Spurious mode suppression in high gain, highly efficient, crossed-field amplifiers

Until recently, the power gain of high power, injected-beam crossed-field amplifiers has been limited by regeneration effects and the inability to obtain adequate cold rf isolation across circuit severs. Both of these effects are caused by the propagation of rf energy in spurious waveguide modes; the so-called "fast-wave" modes characteristic of the two-conductor coaxial waveguide formed by the delay line, tube body surface and the insulated sole electrode. The coupling from the slow-wave mode to these spurious fast-wave modes occurs at the transitions between the waveguide couplers and the slow-wave structure and at the discontinuities caused by imperfections of the slow-wave structure itself. The spurious propagation occurs mainly in the principal TEM coaxial waveguide mode. It causes regeneration in the sections of the amplifier between circuit severs, regeneration due to rf coupling to the gun electrodes and traveling-wave instability due to rf feed-through across the circuit severs. These unwanted effects can be reduced by careful matching to the slow-wave structure and very uniform fabrication of the slow-wave structure. However, to realize the desired very high gain, additional suppression is necessary. It is shown that the judicious placement of rf lossy material can reduce the propagation of the spurious energy to the point where power gains of 30 db or greater may be realized. It is further shown that this technique can be employed in amplifiers producing several kilowatts of cw power with no heating or out-gassing of the rf lossy material. The placement of the loss and the techniques of construction to eliminate heating are discussed.