Energy dump of the ATLAS superconducting system: simulations of electrical and thermal behaviour of magnet system at slow- and fast dump

During the slow dump (discharge) of the Barrel Toroidal (superconducting) magnet of the ATLAS detector, the control system gave an alarm that the differences between the voltages over the conductors were too high. The alarm was not due to any danger, because of some sort of phenomenon observed in the first few seconds after start of the discharge. A possible explanation of the differences of the coil voltages is that the changing current through the conductors may cause induced currents in the coil casing around. The goal was to make a simulation of the electrical behaviour of the magnet system during a slow dump. In this way, an explanation can be found for the start phenomenon of the slow dump of the Barrel Toroid. Some extra analyses on the measurements were performed to describe the energy dissipation during a fast dump. This is done by calculating the resistance of the coils during the dump. With the maximum resistance, the maximum temperature can be estimated, which says something about the enthalpy of the material. There is also found something about the RRR of the conductor and its relation to the properties of the quench. The results of the simulation of the Barrel Toroid as one coil were as expected. The simulation is correct and precise enough to reproduce the measurements. This simulation was extended to a simulation of the eight coils separately. The result was that a change up to 4% in the resistance of the coil c asing can be the cause of the difference in the coil voltages in the beginning of the slow dump. The calculations of the energy dissipation during a fast dump of the BT showed that 73% of the energy was stored in the conductor. The aluminium in the conductor stored 64% of this energy and 17% was stored in the isolation, the casing and the dump unit. Not for all the parts of the magnet the change in enthalpy is calculated. An undefined 10% of the energy is also due to the inaccuracy of the calculations. By analysing a fast dump measurement of the Barrel Toroid, it appeared that a double pancake with a relative low RRR has a relatively higher maximum temperature after the dump, as expected. For both ECTs, the RRR is measured and calculated for all the double pancakes. The ECTs showed that a double pancake quenched later, when it has a higher value for the RRR with respect to other double pancakes.