Experimental Comparison of Beam Excited Modes in Biperiodic Structures

An experiment to study excitations of higher order modes in a coaxial coupled structure and an on-axis coupled structure is in progress. The tuning and assembly of these structures is complete. Calculations have shown that a coaxial coupled structure is 5% less sensitive to beam excitation of its axially symmetric modes than an on-axis coupled structure. Low power tests have identified mode frequencies above the accelerating mode passband showing that higher order modes propagate in an on-axis coupled structure and not in a coaxial coupled structure. High power beam tests are scheduled for later this year.