Varying alpha/lambda in NLC Structures--BNS Damping and Emittance Growth(LCC-0063)

In this note we consider the effect of the varying the iris opening {alpha} in the NLC structures on the beam dynamics and the RF efficiency in the linac. The most important consequence of the variation of the iris openings is the change of the longitudinal and transverse wakefields. Wake as a function of parameter {alpha} for the NLC structures has been previously calculated by K. Bane. Here we will use his result for the scalings of the longitudinal wake w{sub l} and transverse wake w{sub t} with the geometrical dimensions of the structure: w{sub l}(s) = Z{sub 0}c/{pi}{alpha}{sup 2} e{radical}(s/s{sub 0}), dw{sub t}(s)/ds = 2Z{sub 0}c/{pi}{alpha}{sup 4} e{sup -} {radical}(s/s{sub 1}) where for the scaling parameters s{sub 0} and s{sub 1} we have s{sub 0} = 0.41 a{sup 1.8}g{sup 1.6}/L{sup 2.4}, s{sub 1} = ({alpha}[m]){sup 1.96}, with g being the distance between the irises (gap) and L - the cell period (equal to the gap plus the iris thickness). The above formulas were derived assuming 2{pi}/3 phase advance between the structure cells. The wakefields were calculated assuming g = 7.09 mm and L = 8.75 mm and four different ratios of {alpha}/{lambda} = 0.18, 0.17, 0.16, 0.15 ({lambda} =more » 2.62 cm corresponding to the frequency 11.424 GHz). The plots of the longitudinal and transverse wakes are shown in Fig. 1 and 2. The beams dynamics in the NLC lattice (linac.cd1.1.trans) was simulated using the LIAR code for the nominal case with the following parameters: initial beam energy--10 GeV, final beam energy--500 GeV, number of particles in the bunch--10{sup 10}, normalized vertical beam emittance at the entrance to the linac--3 {center_dot} 10{sup -8} m, and the rms bunch length--150 {micro}m.« less