Optimization and measurement for an electron side-coupled linac with longitudinally off-axis coupling cavities

In designing an electron side-coupled linear accelerator, the E-field tilting phenomenon always happens in the low-$\beta$ section, due to the different accelerating cavities (ACs) structures. To improve the field flatness, we decided to translate the longitudinal position of coupling cavities (CCs) away from the centerline. By analyzing the equivalent lumped circuits model for the low-$\beta$ section, the feasibility of the CC off-axis operation is verified; The relationship between the cell–cell coupling coefficient and the corresponding AC E-field magnitude is found out. Besides, the influence of the off-axis CC to the resonant frequency for the half-half unit cell is discussed. Currently, the coupled-cavity linac (CCL) with an even E-field distribution has been simulated and manufactured successfully. We present the cold measurement and tuning procedures from unit cells to the whole linac, including the radio frequency measurement and the bead-pull measurement. The measurement results reveal excellent frequency characteristics and flat E-field distribution for the linac.