Speaker
Description
Our experience with REBCO coils comes primarily from the development of high-field double-pancake/module wound coils for NMR magnet systems. However, many of the conclusions we have reached are quite general. We focus on insulated/insulation coils with a distributed stainless-steel co-wind as the least problematic and thus more promising for our projects, although we also have some experience with no-insulation coils, and we will also touch on that, albeit a little, too. Quench protecting large-stored-energy high-field multi-module REBCO coil magnets is incomparably more complex and difficult than quench protecting any large LTS magnets. In particular, for a number of well-known reasons, it is absolutely necessary to use numerous powerful quench-protection heaters distributed throughout the REBCO windings to quench a significant volume of the coil(s) in a very short time. The difficulty lies in the fact that the magnet design is primarily determined by the requirement not to exceed the maximum permissible value of the strain of the REBCO tapes (including that due to the screening currents), and so the magnet design is not so easy, and sometimes impossible, to change in order to increase the effectiveness of quench protection. As a result, zones appear in the winding that are difficult to quench by the heaters due to too small field angles and thus very high critical currents. This leads to the problem of too high module voltages and thus the magnet voltage-to-ground, which requires additional measures to solve. Also, predictions of the magnet quench behavior and recommendations for the quench protection system design, obtained through comprehensive quench analyses, turned out to be extremely sensitive to the accuracy of information about the parameters of the REBCO tapes used, and first of all - to the one about the value of the critical current (strongly depending on the magnetic field vector and temperature), which varies greatly along the length of the tapes: by up to 10% in a field perpendicular to the tape surface and up to 50% in the parallel field. Properly taking this circumstance into account and achieving good agreement between measurements of module voltages and current in our model coils in the event of a quench and the results of calculations/predictions is a very serious challenge that makes it very difficult, albeit possible, to benchmark the quench code used and its input. The need to take into account significant rotation of REBCO turns due to the screening currents will also be viewed. On the top of it, the magnet systems we design typically include a REBCO insert and an LTS outsert, which requires their protection systems to cooperate.
