M. Modica

Construction and Operation of Cryogen Free

Following a dedicated R&D program, ASG Superconductors has recently developed techniques for designing and constructing open cryogen free MRI magnets, refrigerated by two double stage cryocoolers only. The magnet consists of two coils both made with six double pancakes, each double pancake being obtained reacting and winding 1600 m of multifilamentary, copper-stabilized MgB2 tape supplied by Columbus Superconductors. Here we report the thermal and electromagnetic characterization and the achieved targets of the first prototype, evaluated on a long term activity period. The MRI images, acquired starting from November 2006, further demonstrate the accomplishment of remarkable magnet performances. In parallel to the long term tests on the first prototype, ASG Superconductors has designed and constructed a second MRI magnet with improved characteristics. We present here the related test results and a comparison with the previous ones.

{\hbox{MgB}}_{2}

Since the development of the first MgB2 tapes, ASG Superconductors started an R&D activity aimed to produce magnets using the MgB2 conductor. Reacted and wound magnesium diboride double pancake coils were designed, manufactured (using industrial equipment) and tested, in a cryogen free apparatus, by ASG Superconductors. Each double pancake coil was produced by winding a single length of MgB2, 1600 m long, multifilamentary stabilized tape. The successful achievement of the tests allowed us to produce and test the first coil made with 6 single double pancakes, then the second coil and finally the completion of a cryogen free magnet for MRI application. This paper reports the main features of the coil design and construction, the magnet and the test results.

Magnets for Open MRI Systems

Nickel-sheathed MgB2 tapes were fabricated by means of the powder-in-tube method with a monocore configuration of the superconductor. The metallic tubes were filled with commercial reacted powders and cold-worked, respectively, by groove rolling, drawing, and rolling to flat tapes of about 4 mm in width and 0.35 mm in thickness. Portions of the manufactured conductors were heat-treated in an argon atmosphere at temperatures up to 940 °C. Conductors were characterized by transport measurements to determine the magnetic field dependence of the critical current density, the irreversibility line, as well as to establish an initial relationship between the thermo-mechanical treatment and the current carrying capacity of the manufactured MgB2 samples. Our optimized samples for the low field operation carry a critical current density of at least 4 × 105 A cm−2 at 4.2 K, 1 T. The main outcome of this study, however, is that the optimal heat treatment condition changes if the MgB2 conductor has to be employed respectively for a low- or high-field operation. Finally, the first successful demonstration of the current carrying capability of longer MgB2 tapes is also reported in this paper.

Design, Construction and Tests of MgB2 Coils for the Development of a Cryogen Free Magnet

Although the critical temperature of MgB2 is far below those of HTS materials, its simple composition, its upper critical field, the lack of weak links and the low anisotropy make it an interesting material for magnet applications. The low cost of magnesium diboride and the rather simple conductor manufacturing process triggered the interest of the industry and at present conductors are available in km order lengths. MgB2 is a good candidate for MRI applications and also for particle accelerator magnet applications. Several magnesium diboride coils were constructed and successfully tested in cryogen-free systems. Current values up to 320 A were reached at 14 K for react and wind pancake coils, whose behaviour is described in the paper.

Fabrication and properties of monofilamentary MgB2 superconducting tapes

React & wind magnesium diboride pancake coils and solenoids were constructed and tested. The solenoids (6 layers) were manufactured by winding a single length of MgB/sub 2/ tape (80 m) onto a 15 cm diameter, 15 cm high mandrel. The pancake coils were manufactured starting from a 40 meter long tape: the conductor was wound onto a 12 cm diameter coil former. The magnets were tested both in liquid helium and conduction cooled in a cryogen free apparatus. The results compared with the tape characteristics are reported.

The behaviour of cryogen-free MgB2 react and wind coils

The capability of manufacturing long superconducting MgB2 wires with already remarkable critical currents makes this material a very promising candidate for future applications. Tapes are prepared by the powder-in-tube technique. After the cold working procedure typically carried out by wire drawing and cold rolling, it has been found that a final sintering step carried out in argon atmosphere is a key process for further improving the superconducting properties of the conductors. To study the effect of the deformation and heat treatment processes, we performed neutron scattering experiment. Due to the high penetration depth of neutron inside matter, it was possible to analyse the MgB2 phase still wrapped in the Ni sheath. Our studies were carried out by a full spectra refinement by the Rietveld method. In the starting superconducting powder a large Mg deficiency was observed. In the tapes we found that the large forces applied during the cold working induced a large MgB2 lattice deformation, and that it is partly relaxed during the final sintering process. An important correlation of the residual stress with the critical temperature and the pinning properties was pointed out. We also observed the appearance of detrimental secondary phases during the sintering process. In particular, the MgB2 phase reacted with the nickel sheath and MgB2Ni2.5 was formed at temperatures higher than 850 °C. These results are of basic importance for a further optimization of the transport properties at moderate fields where applications of MgB2 tapes are already envisageable.

Behavior of MgB2 react & wind coils above 10 K

Feasibility of industrial production of MgB2 cables and magnets has been established, thus leading to MRI systems realization. Apart from continuing the development in performances of both cable and magnet, a further important step consists in applying superconductive junctions to windings, to obtain a better field stability. In 2006 a technique to obtain some tens of Ampere in persistent mode operation in a joined MgB2 cable was found. Since then, short windings were repetitively built to test the progress of the performances of the junctions. Among them, a single junction, five meter long windings with a diameter of 260 millimeter were put in persistent mode (i. e. with total resistance less than 10-14 Ohm) with 300 Ampere circulating at 20 Kelvin, self-field; also windings with two junctions and about one meter long with the same diameter were put in persistent mode with 200 Ampere circulating at 20 Kelvin, self-field.

Neutron scattering studies of superconducting MgB2 tapes

A magnesium diboride, cryogen-free, H-dipole magnet with cold iron yoke was constructed and tested. The racetrack coil, 48?cm long, was wound with 350?m of nickel-clad, copper-MgB2 tape. The iron yoke forms a 2.6?cm gap. The magnet was connected to a cryocooler and tested at different temperatures ranging between 8.5 and 24?K. The maximum current, 263?A, was reached, without training, at 8.5?K. The corresponding field in the gap was 2.35?T.

Persistent Mode MgB

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.

_{2}

Magnesium diboride (MgB/sub 2/) pancake coils and solenoids have been constructed and tested. The magnets have been wound using the react and winding technique. The magnets have been tested in liquid helium bath up to the quench currency. In this paper the coils and their behavior is discussed.