WAKEFIELD MONITOR DEVELOPMENT FOR CLIC ACCELERATING STRUCTURE F. Peauger, W. Farabolini, P. Girardot, DSM / Irfu, CEA Saclay, France A. Andersson, G. Riddone, A. Samoshkin, A. Solodko, CERN, Geneva, Switzerland R. Zennaro, PSI, Villigen, Switzerland, R. Ruber, Uppsala University, Sweden Abstract To achieve high luminosity in CLIC, the accelerating structures must be aligned to an accuracy of 5 µm with respect to the beam trajectory. Position detectors called Wakefield Monitors (WFM) are integrated to the structure for a beam based alignment. This paper describes the requirements of such monitors. Detailed RF design and electromagnetic simulations of the WFM itself are presented. In particular, time domain computations are performed and an evaluation of the resolution is done for two higher order modes at 18 and 24 GHz. The mechanical design of a prototype accelerating structure with WFM is also presented as well as the fabrication status of three complete structures. The objective is to implement two of them in CTF3 at CERN for a feasibility demonstration with beam and high power rf. INTRODUCTION The alignment of the accelerating structures in the CLIC main linac is necessary to remove the wakefield effects on the beam. Simulations showed that the emittance growth can be very well improved by aligning the accelerating structures to an accuracy of 5 µm [1]. The full specifications are presented in Table 1 for the commissioning and operation of CLIC in comparison with the CTF3 test conditions. Table 1: Wakefield Monitor Specifications Parameters CLIC commissioning CLIC operation CTF3 Charge / bunch (nC) 0.06 0.6 0.6 Number of bunches 1-312 312 1-226 Bunch length (µm) 45-70 45-70 400 Train length (ns) 156 156 150 Bunch Spacing (ns) 0.5 0.5 0.66 Accuracy (µm) 5 5 Resolution (µm) 5 < 5 Range (mm) ± 2 ± 0.1 ± 2 Beam Aperture (mm) ~5.5 ~5.5 ~5.5 To achieve such accuracy in operation, the beam based alignment is foreseen. It consists of using the transverse higher order modes generated by an offset beam in the accelerating structure in order to evaluate and correct the misalignment. This method has been explored on X-band structures at SLAC [2], at S-band in CTF2 [3] and on superconductive L-band cavities at DESY [4]. The challenge is to adapt and test this technique on the current CLIC accelerating structure and demonstrate its feasibility. WAKEFIELD MONITOR RF DESIGN The basic design of the CLIC accelerating structures consists of 24 tapered and weakly coupled cells working on the 2π/3 mode at 12 GHz, with a mean aperture of 5.5 mm. Each cell has four orthogonal waveguides and rf absorbers in order to damp strongly the higher order modes (HOM) induced by the beam [5]. The dipole modes, usually used for the offset detection are above 16 GHz with a Q factor below 10, making the signal processing particularly challenging. Also, since no length is available in the main linac, the WFM has to be integrated to the structure, without modifying its accelerating and damping properties. The WFM must finally deal with the proximity of high peak power rf signals, which is not the case in a cavity BPM. The rf design of the 12 GHz prototype structure with WFM is presented in Fig. 1. Figure 1: 12 GHz prototype accelerating structure with wakefield monitor (vacuum volume). The wakefield monitor consists of a bent waveguide extending the damping waveguide with the same section. There are four waveguides per structure which are 90° bent for size reasons and connected to the middle cell to have the mean offset of the structure. Two coaxial rf pick- ups are implemented on the WFM waveguide: one on the large side to extract the TM-like modes and the other one on the small side to extract the TE-like modes. The TM- like and TE-like modes are hybrid electromagnetic (HEM) dipole modes in the cavity. They are coupled to the damping waveguides and propagate with the TE 10 mode in the bent waveguide on respectively the vertical (cut-off frequency Fc=13.3 GHz) and horizontal (Fc = 21.2 GHz) polarization. The internal conductor of the pick-up is inserted 1 mm inside the waveguide giving a Proceedings of Linear Accelerator Conference LINAC2010, Tsukuba, Japan TUP098 03 Technology 3G Beam Diagnostics 641