THE PolariX-TDS PROJECT: BEAD-PULL MEASUREMENTS AND HIGH-POWER TEST ON THE PROTOTYPE

A collaboration between DESY, PSI and CERN has been established to develop and build an advanced modular Xband transverse deflection structure (TDS) system with the new feature of providing variable polarization of the deflecting force. The prototype of the novel X-band TDS, the Polarizable X-band (PolariX) TDS, was fabricated at PSI following the high-precision tuning-free production process developed for the C-band Linac of the SwissFEL project. Bead-pull RF measurements were also performed at PSI to verify, in particular, that the polarization of the dipole fields does not have any rotation along the structure. The highpower test was performed at CERN and now the TDS is at DESY and has been installed in FLASHForward, where the first streaking experience with beam will be accomplished. We summarize in this paper the status of the project, the results of the bead-pull measurements and the high power test. INTRODUCTION Several experiments at DESY (FLASH2, FLASHForward, SINBAD) and PSI (ATHOS at SwissFEL) are interested in the utilization of high gradient X-band TDS systems for high resolution longitudinal diagnostics [1]. In this context, a collaboration between DESY, PSI and CERN has been established to develop and build an advanced modular X-Band TDS system with the new feature of providing variable polarization of the deflecting force [2]. The prototype of the novel X-band TDS, the Polarizable X-band (PolariX) TDS, has been fabricated at PSI following the high-precision tuning-free production process developed for the C-band Linac of the SwissFEL project and already used for the fabrication of the tuning-free X-band structure prototypes for CLIC [3–5]. Bead-pull RF measurements has been performed at PSI between December 2018 and January 2019 in order to verify that the polarization of the dipole fields does not have any rotation along the structure. Figure 1 shows the detail of the input and output couplers (left), the whole TDS prototype (middle) and the basic disk geometry (right). The main RF parameters of the TDS and RF pulse ∗ paolo.craievich@psi.ch compressor are summarized in Table 1. High power test on the prototype has been performed at CERN between March and April 2019. In this paper we summarize the status of the collaboration focusing on the results of the bead-pull measurements and the high power tests. Figure 1: Left: detail of the input/output coupler. Middle: whole TDS prototype. Right: basic disk. BEAD-PULL MEASUREMENTS The dipole field flatness and polarization were measured with the bead-pull method. The structure was allocated in a vertical frame and a stepper motor was used to drive along the structure a 2 mm diameter dielectric sphere fixed to a dielectric wire with diameter of 0.16 mm. The bead pulling has been repeated for different offsets, both in the XZ and YZ planes, in ±2 mm range. The measurements were performed with a 4-port VNA and the reflection coefficient resulted to be less than −35 dB with an insertion loss of −5.5 dB. Figure 2 (upper plot) shows results of the bead-pull measurement with offset 0 mm both in X and Y and phase difference between ports of 0 deg. Cell-to-cell phase advance (lower plot) resulted to be 2π/3 with a dispersion of 0.8∘ rms at the working frequency of 11995.2 MHz with a temperature of 33.9∘C. It is worthwhile noting that same results for the phase ad-vance was obtained varying the relative phase between ports, namely for different polarizations. A spherical and dielectric bead perturbs only the electric field, but cannot distinguish between longitudinal and transverse components. However, since the longitudinal component has the largest 39th Free Electron Laser Conf. FEL2019, Hamburg, Germany JACoW Publishing ISBN: 978-3-95450-210-3 doi:10.18429/JACoW-FEL2019-WEP036