ELECTROMAGNETIC-FIELD MEASUREMENTS ON A MILLIMETER-WAVE LINEAR-ACCELERATOR

Perturbational field strength measurements suitable for use on a propo sed 120-GHz 50-MeV electron linear accelerator are described, The meas urements are used to determine the R/Q of the device, where R is the s hunt impedance. The perturbation is achieved by the use of hollow meta llic cylinders with diameters ranging from 25 to 127 mu m which are ap proximately 500 mu m long. The cylinders were fabricated by sputtering aluminum through a shadow mask onto silica optical fibers as well as nylon surgical thread, The perturbational ''form factors'' for such a geometry are experimentally determined using a pillbox cavity, The mea sured values for the form factors are compared to theoretical estimati ons, which result in simple analytical expressions, The measured form factors are also compared to values calculated from a finite differenc e model of the perturbing object, The R/Q for various accelerating mod es is measured on a 12-GHz model of the 120-GHz structure. Results are compared to predictions from a finite difference model of the acceler ating structure.