Numerical study on the impact of errors in a 325 MHz radiofrequency quadrupole and assessing the validity of quasistatic approximation in the analysis

Abstract

The performance of a radiofrequency quadrupole (RFQ) is known to be sensitive to its geometric errors, as well as to the amplitude and phase errors of the input RF. Extensive statistical simulations are typically performed using the available computer codes, for assessing the effect of various errors, to fix the tolerances on these errors. Each of the statistical simulation requires beam dynamics calculations to be performed with a modified electromagnetic (EM) field configuration corresponding to the particular set of geometric errors. Although modified EM field due to geometrical errors should be accurately obtained using EM codes such as CST-MWS, such an approach is time-consuming. It is therefore common practice to obtain the EM fields, under quasistatic approximation, which is faster. Such an approach is routinely followed in codes such as TRACEWIN. In this paper, we first present extensive error studies, using this approach, for a 325 MHz, 3 MeV RFQ, which has been designed as a front-end accelerator for the envisaged Indian Facility for Spallation Research. The validity of quasistatic approximation for such error analysis is explicitly checked in this paper by comparing the results obtained using TRACEWIN, with those obtained using the EM fields derived from the CST-MWS code. To the best of our knowledge, such a comparison has not been reported before. Also, since the available computer codes do not give the option to study the effect of RF phase jitter in an RFQ, techniques evolved for this study are also described in this paper.