High-temperature superconductors (HTS) and MgB₂ may potentially improve the usability of superconducting magnets dramatically owing to their large energy margin. When HTS and MgB₂ wires are used for magnets operated in the persistent current mode, such as in magnetic resonance imaging (MRI) scanners, the electric field generated in the wires must be lower than 10⁻¹⁰ V m⁻¹. In this paper, critical current density, J _c, defined at an electric field criterion of 10⁻¹⁰ V m⁻¹ is evaluated from a magnetisation decay measurement for state-of-the-art monofilamentary MgB₂ wires. By using the obtained J _c, a critical line of our multifilamentary MgB₂ wire is shown on the temperature-magnetic field plane. Here, the critical line is defined as a line on which an electric field of 10⁻¹⁰ V m⁻¹ is generated at a coil current density of 150 A mm⁻². The area inside the critical line is demonstrated to be large enough to fulfil the requirement of 1.5 T MRI scanners operated at 10K–15K. In addition, the iso energy-margin lines are shown on the temperature-magnetic field plane and compared with those of NbTi wires. The MgB₂ wire has an order of magnitude greater energy margin than the NbTi wires in most of the area inside the critical line. This suggests that the MgB₂ wire is highly unlikely to be quenched due to mechanical disturbances.

高温超导体（HTS）和MgB ₂可能由于其较大的能量裕度而极大地提高了超导磁体的可用性。当将HTS和MgB ₂导线用于以恒定电流模式运行的磁体时，例如在磁共振成像（MRI）扫描仪中，导线中产生的电场必须低于10 ^-10 V m ^-1。在本文中，通过最先进的单丝MgB ₂导线的磁化衰减测量评估了在10 ^-10 V m ^-1电场准则下定义的临界电流密度J _c。通过使用获得的J _c ，在温度磁场平面上显示了我们的多丝MgB ₂线的临界线。这里，临界线被定义为在其上的10的电场的线^-10 V M ^-1在150·毫米的线圈电流密度产生的^-2。临界线内的区域被证明足够大，可以满足以10K-15K运行的1.5 T MRI扫描仪的要求。此外，等温能量边界线显示在温度磁场平面上，并与NbTi线进行比较。在临界线内部的大部分区域，MgB ₂线的能量裕度比NbTi线大一个数量级。这表明MgB ₂ 由于机械干扰，焊丝极不可能被淬火。