addition to the fundamental interplay between these two phenomena. The observation of antiferromagnetic ordering in the form of stripes versus the checkerboard pattern observed with the cuprates prompted further comparisons of similarities and differences between the iron-based superconductors and the cuprates. Magnetic correlations appear strongly in these families of materials so for this reason the iron-based superconductors are ideal for the study of the fundamental relationship between magnetism and superconductivity. Since some aspects of high-temperature superconductivity are still under debate a quarter of a century after the discovery of the cuprates, the iron-based superconductors offer another opportunity for the development of a fundamental understanding of the mechanism of high-temperature superconductivity.

The availability of high magnetic fields, particularly the 45 T hybrid dc field magnet in addition to the pulsed-field magnets at LANL, played a critical role in the exploration of many of the interesting characteristics exhibited by these superconductors. Experimental evidence of multiband superconductivity also quickly revealed that the higher-critical-temperature members of this new class of unconventional superconductors exhibited very high upper critical fields comparable to the cuprates. During this flurry of discoveries, it was the fortuitous availability of high magnetic fields in both dc and pulsed modes that sustained the pace of the investigation of these materials. Table 7.1 gives the upper critical temperature and upper critical field for selected iron-based superconductors.

The upper critical field phase diagram of several superconductors having potential for commercialization in high-field magnets, including the LTS and HTS conductors already in production, are shown in Figure 7.4.

Superconducting magnet design also must take into account electrothermal stability, ac losses (magnetic hysteresis) if cycled, quench detection and protection, and stress management. A magnet design that resolves all these issues simultaneously and in an integrated fashion requires a high level of engineering and manufacturing sophistication. This becomes increasingly more difficult as magnetic fields are pushed ever higher. This is fundamental because all of these issues scale with the magnetic field B, or with the magnetic pressure B2.

Clearly, the critical current density Jc decreases with increasing field B. This then requires the use of more superconductor at lower overall winding current density, leading to use of more materials and higher cost. As the size of the coil winding increases, the conductor turns are placed at larger radius, decreasing the effectiveness for generating axial field in the magnet bore. Operation at these high fields also increases the probability and consequences of unstable behavior as operating margins are reduced.

Two of the most significant impacts, though, are (1) difficulty in protecting the magnet from damage in event of a quench and (2) management of the coil stresses from the Lorentz forces. Quench protection becomes more difficult because the



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