Roberto Marabotto

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}

At present, one of the main options for beam bending dipoles of the SIS 300 synchrotron, under design for the FAIR facility at GSI, is a single layer magnet 7.8 m long, 100 mm in bore diameter, generating 4.5 T. This coil has two main features: it is curved, with a curvature radius of 66.67 m (the corresponding sagitta is 114 mm), and shall be ramped at 1 T/s. Both these characteristics demand challenging R&D, aimed at the development of the required conductor and winding technology. The paper discusses both these aspects, in the frame of a general ongoing R&D program at INFN, under the name DISCORAP. Its goal is the construction of a short prototype (3.8 m) dipole, fully integrated into its horizontal cryostat, within three years. The R&D program includes: 1) the activities required to develop low loss superconducting wires and cable; 2) the technological developments (at the industrial level) for defining and optimizing the dipole constructing methods; 3) the construction of curved dipole coil winding models; 4) the construction of the complete curved dipole; 5) the test of the curved dipole in a vertical cryostat; 6) the integration of the curved dipole into a horizontal cryostat, for the final test at GSI.

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.

Development of a Curved Fast Ramped Dipole for FAIR SIS300

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 Facility for Anti-proton and Ion Research (FAIR), under development at GSI, includes the synchrotron SIS300, so called because the magnetic rigidity is 300 Tm. In order to reach the required high intensities of proton and heavy ions beams, the bending dipole magnets have to be pulsed from the injection magnetic field of 1.5 T up to 4.5 T maximum field at the rate of 1 T/s. These 7.8 m long magnets have cos θ shaped coils with a 100 mm bore and the particular characteristic to be geometrically curved, with a sagitta of 112.9 mm. These challenging requirements triggered R&D activities, aimed at the development of suitable construction technologies for fast ramped curved coils. The heart of the R&D program is the construction of a 3.9 m long model. The paper discusses the main problems faced during the design and the construction of the cold mass, mainly covering the aspects related to the manufacture.

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.

The Construction of the Model of the Curved Fast Ramped Superconducting Dipole for FAIR SIS300 Synchrotron

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}

Design activities coupled with conductor R&D and model coil construction are under way for developing a curved fast cycled superconducting dipole suitable for operations of the SIS300 synchrotron at FAIR. The main target is the construction within 2009 of a model magnet (cold mass fully integrated in a horizontal cryostat). This magnet is designed for generating 4.5 T magnetic field in a bore of 100 mm, able to be operated at a field rate of 1 T/s. The magnetic length is 3.8 m with a curvature radius of 66.67 m (the sagitta is 27 mm). The mechanical structure is based on 3 mm thick laminated stainless steel collars, assembled through keys, and 1 mm thick iron yoke laminations, assembled through large Al alloy C-shaped clamps. A 2D finite element analysis has been performed to evaluate stresses and deformations coming out during assembly, cool-down and energization. Particular emphasis has been given to the possible fatigue problems ensuing from the large operating field rate over a large lifetime cycle number, 107. Numerical results are presented and discussed.The FAIR facility, under development at GSI, includes the synchrotron SIS300 (300 Tm rigidity). In order to reach the required high intensities of proton and heavy ion beams, the bending dipole magnets have to be pulsed from the injection magnetic field of 1.5 T up to 4.5 T maximum field at the rate of 1 T/s. These 7.8 m long magnets have cos-theta shaped coils with a 100 mm bore with the particular characteristic to be geometrically curved (the sagitta is 114 mm). The 2D mechanical cross section has been designed in detail: it consists of 3 mm thick laminated stainless steel collars, assembled through keys, and 1 mm thick iron yoke laminations, assembled through large stainless steel C-shaped clamps. This paper analyses some aspects that are intrinsically 3-dimensional: the mechanical behavior of the ends, the effect of the longitudinal pre-stress, the design of the external flange and outer shell.

Short Windings

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.