Lithographically Patterned Cobalt Nanoribbons as Magnetic Artificial Pinning Centers in REBCO-Coated Conductors for Continuous-Wave Accelerator Magnets

Abstract

Abstract

We investigate a lithographically patterned cobalt-nanoribbon array as a magnetic artificial pinning center (APC) architecture for REBCO-coated conductors intended for continuous-wave (CW) superconducting accelerator magnets, using accelerator-driven subcritical (ADS) fission-waste-transmutation injectors as the motivating application. A long-wavelength magnetostatic model (the Wallace/Karlqvist spacing-loss approximation used in magnetic-recording physics) is used to estimate the periodic stray-field harmonic produced above the array as a function of the ribbon fill fraction and cobalt-to-REBCO separation and is combined with the standard flux-lattice matching-field relation Bphi = 2*Phi0/(root3 * p^2) to fix the lithographic pitch required for a given target field. This calculation shows that the 13 nm feature size inherited from a prior magic-angle-graphene proposal corresponds to a matching field of approximately 14 T, which is well outside the 0.4–2 T range of published ADS injector solenoid and corrector designs; a pitch of 45–80 nm is required to place the matching field in the ADS-relevant range, and we adopt that corrected target here. This revision replaces the previously illustrated Figure 3 pinning-force bump with a closed-form calculation: the same periodic stray field is combined with the local (Pearl-length) leading-order approximation to the London-theory vortex-ferromagnet interaction energy U_v-Co(x) and force F_v-Co(x) = -dU_v-Co/dx, yielding a nonfitted estimate of the magnetic pinning-force contribution across the 0.4-2 T design range (Section 5.1). That estimate exceeds typical core-pinning energy and cuprate depairing-current scales, which we read as evidence that the local approximation is an upper bound in this regime — expected, since the calculated Pearl length (~45 nm) is comparable to the design pitch — rather than a validated quantitative prediction; the full nonlocal London-theory treatment of the vortex–magnetic-dipole interaction developed by Erdin, Lyuksyutov, Pokrovsky and Vinokur remains the necessary next calculation to convert this upper bound into a quantitative prediction (Section 9). A Rayleigh-law model of minor-loop hysteresis dissipation under near-DC ripple and a toy superposition model of the ribbon orientation-tunable Jc(theta) are presented explicitly as first-order, nonfitted illustrations of the expected functional behavior, not as predictions of specific measured values. The Rayleigh analysis indicates that hysteresis dissipation should be small under strict DC operation but increases as the cube of any residual field ripple, making the power-supply ripple specification a first-order design constraint rather than a negligible detail. No device has been fabricated, and no finite-element or micromagnetic simulation has been performed; together with the full nonlocal London calculation, these remain the specific open questions identified explicitly in Section 9.