1334
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 3, NO.l, MARCH 1993
CRITICAL CURRENT DEGRADATION
IN
Nb3Sn CABLES UNDER TRANSVERSE
PRESSURE
H.H.J. ten Kate, H.W. Weijers, J.M. van
Oort
Applied Superconductivity Center, University of Twente,
P.O. Box 217,
7500
AE
Enschede,
The Netherlands.
(#)
Lawrence Berkeley Laboratory, USA.
#
Abstract--The critical current degradation of a few
Rutherford type of Nb3Sn cables
are
investigated as
function of transverse pressure. Moreover a comparison
is made between
Nb3Sn
strands produced according to the
Powder-in-Tube, the Bronze and the Modified Jelly Roll
process. The (keystoned) Rutherford cables are charged
at
11
T with transverse pressures up to 250 MPa. Large
differences in critical current reduction are observed,
ranging from
6
to about
60%
at 200 MPa depending
on
the
type of Nb3Sn. It appears that
the
irreversible part is
about
40%
of the total reduction. Moreover, the "irre-
versible" part shows relaxation and a partial recovery
is possible by thermal cycling.
I. INTRODUCTION.
The generation of magnetic fields beyond 10 tesla
requires the application of Nb3Sn cables which material
in general is characterized as very sensitive to
mechanical deformations. Therefore,
in
the framework of
the development of an experimental 11.5 T LHC type of
dipole magnet ([l]), a study of the transverse stress
effects
on
Nb3Sn
was started since in these magnets
stress levels up to 150 MPa can be expected. Further
on
a collaboration with LBL was set up in connection to
the LBL development project for their 13 T magnet D20.
The program is carried out along three routes:
simple 2 dimensional
Nb3Sn
layer or tapes ([2]),
*
study
of
the basic stress tensor by investigating a
*
experiments
on
strand materials ([3]), and
*
the
investigation of complex cables
([4,5]),
which
The execution of these routes in parallel should yield
a better understanding of the basic degradation
mechanism. The electric field E along the supercon-
ductor is a well known function of the current, the
field and the temperature but the additional effect of
the strain tensor has still
to
be clarified in detail.
will be dealt with in this paper.
A lot is known about the effect of axial stress
on
superconducting wires and even the effect of transverse
stress
on
single wires has been studied by many authors
([3,6]). However experimental data about cables are
This work was supported in part by FOM, the Netherlands
Foundation for Fundamental Research on Matter, Utrecht,
the Netherlands. Manuscript received August 24, 1992.
scarce
([4,5,7]).
It is the aim of our research to in-
vestigate the behaviour of Rutherford cables under rea-
listic operation conditions and to study the reproduc-
ibility of the results
as
well
as
to learn about the
differences between various types of
Nb3Sn.
Especially
for this a test facility was developed and during the
last 2 years 11 cable pieces have been investigated.
I1
TEST FACILITY
The test facility consist of an a 16 T/80
mm
magnet
in combination with a current supply and a cryogenic
press. The current supply is a superconducting trans-
former system operated in a feedback mode to generate a
truly stationary current in the sample of
50
kA
maximum. The press, capable of producing 250
kN
which
is equivalent to about
300
MPa transverse pressure onto
the cable, also is a special development. It consists
of a superconducting coil system by which the repulsing
force between the coils is transferred to the pressure
blocks which impresses a prepared section of the cable.
The force put
on
the cable can be easily adjusted by
control of the current in the coil system.
The
cable to be investigated is formed into a
U-shape. The legs are in parallel field and connected
to the current supply.
In
the intermediate section
which is in transverse field, the pressure is applied
across
40
mm
of conductor, see Fig. 1. Both the field
homogeneity and the accuracy of the current measurement
are better then 1%. The uncertainty
in
the applied
pressure is about 2%.
Several nV-meters (Keithley
181/182)
are
used to
measure sample voltages. In combination with the ripple
free sample current, very accurate voltage-current
measurements
on
the cables can be performed with a
resolution of about
50
nV. More details
are
in
([5]).
111.
THE
SAMPLES
Results obtained with 7 new samples are presented
while the former
4
measurements are memorized. The main
parameters of the cables are given in Table 1.
Cable (l), made by ECN in the Netherlands of wires
produced according to the Powder-in-Tube
(PT)
method
([8]), was used by CERN for the construction of an
experimental LHC model mirror magnet.
Cable (2) is
also
made by ECN and it was used for
magnets in the EURATOM
SULTAN
project.
1051-8223/93$03.00
0
1993 EEE
Fig. 1 The central part of
the
pressing arrangement:
(1)
U
shape cable,
(3)
pressure block, (4) pressure pin.
(2) sample holder,
Cables (3) and (4) are experimental conductors made
by LBL of Modified Jelly Roll (MJR) wires of Teledyne
Wah
Chang Albany. Note that cable (4) has strands with
6 sub elements which
is
a relatively old conductor.
Cable
(S),
made by Vacuumschmelze in Germany accor-
ding
to
the Bronze Route (BR), was applied by CERN and
ELIN in an LHC model magnet.
All
samples are prepared and measured using the
same techniques and equipment. A few cables were tested
a few times
to
check reproducibility. The samples
are
marked by
an
abbreviation indicating the production
technique preceded by a serial number. For the MJR
samples also the number of strands is indicated.
The heat Qeatments of the cable were
as
specified:
ECN
PT:
64
hrs.
at 67S°C; VAC-BR: 144
hrs.
at 65O0C;
TWCA-MJR:
100
hrs.
at 200OC, 24
hrs.
at 34OoC,
48
hrs.
at 58OoC,
48
hrs.
at 65OoC.
Except lPT,
all
the samples were impregnated with resin
(STYCAST285OFI'). Sample 1PT was covered only partly.
100
MPa caused by a bad
i
the pressure block was
in
surface which prohibited a
cable. Therefore, also
large degradations of
improvements the IC
which data
are
confirmed by
the
re
A.
The shape
of
the voltage-current
CUN
example of the differences
are
which also the
IC
criterion lin
indicated. It was
not
possible
to
tages of strands with their posi
example, taps #4 showing the
which lies at the thin edge
would expect
the
highest vol
contact gives .about the av
of
5
pV
all
taps give the
Further conclusions
are
voltages slightly increases at
sequence is not changed. This
pressure is about uniform.
MO
that
the
self field and the
force
on
the
test section
In Fig.3 an example
function
of
the applied tr
shifting of the voltage for
Table
1.
Characteristics of the
Nb3Sn
Rutherford type of cabled conduc
Production: powder in tube powder in tube
MJR
26
Model: keystoned rectangular keystoned
Dimensions
[mm]:
(1.53/1.85)16.6 (1.82/1.82)18.2 (2.21/2.5 1)17.0 (1.07/1.29)15.8
Number of strands: 36
Number
of
filaments: 192 192 6 subelements 36 subelements
Diameter strand [mm]:0.90
Matrix:
cu
cu CuICuSnbarr. Cu/CuSnbarr.
Pitch cable
[mm]
120
Measured Ic@11T w]:16.5
Samples:
1PT
2,3,4,5 PT
7,8,9
MJR26
6
MJR48
1336
2-
--
5e-14
Ohm
m
9
12
Is
[kA]
15
Fig. 2 Example of the spread in voltages measured
on
strands in the cable. #l=global taps, #2,3,4,5
are
various strands under the pressure block.
0
5
10
15
I
[kAl
Fig.
3
U-I curves at 11 T and a stress of 5-200
MPa.
kUe5 pV, 0<1<15
U.
Sample lOBR (VAC).
samples has yielded the following conclusions.
The U-I curves can not be described by the n-power
law U(1)
=
constant
x
I", as usually applied satisfac-
tory
in the case of monolithic wires. For example, in
the case of Fig. 3, the
n
value
is
4 at 10 kA,SMPa and
increases to 40 at the voltage level
of
5
pV.
The applied stress does not influence the current
sharing part
of
the U-I curve but only the superconduc-
ting part. The shape of the transition remains within
the
available accuracy unchanged.
B.
Critical current versus strain, summary
of
results
The main differences between the three types of
Nb$n
are presented in Fig.
4
in which the critical
cunrent reduction (applying the
1
pV/cm criterion) is
Ic/Ico
[-I
I
8
6
4
given as function of the applied transverse stress up
250 MPa. This picture is based
on
the samples 5-12.
Within the measuring error of about 2% the results have
been reproduced by second samples. The conclusions that
can be drawn from the picture are collected in Table
2.
Table 2. Summary critical current reductions vs stress.
B=ll
T.
and
1
pV/cm criterion:
100
MPa
200 MPa
ECN type powder-in-tube: 2-4
%
5-8
%
18-22%
VAC type bronze route: 7-10%
TWCA MJR route, 6 subelements:20-25% 55-60%
,36 subelements: 10-12% 40-45%
It appears that the PT conductor, characterized by the
thick Nb tubes that enclose the
Nb3Sn
layers, shows the
best strain resistance. The
MJR
wires as far as tested
here are extremely sensitive to stress. Especially the
"old" conductor with 6 sub elements shows a severe
degradation. It is recommended to investigate the more
recently produced MJR wires with 120 and 378 sub
elements to check possible improvements.
All samples are also investigated at
9
T. The
qualitative behaviour is the same but the critical
reductions at
9
T
are
about 75%
of
those at 11
T.
Sample 8MJR26 is tested with a pressure block not
matching the keystone angle of the cable.
In
this
case
the pressure is not uniform and consequently the
measured degradations
are
50
and
90%
at 100 and 200 MPa
respectively. Also the irreversible part of the IC
degradation has increased compared to the normal case.
C.
Relaxation
of
the "irreversible" degradation
Another important observation concems the
"ir-
when the applied stress is made zero again. It appears
that the irreversible part after applying a certain
stress amounts
to
30-50% of the actual degradation
under stress. This is valid for all the samples 4-11.
However by chance it was discovered that in the
case of sample llBR the irreversible part is not
per-
sistent. After thermal cycling,
i.e.
a repeat
of
the
measurement
after
15
days and a
warming
up
to
room
temperature, the critical current of this particular
sample has attained its virgin value again (even a
little bit more).
In
Fig. 5 this "relaxation" is demon-
strated for the llBR sample. After an interval and a
+VIRGIN
1
I
rl
6
++15
MIN
-v-
1
DAY
+VIRGIN
2
41
HOUR
+VIRGIN
3
10
11
12
13
14
15
I
[kAl
the "irreversible" part of the critical current.
Fig. 5 U-I curves of sample llBR showing relaxation of
thermal cycle the virgin critical current could be
restored.
This
observation proves that (at least) in
the
BR
sample up
to
200 ma,
no
permanent damage has
occurred.
To
check this the filaments in the sample
have been examined with SEM and it appeared that there
is
no
difference
in
the small number
of
interrupts
as
present
in
unpressed and pressed strands.
Obviously the
IC
reduction
in
the BR sample
of
20-25%
is
completely reversible by relaxation and
consequently the
IC
reduction under transverse stress
of
200 MPa is
nat
caused by damage
to
the
Nb3Sn
filaments. Quite a different behaviour occurs in the
MJR
conductor. The irreversible reduction is
20
to
50%
and is persistent. A
SEM
micro analysis of the strands
clearly shows visible damage ([9]).
CONCLUSIONS
For the first time a systematic study of the IC
reduction
of
impregnated
Nb3Sn
Rutherford cables caused
by transverse stress has been carried out. Remarkable
and reproducible differences between three types of
Nb3Sn
are found.
At a stress level of 200
MP
7,
20
and 40-60%
are
found for
Bronze type and the Modified
About
40%
of
"irreversible". It is
ments due to applying the
The major part of the
(when cold) and perman
other hand, the bronze con
reduction and after thermal cycling the
current could
be
restored.
H.H.J. ten Kate, e.a.,"De
ental 10 T
Nb3Sn
Di
IEEE,
Tr.
on
Magn.,
B.
ten Haken,
A.E.
reduction
of
the
mtrrns
sive stress",
Proc.
H.H.J. ten Kate,
e
cal current in
B. Jakob, e.a.
'
sel,
H.H.J.
ten Ka
cal current
in
Nb3Sn
c
loads",
Proc.
ASC
1992,