Design of a 30kA pulsed current generator for a new eddy current septum in the CERN PS extraction.

The presently used direct drive septum magnet (PE.SMH16) deflects protons and ions into the extraction transfer line TT2 from PS ring SS16 towards the experimental areas of the PS complex and the SPS. It has been in operation since 1994 and as a result of an ever-increasing extraction frequency the original average lifetime of 5 years has been reduced to about 2 years. Subsequently it will be replaced with a new and more robust eddy current septum that will require far less maintenance. Together with the new septum a pulsed current generator with 30kA peak current must be designed and built. The pulsed current has a flat top that is generated by adding a 2. 5kHz fundamental sinewave and a third harmonic with 11% of the amplitude of the fundamental. This paper presents two generator concepts. In the first one the energy of the capacitor bank is dissipated in a dump resistor and capacitor banks must be recharged from zero for every pulse. This is a common approach for pulsed power generators. In the second concept most of the energy is recovered by letting the current flow in reverse direction through the septum magnet. We estimate to recover >70% of the energy and thus reducing the energy cost substantially. The design of energy recovery presents a number of design challenges resulting from non-ideal behavior of GTO thyristors and power diodes which are outlined in this publication. At the end of the energy recovery the current flow must be stopped safely. Diodes with fast reverse recovery must be used and GTO $dv/dt$ limitations must be respected. Snubber circuits are placed in the circuit to absorb energy transients. Gate drive requirements for safe operation of the GTOs are described.