Broadband Frequency Electromagnetic Characterisation of Coating Materials

In the new generation of particle accelerators and storage rings, collective effects have to be carefully analyzed. In particular, the finite conductivity of the beam pipe walls is a major source of impedance and instabilities. A reliable electromagnetic (EM) characterisation of different coating materials is required up to hundreds of GHz due to very short bunches. We propose two different measurement techniques for an extended frequency characterization: (i) a THz time domain setup based on the signal transmission response of a tailored waveguide to infer the coating EM properties from 100 to 300 GHz or even further. This technique has been tested both on NEG and amorphous Carbon films. (ii) a resonant method, based on dielectric cavities, to evaluate the surface resistance Rs of thin conducting samples at low (GHz) frequencies. Due to its high sensitivity, Rs values can be obtained for very thin (nanometric) coatings or for copper samples with a laser treated surface, since they have an expected conductivity very close to bulk copper. INTRODUCTION Next generation of light sources and particle accelerators requires vacuum chamber treatments with sundry surface materials for both avoiding electron cloud (e-cloud) effects and enhancing the ultra-high vacuum condition, in order to prevent machine performance degradation and luminosity limitations. The reduction of Secondary Electron Yield (SEY) threshold value, for the mitigation of the e-cloud mechanism, has been a major subject of recent studies for finding the best candidate as pipe coating material. However, the use of a coating material unavoidably changes the overall machine surface impedance. Therefore, an accurate EM characterization is required. In this work, three coating materials are the subject of our study: Non Evaporable Getter (NEG) alloys [1], that transform the vacuum chamber into an effective pump reducing the induced gas desorption and SEY; Amorphous Carbon (a-C), tested [2] at different CERN facilities with very effective results on SEY reduction; Laser-engineered surface structures (LESS) [3,4] produced on copper surfaces, considered as a possible alternative mostly to scale down the SEY threshold. Recently, we developed a time domain waveguide spectroscopy method by using a tailored guiding structure. The design allows to measure large area homoge∗ andrea.passarelli@na.infn. it neous coating deposited on removable metallic plates and to re-use the test system for different measurements. The EM characterization is performed by resorting to an adhoc analytical tool that is used for the experimental data interpolation. This technique has been successfully used to characterise NEG coatings deposited on both sides of thin copper slabs inserted in a waveguide. In the following, we summarize the EM characterization performed on NEG coating and already published [5, 6]. We also report, for the first time, the preliminary results on the EM characterisation of a-C coating. Lastly, for the EM response of nanometric laser treated surface structures (with an estimated conductivity close to copper), we introduce a resonant technique approach, based on a dielectric cavity, in order to improve the overall sensitivity of the characterisation method. ELECTROMAGNETIC CHARACTERIZATION OF NEG AND a-C COATINGS For the measurements, we used a tailored waveguide with a removable internal part where the coating is deposited. The guiding device is a 16 × 12 × 140 mm3 parallelepiped of gold plated brass. It consists of a central waveguide having a square section rotated by 45∘, side 1.1 mm and length 62 mm, connected to two pyramidal horn antennas 39 mm long, with side width from 1.1 mm to 6 mm (see Fig. 1). This configuration has the advantage to optimise the freespace to waveguide transition, reducing spurious contributions and improving the EM signal transmitted through the structure [7]. Figure 1: Device consisting of a waveguide with two pyramidal horn antennas. (a) Front view; (b) open view. The single mode (sum of TE1,0 and TE0,1) frequency window ranges from 135 GHz to 300 GHz, limited by the second propagating mode [8]. Measurements are carried out using a Time Domain Spectrometer (TDS) operating in transmission mode, based on a commercial THz-TDS 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW Publishing ISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-MOPAB346 MOPAB346 C on te nt fr om th is w or k m ay be us ed un de rt he te rm s of th e C C B Y 3. 0 lic en ce (© 20 21 ). A ny di st ri bu tio n of th is w or k m us tm ai nt ai n at tr ib ut io n to th e au th or (s ), tit le of th e w or k, pu bl is he r, an d D O I 1076 MC7: Accelerator Technology T06 Room Temperature RF system (TERA K15 by Menlo Systems) customized for the characterization. The signal is recorded for each sample by averaging 1000 pulses for an overall acquisition time of 10 minutes. Frequency dependent transmission curves are obtained through the application of a standard FFT algorithm. In the experiment, the frequency resolution is about 8 GHz, determined by the scanning range of the delay line. The conductivity value of the coated material is obtained from the comparison between the signal amplitude transmitted through the waveguide with and without coating, as reference. The relative attenuation along the entire structure has been analytically calculated as detailed in [6]. An analytical tool is used for the conductivity retrieval of the material under test. NEG Coating Results Two Ti-Zr-V NEG samples, grown at the CERN deposition facilities [2], with 3.8 μm and 4.3 μm thickness, have been deposited on both sides of a thin copper slab, same length of the guiding device (140 mm) and 0.050 mm thick, that has been placed in the center of the device. In Fig. 2, the experimental relative attenuation (with respect to a bare copper slab used as a reference) for both NEG coatings are shown. Data below 200 GHz have been discarded to avoid artifacts in the spectrum due to group and phase velocity dispersion. 0.2 0.22 0.24 0.26 0.28 0.3 3 4 5 6 7 Frequency [THz] A tt e n u a ti o n [ d B ] σ coat = 7.7 × 10 5 S/m