Davide Nardelli

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

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.

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

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

A high-purity MgB4 phase has been synthesized and used as the precursor powder for the realization of in situ wires. Various final heat treatments, from 550 to 1100 ° C for 20 min each, have been carried out to convert the inner mixture to MgB2. Critical current densities up to 1.75 × 106 and 0.98 × 106 A cm−2 at 4.2 and 20 K, with 0.2 T, up to 0.76 × 105 A cm−2 at 4.2 K and 4 T, and up to 0.78 × 105 A cm−2 at 20 K and 2 T were measured. Critical temperatures up to 38.5 K were measured. These results suggest that this could be a good alternative to both ex situ and classical in situ routes.

Behavior of MgB2 react & wind coils above 10 K

A nonmagnetic sheath, namely a Ni/Cr alloy (Ni80/Cr20), was used to fabricate MgB/sub 2/ tapes through the powder-in-tube (PIT) ex-situ technique. A series of samples sintered at different temperatures between 200/spl deg/C and 950/spl deg/C were prepared in order to correlate all the superconducting properties between them and with the sintering temperature. The possibility of using a nonmagnetic but at the same time hard sheath chemically compatible with the MgB/sub 2/ phase made possible a simpler interpretation of transport and magnetic measurements in magnetic field. With the chosen cold working parameters, we found that the amount of MgB/sub 2/ lattice strain was not sufficient to produce any enhancement of the upper critical field. The effect of the heat treatment temperature was on one hand to increase the grain connectivity but also to reduce the pinning properties of the superconductor.

Large critical current density in MgB2 wire using MgB4 as precursor

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.

MgB/sub 2/ tapes with non-magnetic sheath: effect of the sintering temperature on the superconducting properties

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.

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In the present paper we report an in?situ high-energy x-ray diffraction analysis of MgB2 tapes during the preparation process. The experiment was performed in a specifically designed furnace working in a reducing atmosphere, compatible with the Laue diffraction condition. The? MgB2 synthesis was realized starting from MgH2 and amorphous B in powder form as precursors, varying reaction temperature and testing different cooling processes. We analysed both the MgB2 synthesis and the sintering process of tapes prepared with these powders. Phase evolution, micro-?and crystallographic structure were monitored during the different thermal treatments. Among the main results we observed the formation of MgB2 at an extraordinary low temperature (300??C), probably as a result of a solid-state reaction between MgH2 and B. Furthermore, we studied the dependence of the microstructure upon the thermal treatment and its effect on the critical current performance of the superconducting tapes.