Download Behavior of a 14 cm Bore Solenoid with Multifilament MgB2 Tape

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Behavior of a 14 cm Bore Solenoid with
Multifilament MgB2 Tape
M. Alessandrini, R. Musenich, R. Penco, G. Grasso, D. Nardelli, R. Marabotto, M. Modica, M.
Tassisto, H. Fang, G. Liang, F.R. Chang Díaz, and K.Salama
Abstract - The properties of MgB2 have the potential to make
this material a viable solution for applications in which
temperature, costs or weight are considered relevant constraints.
In order to realize large scale applications, it is important to
investigate the material, but also the winding process for MgB2
wires and tapes. In the literature small coils have already
demonstrated central magnetic flux density above 2 tesla,
overcoming common winding problems related to MgB2 wires. In
this current research, efforts are being made in order to improve
the performance of solenoid coils, which are of particular interest
for many applications, e.g. for space propulsion systems such as
the VASIMR engine. A number of coils with MgB2 tapes are
being built. In this paper we present results of the test of a 14 cm
bore solenoid wound with 400 meters of multifilament, copper
stabilized tape. The magnet was tested in a cryocooled vacuum
chamber and it reached 175 A at 16 K with a central B0 of 1 tesla.
Index Terms - MgB2, cryocooler, superconducting magnet.
M
I. INTRODUCTION
diboride is an apparently simple binary
compound that was discovered to be superconducting
only in 2001 [1].
The general process of tape and wire fabrication is the
powder-in-tube method (PIT), which is characterized by
filling metallic tubes with powder and then drawing and
rolling into tapes. Two methods have been used so far for wire
application. One is the in-situ process: Mg and B powders are
deposited in a metallic tube that undergoes deformation
(drawing, rolling); a final heat treatment is necessary and it is
applied only when the wire has reached its final assembly, e.g.
wind & react magnets. Another is the ex-situ process, which
consists of filling a metallic tube directly with MgB2 powder,
AGNESIUM
Manuscript received August 29, 2006. This work was carried out and
supported by ASG Superconductors, Columbus Superconductors, the National
Institute of Nuclear Physics-INFN (Italy), and the Texas Center for
Superconductivity-University of Houston.
M. Alessandrini and K. Salama are with the Texas Center for
Superconductivity and the Mechanical Engineering Department at University
of Houston, Houston, TX 77204-4006 USA (e-mail: [email protected]).
R. Musenich is with INFN, Genoa, Italy.
G. Liang and H. Fang are with the Department of Physics, Sam Houston
State University, Huntsville TX, USA.
R. Penco, D. Nardelli, R. Marabotto, M. Modica and M. Tassisto are with
ASG Superconductors, Genoa, Italy; G. Grasso is with Columbus
Superconductors, Genoa, Italy.
F. R. Chang Díaz is with Ad Astra Rocket Company, Houston TX, USA.
drawing the tube and annealing at high temperature the wire,
before using it, e.g. react & wind magnets.
The activity carried out by ASG Superconductors and the
Institute of Nuclear Physics-INFN (Genoa, Italy) on MgB2
solenoids and pancakes has demonstrated the feasibility of
magnesium diboride magnets with ex-situ tape [2-6], that
brings some difficulties in the winding operations, but solve
other problems related to the high temperature heat treatment
of wind & react magnets.
The use of pure magnesium diboride in high magnetic field
is still considered a limit. In fact, its critical current density
drops rapidly due to the weak pinning centers and low upper
critical field. However, during the past years different
techniques have contributed to increase the usability of MgB2
(thermal treatment [7], mechanical process [8, 9], irradiation
[10], doping [11], magnetic shielding [12, 13], etc.) and
current research offers to this material an interesting future in
a large variety of magnet applications [14, 15]. Currently,
superconducting magnets in MgB2 are considered to be very
competitive at about 20-25 K for fields lower than 4 tesla and,
at lower temperature, when a large temperature margin is
required [16]. For instance, in high energy experiments, MgB2
magnets might be considerably useful in the interaction zone,
where magnetic field and radiation are relatively high. In
MRI, this material could considerably lower the price of both
installation and maintenance. In space applications, great
interest is growing in the use of this lightweight compound
[17, 18].
In the last years, small coils with monocore MgB2 wires
were made at University of Houston and tested [19, 20].
Results were compared with properties of short wires with the
aim of identifying and reducing the influence of long length,
current lead connections, bending strain and magnet
assembling procedure on the final current densities.
Although many promising results have been obtained in
metal sheathed MgB2 wires and tapes, expected results are not
easily obtained in wound coils. The mechanical engineering
department at University of Houston is broadening its own
expertise in the fields of MgB2 magnet design, winding and
testing through the recent collaboration with ASG
Superconductors and INFN-Genoa. At the Houston Advanced
Research Center-HARC and at Sam Houston State University,
other groups are participating in these research efforts with
Texas Center for Superconductivity.
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Fig. 2.
Fig. 1.
VASIMR, schematic view.
In this paper we present the results relative to a solenoid
wound with about 400 m of a 14 filaments, copper-stabilized
tape produced by Columbus Superconductors (Genoa, Italy).
A large 14 cm bore was chosen because of the potential
space application for this kind of magnet into the Variable
Specific Impulse Magnetoplasma Rocket (VASIMR), an
electric space thruster under development at Ad Astra Rocket
Company [21], located within the confines of the NASA
Johnson Space Center in Houston.
II. MGB2 FOR SPACE PROPULSION
Chemical rockets have limited efficiency for in-space
applications due to the fact that the exhaust fuel is slow (low
Specific Impulse) relative to the speeds needed to move about
the solar system. As a result, chemical systems are massive
and carry a large amount of fuel, preventing from short trip
missions. Electric space propulsion systems are the best
candidate to reduce mission time and costs, due to the higher
efficiency and higher exhaust speed (at least one order of
magnitude). In the last ten years, electric space propulsion has
been demonstrated as a key technology for robotic exploration
of the solar system. Among the many different concepts, Ion
Engine, Hall Effect Thrusters and magneto-plasmadynamic
MPDs seem to have the best performances.
An application of superconducting magnets in electric
thrusters was reported in a paper more than 30 years ago by
NASA [22], and more recently in other works [23].
The VASIMR engine is the first electromagnetic thruster
with the need of large bore coils and high magnetic field, thus
it is the best candidate for the use of superconducting
magnets. This engine is a high power, radio frequency-driven
magnetoplasma rocket, capable of Specific Impulse over
Thrust modulation at constant power. The physics and
engineering of this device have been under study since 1980.
A simplified schematic of the engine is shown in Fig. 1.
This electric thruster consists of three main sections: a
helicon plasma source, an ICRH plasma accelerator, and a
magnetic nozzle.
One key aspect of this concept is its electrode-less design,
which makes it suitable for high power density and long
Solenoid, size comparison.
TABLE I
COIL CHARACTERISTICS
Inner Diameter
Outer Diameter
Height
Number of turns per layer
Number of layers
Total number of turns
Weight
Total tape length
Max central magnetic flux density B0
137 [mm]
195 [mm]
135 [mm]
35
28
980
13 [kg]
≈ 400 [m]
≈ 1 [T]
component life by reducing plasma erosion and other
materials complications.
Commercial VASIMR applications may include re-boost of
large orbiting platforms, satellite delivery and repositioning,
as well as cargo delivery to the Moon. Also this technology
could lead to higher-power plasma propulsion for future
interplanetary human and robotic missions.
If we take into account that today the cost of transport to
low Earth orbit can reach 15,000 $/kg, superconductor
technology becomes an important aspect for this device not
only because of high magnetic fields, but also because of the
relatively light weights of superconducting magnets.
The use of magnesium diboride in the VASIMR project is
mainly suggested by three factors:
1- magnesium diboride is intrinsically a very lightweight
superconducting material;
2- the prototype engine is currently using copper solenoid
magnets with a bore of about 30 cm and with a central
magnetic flux density lower than 1 tesla;
3- the flow of high density hot plasma through the magnet
bores suggest the use of a superconducting material
with high critical temperature.
III. COIL DESCRIPTION
To the best of our knowledge this solenoid (shown in Fig.
2) is the first one with a large 14 cm bore wound with the
multifilament copper-stabilized tape. This tape has been
fabricated by powder-in-tube method, through the ex-situ
process; MgB2 powder is packed inside nickel tubes, which
are then drawn into long wires and restacked inside another
nickel tube together with a copper core and an iron barrier
preventing diffusion of copper into nickel and MgB2.
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Fig. 3.
Fig. 4.
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Magnetic flux density vs. radial distance from the center, at 172 A.
Fig. 5.
Scheme of the vacuum chamber set-up for magnet testing.
Fig. 6.
Solenoid inside the vacuum chamber before cooling down.
Magnetic flux density vs. axial distance from the center, at 172 A.
Final shape is obtained through drawing and rolling. The
superconducting properties are improved by an in-line heat
treatment.
Characteristics of this solenoid are in Table I. The coil was
manufactured by ASG Superconductors wrapping a glass tape
on the conductor and using a wet-winding technique to
maximize turn-to-turn insulation with a resin cured at room
temperature. The copper coil form has been cut to avoid losses
due to the current ramp rate, it is 2 mm thick, and insulated by
a glass sheet. The mandrel is about 14 cm high, so that 27
layers of 35 turns each are wound on it. The layer transitions
are about one turn long, i.e. 43 cm, so that bending radius in
the direction of larger inertia is about 24 m. The resulting
bending strain is in the order of 10-5, low enough for a safe
joggle.
Taking into account the non-linear behavior of iron and
nickel, a FEM computation of a solenoid with the same
geometry carrying 172 A (that is a value reached during
experiments at 16 K) indicates a maximum magnetic flux
density of about 1.2 tesla close to the conductor on the inner
layer. Results of the FEM analysis are given in Fig. 3 and Fig.
4, but the magnetic flux density inside the tape is not shown.
IV. EXPERIMENTAL SET-UP
The cryogen free facility, schematically shown in Fig. 5, is
composed of a vacuum chamber about half a meter in
diameter, equipped with a cryocooler G-M Sumitomo having
a cooling power of 1.5 W at 4 K.
The first stage runs at about 30-35 K and is used to cool
down the HTS current leads via an electrically insulated heat
exchanger, whereas the second stage is connected to a copper
plate and is designed to run down to 4 K. Several brass
spacers are inserted between the magnet copper plate and the
cryocooler in order to limit the power into the heater that
regulates the temperature.
Heaters and temperature probes are located on the magnet,
on the copper plate and on the current leads. The magnet was
kept inside an aluminum radiation shield (Fig. 6), cooled by
the first stage of the cryocooler.
Due to the heat load and to electrical insulation, the
temperature of the magnet during the experimental activity
was not lower than 16 K.
In order to measure the quench propagation, the solenoid
was equipped with six voltage taps. The voltage taps are
located as follows: first one at the inner electrical exit, second
one at the end of the second layer, third one at the fifth layer,
fourth one at the tenth layer, fifth one at the nineteenth layer
and the last one at the outer electrical exit.
The signals from the fifth tap, together with the two
electrical exits, were sent to the quench detection system
(balance method).
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Fig. 7.
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Spontaneous quenches at temperatures between 16 and 20.5 K.
Fig. 8. Magnet load line and wire critical current lines (at 20K and at 16K).
B is the maximum magnetic flux density on the inner layer, but outside the
nickel clad. Black dots represent three of the spontaneous quenches.
Fig. 9. Evolution of current and voltage during a spontaneous quench at
20.5 K, Ic=137 A.
Fig. 10. Evolution of internal resistance of the first two segments (R21, R32) at
20.5 K, Ic=137 A.
V. RESULTS AND DISCUSSION
The quench propagation analysis was performed throughout
seven spontaneous quenches at temperatures ranging between
16 and 20.5 K. All the quench current values lay on a straight
line between the highest one of 175 A at 16 K and the lowest
one of 137 A at 20.5 K (Fig. 7).
Assuming that the electromagnetic model (partially showed
in Fig. 3) of this solenoid could reasonably fit the real
magnetic field on the tape, a comparison with the performance
of short tapes, taken from the same batch of the wound tape,
shows a decrease of the critical current below 20% (Fig. 8),
which can be considered a normal degradation for long and
wound ex-situ MgB2 wires.
In particular, with the magnet at 16 K and current of 175 A,
the calculation of the maximum magnetic flux density on the
tape, but outside the nickel sheath, is about 1.2 tesla, whereas
short samples of the same batch could carry over 200 A
during tests in applied field.
Also, it is interesting to note that the electromagnetic model
indicates a maximum flux density of 1.6 tesla inside the nickel
sheath of inner layers of the solenoid when carrying 175 A.
Fig. 11. Comparison between the experimentally measured total magnet
resistance (at 20.5 K, Ic=137 A) and the resistance computed assuming a
longitudinal quench propagation rate of 11.5 cm/s.
Anyway, a direct comparison with the measurements on the
short sample can hardly be precise due to the strong
inhomogeneity of the magnetic field of both sample and coil,
especially when the matrix is ferromagnetic [5, 24].
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study stability problems.
A flexible Minco thermofoil heater (3 cm long, 0.7 cm wide
and with resistance of 11 ohm at 20 K) was wound together
with the first two turns of the solenoid in order to study
induced quench by releasing heat loads on the tape. Fig. 12
shows a typical evolution of the voltages after the release of
0.15 J in 190 ms while carrying 101 A at 20.1 K. The quench
starts right after and it propagates on the second element (taps
2-3) after 1.4 s.
VI. CONCLUSION
Fig. 12. Quench induced by a MINCO thermofoil heater in contact with the
first turn of the magnet. The tape was carrying 101 A at 20.1 K when the
heater released 0.15 J in 190 ms.
In Fig. 9 the evolution of current and voltage is reported for
the quench test at 20.5 K. The quench takes place at the inner
layer, as expected, where the magnetic field is stronger and
the protection system activates the breaker at t = 0. The
current runs at 137 A and starts to decrease when the
resistance within the voltage taps 1-2 (and successively 2-3)
rapidly rises up. As a consequence, the voltages between taps
3-4, 4-5 and 5-6 decrease with current. Taking into account
the tape inductance and the decreasing current, Fig. 10 shows
the evolution of the resistance of the first two parts transiting
into normal conducting state, within voltage taps 1-2 and 2-3.
The resistance R of the i-th element is given by formula (1):
dI ⎞
⎛
Ri = ⎜ Vi − Li ⎟ / I
dt ⎠
⎝
(1)
where V is the voltage, L is the inductance of the i-th element
of the magnet measured during the ramp-up and I is the
current.
When the transition starts in the second element (voltage
taps 2-3), the value of the resistance R21 of the first element
indicates that a part of the tape not longer than 50 cm has
already turned normal. The transition of the second element
starts about 0.4 s after the quench started in the first element.
As a comparison, in Fig. 11 the evolution of total magnet
resistance is computed with Wilson’s QUENCH [25] and it is
compared with the data collected during the experiment. We
assumed the following parameters in order to obtain a curve
that fits experimental data:
- longitudinal propagation rate of 11.5 cm/s,
- alpha ratio (transverse prop. rate / longitudinal. prop.
rate) of 0.1.
Anyway, this model is a poor approximation since it only
considers one transverse propagation rate (i.e. axialsymmetrical wires), whereas a tape has two different
transverse propagation rate. More work is needed to better
define the quench propagation phenomena in this solenoid.
Another study will be carried out soon by analysing other
data; in fact other eight quenches were induced in order to
These results show a degradation of the current carrying
capability below 20% for long and wound ex-situ MgB2
superconductors compared to short samples, thus indicating
that these tapes can be used to prepare solenoid coils and that
the layer jump is not a problem for a 14 cm bore magnet form.
Our tests demonstrated the robustness of these tapes, the
solenoid was not damaged by quench.
This magnet is only a prototype, but the achievement of a 1
tesla central B0 at 16 K in such a large solenoid demonstrates
the usability of MgB2 conductors for applications such as
electric space propulsion systems and the aforementioned
VASIMR. Critical current, weight and size will be further
improved in the next steps of our research activity. Also other
wires will be tested in order to identify the best configuration
for this kind of magnets.
ACKNOWLEDGMENT
The authors thank the Laboratory for Accelerator and
Applied Superconductivity of the National Institute of Nuclear
Physics (Milan, Italy) for the time and efforts allotted to this
project.
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