The Depairing-Current Ceiling: How Much of the Fundamental Current Limit Pinning and Microstructure Have Captured in NbTi, Nb3Sn, MgB2 and REBCO Conductors

The depairing current density, at which the kinetic energy of the flowing condensate itself destroys superconductivity, is derived from Ginzburg-Landau theory as J_d = Phi0/(3 sqrt(3) pi mu0 lambda^2 xi) and evaluated at zero temperature from published penetration depths and coherence lengths: NbTi 40.54 MA/cm2, Nb3Sn 182.11 MA/cm2, MgB2 135.45 MA/cm2, and REBCO 224.01 MA/cm2. Compared with commercial critical current densities against a temperature-matched ceiling J_d(T) = J_d(0)(1-T/Tc)^(3/2), the captured fractions are 1.743% for NbTi (3000 A/mm2 at 4.2 K, 5 T), 0.235% for Nb3Sn (2900 A/mm2 at 4.2 K, 12 T), 0.088% for MgB2 wire (1.0e5 A/cm2 at 4.2 K, self-field), and 20.343% for the REBCO layer (3.0 MA/cm2 at 77 K, self-field). The REBCO layer is therefore closest to its fundamental limit, and MgB2 wire has the most headroom, with the qualification that its gap is dominated by porosity and intergrain connectivity rather than pinning alone. All numbers are produced by the deposited compute.py; the calculations are simulated, not measured. AI use is disclosed: this paper was produced by Dave, an AI system, at the direction of the author. The author, William Alexander Aldred, directed the work and is sole author of record.