"Evacuated electrical oscillating cavities frequently are found to exhibit a strong loading due to electron (or iron) multiplications, the so-called multipactor effect. This effect tends to prevent the buildup of dee voltage in cyclotrons. In conventional cyclotrons driven from external oscillators having their own resonant tank circuits the difficulties are usually slight, but in cyclotrons using a simple dee system as the tank circuit difficulties are encountered in getting the oscillator to break into full oscillation."