Quadrupole effects in on-axis coupled linacs

Elliptical beam shapes have been seen at the exit of a number of on-axis coupled electron linacs. These noncircular shapes have been attributed to the quadrupole effects produced by the coupling slots in the cell walls. Qualitative and detailed cavity calculations predict that the quadrupole effects cancel if the coupling slots are aligned across the accelerating cavities, as opposed to rotated through 90 degrees as has normally been the case. Two short S-band linacs, identical except for the orientation of the coupling slots, have been built to test the predictions. The results of detailed measurements of the beam profiles at the exit of the two linacs are discussed.<>