RESISTIVE WALL WAKES AND CLEARING ELECTRODES *

One electron and one positron damping ring are foreseen for the International Linear Collider (ILC). Damping rings are necessary to reduce the emittances delivered by the particle sources to the small values required for luminosity production. In this paper the effect of resistive wall wakes on the clearing electrodes, which are one option to mitigate or suppress the effects due to electron clouds, are investigated. Furthermore the resistive wall effects of NEG coating and the implications on transverse mode coupling instabilities are presented. INTRODUCTION Damping rings are necessary to reduce the emittances obtained from the particle sources to the design values of the linear collider. In the recent reference design of the International Linear Collider (ILC) [1] one electron and one positron damping ring, each about 6.7 km long, are foreseen. The main parameters of the damping rings are listed in Table 1. Table 1: ILC DR Parameters Parameter ILC DR Energy / GeV 5 Circumference /m 6695 RF Frequency / MHz 650 RF harmonic number 14516 RF Voltage / MV 24 Momentum compaction 1.22 10-3 Synchrotron tune 0.067 Total current / mA 376 Number of bunches 2625 Bunch population / 10 2.0 Bunch separation / ns 7.69 Emittance (horz.) /nm 0.51 Bunch length / mm 9 Damping time H/V/L / ms 25.7 / 25.7 / 12.9 In positron storage rings electrons produced by photoemission, ionization and secondary emission accumulate in the vacuum chamber forming an “electron cloud”. The interaction of the electron cloud with the positron beam can cause transverse single bunch instabilities [2] leading finally to an emittance blow up. These effects have been studied in [3, 4] using computer simulations and analytical estimates. Simulations indicate that the use of clearing electrodes is an effective method to suppress the electron effects [5, 6, 1]. Furthermore it has been proposed to use TiVrZr films as an effective getter material (NEG pump) with a low secondary emission coefficient inside the vacuum system of the damping rings [7]. CLEARING ELECTRODES There exist different designs for the clearing electrodes to minimize the effect of the geometric impedance. A schematic layout of a clearing electrode is shown in Fig. 1. A metallic electrode is mounted in the vacuum pipe using feed-throughs. Figure 1: Schematic layout of a clearing electrode. In addition, other designs with continuous clearing electrodes based on double layers of enamel coating have been proposed [8,9,10]. The calculation of the resistive wall wakefields, which are presented in this paper, are based on metallic surfaces in a round beam pipe. It is assumed that enamel layers are not visible for the beam, which is a rather crude approximation. RESISTIVE WALL WAKEFIELD The wake field due to the resistivity of the metallic wall has been calculated for various geometries [11, 12, 13, 14, 15]. To estimate the resistive wall wake fields for the damping ring the results for a round vacuum chamber have been used [12]. The normalized longitudinal and transverse wake potential for a Gaussian bunch are shown in Fig. 2. From the wake potential the total loss parameter and the kick parameter are calculated: