Design and measurements of the high gradient accelerating structures

The purpose of this thesis was to study on design and measurements of the high gradient accelerating structures. After introducing the main parameters to characterize Linacs we explained the application of the periodic accelerating structure. Then we studied TW accelerating structure operating at K-band frequency in order to linearize longitudinal space phase to increase beam brightness in the framework of the Compact Light XLS project in order to produce hard x-ray. We estimated group velocity as a function of frequency both analytically and numerically. Analytical results have a good agreement with the numerical results. The main parameters such as shunt impedance, quality factor (Geometric factor) and R/Q independently from the operating frequency for the TM010, TM110 and TM011 for a single cylindrical “pill-box” have been determined analytically as they provide accurate model for the accelerating structures. In order to characterize a normal conducting high accelerating structure with maximum gradients operating at X-band with extremely low probability of RF breakdown, an electroformed SW structures has been fabricated and characterized by SLAC and INFN with collaboration of other institute around the world at 11.424 GHz, coated with Au-Ni. We designed a gold plate RF high gradient structure operating at the X- band coated with Au-Ni. Bench measurements have been performed in the Department of SBAI of the University of Rome “La Sapienza”. The Slater method for the SW cavity has been employed in order to quantify the electric field inside the structure. Comparing the results with the results exposed from HFSS we report the features that have been quantified, showing good agreement. We continued working on the perturbation effect due to the aperture coupled between a waveguide and a cavity but for our application in SW multi-cell high gradient accelerating structure we studied on theoretical approach for reflection coefficient calculation in a SW cavity coupled to a waveguide. One method was based on circuit theory in which we found the overall Q of a resonant circuit for a cavity coupled to an external waveguide containing the RF generator. Q calculation led to the determining of the shunt impedance and consequently the reflection coefficient calculation. Comparison of the results shows a good agreement with the numerical results carried out by using the numerical code, HFSS. Another method of reflection coefficient calculation has been accomplished. We applied the modified Bethe’s theory presented by Collin and developed by De santis, Mostacci and L.Palumbo for TM01 mode cavities coupled by a small hole with a thickness size comparable to the wavelength. The amplitudes of forward and backward waves due to polarizabilites have been determined and we found equations for reflection and transmission coefficients. We demonstrated that our equation for reflection coefficient calculation is an analogous of the reflection coefficient obtained by Collin for TE10 using the circuit theory.