An electron acceleration system includes a first RF cavity, and a second RF cavity whose center is located at a distance not more than 1.5 inch from the center of the first RF cavity, along an axis. The first RF cavity has a length less than about 0.25 inches. The on-axis coupling between the first and second RF cavities along the axis, which is primarily electric, is cancelled out by an off-axis coupling between the RF cavities off the axis, which is primarily magnetic. In this way, the net RF coupling between the RF cavities is zero. The phase and amplitude of the first and second RF cavities are each independently adjustable.