Description
IOTs have been described as a cross between a klystron and a triode, hence Eimac's trade name for them, Klystrode. They have a cathode with a control grid 0.1 mm in front of it like a triode. They then use high voltage DC and a magnetic lens to focus a modulated high energy electron beam through a small drift tube like a klystron. This drift tube prevents backflow of electromagnetic radiation. The bunched electron beam passes through a resonant cavity, equivalent to the output cavity of a klystron. The electron bunches excite the cavity, and the electromagnetic energy of the beam is extracted by a coaxial transmission line.
The highest frequency achievable in an IOT is limited by the grid-to-cathode spacing. The electrons must be accelerated off the cathode and pass the grid before the RF electric field reverses direction. The upper limit on frequency is approximately 1300 MHz.
Thermal radiation from the cathode also heats the grid. As a result, low-work-function cathode material evaporates and condenses on the grid. This eventually leads to a short between cathode and grid, as the material accreting on the grid narrows the gap between it and the cathode. In addition, the emissive cathode material on the grid causes a negative grid current (reverse electron flow from the grid to the cathode). This can swamp the grid power supply if this reverse current gets too high, changing the grid (bias) voltage and, consequently, the operating point of the tube. Today's IOTs are equipped with coated cathodes that work at relatively low operating temperatures, and hence have slower evaporation rates, minimizing this effect. IOTs, like most linear beam tubes having external tuning cavities to achieve bandwidth, are equipped with arc detectors located in the output cavities that trigger a crowbar circuit based on a hydrogen thyratron or a triggered spark gap in the high-voltage supply. The purpose of the crowbar circuit is to instantly dump the massive electrical charge stored in the high voltage beam supply before this energy can damage the tube assembly during an uncontrolled cavity, collector or cathode arc.
The latest versions of IOTs achieve even higher efficiencies (60%-70%) through the use of a Multistage Depressed Collector (MSDC). One manufacturer's version is called the Constant Efficiency Amplifier (CEA), while another manufacturer markets their version as the ESCIOT (Energy Saving Collector IOT). The initial design difficulties of MSDCIOTs were overcome through the use of recirculating high dielectric transformer oil as a combined coolant and insulation medium to prevent arcing and erosion between the closely spaced collector stages and to provide reliable low-maintenance collector cooling for the life of the tube. Earlier MSDC versions had to be air cooled (limited power) or used de-inonized water that had to be filtered, regularly exchanged and provided no freezing or corrosion protection.
Read more about this topic: Inductive Output Tube
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