A racetrack geometry to avoid undesirable azimuthal variations of the electric field gradient in high power coupling cavities for TW structures

Abstract

The high levels of peak RF input power required for linear collider TW accelerator structures, and the need to prevent undesirable transverse momentum contributions being imparted to the beam during traversal of the linac coupler cavities, have led to the use of symmetric dual feed cylindrical cavities having diametrically opposed side wall coupling apertures. Analyses of these cavities have shown that the coupling apertures in the wall of the cavity cause azimuthal variations of the longitudinal electric field that can enhance the maximum surface gradient (and secondary electron emission) and reduce the peak power rating of the structure by 10 to 20 percent. These field variations also produce transverse gradients of longitudinal electric field amplitude and phase that can seriously degrade the beam emittance due to quadrupolar transverse momentum contributions. The racetrack shaped coupler cavity, described herein, avoids many of these undesirable features while still allowing the correct cavity match and tuning condition to be readily achieved.