On noise generated in ignitors of TR tubes

The noise generated by a dc glow discharge inside a coaxial ignitor embedded in a standard X-band TR stage has been studied experimentally and the results analyzed. The noise variation due to changes in physical dimensions, notably the dc and RF gap length, have been experimentally measured and a physical explanation presented to account for the phenomenon. Dependence on dc current, pressure, and gas type was also measured in controlled experiments. It is shown that the generated noise and the coupling of this noise to the microwave transmission line is a critical function of the tip shape and insertion depth of the solid cone into the coaxial ignitor cone. This effect is due to the interception of the charge carriers by the apex of the solid cone. This interception creates a noise current whose axis makes an angle with the vector of a propagating mode(s). When this angle is small and the discharge current large, the noise energy in the transmission line becomes large. Data showing a linear dependence of generated noise with ignitor current provides strong evidence that this is shot noise. Calculations of the available thermal power per cycle and the shot noise power per cycle are summed and compared with each other and with experimental values. These calculations further indicate the dominance of shot noise power under experimental conditions simulating a practical TR tube. Conclusions regarding techniques for the development of ultralow noise TR\´s are given.