No-insulation multi-width winding technique for high temperature superconducting magnet

Abstract

We present a No-Insulation (NI) Multi-Width (MW) winding technique for an HTS (high temperature superconductor) magnet consisting of double-pancake (DP) coils. The NI enables an HTS magnet self-protecting and the MW minimizes the detrimental anisotropy in current-carrying capacity of HTS tape by assigning tapes of multiple widths to DP coils within a stack, widest tape to the top and bottom sections and the narrowest in the midplane section. 1,2 In the event of a quench, we confirmed that the magnet current in an NI winding automatically bypassed the quench spot through turn-to-turn contacts from its original spiral path and that the magnet remained stable even though its operating current was pushed to twice the magnet critical current.1 Due to this self-protecting feature, an NI magnet requires a minimum amount of stabilizer, just enough for ease of splicing and handling.3-8 The absence of both turn-to-turn insulation and the extra stabilizer needed in its insulated (INS) counterpart makes the NI magnet highly compact and enhances its overall current density. 9-18Commercial 2G conductor comes as tape with width/ thickness ratio in a range of 5-40. 19,20 Unlike a conventional assembly of DP coils, in which the DP coils are wound with the same-width 2G tape, [21][22][23] in our MW technique, we place DP coils of the narrowest tape width in the magnet midplane section, placing DP coils of gradually wider tapes away from the midplane, with the widest-tape DP coils at the top and bottom sections, where the normal field that limits 2G tape performance is at its peak. This MW technique significantly enhances the overall current density of such a DP coil assembly at a given operating current.On one hand, the NI technique enables an HTS magnet to be self-protecting and thus to operate at a high current density (>150 kA/cm 2 ), 2 both features are not possible with the conventional HTS magnet. On the other hand, the MW technique is the most suitable and effective approach to "conductor-grade" 24 DP coils wound with highly anisotropic 2G HTS tape. A combination of NI and MW techniques (NI-MW) thus not only satisfies key operation requirements in protection and stability but also enables HTS magnets to be highly compact, which will lead to significant reduction in magnet price, one of the decisive factors in the marketplace.To demonstrate the NI-MW concept, we have designed and constructed a test NI-MW magnet as seen in Fig. 1(a). It consists of a stack of seven DP coils wound with "bare (no copper stabilizer)" 2G conductor, manufactured by AMSC without any turn-to-turn insulation (NI technique). The original conductor was 46-mm wide and then was mechanically slit to have a target width. The conductor width is 2.5 mm for the center DP coil (DP4 in Fig. 1) and increases up to 4.0 mm for the top and bottom DP coils (DP1 and DP7). As a result, this MW magnet generates 22% greater field than its single-width (SW) counterpart with the same overall dimensions (inner diameter, outer diameter, and height) as those of the SW mag...

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“…This magnet features the multi-width (MW) design consisting of 26 doublepancake coils with 5 various widths wound up with second generation (2G) GdBCO HTS tapes, which allows for a higher overall current density for the same operating current [13]. It is also incorporated with no-insulation (NI) winding technique enabling HTS magnets to be self-protecting against potentially destructive quenches [14].…”

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“…Alternatively, the conductors could be encased or wrapped in a ceramic insulation material. In addition recent work [30] has started to explore high-field magnets that do not use ceramic insulators, but rather rely on the steel structure of the tape to act as a form of insulator at cryogenic temperatures.…”

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“…One of these, the multi-width (MW) winding technique [43], is applicable to a magnet comprising an assembly of double-pancake coils. It consists in grading the conductor width of the double pancake coils, assigning the narrowest to magnet regions near the magnet midplane and the widest to those at and near the magnet ends where, as stated above, the tape performance degradation by perpendicular field components is greatest.…”