Transverse Wakefields And Means To Suppress Wakefields In High Gradient Linear Colliders

The symposium held at Tsinghua University, in honor of Dr. Juwen Wang, is a testament to his engagement with diverse areas of linear accelerators. My work has overlapped with his during the period of the Next Linear Collider/Japanese linear Collider (NLC/JLC) programe in particular. Here I report on the beam-exited long-range wakefield in these linacs, and also on subsequent developments in the context of the Compact Linear Collider programme (CLIC). This self-excited wakefield, if left unchecked, can at the very least cause a marked dilution in the beam emittance, and in the worst case can cause a catastrophic beam break up (BBU) instability rendering the particle beam unusable. To ameliorate this affect, there are two main strategies: one can either aim at heavily damping the wakefield (in practise requiring a Q value as low as similar to 10) or one can detune each of the accelerator's cells to ensure that each mode is excited at slightly different frequency, in a precise manner. The former approach entails placing damping waveguides and damping materials in relatively close proximity to the beam, and this is the approach adopted by the CLIC collaboration. For the NLC/JLC we collectively followed the latter approach, in which the dipole mode of each cell is detuned with an erf function profile along each accelerator structure. Eventually the modes, which form the wakefield, recohere and, to ensure the wakefield remains below a specified level a portion is coupled out through slots cut into each cell to an attached waveguide-like manifold. Typically medium Q values are aimed at, between 500 and 1000. This scheme entails suppressing the modes which comprise the wakefield -using damped and detuned structures (DDS). Sampling an attenuated portion of this manifold radiation also provides both a beam and structure diagnostic. A similar DDS design, but with more stringent wakefield suppression requirements, has been followed as an alternative to the CLIC damping scheme. Wakefield suppression for the NLC/JLC and CLIC is reported on, with a focus on the DDS approach.