ATF2 BEAM HALO COLLIMATION SYSTEM BACKGROUND AND WAKEFIELD MEASUREMENTS IN THE 2016 RUNS N. Fuster-Martínez, IFIC (CSIC-UV) * , Valencia, Spain A. Faus-Golfe, P. Bambade, R. Yang, S. Wallon, LAL, Université Paris-Sud, Orsay, France F. Toral, I. Podadera, CIEMAT, Madrid, Spain G. White, SLAC, California, USA K. Kubo, N. Terunuma, T. Okugi, T. Tauchi, KEK and SOKENDAI, Tsukuba, Japan Abstract A single vertical beam halo collimation system has been installed in ATF2 in March 2016 to reduce the background in the IP and Post-IP region. In this paper, we present the results of an experimental program carried out during 2016 in order to demonstrate the efficiency of the vertical colli- mation system and measure the wakefields induced by such a system. Furthermore, a comparison of the measurements of the collimation system wakefield impact with CST PS numerical simulations and analytical calculations is also presented. INTRODUCTION The ATF2 [1] facility was constructed to address two ma- jor challenges of the Future Linear Collider (FLC): focusing the beam to the nanometer scale using the International Lin- ear Collider (ILC) Final Focus System (FFS) and proving nanometer beam stability. Undesired background due to beam halo hitting the beam pipe of some machine compo- nents could limit the performance and experiments of the machine. In order to control the beam halo and the losses, beam halo collimation systems are necessary. The design of such a systems is a complex balance between the efficiency needed, the wakefields induced which can compromise the beam stability and self-preservation. In ATF2, background photons generated in the Post-IP showed to be limiting the performance of the Post-IP di- agnostics. There was no dedicated beam halo collimation system in ATF2 although some apertures and a Tapered beam Pipe (TBP) installed in the high β-region were inter- cepting part of it. We have performed a feasibility design study of a vertical collimation system [2, 3] with the main objective of reducing the background photons in the ATF2 Post-IP. The system was constructed at LAL and installed in ATF2 in March 2016 [4]. In this paper, we present the results of an experimental program carried out during 2016 in order to demonstrate the efficiency of the vertical collimation system and measure the wakefields induced by such a system. These wakefield measurements were done to investigate the optimum oper- ation mode of the vertical collimation system in terms of efficiency and acceptable wakefield impact. In addition, we performed a systematic benchmarking study of theoretical models, numerical simulations and measurements. The fact * Work supported by FPA2013-47883-C2-1-P and ANR-11-IDEX-0003-02 that there are discrepancies in the wakefield kick described in different analytical models for the same regime, in the models implemented in the tracking codes and between sim- ulations and measurements (ESA (SLAC) 2001-2007 [5]) motivated our study. In addition, there are different analyti- cal regimes (inductive, intermediate, diffractive) depending on the geometry of the jaws and beam parameters and when the parameters of the problem sit close to the limits the es- timations are not accurate and numerical simulations and measurements are crucial. These benchmarking studies have contributed to the understanding of the applicability of the tools used to estimate such effect being essential for the de- sign of the FLC collimation systems. The study presented on this paper applies for structures laying in the inductive geometric wakefield regime and long range resistive one. WAKEFIELD MEASUREMENTS Experimental Set-Up and Data Analysis Descrip- tion Wakefields are induced when a beam is passing through an accelerator component with an offset, Δy c (see Fig. 1). In our experiment, in order to simulate this condition we move the vertical collimation system around the beam in a symmetric way keeping constant the half aperture, a, of the system but changing the collimator-center-beam offset and we measure the induced orbit changed in the downstream BPMs. This could be done because the collimation system jaws can be moved independently. Upstream C-BPMs Downstream C-BPMs Vertical Collimation system e- Beam κ y Δy BPM Δy c a Reference orbit Figure 1: Experiment set up scheme for wakefield measure- ments. The orbit variation at a downstream BPM, Δy BPM , is related with the offset at the collimation system, Δy c as: Δy BPM = R 34 eq E κ y Δy c (1) TUPIK075 Proceedings of IPAC2017, Copenhagen, Denmark ISBN 978-3-95450-182-3 1864 Copyright © 2017 CC-BY-3.0 and by the respective authors 01 Circular and Linear Colliders A03 Linear Colliders