Cold Cathode Magnetrons: Yesterday, Today and Tomorrow
One of the most important achievements in the microwave vacuum devices field over the last 30 years is the design and mass production of magnetrons with non-incandescent (cold) cathodes. In the U.S., these magnetrons are known as ‘magnetrons with field cathode emission,’ although they do also have secondary-electron emission.
The usage of the cold cathode magnetrons has particular advantages with regards to designing and developing new equipment which has certain advantages, such as:
- Instant availability without using the attendant and forced incandescence modes, which reduces reliability and shortens the equipment’s lifetime.
- Possibility of short-range commutation and switching off equipment modes, including repeated changes of duty cycles as much as 100 times or more.
- Improved lifetime and dependability of the equipment, even under increased cathode loads because of the utilization of ‘cold’ cathodes.
- No heater chain failures, heater transformers or heater relays and switches means greater reliability and extended life.
- As there is no heater current and therefore no electromagnetic field, there is no modulation of the magnetron’s electron stream and its frequency spectrum modulation.
- Simplified design results in reduced weight and size and subsequent cost savings.
JSC Pluton began to develop cold cathode magnetrons in 1974, with serial production starting in 1984. It seems strange that in 25 years of industrial production of magnetrons, the technology of non-incandescent magnetrons has not expanded greatly while other similar breakthroughs (such as multi-beam klystrons) have.
One reason is that the first cold cathode magnetrons did not have a long lifetime because they were built using impregnated secondary-emission emitters, which failed to work properly even in incandescent magnetrons when the temperature was lower than 900°C. In 1990, impregnated emitters were substituted for new palladium-barium emitters to provide a longer lifespan and greater stability. Until 2000, about 10 different types of non-incandescent magnetrons in the 2 and 3 cm wave range were built, some of which have a lifetime of 5,000 to 10,000 hours.
The development of the first 8 mm wave range magnetron with a cold cathode was completed in 2002. The testing of this model showed that the magnetron could have a life span of up to 5,000 hours. Up until now, JSC Pluton has developed and manufactured more than five different models of magnetron in the 8 mm wave range (12 inclusive of all modifications) with different output power levels.
Now, different types of non-incandescent magnetrons are produced. They have a frequency range of 8 to 40 GHz, pulse power of 1.5 to 50 kW and anode voltage of 4.5 to 12.5 kW. Modern magnetrons exhibit a great difference in the duty modes: the pulse length of new models is 0.05 to 6 µs, with a duty cycle from 0.005 to 0.0002.
The characteristics of cathode magnetrons mean they can compete with incandescent magnetrons and also have the advantage of instant readiness, longer lifetime, good reliability and economical operation. Presently, coaxial, ligament and rising-sun non-incandescent magnetrons with fixed or agile (tunable) frequency are manufactured.
The design and production technology of magnetrons are constantly being enhanced. Considerable investment and effort is required to support and develop the complex cathode technology, which is necessary for building more powerful and more short-wave non-incandescent magnetrons. Therefore, it should be noted that the complicity of the cathode technology, its connection to construction parameters of magnetrons and interaction space peculiarity do not allow the license-free production of non-incandescent magnetrons. Figure 1 shows a Ka-Band non-incandescent magnetron with a minimal pulse power of 16 kW and the maximum weight of 360 g.
The growth in the production of cold cathode magnetrons can be achieved in several ways. The first is the development of more powerful magnetrons in frequency ranges that have already seen magnetron development. By increasing a magnetron’s output power, the anode voltage will also be increased, providing better conditions for cathode field emission. However, due to their heavy weight, high cost and complexity of use, along with the high power anode voltage, the demand for such powerful magnetrons is quite low.
The second way is to develop magnetrons in the 80 to 150 GHz frequency range. This approach has great potential but needs significant investment in the development of a cathode which will be able to produce the necessary emission current density.
The third and probably the most feasible option is to find new, effective methods of cold cathode magnetron application that exploit its advantages of instant readiness, high pulse power allied to low supply voltage, small dimensions and weight, mass production, easy operation, high efficiency and low cost of operation.
A prime example of this is the production of millions of microwave oven magnetrons in different countries all over the world. In the author’s opinion, a good prospect would be the use of cold cathode magnetrons for fuel ignition in car engines. According to the Patent Appl. U.S. 2007/0240660 A1, the CO blowout will decrease by ten times and the fuel flow will decrease by three times when using the non-incandescent magnetron for fuel ignition in the engine combustion chamber.
It is estimated that in big cities alone Light Motor Vehicles (LMV), will have an economy of 500 liters of fuel in the first 10,000 km, which equates to approximately $1,000. Through mass production of such cold cathode magnetrons (with the power supply included) the manufacturing cost will be much lower and the magnetron’s life span of 5,000 to 10,000 hours would result in an expectancy of journeys of around 200,000 to 500,000 km without magnetron replacement.
Thus, if LMVs can be built using the energy efficiency and ecological engines utilizing the microwave energy of cold cathode magnetrons, they would make a considerable contribution to addressing big cities’ ecological problems. On top of that, the necessary investment would be much lower than those required for making all vehicles electric and generating the associated extra power that will be needed to run them.
Examples such as these illustrate the potential for developing highly efficient non-incandescent magnetrons for different projects in various fields with the design of more powerful and high frequency cold cathode magnetrons and their application remaining of major significance.