No-insulation REBCO Magnets Research Articles

Quench analysis of a no-insulation REBCO magnet based on the ADI method considering the coupling effect of the cryostat

A no-insulation (NI) REBCO superconducting magnet is under development at Wuhan National High Magnetic Field Center, China. The magnet with the liquid helium cryostat system has a compact structure to reduce the space required for operation. During a quench, the fast-changing spatial magnetic field around the NI magnet may induce a strong eddy current in the conductive parts of the cryostat. The eddy current and its associated Lorentz force will generate mechanical stress on the cryostat, especially on the thermal shield (TS). The mechanical strength of the cryostat needs verification in the preliminary design. Furthermore, the degree to which the electromagnetic coupling between the cryostat and NI magnet might impact the quench behaviors of the NI magnet remains uncertain. In this paper, a multi-physics quench model is newly developed for the NI REBCO magnet, and the alternating direction implicit method is employed for the solver of the thermal model to improve computational efficiency. This simulation model can consider the electromagnetic coupling effect between the NI magnet and cryostat by constructing a partial element equivalent circuit. A quench analysis has been performed and we found that: (1) The cryostat can function as a secondary shorted circuit to the NI magnet and slow down the quench speed to a certain extent. (2) During the rather fast inductive quench phase, the cryostat will experience an attraction force towards the quench propagation frontier. (3) A quench propagation from one end of the magnet can cause a significant z-axis unbalanced force on the TS. (4) Cryostat materials with drastically changed electrical conductivity can significantly affect their mechanical responses during a quench. However, the eddy current density and maximum Von Mises stress on the TS are barely affected by the thickness of the TS and the contact resistance of the NI REBCO magnet.

Unbalanced radial current flow simulation of no-insulation REBCO pancake coils during normal state transition

The no-insulation (NI) winding technique greatly enhances the thermal stability of REBCO (REBa2Cu3O x , RE = Rare Earth) pancake coils by avoiding burn-out and thermal runaway. The complicated electrical behaviors in NI REBCO pancake coils produce convoluted mechanical behaviors, so that some journal papers reported that high-field NI REBCO pancake coils were mechanically damaged during quench. To apply NI REBCO magnets for practical commercial use, it is important to understand the electromagnetic and mechanical behaviors of NI REBCO pancake coils in detail with both experiments and simulations. To clarify the electrical behaviors, a few simulation methods for NI REBCO pancake coils have been proposed; such as a simple RL parallel equivalent circuit. In these previous models, the radial current paths along the top and bottom of the pancake coils are represented as one current circuit path. However, since the radial current path of the bottom of one pancake coil is actually very close to that of the top of the next lowest coil, inductive behavior between these two paths appears. The simulation results show a probability that different amounts of radial current on the top and bottom of one pancake coil are carried during quench.

Quench Analyses of the MIT 1.3-GHz LTS/HTS NMR Magnet.

The MIT 1.3-GHz LTS/HTS NMR magnet is currently under development. The unique features of this magnet include a 3-nested formation for an 800-MHz REBCO insert (H800) and the no-insulation (NI) winding technique for H800 coils. Because when it is driven to the normal state, an NI REBCO magnet will respond electromagnetically, thermally, and mechanically that may result in permanent magnet damage, analysis of a quenching magnet is a key aspect of HTS magnet protection. We have developed a partial element equivalent circuit method coupled to a thermal and stress finite element method to analyze electromagnetic and mechanical responses of a nested-coil REBCO magnet each a stack of NI pancake coils. Using this method, quench simulations of the MIT 1.3-GHz LTS (L500)/HTS (H800) NMR magnet (1.3G), we have evaluated currents, strains, and torques of H800 Coil 1 to Coil 3 and L500, and center fields of 1.3G, L500, and H800. Our analyses show H800 is vulnerable to mechanical damage.

Contact resistance between two REBCO tapes: the effects of cyclic loading and surface coating

No-insulation (NI) superconducting REBCO magnets have advantages of self-quench-protection, a very high engineering current density and high mechanical strength, and the potential to reach very high magnetic fields. However, NI REBCO magnets have drawbacks of a long magnet charging time and high field ramp losses. These can be mitigated by controlling the turn-to-turn contact resistivity (Rc). In an effort to control Rc, we consider two approaches. One is coating a REBCO conductor with various resistive thin films, and the other is to use a stainless steel (SS) tape as an interlayer which is also coated with different metallic films. We present experimental results of Rc of an as-received sample under cyclic contact pressure of 2.5–25 MPa up to 30 000 cycles. After an initial increase in Rc for the first 10–20 cycles, Rc decreases to about one tenth of its initial value after a few hundred cycles. A warm-up and cool-down thermal cycle does not significantly change the low Rc resulting from a previously high number of load cycles. We also studied Rc of REBCO tapes that are coated with different resistive layers and interlayers. In order to increase Rc, we experimented with electro- and electroless plating of Ni, Cr, and Ni–P. We also measured Rc with a thin metallic interlayer as a coil co-winding material which included Cu, SS, and SS plated with Ni and Cu. A SS interlayer increases Rc by about three orders of magnitude; while the Cu plated SS interlayer only increases Rc by one order of magnitude. Finally, we treated the as-received REBCO surface by oxidation using an Ebonol® C solution. This controlled oxidation allowed the Rc to be controlled over a wide range.

Simulation on Electrical Field Generation by Hall Effect in No-Insulation REBCO Pancake Coils

The no-insulation (NI) winding technique is one of the most promising high-temperature superconducting (HTS) coil protection methods to realize the practical use of HTS magnets. Many NI REBa $_{2}$ Cu $_{3}$ O $_{7-x}$ (REBCO, RE = Rare Earth) magnets have been developed to generate ultrahigh magnetic fields for NMR or MRI magnets. When NI REBCO magnets are operated in a high magnetic field, uncommon phenomena can be observed, because the NI REBCO magnets have multicomplicated current paths. An unexpected voltage rise was experimentally observed during charging NI coils or after quench. The cause of such a voltage rise cannot be explained by means of a partial element equivalent circuit method, which was proposed to clarify such complicated current behavior. We think one of causes is an electric field generation by Hall effect. In this paper, therefore, we will investigate the electrical field behavior of NI REBCO coils by Hall effect. An unforeseen voltage would appear between terminals.

Thermal and Electromagnetic Simulation of Multistacked No-Insulation REBCO Pancake Coils on Normal-State Transition by PEEC Method

This paper presents the thermal and electromagnetic behaviors of multistacked no-insulation (NI) REBa 2Cu3O7− x (REBCO, RE = rare earth) pancake coils. The NI winding technique gives a high enough thermal stability not to damage REBCO coils even though a normal-state transition occurs. The high thermal stability has been verified through overcurrent tests. Moreover, the numerical simulation of multistacked NI REBCO pancake coils has been performed to investigate the electromagnetic behaviors in detail. The simulation results also confirmed the high thermal stability of NI REBCO pancake coils. However, it does not mean that an NI REBCO magnet never quenches. In experiments, it was observed that a quench in one of the pancake coils propagated to the other pancake coils sequentially. The above sequential quench from pancake to pancake has not been confirmed in the numerical simulation since the thermal behavior is not considered. For the more reliable verification, we have developed a numerical simulation to investigate the thermal and electromagnetic behaviors by the partial element equivalent circuit method and the two-dimensional thermal finite-element method. In this paper, the six-stacked NI REBCO pancake coil is simulated with several operating temperatures, and the sequential quench is reproduced. The velocity of the sequential quench is also shown.

Overall Thermal Conductance and Thermal Contact Resistance in No-Insulation REBCO Magnet

In no-insulation (NI) REBCO coils, thermal contact resistance (TCR) between winding turns has important influence on thermal stability of REBCO magnets under varying magnetic field or even in case of quench. Reduction of TCR between winding turns is one of the crucial factors to develop thermally stable magnets cooled by conduction. To avoid a delamination problem, epoxy is not usually used between winding turns and the significant TCR exists in NI magnets. Then, thin epoxy layers between coil surfaces and metallic conduction plates are used to enhance thermal conduction in conduction cooled magnets. To design thermally stable NI REBCO magnets cooled by conduction, TCR, induced by dry contacts of winding turns, should be properly evaluated. This paper describes measurements of overall thermal conductance using a model coil without conduction plates and epoxy layers from 10 to 40 K in a conduction test apparatus. TCR is evaluated using the measured overall thermal conductance and previously reported transverse thermal conductivity of a REBCO wire. In another model coil with conduction plates and epoxy layers, enhanced overall thermal conductance is measured and compared with finite element analysis results that include the measured TCR.

Quench behavior of a no-insulation coil wound with stainless steel cladding REBCO tape at 4.2 K

A single pancake no-insulation (NI) coil was wound in a hermetic way with REBCO tape having a thin (1–2 μm) cladding layer of stainless steel. With an electric heater embedded at the innermost turn of the coil, heater-induced quench tests were performed under a background field of 15 T in a bath of liquid helium at 4.2 K. Despite the large average contact surface resistance of 1.63 cm2 times larger than those of pure NI coils, ‘thermal recovery’ was observed at a coil current density (Je) below 700 A mm−2 mainly due to the local current sharing. The coil experienced nine consecutive ‘thermal runaway’ quenches at Je of 700–820 A mm−2 (power supply limit), yet no discernible coil degradation or damage was observed. The results demonstrated that the stainless steel cladding REBCO tape may be effective at reducing the charging delay of high field NI REBCO magnets without sacrificing the self-protecting feature.

Contact resistance between two REBCO tapes under load and load cycles

No-insulation (NI) REBCO magnets have many advantages. They are self-protecting, and therefore do not need quench detection and protection, which can be very challenging to implement in a high Tc superconducting magnet. Moreover, by removing the insulation and allowing thinner copper stabilizers, NI REBCO magnets have significantly higher engineering current density and higher mechanical strength. On the other hand, NI REBCO magnets have the drawbacks of long magnet charging time and high field ramp loss. In principle, these drawbacks can be mitigated by managing the turn-to-turn contact resistivity (Rc). Evidently, the first step toward managing Rc is to establish a reliable method of accurate Rc measurement. In this paper, we present experimental Rc measurements for REBCO tapes as a function of mechanical load up to 144 MPa and load cycles up to 14. We find that Rc is in the range of 26–100 μΩ·cm2, and that it decreases with increasing pressure, and gradually increases with the number of load cycles. The results are discussed in the framework of Holm’s electric contact theory.

‘Defect-irrelevant’ behavior of a no-insulation pancake coil wound with REBCO tapes containing multiple defects

A no-insulation (NI) pancake coil was wound with a 23 m long REBCO tape containing multiple ‘defects’, at which local critical currents are substantially lower (<80%) than the tape’s lengthwise average. During a charging test up to 60 A in a bath of liquid nitrogen at 77 K, the test coil showed ‘defect-irrelevant’ behaviors, i.e. coil terminal voltages and center fields in steady-state operations were barely discernible from those of an ideal ‘defect-free’ coil. Also, the critical current (Ic) of the coil was measured to be 68 A, close to 72 A estimated from the measured Ic angular dependency data of a defect-free sample tape that was used for construction of the coil. The results demonstrate a potential to build a pancake coil with REBCO tapes regardless of their defects, which may lead to a substantial reduction in the construction cost of high field NI REBCO magnets.