and filament quality can be optimized to yield a target Jc of 3000
A/mm2, and a minimum value Jc of 2000 A/mm2. Also, the quality of the
Sn-Ti alloy must be improved to prevent wire breakage due to large
intermetallic particles of Ti3Sn. For the internal tin process, it must
be demonstrated that a bundled rod approach can be cold drawn to yield
long piece lengths, or an alternative process such as MEIT that does not
require bundled rods must be adopted. Also, a continuous barrier
wrapping process must be developed. These scale-up efforts will require
about \$1M/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.

Fig. 1

![](media/image1.wmf)

![](media/image2.wmf)Table 1.