Loss factor Dependence on Group Velocity in Disk-Loaded Travelling-Wave Structures

The loss factor, a quantity linked to the energy lost by a point-like charge when traversing an accelerating (or decelerating) structure, can be computed using programs which solve Maxwell's equations in time domain and provide the correct result within the limitations inherent to the numerical simulation process. An alternative method, commonly used, consists in the derivation of the loss factor from the parameter R/Q, which is computed using codes operating in frequency-domain. Recent calculations of the loss factors for disk-loaded structures performed with the two methods have produced diverging results. The discrepancy of the results is a function of the group velocity and can be eliminated by introducing a correction term in the formula linking the loss factor to the R/Q obtained from frequency-domain calculations.