Investigations Into X-Band Dielectric Assist Accelerating Structures for Future Linear Accelerators

Dielectric assist accelerating (DAA) structures are being studied as an alternative to conventional disk-loaded copper structures. This article investigates numerically an efficient <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$X$ </tex-math></inline-formula>-band DAA structure operating in a higher order mode of TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub>-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula>. This accelerating structure consists of dielectric disks with irises arranged periodically in a metallic enclosure. Through optimizations, the radio frequency (RF) power loss on the metallic wall can be significantly reduced, resulting in an extremely high quality factor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q_{0}=134\,525$ </tex-math></inline-formula>and a very high shunt impedance <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$r'=781\,\,\text{M}\Omega $ </tex-math></inline-formula>/m. The RF-to-beam power efficiency reaches 51% which is significantly higher than previously-reported Compact Linear Collider (CLIC)-G structures with an efficiency of only 33.5%. The optimum geometry of the regular and the end cells is described in detail. Due to the wide bandwidth from the dispersion relation of the accelerating mode, the DAA structure is allowed to have a maximum number of 72 regular cells with a frequency separation of 1.0 MHz, which is superior to that of conventional disk-loaded copper structures. In addition, the DAA structure is found to have a short-range transverse wakefield lower than that of the CLIC-G structure.