ILC Damping Rings : Benefit of the Antechamber or : Antechamber vs . SEY ∗

We present simulation results of the build-up of the electron-cloud density ne for the two proposed ILC damping ring lattices, DC04 and DSB3, with particular attention to the potential benefit of an antechamber. We examine a field-free region and a dipole bending magnet, with or without an antechamber. We assume a secondary electron emission model for the chamber surface based on approximate fits to measured data for TiN, except that we let the peak value of the secondary emission yield (SEY), δmax, be a variable. We conclude that there is a critical value of δmax below which the antechamber provides a substantial benefit, roughly a factor ∼ 40 reduction in ne relative to the case in which δmax exceeds the critical value. We estimate the steady-state value of ne as a function of δmax, and thereby obtain the critical value of δmax for all cases considered. Thus, from the perspective of the electron-cloud effect, the inclusion of an antechamber in the design is justified only if δmax is below the critical value. The results presented here constitute a slight extension of those previously presented in March and September, 2010 [1, 2]. INTRODUCTION AND ASSUMPTIONS The desire to limit the potentially serious adverse consequences from the electron cloud effect (ECE) in the proposed ILC positron damping ring has led to the consideration of adding an antechamber to the vacuum chamber [3], a design decision similar to the one adopted many years ago for the positron ring of the PEP-II collider [4]. The antechamber provides the obvious benefit of extracting from the vacuum chamber a large fraction η (η =antechamber clearing efficiency) of the synchrotron-radiated photons, which are therefore unavaliable to generate photoelectrons. Fighting against the photon clearing effect of the antechamber is the process of secondary electron emission off the walls of the chamber. The number of secondary electrons grows in time in a compound fashion, and can therefore readily negate the clearing effect of the antechamber. The secondary electron density is a nonlinear function of bunch intensity and of δmax, and exhibits threshold behavior in both of these variables, hence the resulting balance between the antechamber and the SEY of the chamber material is non-trivial. ∗Work supported by the US DOE under contract DE-AC0205CH11231 and by the CESRTA program. Invited talk presented at the ECLOUD10 Workshop (Cornell Univerity, Oct. 8-12, 2010). †[email protected] We consider both proposed lattices, DC04 (C = 6 km) and DSB3 (C = 3 km), and for each of these we examine field-free regions and dipole bending magnets. For each case, we simulate the build-up with and without an antechamber of clearing efficiency η = 98% (Fig. 1). In all cases we set the bunch spacing tb = 6 ns, and then repeat the analysis for most cases for tb = 3 ns. The beam energy and bunch intensity are fixed throughout. The SEY function δ(E0) used here is shown in Fig. 2. The emission spectrum corresponds, approximately, to that of TiN, but we let δmax be an adjustable input parameter on the range 0−1.4. A detailed set of parameters is listed in Tables 1-2. This being a build-up simulation, the beam is a prescribed (non-dynamical) function of space and time, with bunches of specified sizes, intensity and spacing. The fill pattern simulated consists of 5 trains, as defined in Table 1, whether the bunch spacing is 3 or 6 ns. The electrons, on the other hand, are fully dynamical. The analysis is carried out with the electron-cloud build-up code POSINST [5–8]. Figure 1: Cross section of the vacuum chamber, without and with an antechamber. The red dot at the center represents the approximate one-sigma beam profile.