Development of Bi(2212)-based wire for innovative magnets

In the last years, the development of high field magnets involving an innovative layout is taking place. This layout is based on the winding of wires shaped as tilted solenoidal layers, fed with opposite current, causing the generation of a transverse dipolar or multipolar magnetic field. This special class of magnets, called Canted Cosine Theta, is widely studied especially in view of the developments of high field magnets for future high energy colliders, as well as applications in medical and electrical fields. In the Genoa area, a project called BISCOTTO, an acronym for BiSCCO Cosine Theta Tilted sOlenoid, is going on with the aim to develop CCT technologies. In particular, the final aim of the BISCOTTO project, born from the collaboration between the Genoan and Milan sections of INFN and CNR-SPIN Genoa, is the development of the key technologies to be involved in the design and construction of a superconducting canted cosine theta magnet involving HTS conductors. Since the present technology on HTS conductors shows that different types of promising conductors could be used ( BiSCCO-2212 wires, ReBCO tapes and MgB_2 wires) and since the development of the magnet cannot be separated from the conductor, both a CCT with BiSCCO wire and with MgB_2 wire are studied. It is worth mentioning that the development of technologies for HTS magnets has a more general value also beyond this specific objective because the use of HTS material for any kind of magnet layout would have a significant impact in much wider fields. As already said, the design of a CCT HTS magnet cannot be decoupled from the construction techniques, which have many critical aspects. The Wind&React technique for BiSCCO is needed. This means that the winding operation is not so problematic because the conductor is not heat treated yet, but the heat treatment of the whole magnet is critical for possible degradation due to thermomechanical reasons. Moreover, the design of the HTS magnet requires detailed knowledge of the critical current density of the conductor as function of temperature, magnetic field and strain (see Chapters 2 and 3 for more details about those behaviours in BiSCCO wires). Due to these difficulties, the construction of a full magnet is too premature and, in any case, would require a long length of conductor not developed yet. The choice of the MgB_2 beside BiSCCO was took to start the study of this problem with a well-known and cheap conductor. The construction of a small model with two layers of a BiSCCO wire and/or an MgB_2 wire is feasible and would allow to understand and to solve some very basic problems. This work is mainly focused on the development of high performances Bi(2212) wires, manufactured avoiding the use of any Over Pressure (OP) treatment, to be involved in the design and construction of a Canted Cosine Theta (CCT) solenoids. There are several important motivations and interesting application fields for this kind of magnet, from particle physics to medical or electrical applications, and the development of CCT solenoids based on HTS conductors would open new horizons in those areas. Coming to the structure of this thesis, after a brief and general introduction on superconducting wires and magnets (Chapter 1), the manufacturing and optimization of Bi(2212) wires through the easily industrial scalable PIT technique, specifically with the “Groove Die Groove” (GDG) process developed at CNR-SPIN in Genoa, is presented in detail in Chapter 2. Besides, a complete characterization of the transport properties of wires with useful performances for application is reported both vs the applied magnetic field (up to 7 T) and the temperature, in a range between 4.2 K up to 20 K. Regarding magnet development, the most common solution to achieve the desired overall current in superconducting magnets is to arrange the superconducting wires into cables. One of the most used designs for cables is the Rutherford architecture. This kind of cable is a good balance between a limited wire deformation and good cable compaction. Nevertheless, some superconducting materials, such as Nb_3 Sn or Bi(2212), suffer the cabling operation in reducing J_C carried by single wires. One of the challenges we face is trying to understand if Bi(2212) wires made with an innovative process developed at CNR-SPIN and called GDG process can be arranged into a Rutherford-like cable without a remarkable properties degradation. In Chapter 3, the effects of the mechanical deformations induced on wires by the cabling process are studied. In order to get that, a series of wires has been flattened by flat rolling. The relative transport properties have been measured as a function of the level of applied deformation. Moreover, the correlation between the distribution of the force accumulation inside the wires, evaluated through finite element simulations made with Ansys™ code, and the experimental obtained critical current densities is described. Understanding the interplay between the distribution of the forces and the superconductive properties of the wires is of utmost importance for the development of high-quality cables. In the end, Chapter 4 is dedicated to the first steps done in the design and construction of a CCT solenoid prototype made of two different HTS conductors.