Many models of heat engines have been studied with the tools of finite-time thermodynamics and an ensemble of independent quantum systems as the working fluid. Because of their convenient analytical properties, harmonic oscillators are the most frequently used example of a quantum system. We analyze different thermodynamical aspects with the final aim of the optimization of the performance of the engine in terms of the mechanical power provided during a finite-time Otto cycle. The heat exchange mechanism between the working fluid and the thermal reservoirs is provided by the Lindblad formalism. We describe an analytical method to find the limit cycle and give conditions for a stable limit cycle to exist. We explore the power production landscape as the duration of the four branches of the cycle are varied for short times, intermediate times, and special frictionless times. For short times we find a periodic structure with atolls of purely dissipative operation surrounding islands of divergent behavior where, rather than tending to a limit cycle, the working fluid accumulates more and more energy. For frictionless times the periodic structure is gone and we come very close to the global optimal operation. The global optimum is found and interestingly comes with a particular value of the cycle time.

High-temperature superconducting coated conductors (CCs) are considered an enabling ultra-high field (UHF) tape-conductor technology due to their extremely high engineering current densities at very high magnetic fields and low temperatures, and high mechanical strength. A major challenge is however related to induction of problematically large superconducting screening currents (as an effect of the large width-to-thickness ratio) when the wide tape conductors are exposed to strong transverse magnetic fields above 20 T, which is the case in many UHF magnet systems. Subdividing the superconducting layer into narrow parallel decoupled filaments has been shown to effectively reduce superconducting screening currents by a factor comparable to the number of filaments. The filamentization is however not effective until the induced coupling currents flowing across the filaments have decayed. The effectiveness of the multifilamentary structure in suppressing coupling currents and reducing the decay time constants is directly linked to potential current paths between filaments. Very recent experimental and numerical studies have examined both the challenge of magnet precision, caused by screening-current-induced fields, and the fatal consequences of local uneven tape stresses exceeding the irreversible limits for commercial CCs. These studies have conclusively revealed that screening currents must not be ignored in the mechanical design and other studies have introduced multifilamentary CCs as a viable solution. This paper aims to review the efforts made in developing and investigating multifilamentary CCs for ultra-high field applications focusing on the screening-current-related mechanisms, critical system-level effects, effectiveness of filamentization in UHFs, fabrication and large-scale analysis of multifilamentary CCs, in addition to providing cost estimates of previously studied filamentized CC fabrication techniques.

We show that the inability of a quantum Otto cycle to reach a limit cycle is connected with the propagator of the cycle being noncompact. For a working fluid consisting of quantum harmonic oscillators, the transition point in parameter space where this instability occurs is associated with a non-Hermitian degeneracy (exceptional point) of the eigenvalues of the propagator. In particular, a third-order exceptional point is observed at the transition from the region where the eigenvalues are complex numbers to the region where all the eigenvalues are real. Within this region we find another exceptional point, this time of second order, at which the trajectory becomes divergent. The onset of the divergent behavior corresponds to the modulus of one of the eigenvalues becoming larger than one. The physical origin of this phenomenon is that the hot and cold heat baths are unable to dissipate the frictional internal heat generated in the adiabatic strokes of the cycle. This behavior is contrasted with that of quantum spins as working fluid which have a compact Hamiltonian and thus no exceptional points. All arguments are rigorously proved in terms of the systems' associated Lie algebras.