Dipole Mode Detuning in the Nlc Injector Linacs

A major consideration in the design of the accelerator structures in the injector linacs of the JLC/NLC[1] is to keep the wakefield effects within tolerances for both the nominal (2.8 ns) and alternate (1.4 ns) bunch spacings. One important multi-bunch wakefield effect is beam break-up (BBU), where a jitter in injection conditions of a bunch train is amplified in the linac; another is static emittance growth caused by structure misalignments. The injector linacs comprise the prelinac, the e drive linac, the e− booster, and the e booster. The first three will operate at S-band, the last one, at L-band. Compared to the main (X–band) linac, the wakes will tend to be smaller by a factor 1/64 and 1/512, respectively, for the S– and L– band linacs. This reduction, however,—especially for the S-band machines—, by itself, is not sufficient. Two ways of reducing the wake effects further are to detune the first pass-band dipole modes and to damp them. In this report our goal is to design the accelerator structures for the injector linacs using detuning alone, an option that is simpler than including damping. We will consider only the effects of modes in the first dipole pass-band, whose strengths overwhelmingly dominate. The effects of the higher passband modes, however, will need to be addressed in the future. For a more detailed version of this work see Ref. [2]. Note that the design of the e booster structure, which is straightforward, will not be discussed here. Machine properties for the injector linacs are given in Table 1. Shown are the initial and final energies E0, Ef , the machine length L, the initial (vertical) beta function averaged over a lattice cell β̄0, and the parameter ζ for a rough fitting of the beta function to β̄ ∼ E . The rf frequencies are sub–harmonics of 11.424 GHz. As for beam properties, for the nominal bunch train configuration (95 bunches spaced at 2.8 ns), the particles per bunch N = 1.20, 1.45, 1.45, 1.60 × 10 and normalized emittance ǫyn = 3×10−8, 10, 10, .06 rm, for the prelinac, e drive, e booster, and e booster, respectively. For the alternate configuration (190 bunches spaced at 1.4 ns) N is reduced by 1/ √ 2.