Inductive Adder for Driving Kicker Magnets Terminated in a Short-Circuit

In the CERN particle accelerator complex, fast pulsed magnet systems are used to transfer the particle beam between accelerator stages. Transmission line type kicker magnets are used in many of these systems to deflect the charged particles using a magnetic field pulse with a well-defined flat top and a fast rise and fall time. Some kicker magnets are terminated with a short-circuit for the advantage of twice the kick-strength for a given system impedance, aperture size, and magnet length, due to the doubling of the current in the kicker magnet. The pulse generators based on pulse forming lines or pulse forming networks and thyratron switches, currently used to drive these kicker magnets, should be replaced in the near future, due to the obsolescence of the thyratron switches employed and because of the environmental impact of $\mathrm{SF_6}$ gas which is used in the pulse forming lines. As a promising alternative, an inductive adder for driving these kicker magnets terminated in a short-circuit is investigated in this work. In order to deal with the wave reflected from the short-circuit back into the inductive adder, as part of a new design approach, a specifically tailored branch module has been designed and built. To account for the reflection at the short- circuit, the branch module features a novel topology with two independently controlled semiconductor switches. This allows to first inject energy into the connecting cable and the kicker magnet, then to circulate the resulting current in a free-wheeling interval, and, finally, to absorb the energy at the end of the pulse. This new mode of operation makes it possible, to design the cross-sectional area of the magnetic cores of the inductive adder according to only twice the signal propagation time along the connecting cable and the kicker magnet, rather than according to the whole pulse length as in a conventional design. This allows for a significant reduction in the cross-sectional area of the magnetic cores for the usual operating cases. In addition, in contrast to a conventional design, an operation at a higher pulse repetition rate is possible, as a reset of the core is not required with the new mode of operation. It is essential for the application that the operation of the switches for the pulsed energy injection and extraction by the inductive adder results in steep edges at exactly defined times, which are determined by the signal propagation times in the connecting cable and the kicker magnet. These requirements have been met by implementing the gate boosting operation of SiC MOSFETs, resulting in a rise time of both voltage and current of 5 ns for a voltage of 1.2 kV and a current of 120 A per branch module. The branch module and the new mode of operation have been tested successfully with a short-circuit terminated replacement load. The experimental results confirm the advantages of the novel design, the circuit concept of the branch module and the new mode of operation.