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Oil Cooling of MSDC IOTs

Introduction
The Constant Efficiency Amplifier (CEA) is the most efficient high-power UHF amplifier available for use in terrestrial broadcasting. In 1997, when L-3 Communications Electron Devices first proposed reducing the CEA to practice, it was assumed that cooling the device would present a formidable challenge. The CEA is a Multiple Stage Depressed Collector Inductive Output Tube (MSDC IOT). Incorporating multiple stages of collector depression can increase the efficiency of the device by more than 50%.

The IOT consists of three major elements: the electron gun, the RF circuit, and the collector. Electron energy from the gun that is not converted to RF power is dissipated in the collector as heat. Since the collector of the IOT resides at ground potential, water can be used for cooling. The CEA’s increased efficiency is derived from having multiple collector stages, which operate at voltages that incrementally rise towards cathode potential. With reduced potentials on collector stages, electrons emerging from the output cavity gap are sorted and collected by their respective energy levels. Although this makes the device more efficient, it presents a cooling challenge, since one has to choose a cooling medium that has a dielectric strength high enough to prevent high-voltage breakdown between the closely spaced collector stages.

The Challenge of Air Cooling
In 1997, the expectation for average RF output power, in digital operation, from a single tube was approximately 15 kW. At that power level, air cooling was determined to be suitable for cooling the CEA. However, air cooling presents unique challenges:

• Air handling systems are noisy.
• The air has to be filtered to avoid contamination of the collector’s high-voltage standoff ceramics.
• High-voltage breakdown in air between the collector stages becomes a problem and is even more pronounced at high altitude.
• Air cooling has practical limitations related to the principle that as air is compressed (in developing backpressure for passage through cooling fins) its temperature rises making it less useful as a cooling medium.

The Move to Liquid Cooling
In 1999, L-3 engineers became aware that the average power level expectations derived from a single tube had risen to 30 kW. Liquid cooling became necessary due to higher thermal dissipation levels in the collectors. The chosen cooling medium had to have excellent dielectric properties. The first choice that came to mind was pure or deionized (DI) water. However, using DI water creates some challenges:

• The metal surfaces of the cooling loop must be meticulously scrubbed to keep them free of ions. Otherwise, corrosion (electrolysis) will occur.
• Long insulated hoses are needed between collector stages to prevent high-voltage breakdown.
• It creates a potential freezing and boiling problem.

The Need for Oil Cooling
To solve the problems associated with air- and pure water-cooling systems, L-3 engineers chose to use commercially available insulating oil made from 100% synthetic hydrocarbon oils. The chemical family designation for this type of oil is Polyalpha Olefin (PAO) Hydrocarbon. This type of oil is biodegradable and has excellent dielectric properties (23kV/mm) and heat transfer characteristics. Although the oil’s ability to transport heat is diminished in comparison to water, it is not a problem with the CEA thanks to the amplifier’s decreased collector dissipation and subsequent increase in efficiency. Alpha-1® oil, manufactured by Dielectric Systems Inc., is currently used for cooling the CEA. This oil is used extensively by the transformer industry for retrofitting transformers filled with mineral-based dielectric oils. Using a dielectric fluid as the CEA’s cooling agent presents several advantages:

• The entire collector assembly can be immersed in the same oil bath, eliminating the need for insulating cooling hoses between collector stages and thereby simplifying the design of the collector.
• The oil provides long-term corrosion protection to the collector surfaces.
• The cooling loop can utilize a standard pump, standard plumbing components, and standard hydraulic oil filters.
• The oil is biodegradable and does not need to be treated as hazardous waste.
• Dielectric oil is expected to last for the life of the tube, resulting in dramatically reduced cost of ownership.

The Truth About Dielectric Oil
Recently, there has been some negative press regarding the use of dielectric oil to cool MSDC IOTs. Statements were made containing the words unproven and hazardous. The truth is that dielectric oils are and have for decades been used in many applications to cool and insulate high-power equipment. Listed below are a few of the known applications:

• Shell’s Diala® and Exxon’s Univolt are electrical insulating oils that are commonly used in most of the linear high-voltage power supplies used to power IOTs and Klystrons used in broadcast service.
• Electrical insulating oils are used to insulate and cool high-power microwave devices. Some examples are products L-3 Communications currently manufactures, including the Cross Field Amplifier (CFA) used in many airport surveillance radars, and a megawatt S-band Klystron used in ground-based radars.
• In military airborne applications where space is limited and temperatures can go down as low as -54°C, insulating oils like Exxon’s Coolanol function as a coolant, hydraulic fluid, and dielectric fluid. Coolanol is used on the ALQ-99 radar aboard the EA-6B Prowler and the APG-65, on the AV-8 radar aboard the F-18 and the AWG-9, and on the AIM-54 radar aboard the F-14. The transmitters and radars used on the B-2 bomber also depend on Coolanol because they require a coolant that has high dielectric properties and thermal transport characteristics.

What is also interesting is that due to high cost, maintenance, and performance issues, Polyalpha Olefin is replacing Coolanol. Section 5.1.d of the Joint Oil Analysis Program Manual, Version II, Published in March of 1999 by direction of Commander, Naval Air Systems Command, states: “Coolant Fluid Contamination. Silicate ester fluids (i.e., Coolanol 25R) are very effective in transferring heat and providing high-voltage insulation. However, they have two undesirable properties: a hygroscopic nature and poor hydrolytic stability. Coolanol fluid is being replaced by Polyalpha Olefin (PAO) dielectric coolant fluid, which does not exhibit these properties.”

The PAO Advantage
L-3 has been processing CEAs with no failures induced by the PAO coolant (Alpha-1®) since 1999. And Thales Broadcast and Multimedia has been using Alpha-1® in its Paragon MSDC IOT transmitter without a problem since February of 2001.