Optimized Wakefield suppression & emittance dilution-imposed alignment tolerances in X-band accelerating structures for the JLC/NLC

Abstract

In order to prevent electrical breakdown occurring in the JLC/NLC (Japanese Linear Collider/Next Linear Collider) X-band structures several new structures are under investigation. These accelerating structures represent an evolutionary design of the DDS series of structures. The phase advance per cell has been varied and the detailed elliptical shape of the cell has been varied in order to simultaneously minimize the group velocity, the surface electromagnetic fields and the pulse temperature rise on the copper surface. It is also important to ensure that the wakefield induced by multiple bunches traversing the accelerating structures does not disrupt trailing bunches. The long-range wakefield must be decreased adequately in order to prevent a BBU (beam break up) instability occurring and to ensure that emittance dilution due to the higher order modes is kept to acceptable levels. The long-range wakefield is forced to decohere by detuning all of the frequencies such that the mode density of frequencies is approximately Gaussian. In order to minimize the impact of the wakefield on the beam dynamics we change the bandwidth and the standard deviation of the Gaussian distribution of frequencies such that a cost function is minimized. Interleaving of cell frequencies of adjacent structures is required to adequately damp the wakefield of each particular structure under consideration. The resulting alignment tolerances imposed on the cells and structures is significantly looser alignment tolerances with the use of the code.