Minimization of Transverse Wakefields in the NLC Accelerator Structures

Abstract

The progress of a multiple bunches of electrons through several thousand accelerator structures results in a wakefield which if left unchecked will kick successive bunches off the axis of the accelerator and can at the very least dilute the final luminosity of the final colliding beams, or at worst can lead to a BBU (Beam Break Up) instability. In order to damp the wakefields to acceptable levels for travelling wave structures we detune the frequencies of the cells and we couple out the field to four adjacent manifolds. Optimizing the manifold-cell coupling for several hundred cells and changing the bandwidth parameters of the distribution has in previous structures been achieved by a process of trial and error. Here, we report on an optimized Fortran code that has been specifically written with the aim minimizing the sum of the squares of the RMS and standard deviation of the sum wakefield. Sparse matrix techniques are employed to reduce the computational time required for each frequency step. The wakefield is minimized whilst ensuring that no significant local surface heating occurs due to slots cuts into the accelerator cells to couple out the wakefield.