Microwave Breakdown in Inhomogeneous Fields

Licentiate thesis, 2010

Microwave breakdown continues to be a risk in many applications involving the transmission and reception of microwave frequency signals. Satellite communication is especially interesting from the research point of view because of the extreme costs, and limited testing possibilities. Microwave breakdown is generally divided in two types, multipactor and corona. Multipactor breakdown appears in vacuum, e.g. space environment, under certain electron-field resonance and power conditions. It is caused by free electrons in the system that multiply themselves exponentially, causing disturbances ranging from noise to physical damage on the hardware. On Earth, the microwave system is typically filled with, and surrounded by Air, or possibly some other gas. In the presence of a microwave field it runs the risk of corona breakdown, i.e. the electrical breakdown of the gas, and the creation of a breakdown plasma. The deleterious effects of this plasma range from disturbances on the transmission, to damage on the hardware. To avoid microwave breakdown it is necessary both to make tests and to calculate the critical electric field strength at which breakdown occurs. When the fields are inhomogeneous, this is quite complicated. Several effects not present in the homogeneous case needs to be considered. It is also of interest to consider the evolution of the plasma after the initial breakdown. When the field is intensified locally, the initial breakdown volume may be small, and the subsequent temporal evolution of the plasma is a question of thermodynamic balance. This thesis presents research carried out on corona breakdown in a rectangular resonant cavity and around a hemispherical boss. It is seen that for high pressures, the initial breakdown volume in these systems is comparatively small. An investigation of multipactor breakdown between two parallel cylinders shows that the ponderomotive force, and the geometrical spreading of electrons serves to raise the breakdown threshold substantially. Finally the subsequent evolution of a small plasma sphere in homogeneous field is analyzed, and it is found that it is thermodynamically unstable.