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 is under development at Forschungszentrum Karlsruhe in cooperation with the office of engineering IbK at Karlsruhe. The requirements were the supply of two klystrons of 5 MW output and, alternatively, one multibeam klystron of 10 MW output. Including a pulse transformer (1:13/11.5), the system will deliver pulses of 130 kV or 115 kV, 1.7 ms pulse length with a flat top of ± 0.5 %, at a repetition rate up to 10 Hz. This new system's main features are the SMES, a capacitive storage, a low voltage/ high current power supply (rated 27 V/2.6 kA) for the magnet, a high voltage/low current switched-mode power supply (rated 14 kV/45 A) for the capacitor, a fast thyristor power switch for 2.6 kA approx. continuous current and 2 ms break, a new IGCT power switch made of ten 4.5 kV IGCT's with rated 2.6 kA/14 kV, and a safety switch unit. The goal of the development of the prototype is the demonstration of this alternate design. A significant part of the components of the system has been built. A first set of the components had been arranged to form a model of the modulator and 1 MW pulses were generated.Presenting Author : Juengst K.