A new concept for fast pulse transformers is described. The transformers have been used in two applications with excimer lasers: a) To produce a short 100 MW/90 kV/100 ns pulse for driving a preionization X-ray source. b) To step up the voltage in a command charging circuit for charging a water pulse forming network within a few microseconds to an energy level of 100 Joule electrical energy per pulse. A report is given on the performance characteristics at repetition rates up to 100 Hz. A coupling factor close to unity and nearly ideal transformer efficiencies have been achieved.