Evolution of the Coaxial Cavity from the Cylindrical Cavity
College
College of Sciences
Department
Physics
Graduate Level
Doctoral
Graduate Program/Concentration
Physics
Presentation Type
Oral Presentation
Abstract
In our attempt to increase effectiveness of superconducting RF cavities, Bardeen-Cooper-Schrieffer (BCS) theory serves as a foundation. It optimizes the design of SRF cavities to achieve lower surface resistance. Previous studies have suggested for the RF field dependence of surface resistance, coaxial cavities have proven to be a superior choice. A coaxial half-wave superconducting cavity has been designed and built to measure properties of surface resistance, i.e. frequency, temperature, and RF field dependence. The cavity resonates over a broad frequency range (325 – 1300 MHz), allowing the study of frequency dependence on the same surface, whereas a cylindrical pillbox cavity supports only a single fundamental mode. The transverse magnetic (TM) modes of a cylindrical cavity transition into transverse electric and magnetic (TEM) modes in the coaxial cavity, which is of our primary interest. In a cylindrical cavity, the peak magnetic field is concentrated on the surface and peak electric field along the cavity axis, whereas in a coaxial half-wave cavity the field distribution is strong in the center conductor decaying exponentially towards the outer conductor. Our investigation focuses on this key distinction. We designed several superconducting RF cavities using CST studio suite simulation software, modifying the gap length inside the cylindrical structure. Gradually increased gap length of the cylindrical geometry transition into coaxial structure. TM011 mode of the cylindrical cavity evolves into the TEM1 mode. Other modes, TM012, TM013 and TM014 transforms into TEM2, TEM3 and TEM4 mode respectively. We calculated the peak electric and magnetic fields, geometrical factor (G) and other properties using CST to have a better understanding of the TEM modes to decide further necessary surface treatments. These treatments include nitrogen doping and low temperature (~120˚C) baking to reduce BCS resistance. These finally results in improving the cavity quality factor (Q0).
Keywords
Accelerator physics
Evolution of the Coaxial Cavity from the Cylindrical Cavity
In our attempt to increase effectiveness of superconducting RF cavities, Bardeen-Cooper-Schrieffer (BCS) theory serves as a foundation. It optimizes the design of SRF cavities to achieve lower surface resistance. Previous studies have suggested for the RF field dependence of surface resistance, coaxial cavities have proven to be a superior choice. A coaxial half-wave superconducting cavity has been designed and built to measure properties of surface resistance, i.e. frequency, temperature, and RF field dependence. The cavity resonates over a broad frequency range (325 – 1300 MHz), allowing the study of frequency dependence on the same surface, whereas a cylindrical pillbox cavity supports only a single fundamental mode. The transverse magnetic (TM) modes of a cylindrical cavity transition into transverse electric and magnetic (TEM) modes in the coaxial cavity, which is of our primary interest. In a cylindrical cavity, the peak magnetic field is concentrated on the surface and peak electric field along the cavity axis, whereas in a coaxial half-wave cavity the field distribution is strong in the center conductor decaying exponentially towards the outer conductor. Our investigation focuses on this key distinction. We designed several superconducting RF cavities using CST studio suite simulation software, modifying the gap length inside the cylindrical structure. Gradually increased gap length of the cylindrical geometry transition into coaxial structure. TM011 mode of the cylindrical cavity evolves into the TEM1 mode. Other modes, TM012, TM013 and TM014 transforms into TEM2, TEM3 and TEM4 mode respectively. We calculated the peak electric and magnetic fields, geometrical factor (G) and other properties using CST to have a better understanding of the TEM modes to decide further necessary surface treatments. These treatments include nitrogen doping and low temperature (~120˚C) baking to reduce BCS resistance. These finally results in improving the cavity quality factor (Q0).