11-09-2013, 04:16 PM
A versatile cryocooled 15 T superconducting magnet with a room-temperature bore and an optical window
A versatile cryocooled .pdf (Size: 1.04 MB / Downloads: 39)
ABSTRACT
A close-cycle cryocooled Nb3Sn superconducting
magnet, using a single coil along with leads of the
2223 bismuth cuprate high-temperature supercon-
ductor, has been designed and fabricated. The mag-
net achieves a field up to 15 T and has a
roomtemperature bore of 52 mm as well as an opti-
cal window. The cooling-down time from room tem-
perature to 3.5 K is 13 h. The maximum field attain-
able is 15 T for 3 h and 14.5 T for continuous use.
This magnet will be specially useful in laboratories
where liquid helium is not readily available.
INTRODUCTION
SUPERCONDUCTING magnets employing Nb3Sn coils
immersed in liquid helium are continuously used to at-
tain high magnetic fields 1. The connecting leads in these
magnets are made up of copper wires. Due to heat losses
through the leads, these magnets consume large quanti-
ties of liquid helium. After the discovery of high T c su-
perconducting materials, the copper leads are being re-
placed by the high T c ceramic superconductor leads, to
reduce thermal losses up to 90%. Close-cycle refrig-
erator systems reaching 4.2 K with a cooling capacity
of 1.0 W at 4.2 K have been fabricated by employing
solid state conduction cooling. From the engineering
point of view, one can design a superconducting magnet
by employing the conventional Nb3Sn coils and high T c
superconductor leads so that the magnet can work with-
out liquid helium. An important requirement in super-
conducting magnets is a room-temperature bore, essen-
tial for researchers working at high temperatures, but
this is difficult to achieve in the conventional immer-
sion type technology. What would be most useful is to
have a magnet that can produce high magnetic fields
without liquid helium, and which also has a
roomtemperature bore. Such a magnet will also allevi-
ate problems faced with the non-availability of liquid
helium in many countries.
Performance of the magnet
In order to test the performance of the magnet, we have
measured the magnetoresistance of polycrystalline as
well as thin film samples of rare earth manganates at
different fields and at different temperatures. In Figure
4, we show the temperature variation of resistance of
an epitaxil thin film of Pr 0.7 Ca 0.3MnO 3 deposited on a
LaA1O 3 (001) substrate at different magnetic fields.
Pr 0.7Ca 0.3 MnO 3 is a charge-ordered insulator, with the
charge-ordering temperature around 230 K (ref. 8). We
see that the magnetic field induces an insulator-metal
transition in the film. The material becomes metallic
starting from ~190 K or down at 14.5 T.
We have operated the magnet for 1700 h. or more
continuously and found the performance of the magnet
to be satisfactory. Since we completed the fabrication
of this magnet, we have come across a paper by
Watanabe et al. 9 , who have achieved a field of 15 T by
employing a three-coil system. The pre-cooling time of
the magnet (from room temperature to 3.4 K) was 110
h, while the magnet designed by us takes only 13 h to
reach the same temperature.