TD-98-035

May 12, 1998

NAPA VALLEY WORKSHOP ON HTS/LTS FOR HEP

(03/30-04/1/1998)

Emanuela Barzi, Peter Limon, Yuri Terechkine, Sasha Zlobin

[Abstract:]{.underline}

A summary is given for a majority of the presentations delivered at the
workshop by the U.S. research and industrial communities. Most of the
presentations were related to superconducting materials, wires and
cables and their applications in high energy physics.

# **SESSION 1: SUPERCONDUCTOR NEEDS FOR HEP**

# **Peter Limon (KS^\*^) -- The HEP Magnet Challenges**

Accelerators have to be built through the design and production of
cost-effective magnets. Higher energy machines are needed to investigate
the source of electroweak symmetry breaking, the origin of the
difference in mass of elementary particles and force mediators, the
reason why matter predominates over anti-matter. New observations might
lead to understand what is the composition of dark matter in the
universe. Besides completing our present understanding of the Standard
Model, future observations will also hopefully lead to extend the theory
and eventually reach the unification of gravity with the other forces.

The Tevatron is the world's highest energy machine. In 2005 the LHC, a
proton-proton collider, will start operating at CERN with a center of
mass energy of 14 TeV and a luminosity of 10^34^ cm^-2^s^-1^. LHC will
probe mass scales of about 1 TeV. The U.S. is participating in the
machine and the detectors. Post-LHC machines will be built by global
collaborations in which US must play a major role.

A new national VLHC steering committee has been founded to coordinate
and communicate scientific work. At the Snowmass_96 Summer Study on New
Directions for HEP, a 50 TeV x 50 TeV proton-proton collider with a 3
TeV injector was foreseen in both a high and a low-field option. The
major advantage of a sufficiently high-field choice is the independence
of luminosity of the initial emittance due to synchrotron radiation
damping. This phenomenon becomes effective at around 10 T.

\* KS = Keynote Speaker

Aiming at LHC IR quadrupoles and future machines, an R&D program with
companies has improved the jc at 10.5 T and 1.9 K in the binary (NbTi)
by 7%, and a collaboration is going on in an SBIR with industry on
ternary (NbTiTa) for a better performance. Contracts are being initiated
for A15 conductor R&D, where the goals are lower cost, small d~eff~, and
higher jc at 12T.

The HTS field has been entered with binary power leads for the Tevatron
magnets. A successful 6 kA lead has been tested. Encouragement and
possible collaboration with industry in HTS R&D are foreseen in a near
future.

# **Ron Scanlan, LBNL (KS) - The HEP Superconductor Challenges**

\* *HTS*

Development of:

-   Bi2212 19-strand cables with NiCr cores (R&D going on at LBNL, OST,
    IGC, OSU)

-   Bi-tape coils (LBNL, BNL)

-   Racetrack coils: common coil and TAMU (LBNL, BNL)

-   HTS binary current leads (Fermilab, IGC, ASC, Eurus)

-   HTS cable in conduit (TU)

\* *Nb3Sn*

Development of:

-   Nb3Sn cable with SS core

-   Nb3Sn racetrack coils (LBNL)

\* *Nb3Al*

Development of:

-   Rapid quench process (Hitachi/ NRIM, facility under construction at
    OSU)

-   Conventional process (Sumitomo/JAERI, Hitachi)

> For Nb3Al a collaboration is foreseen between LBNL, OSU, IGC and OST.

\* *NbTi alloys*

Development of:

-   Fe-doped

-   NbTiTa

-   Artificial Pinning Centers (APC)

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-   Muon collider: the cost of the conductor is not a driving issue,
    Nb3Sn, Nb3Al, or HTS could be used, as the cable-in-conduit
    approach.

-   VLHC: the main issue is a cost effective design.

### Ted Collings, Ohio State University, et.al. -- Coupling Current Control in Cables

*Copies of the transparencies are available.*

## **SESSION 2: NbTi**

### Peter Lee, ASC-UW (KS) - Conventional NbTi: How far to go?

State of the art: H~c2~ = 11.5 T @ 4.2K

H~c2~ = 14 T @ 1.9K

Jc = 3070 A/mm^2^ @ 4.2K, 5 T

Jc = 1420 A/mm^2^ @ 1.9K, 10.5 T

The addition of 500 ppm of iron in NbTi refines the precipitates size,
possibly because the iron is a β-stabilizer and therefore lowers the
temperature at which the phase change occurs.

### Hem Kanithi, IGC -- NbTi with Fe Additions

This approach is called Engineered Pinning Centers (EPC). The hi-Fe
material had smaller and more uniform precipitates. The best performance
in the jc at 4.2 K was measured to be 1843 A/mm^2^ @ 7 T and 1210
A/mm^2^ @ 8 T.

At ASC-UW the following values were obtained in the jc measurements:

1463 A/mm^2^ @ 7.5 T, @ 4.2 K, 1419 A/mm^2^ @ 10.5 T, 1.9 K.

At 1.9K and 10.5 T, the jc in the non-copper has improved by 6.9% with
respect to what was achieved with the wires developed for SSC. There has
been no cost reduction in the meantime.

### Eric Gregory and Taeyoung Pyon, IGC -- Ternary NbTiTa

The method used to make the ternary alloy was plasma melting, which is
the best for homogeneity (although it may give problems after
remelting). The required level of homogeneity and prestrain was reached
in two different alloy compositions (Nb17wt.%Ta44wt.%Ti and
Nb28wt.%Ta41wt.%Ti) which were made into monofilaments.

The heat treatments (HT) were made at 450, 475, 485 and 500^o^C and were
followed by a quench. The best HT is a high temperature one (485^o^C).
Multiple HTs do not necessarily help.

The best jc was obtained in the 17/44 alloy. At 4.2K, it was measured to
be 2500 A/mm^2^ at 5T and 1200 A/mm^2^ at 8 T. Jc measurements made @
4.2K, 8 T were assumed to be equivalent to jc values @ 1.9K, 11.5 T.

The multifilament had 3000 filaments. H~c2~ can be measured by the
resistive method or by extrapolation of the irreversibility field.

### Lance Cooley, ASC-UW (KS) -- Ultimate Pinning with Multilayers

A Magnetic-pinning model was proposed to explain the behavior of
superconducting multilayers. The pinning layers were treated as planar
defects with a high critical current density (nonlocal Josephson
junctions). Abrikosov vortices can be pinned by these defects and become
AJ vortices (Abrikosov vortices with anisotropic Josephson cores). These
AJ vortices are prevented from moving along a defect by inhomogeneities.
The AJ vortices interact with neighboring Abrikosov vortices, causing a
collective pinning effect. The major difference between this model and
the usual Core-pinning model is that the proximity length, rather than
the coherence length, is the important length scale with respect to the
pinning force. The main claim was that *dirty* (short mean-free path)
pinning layers are better than *clean* (long mean-free path) ones.
However, the experimental evidence seems to be inconclusive at best.

**Bill Warnes/ Kevin McLaughlin -- NbTi Thin Film APC**

**Lesh Motowidlo, IGC -- APC NbTi with Ni Pins**

Ferromagnetic pins like Ni require a smaller percentage volume of
material (4÷6 vol.%) than Nb pins to be effective. This leads to a
reduced proximity effect between the filaments and a pinning force of 10
GN/mm^2^ @ 5T. Jc's greater than 3700 A/mm^2^ were reached @ 4.2K, 5T.

Nevertheless, APC cause a reduction in both H~c2~ and Tc, as well as in
jc at high fields.

**Terry Wong, Supercon -- APC NbTi with Nb and Ni Pins**

###### 4 vol.% Nb pins were added to a Nb47%wt.Ti matrix to give an overall composition of Nb44.3wt.%Ti. Three extrusions were needed to reach the desired composite size. Pins sizes of the order of 1nm were found to be better at fields above 7 T. A further claim was that no H~c2~ nor Tc depressions were observed up to pins sizes of 30 nm. 

**Rob Heussner/ David Larbalestier, ASC-UW -- APC with Nb and Ti Pins**

24 vol.% Nb pins were added to a Nb47%wt.Ti alloy. The pins size was 20
nm and the jc \@5 T reached 4700 A/mm^2^. No satisfying results were
achieved at higher fields.

#### SESSION 3: A15 CONDUCTORS

### Joe Minervini (KS), MIT -- ITER Production Lessons

25 tons of Nb3Sn strand for the 13 T central solenoid (CS) model coil
and 3.6 tons for the 11 T toroidal field (TF) model coil were needed.

ITER main requirements were on the jc (greater than 700 A/mm^2^ @ 12 T,
4.2 K in the High Performance I strand), the hysteretic losses (\< 200
mJ/cm^3^ for ±3 T cycle @ 4.2 K in the High Performance II strand), the
copper RRR (\>100), and the piece length (\~ 5km). The wire had to be Cr
plated to prevent sintering and to provide interstrand resistance (oil
residuals were sometimes found on the wire).

The companies that participitated in the R&D program were the following:
Europa Metalli, Vacuum Schmeltze, Furukawa, Hitachi, Mitsubishi,
Bochvar, IGC, TWCA. Nb3Sn costed 1000÷2000 \$/kg on average. 1 milion
dollars were spent per company.

Recommendations on the measurements:

\* measure Ic instead of jc

\* measure R/L instead of RRR

\* consider Tc or I(T) measurements for temperature margins

The cost of the wire is not expected to lower.

### Dan Dietderich/ Ron Scanlan -- Engineered Pinning Centers

An effect of the EPC approach is to inhibit grain growth (another is to
increase reaction rates). Pinning centers in Nb3Sn films give an
engineered microstructure with 20÷25 nm in grain size. Jc can be greater
that 10000 A/mm^2^ @ 4.2K, 12 T. This is a factor 2÷3 higher than in an
undoped material. The next step will be to apply EPC to round wires
processed through the powder-in-tube (PIT) and internal tin methods.

### Eric Gregory -- Internal tin Nb3Sn

The variables involved in the process are:

\* Alloy composition

\* Alloy quantity

\* Tin quantity

\* Barrier composition, thickness and location

\* Subelement number

\* Subelement design

\* Spacers (yes or no)

\* HT conditions

\* Copper percentage (25% ≤ Cu ≤ 60%)

In D20, an IGC multifilamentary material with 30% copper and a jc of
800A/mm^2^ @ 12 T, 4.2 K was used. Then two new low-loss materials were
developed: one had a jc of 900A/mm^2^ \@12 T, 4.2 K and losses on the
order of 200 mJ/cm^3^ (corresponding to 6μm filaments^♣^), the other had
a jc of 1100A/mm^2^ @ 12 T, 4.2 K and losses of about 500 mJ/cm^3^
(corresponding to 20μm filaments^♣^).

### Jim McKinnell, OST -- Nb3Sn and NbTi at OST

NbTi was doped with 520 ppm of iron.

At ASC-UW the following values were obtained in the jc measurements:

1466 A/mm^2^ @ 7.5 T, @ 4.2 K, 1422 A/mm^2^ @ 10.5 T, 1.9 K.

At 1.9K and 10.5 T, the jc in the non-copper has improved by 7.1% with
respect to what was achieved with the wires developed for SSC. There has
been no cost reduction in the meantime.

### Shahin Pourrahimi, MIT -- Flexible Nb3Sn Cables

The approach proposed is of the react and wind type. The cable is made
out of the appropriate relative amount of copper and superconducting
strands. The strands diameter is small enough to minimize the bending
strain, and the wires are oxidized in order to prevent sintering. The
oxidation of the wire is carried out in a tube furnace at 400^o^C for
10s in a pressurized O~2~ environment.

### Jack Ekin (KS) -- Strain Effects in HTS and LTS

> $$$\frac{j_{c}}{j_{\text{cm}}} = \frac{F_{p}(\varepsilon)}{F_{\text{pm}}}\left\lbrack \frac{B^{{}_{c2}}(\varepsilon)}{B^{{}_{c2m}}} \right\rbrack^{- p} \cdot \left\lbrack \frac{1 - \frac{B}{B^{{}_{c2}}(\varepsilon)}}{1 - \frac{B}{B^{{}_{c2m}}}} \right\rbrack^{q}$A15
> and HTS superconductors are sensitive to both axial (hoop stress) and
> transverse stress. The effect of the axial strain on the jc can be
> parametrized as follow:

where the *m* index indicate values at zero strain. The first factor
dominates at low fields, whereas the second prevails at high fields. The
hoop strain at which an irreversible degradation occurs in the
superconductor is a function of the filament diameter. For
multifilamentary Nb3Sn wires with filament diameters from 5μm to 50μm,
the irreversible hoop strain is ±(0.5÷0.7)%, for BSCCOs it is
--0.1÷0.2%, for YBCO it is --1.0%÷0.5%.

The maximum transverse stress that can be applied to Nb3Sn is 100MPa, to
Nb3Al it is 300÷500 MPa, to Bi2223 with a pure Ag matrix it is 20 MPa.

### Alex Gourevich, ASC-UW -- Theory of Strain Effects in A15s

Using the BCS theory to explain how a pressure applied to an A15
compound affects its Tc. Chain models predict, from the band theory of
electrons, that the elastic effect produces peaks in the electrons
density. A structural transition from cubic (c = a) to tetragonal (c \<
a) symmetry occurs just above Tc, with a subsequent softening of the
shear modulus and an antiphase domain structure. The structural and
superconducting transitions are coupled and coexist together. These
transitions are described using the Ginzburg-Landau theory.

-   Arnaud Devred, private communication.

### Eric Gregory - Nb3Al SBIR Results

### **Nb3Al H~c2~ is \~ 36÷41 T, to be compared to Nb3Sn H~c2~ which is \~ 24÷30 T. There exist two methods to obtain superconducting Nb3Al:**

-   the low temperature one at less than 1000^o^C by means of
    solid-state diffusion

-   a melt quench at about 2000^o^C followed by a low temperature heat
    treatment at about 800^o^C

To develop the precursor strand for the rapid quench process, a series
of different designs in short lengths were made. Long lengths (\~ 300m)
were made only out of the best design.

The rod-in-tube method where a monofilament is to be swatted and drawn,
is more difficult than the jelly roll, where 19 monofilaments (three
rolled up foils of Nb and one of Al) are restacked and hydrostatically
extruded.

##### Mike Sumption, Ohio State University, et. al. -- Nb3Al Strands by Ohmic Heating

Nb3Al (precursor strand design and fabrication by IGC) is heated in less
than 0.5s to 1900^o^C and held at 1900^o^C for 0.1s by making a current
flow in the wire. Then the wire is given a rapid (4÷5s) quench in liquid
gallium, followed by a heat treatment to make ordered metastable A15
phase.

Besides magnetization measurements, the DC susceptibility and the Tc
were measured on short samples at Yamagushi University in order to check
if the right phase was attained.

At Plastronics the process above is carried out as a reel-to-reel
quench.

Within a 5-year program, the following achievements are planned: wires
of 1km length, improvements in the reel-to-reel processor, jc and
stability measurements, and the fabrication of a Rutherford type cable.

*Copies of the transparencies are available for more information.*

### Peter Lee -- Microstructure of A15s

#### SESSION 4: BSCCO 2212

**Eric Hellstrom, ASC-UW (KS) -- 2212 Key Issues**

The most relevant drawback in Bi-tapes is that the critical current
decreases nearly exponentially with length.

The main problems in the processing are the bubbling due to CO~2~ and
vapor, reactions with the insulation, and the high sensitivity of the
process to temperature. Usual heat treatments involve 1 day in vacuum at
700^o^C to remove vapor, followed by 2 days at 835^o^C to remove CO~2~.
In order to reduce porosity, an overpressure technology was developed
together with a careful thermal processing and an appropriate conductor
design.

**Steve Van Sciver, NHMFL -- HTS Wire Program**

Studies on HTS conductors, insulation materials and coil designs are
carried out in order to build a 25 T solenoid magnet at 4.2K for NMR at
1.1 GHz.

The electro-mechanical properties of Bi-2212 are studied by means of 1÷3
T insert coils made out of Bi-wires. The 165mm insert coil is kept at
4.2K and it is located in the 200mm bore of a 20 T resistive solenoid
magnet which is water cooled. The jc measured on a 50m short sample of
Bi-wire is about 1000 A/mm^2^. The length of Bi-wire needed for the HTS
insert coil is 1÷2 km. A wind, react and sintering process that involves
a heat treatment at 892^o^C for two days is used. Some of the insulator
materials under study are organozirconium and isopropanol, with
insulation thicknesses of 5÷10 μm.

**Justin Schwartz, NHMFL -- HTS Conductor Issues for High Field
Magnets**

**Ken Marken, OST -- 2212 Conductors and Leads**

The melting process is used. The factors that limit the conductor length
are:

-   filament sausaging

-   bubbles (carbon reduction, gas minimization, and heat treatments can
    be used as counteractions)

-   leakage through sheath

-   Bi2212 melting behavior

-   heat treatment temperature (sensitivity to ± 1^o^C)

19-filament tapes with Ag/SC \~ 3/1 and 121-filament round wires with a
diameter of 0.8mm and Ag/SC \~ (2÷3)/1 are made. The following jc's were
measured on 200m long samples at 4.2K: 930 A/mm^2^ @ 0 T, 330 A/mm^2^ @
6 T, 280 A/mm^2^ @ 15 T.

Cables that carry 920÷950A @ 6 T are made out of 18÷19 round wires and a
jc comparison is carried out after the wires are extracted from a cable.

HTS current leads of up to 5000A are being designed and built, and
manufacturing of a prototype of 13kA HTS current leads is underway.

### Lesh Motowidlo -- 2212 Conductors and Leads

Round wires that reach a maximum jc of \~ 1000 A/mm^2^ @ 4.2K and 12 T
for a filament diameter of 10 μm are made. The jc in Bi-2223 tapes
undergoes a 60% degradation under thermal cycling between 4.2 and 77K.
In the short sample measurements the standard ITER mandrel is used.
Cables with a NiCr core are being built.

### Dan Dietderich/ Ron Scanlan -- 2212 Rutherford Cables

Cables are made out of IGC and OST strands of 0.8 mm in diameter. A
0.4÷1 mm NiCr core in the cables is used to reduce the bending radius of
the strands. 1000 A @ 4.2K and 5 T in 18-strand cables, and 650 A @ 4.2K
and 8 T in 19-strand cables are reached. The next step will be
30÷36-strand cables. No degradation was observed between the virgin and
the extracted strands from a cable.

### Peter McIntyre/ Rainer Soika, Texas University -- Spring Core 2212 Cables

An Inconel spring core cable was made out of Bi-2212 strands. The spring
core diameter is 1 mm. Approximately 75% of the structural support is
provided by an outer Inconel shell.

#### SESSION 5: BSCCO 2223

**Bart Riley (KS) -- 2223 Key Issues**

Most of the (BiPb)~2~Sr~2~Ca~2~Cu~3~O~x~/Ag composites are made by the
powder-in-tube (PIT) method. Powders with a nominal composition of
Bi-2223 are calcined into a precursor powder containing mainly the
Bi-2212 phase. The calcined powder is filled into an Ag alloy tube,
which is then mechanically deformed by extrusion, drawing, rolling and
pressing into thin wires or tapes. The as-deformed tapes need further
annealing (sintering) in order to form the Bi-2223 phase and texturize
the material.

The main difficulty in producing high-quality Bi-2223/Ag composites in
long lengths is that the mechanical properties of the ceramic powder are
very different from those of the Ag sheath. Key parameters to maximize
the current-carrying capability are the homogeneity of the tape/wire
cross section and a high density and a textured core.

Jc's of 700 A/mm^2^ have been obtained in Bi-tapes @ 77K and in
self-field, and 1400 A/mm^2^ @ 4.2K and 12 T.

**Pradeep Haldar -- 2223 at IGC**

**Mike Tomsic -- Conductors at Eurus**

**Bill Sampson, BNL -- Measurements at 4K on 2212 and 2223**

#### SESSION 6: COATED CONDUCTORS

### Masaki Suenaga, BNL (KS) -- Coated Conductor Issues

There are two methods: Ion Beam-Assisted Deposition (IBAD) and
Rolling-Assisted Biaxially Textured Substrates (RABITS).

### **Don Kroeger, LANL -- Present** Position on RABITS Development

**Dean Peterson, LANL** **-- IBAD coated conductors**

In IBAD coated conductors the jc has reached 2000 A/mm^2^ @ 77K and 5 T.

#### SESSION 7: CURRENT LEADS

### **John Miller (KS) -- Current Leads**

**Mike Green -- Design of HTS Current Leads for Magnet Systems**

### Pradeep Haldar/ Lesh Motowidlo -- HTS Current Lead Development at IGC

5kA binary current leads for Fermilab and 13kA leads for CERN are being
built.

The upper section of the 5kA lead carries the current from 300K to 80K
by means of ten parallel copper tubes internally cooled by cold He gas
and externally by cold N~2~ vapor. The HTS section, which carries the
current from 80K to 4K, is made with Bi-2223 parallel tapes that are
stacked and incorporated in a fully impregnated glass-epoxy structure.
He vapor is made to flow over the tape surfaces. The copper and HTS
sections are jointed together at a copper station whose temperature is
kept at 80K by means of a liquid N~2~ heat exchanger. At the nominal
operating current, the estimated liquid N~2~ flow rate per lead is 3.7
l/hr, the He mass flow rate is 0.032 g/s and the heat leak to the He
bath is 0.7 W.

###### The heat leak of the 13kA lead is 0.5 W at zero current, and 1.7 W with a current of 13kA.

**Gregory Schnitchler, ASC -- HTS Current Lead Development at ASC**

######  5kA binary current leads for Fermilab and 13kA leads for CERN are being built. 

###### The upper section of the 5kA leads is made of a rolled copper screen, which is constructed with a square mesh of very thin wire. A transfer line is inserted in the copper tube, and liquid N~2~ directly cools the copper joint to the HTS section. The HTS section is made with multifilamentary Bi-2223 tapes that are sintered together and stacked. The tape stacks are then bonded directly to epoxy-fiberglass composites. 5kA leads are expected to operate in a magnetic self-field of approximately one hundred gauss. In order to shape the self-field into concetric lines parallel to the tapes, which allows for better current tolerance, a circular array of tapes is used. The HTS section is a stepped lead. The cross section of the HTS tape decreases along the length of the lead from the 80K end to the 4.5K end. He vapor is made to flow over the tape surfaces. At the nominal operating current, the estimated liquid N~2~ flow rate per lead is 2.6 l/hr, the He mass flow rate is 0.028 g/s and the heat leak to the He bath is 0.6 W. 

###### The heat leak of the 13kA lead is only 1 W with a current of 13kA.

**Sandor Feher -- HTS Power Lead Test Results**

Binary copper/HTS power leads for the Tevatron magnets are being
considered by Fermilab in place of the conventional 5kA and 6kA
He-cooled copper leads in order to save cryo cooling capacity. This
saving would allow to lower the temperature of the magnets and increase
the magnetic field, therefore increasing the beam energy from 900 GeV to
1 TeV.

Tests of ASC and IGC binary power leads delivered to Fermilab in fall
1997 were carried out using new instrumentation and new software for the
Vertical Magnet Test Facility (VMTF) data acquisition system. A new
dewar was procured and associated hardware were designed and fabricated.

The first version of ASC power leads was tested at the end of 1997.
After a slow cool-down, the quench protection system was fully checked
out and the DC tests were started. The current was increased in 500A
steps. Above some current, both the voltage drops across the copper
section and the upper HTS segments went unstable at constant current.
The problem was overcome on the copper section by operating the flag
heaters at the top of the power leads in order to keep their temperature
at room values. However, above 3000A the quench protection system would
detect the voltage rise on the HTS segments and ramp down the current.
AC ramp rate studies were also performed, as well as operation under
either reduced LN~2~ or LHe cooling conditions. While the formers were
successful up to 3000A, in both reduced cooling conditions the
temperature of the copper-HTS joint increased above 85K causing the
system to trip.

Tests of the IGC leads were underway at the time of the workshop and
were successfully completed shortly afterwards after in-situ
recalibration of the temperature sensors.

**John Pfotenhauer -- Fault Mode Operation Characteristics of HTS
Leads**

#### SESSION 8: COST PERFORMANCE

Paul Grant (EPRI), David Larbalestier, Dean Peterson, Hans
Schneider-Muntau, Bruce Strauss (DOE), Mike Harrison, Jim McKinnel,
Steve Gourlay, Balu Balachanan.

DOE goal for HTS wire cost is 10\$/(kAm), which is 1÷2 orders of
magnitude less than the present cost. This is to be compared to NbTi
cost which is 1 \$/(kAm) and to Nb3Sn cost which is 10 \$/(kAm).