Fast Totem-Pole Grid-Catch Mod-Anode Modulator for the Indiana University "LENS" Klystron RF Amplifier System

This paper provides the electrical design and test results of the newly designed mod-anode modulators for the Indiana University Low Energy Neutron Source (LENS) klystron RF amplifier system. The design is based on totem-pole connected floating-deck vacuum tube amplifier systems, the "on-deck" turns on the klystron (via the mod-anode), the "off-deck" turns off the klystron. Via feedback techniques, the klystron mod-anode voltage tracks the klystron cathode voltage at a predetermined set-point. The compensated mod-anode to cathode tracking provides for a stable klystron RF amplitude characteristic as it provides a constant klystron beam current. This constant klystron beam current is provided even as the klystron cathode voltage slumps from the (-100 kV) capacitive energy store. All klystrons operate from a common capacitor bank and power supply system. The on-deck uses a magnetron injection gun switch tube (an Injectron) as a switching and regulating element that has a pentode-like characteristic curve. The klystron mod-anode set point is determined by a voltage divider that provides a voltage reference to the on-deck modulator. The output of the on-deck connects to the klystron mod-anode. Also connected to the klystron mod-anode and on-deck is the off-deck. The off-deck uses a H.V. planar triode as a simple switch in a cascode circuit configuration, a MOSFET is in series with the tube cathode as the on-off control element. Timing signals to the on and off decks is via fiber optics.