ANALYSIS OF INVERTER CHARGING WAVEFORM FOR ULTRA STABLE SCSS MODULATOR

The SCSS (SPring-8 Compact SASE Source) XFEL [1] requires ultra stable RF sources. The SCSS smart modulator driving a klystron RF source uses an inverter charging system. Therefore, the stability of RF sources is directly determined by the one of inverter power supplies. The regulation and the stability of an inverter depend on not only the structure of inverter topology but also the fidelity of a signal monitoring. For better stability, we need stable and adequate monitoring of a charging voltage. The charging waveform is composed of a net PFN (pulse forming network) voltage and ripple components that is related to the system-dependent circuit parameters. This ripple is proportional to the ratio of the PFN capacitance to the stray capacitance of a pulse transformer and a klystron load. We can manipulate the feedback signal with suitable filtering but it is shown that the charging stability depends on the way of signal conditioning also. The longterm drift has to be minimized by the temperature stabilization of the probe and feedback circuits.