Multipactor in low pressure gas and in nonuniform rf field structures

Doctoral thesis, 2007

Resonant electron multiplication in vacuum, multipactor, is analysed for several geometries where the RF electric field is nonuniform. In particular, it is shown that the multipactor behaviour in a coaxial line is both qualitatively and quantitatively different from that observed with the conventionally used simple parallel-plate model. Analytical estimates based on an approximate solution of the non-linear differential equation of motion for the multipacting electrons are supported by extensive particle-in-cell simulations. Furthermore, in a microwave iris the electrons tend to perform a random walk in the axial direction of the waveguide due to the initial velocity distribution. The effects of this phenomenon on the breakdown threshold are analysed. The study shows that the threshold is a function of the height-to-length ratio of the iris and for a fixed value of this ratio, the multipactor susceptibility charts can be generated in the classical engineering units. Using the parallelplate concept, the multipactor threshold in low pressure gases has been analysed using a model for the electron motion that takes into account three important effects of electron-neutral collisions, viz. the friction force, electron thermalisation, and impact ionisation. It is found that all three effects play important roles, but the degree of influence depends on parameters such as order of resonance and secondary emission properties. In addition, a new method for detection of multipactor is presented. By applying a weak amplitude modulation to the input signal and performing a fast Fourier transform on the detected signal, accurate and unambiguous measurement results can be obtained. It is demonstrated how the method can be used in both single and multicarrier operation.