Superconducting Accelerator Magnets Based on High-Temperature Superconducting Bi-2212 Round Wires

Abstract

Superconducting magnets are an invaluable tool for scientific discovery, energy research, and medical diagnosis. To date, virtually all superconducting magnets have been made from two Nb-based low-temperature superconductors (Nb-Ti with a superconducting transition temperature Tc of 9.2 K and Nb3Sn with a Tc of 18.3 K). The 8.33 T Nb-Ti accelerator dipole magnets of the large hadron collider (LHC) at CERN enabled the discovery of the Higgs Boson and the ongoing search for physics beyond the standard model of high energy physics. The 12 T class Nb3Sn magnets are key to the International Thermonuclear Experimental Reactor (ITER) Tokamak and to the high-luminosity upgrade of the LHC that aims to increase the luminosity by a factor of 5–10. In this paper, we discuss opportunities with a high-temperature superconducting material Bi-2212 with a Tc of 80–92 K for building more powerful magnets for high energy circular colliders. The development of a superconducting accelerator magnet could not succeed without a parallel development of a high performance conductor. We will review triumphs of developing Bi-2212 round wires into a magnet grade conductor and technologies that enable them. Then, we will discuss the challenges associated with constructing a high-field accelerator magnet using Bi-2212 wires, especially those dipoles of 15–20 T class with a significant value for future physics colliders, potential technology paths forward, and progress made so far with subscale magnet development based on racetrack coils and a canted-cosine-theta magnet design that uniquely addresses the mechanical weaknesses of Bi-2212 cables. Additionally, a roadmap being implemented by the US Magnet Development Program for demonstrating high-field Bi-2212 accelerator dipole technologies is presented.

Conceptual Design of a HTS Dipole Insert Based on Bi2212 Rutherford Cable

“…Km-length quantities of Bi2212 round wires have been commercially produced since the 1990s. The critical current density J c and the engineering wire current density J e at 20 T and 4.2 K of the round wire increased from 1500 and 400 A/mm 2 , or less in the 2000s, respectively, to the J e values shown in Figure 7 [10]. The range of the engineering wire current density for the wire used in the Rutherford cable provided by LBNL at 5 T is shown in the plot by the red double arrow, which is consistent with the parameters of the 2015 generation Bi2212 wire.…”

“…The design concept of the Bi2212 insert is based on a two-layer cos-theta coil with a stress management concept which was developed at Fermilab for high-field large-aperture Nb 3 Sn accelerator magnets [6]. The coil uses a Rutherford cable with rectangular cross-section 7.8 mm wide and 1.44 mm thick, wrapped with 0.15 mm thick insulation [10].…”

“…For the first insert coil, the coil materials and the current procedures developed at LBNL [10,14] and at Fermilab will be used. The mandrel and the parts of the coil structure will be made of Aluminum-bronze-954 or Inconel-600, which is more expensive and challenging to machine, or some other appropriate material which can accept the high-temperature heat treatment in an oxygen atmosphere.…”

“…Unfortunately, the Bi2212 leaks that were observed at 1 bar reaction in oxygen do not desist when performing the 50 bar heat treatment. Spots or discolorations form where Bi2212 liquid leaks through the encasing Ag alloy metal at high temperatures and react with surrounding materials, including the insulation [10]. Most leakages in small racetracks produced at LBNL occur at the Rutherford cable edges, and they degrade the J e locally.…”

“…To reduce Bi2212 leaks, before insulating the Rutherford cable with the mullite sleeve, the cable is brushed with a thin layer of TiO 2 -polymer slurry, i.e., TiO 2 powder mixed with ethanol. Once this is of completed, this coating is also applied to the insulated cable onto the mullite sleeve [10,14]. This method does not eliminate leaks, but at least reduces their number.…”