Behavior of a 14 cm Bore Solenoid with Multifilament MgB 2 Tape

Transcription

1 > Behavior of a 14 cm Bore Solenoid with Multifilament MgB 2 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 MgB 2 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 MgB 2 wires and tapes. In the literature small coils have already demonstrated central magnetic flux density above 2 tesla, overcoming common winding problems related to MgB 2 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 MgB 2 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 B 0 of 1 tesla. Index Terms - MgB 2, cryocooler, superconducting magnet. M I. INTRODUCTION AGNESIUM 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 MgB 2 powder, Manuscript received August 29, 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 USA ( amatteo@uh.edu). 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 MgB 2 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 MgB 2 (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 MgB 2 are considered to be very competitive at about 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, MgB 2 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 MgB 2 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 MgB 2 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 MgB 2 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.

2 > Fig. 2. Solenoid, size comparison. 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. MGB 2 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 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 TABLE I COIL CHARACTERISTICS Inner Diameter 137 [mm] Outer Diameter 195 [mm] Height 135 [mm] Number of turns per layer 35 Number of layers 28 Total number of turns 980 Weight 13 [kg] Total tape length 400 [m] Max central magnetic flux density B 0 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; MgB 2 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 MgB 2.

3 > Fig. 3. 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. 4. 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. Fig. 6. Solenoid inside the vacuum chamber before cooling down. The first stage runs at about 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).

4 > Fig. 7. Spontaneous quenches at temperatures between 16 and 20.5 K. Fig. 9. Evolution of current and voltage during a spontaneous quench at 20.5 K, I c =137 A. 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. 10. Evolution of internal resistance of the first two segments (R 21, R 32 ) at 20.5 K, I c =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 MgB 2 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, I c =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].

5 > 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. 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 (1) dt 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 R 21 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 VI. CONCLUSION These results show a degradation of the current carrying capability below 20% for long and wound ex-situ MgB 2 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 B 0 at 16 K in such a large solenoid demonstrates the usability of MgB 2 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. REFERENCES [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Superconductivity at 39 K in magnesium diboride, Nature, 410 (2001) 63. [2] R. Musenich, P. Fabbricatore, C. Fanciulli, C. Ferdeghini, G. Grasso et al., Construction and Tests of MgB 2 React & Wind Coils, IEEE Trans. Appl. Supercond. 14 (2), , Jun [3] R. Musenich, P. Fabbricatore, S. Farinon, C. Ferdeghini, G. Grasso et al., Behavior of MgB 2 React & Wind Coils Above 10 K, IEEE Trans. Appl. Supercond. 15 (2), , Jun [4] M. Modica, G. Grasso, M. Greco, R. Marabotto, R. Musenich et al., Behavior of MgB 2 Reacted and Wound Coils From 14 K to 32 K in a Cryogen Free Apparatus, IEEE Trans. Appl. Supercond. 16 (2), , Jun [5] R. Musenich, P. Fabbricatore, S. Farinon, M. Greco, M. Modica et al., The behaviour of cryogen-free MgB 2 react and wind coils, 2006 Supercond. Sci. Technol. 19 (3), S126-S131. [6] G.Bellomo, R.Musenich, M.Sorbi and G.Volpini, MgB 2 Coils for Particle Accelerators, IEEE Trans. Appl. Supercond. 16 (2), , Jun [7] A. Matsumoto, H. Kumakura, H. Kitaguchi, H. Fujii and K. Togano, The annealing effects of MgB 2 superconducting tapes, Physica C 382 (2-3), , Nov [8] S. H. Zhou, A. V. Pan, H. K. Liu, J. Horvat and S. X. Dou, Effect of various mechanical deformation processes on critical current density and

6 > microstructure in MgB 2 tapes and wires, 2002 Supercond. Sci. Technol. 15 (11), [9] K. Salama, Y. X. Zhou, M. Hanna, M. Alessandrini, P. T. Putman and H. Fang, Electromechanical properties of superconducting MgB 2 wire, 2005 Supercond. Sci. Technol. 18 (12), S [10] M. Eisterer, M. Zehetmayer, S. Tonies, H. W. Weber, M. Kambara et al., Neutron irradiation of MgB 2 bulk superconductors, 2002 Supercond. Sci. Technol. 15 (2), L9-L12. [11] S. X. Dou, S. Soltanian, J. Horvat, X. L. Wang, S. H. Zhou et al., Enhancement of the critical current density and flux pinning of MgB 2 superconductor by nanoparticle SiC doping, Appl. Phys. Lett. 81 (18), , Oct [12] J. Horvat, X. L. Wang, S. Soltanian, and S. X. Dou, Improvement of critical current in MgB 2 /Fe superconducting wires by a ferromagnetic sheath, Appl. Phys. Lett. 80 (5), , Feb [13] J. Horvat, S. Soltanian, X. L. Wang and S. X. Dou, Magnetic shielding in MgB 2 /Fe superconducting wires, IEEE Trans. Appl. Supercond. 13 (2), , Jun [14] E. W. Collings, S. Kawabata, M. Bhatia, M. Tomsic, and M. D. Sumption, Magnesium Diboride Superconducting Strand for Accelerator and Light Source Applications, IEEE Trans. Appl. Supercond. 16 (2), , Jun [15] G. Masullo, A. Matrone, A. Santoro, S. Besio, S. Pittaluga et al., Analysis of advantages obtainable using MgB 2 in the design of electromagnets with iron yoke for MRI, Int. J. Mod. Phys. B 17 (4-6), Part 2, Mar [16] A. Devred, S. A. Gourlay, and A. Yamamoto, Future Accelerator Magnet Needs, IEEE Trans. Appl. Supercond. 15 (2), , Jun [17] S. I. Schlachter, W. Goldacker, A. Frank, B. Ringsdorf and H. Orschulko, Properties of MgB 2 superconductors with regard to space applications, Cryogenics 46 (2-3), , [18] Superconductor Week, Vol. 20, No. 10, [19] H. Fang, P. T. Putman, S. Padmanabhan, Y. X. Zhou and K. Salama, Transport critical current on Fe-sheathed MgB 2 coils, 2004 Supercond. Sci. Technol. 17 (4), [20] M. Alessandrini, H. Fang, M. Hanna, P. Putman, Y. X. Zhou et al., The Potential Application of MgB 2 Superconducting Magnets in Space, IEEE Trans. Appl. Supercond. 16 (2), , Jun [21] E. A. Bering, M. Brukardt, J. P. Squire, T. W. Glover, V. Jacobson and G. McCaskill, Recent Improvements in Ionization Costs and Ion Cyclotron Heating Efficiency in the VASIMR Engine, 44 th Aerospace Sciences Meeting and Exhibit, AIAA , Reno, NV, Jan [22] D. J. Connolly, A. R. Bishop, G. R. Seikel, Test of permanent magnet and superconducting magnet MPD thrusters, AIAA/SAE 7 th Propulsion Joint Specialist Conference, AIAA , Salt Lake City, UT, Jun [23] C. Bruno and D. Casali, Superconducting Materials Applied to Electric Propulsion Systems, Journal of spacecraft and rockets, 41, 4, [24] P. Fabbricatore, R. Musenich, R. Parodi, S. Pepe and R. Vaccarone, Self field effects in the critical current measurements of superconducting wires and cables, 1989 Cryogenics 29, [25] M. N. Wilson, Superconducting Magnets, Oxford University Press.