Power/frequency goals of 1 MW continuous wave at 140 GHz are being pursued.
Power/frequency goals of 1 MW continuous wave at 140 GHz are being pursued.Large performance improvements have been achieved through the application of new materials and processes in microwave generators.
In addition to power and frequency, other performance factors are important to specific applications.
Gain, linearity, noise, phase and amplitude stability, coherence, size, weight, and cost must also be considered.
Table 2-1 shows the most likely candidate tubes, together with a few salient characteristics, including device cost and cost per watt of power generated.
The cost of the ancillary equipment such as power conditioning, control circuitry, transmission line, and applicator must be added to the numbers shown.
Large quantifies often lead to lower cost, and thus for many microwave heating and processing applications the magnetron is the device of choice, with advantages in size, weight, efficiency, and cost.
The magnetron is the major player in a class of tubes termed "crossed field," so named because the basic interaction depends upon electron motion in electric and magnetic fields that are perpendicular to one another and thus "crossed." In its most familiar embodiment, shown schematically in Figure 2-4, a cylindrical electron emitter, or cathode, is surrounded by a cylindrical structure, or anode, at high potential and capable of supporting microwave fields.When the time of transit became an appreciable part of a microwave frequency cycle, performance degraded, forcing the designer to smaller and smaller sizes to achieve higher frequency.The invention of the kylstron obviated this limitation by utilizing space-charge effects At even higher frequency and higher power levels, limitations associated with voltage and size were encountered.Although there is a broad range of materials that can be processed using microwaves, there are fundamental characteristics and properties that make some materials particularly conducive to microwave processing and others difficult.While an empirical understanding of microwave processing is important in moving developmental processes into production, a more fundamental approach is required for development of optimized process cycles, equipment, and controls.These are the tubes used in conventional microwave ovens found in almost every home (with power on the order of a kilowatt in the 2—3 GHz range) and in industrial ovens with output to a megawatt.Radars employing magnetrons number in the tens of thousands, and household ovens employing the so-called "cooker magnetron" number in the tens of millions.The application of these fundamental concepts for design or selection of a practical processing system is discussed in Chapter 3.The unique performance characteristics that arise from the interactions of microwaves and materials and how they may be used to develop application criteria are described in Chapter 4.Figure 2-2 traces, in time, that beginning and the inventions and developments that followed.The dependence of one tube on the understanding and development of others is illustrated.