Multipacting electron signal behavior

Electron multipacting in a coaxial waveguide of the cold part of TTF-III power coupler has been observed at SLAC to be correlated with vacuum pressure growth and delayed with respect to the start of the rf power pulse. Analysis of the electron signal, gas pressure and rf power has been performed. It is found that at a given rf power level, the multipacting electron saturation current is proportional to the Kth power of gas pressure, where K is positive and only determined by the applied rf power and gas species. The K value is found much larger at the resonant mulipacting power than that at the non-resonant power. A model of the physics explains the delay time of the electron signal. All of the model predictions have been experimentally demonstrated.