Modeling of the Voltage Waves in the LHC Main Dipole Circuits

Abstract

When a fast power abort is triggered in the LHC main dipole chain, voltage transients are generated at the output of the power converter and across the energy-extraction switches. The voltage waves propagate through the chain of 154 superconducting dipoles and can have undesired effects leading to spurious triggering of the quench protection system and firing of the quench heaters. The phase velocity of the waves travelling along the chain changes due to the inhomogeneous AC behavior of the dipoles. Furthermore, complex phenomena of reflection and superposition are present in the circuit. For these reasons analytical calculations are not sufficient for properly analysing the circuit behavior after a fast power abort. The transients following the switch-off of the power converter and the opening of the switches are analysed by means of a complete electrical model, developed with the Cadence © suite (PSpice © based). The model comprises all the electrical components of the circuit, additional components simulating the dipole AC behavior, and the ground lines of the circuit including its parasitic capacitances. The simulation results are presented in order to illustrate the behavior of the circuit and to assess its performance under different operating conditions. The comparison between measurement data and simulations shows a very good agreement.