Based on a superconducting magnetic energy storage (SMES) a long-pulse klystron modulator has been designed for use in the TESLA Test facility (TTF) at DESY, Hamburg. A prototype with an output power of 25 MW is under development at Forschungszentrum Karlsruhe in cooperation with the office of engineering IbK at Karlsruhe. The system will deliver pulses of up to 130 kV and 1.7 ms pulse length with a flat top of ±0.5% at a repetition rate up to 10 Hz. Some of this new system's main features are the highly dynamic SMES (100 T/s), a switched-mode high voltage power supply (rated 14 kV/45 A), a fast thyristor power switch and an IGCT power switch rated 2.6 kA/14 kV. This demonstration system is to serve alternatively two klystrons of 5 MW RF output or one multibeam klystron of 10 MW RF output. A first set of the system components had been arranged to form a model of the modulator and 1 MW pulses were generated. The SMES and its cryostat have undergone initial testing at DC operation. The data acquisition and control system DOOCS under development at DESY has been adapted to the authors' computer and experimental environment.