Helix and coupled-cavity traveling-wave tubes

After 28 years of considerable research and development, two types of traveling-wave tube (TWT) circuits have captured the major portion of the production business. For broad-band applications, the helix is almost exclusively used, while for high-average-power applications, such as radar transmitters, the coupled cavity circuit has gained the ascendancy. Recent advances have been stimulated and supported by the ever increasing needs of electronic countermeasures (ECM) where average power and bandwidth are of critical importance. For airborne systems where the limitations on weight and size are stringent, periodic magnetic focusing has been utilized along with special gridded electron guns which make possible simple modulation circuits. Although relatively few TWT's have been manufactured in large quantitites, there are many refined prototypes covering the frequency range from L band through Ku band at power levels of from a few hundred watts to several hundred kilowatts. Very impressive life data have now been accumulated for the TWT amplifiers currently operating in all of the communication space satellites. Although this particular application is less than 10 years old, there are hundreds of TWT's continuously transmitting in orbit today. Significant advances have been recently achieved in the maximum obtainable efficiency from a TWT designed for system use. With careful design of the electron optics and the employment of velocity tapering, efficiencies in excess of 50 percent are quite common in a practical transmitter operating in the X-band region. Broad-band dual-mode tubes are now in the development stage which permit efficient operation at two different power levels separated by as much as 7 dB. Such versatility is quite important to modern airborne countermeasure techniques. In the foreseeable future, the total TWT market will likely remain constant at the 80-million dollar level in spite of the continued erosion of the present tube applications caused by solid-state microwave devices. As power levels increase and the higher frequency bands become more popular, TWT's will remain unchallenged in many situations. No dramatic breakthroughs are predicted for this mature technology, but there will be increasing emphasis on life, reliability, and cost.