Optimization of CLIC transfer structure (CTS) design to meet new drive beam parameters

In the original CTS design, the 2.8 mm-wide slit joining the cylindrical beam chamber to the teeth-loaded rectangular waveguides provided a sufficient coupling for the 160 nC drive-beam bunches to deposit 40 MW of 30 GHz power in the waveguides. The drive-beam generation studies have recently evolved towards schemes which envisage an increased number of bunches at reduced charge per bunch. In order to cope with the reduced beam intensity while maintaining the correct power output level, the CTS design has been modified to increase the beam coupling parameter. Moreover, the new CLIC Test Facility (CTF2) requires transfer structures with even higher beam coupling because of the reduced bunch frequency (3 GHz). Additional modifications to the original design were required by manufacturing, which imposed the presence of round lips at the slit edges. We have simulated the new CTS by means of MAFIA in order to explore the range of variation of the coupling parameter with slit aperture, and optimized the design to meet the requirements of both the new drive beam and the CTF2. We report here the results of the simulation studies