Investigation of Electromagnetic Field Perturbation With Respect to Mechanical Imperfections in Radio-Frequency Quadrupole (RFQ) Structure

Electromagnetic (EM) fields near the beam axis in a radio frequency quadrupole (RFQ) are very sensitive to dimensional changes especially on vane tip geometries. Due to measurement difficulties, field distribution around the beam axis is generally assumed to be equal to that measured outside of vane gaps in quadrants using a bead-pull technique. If an RFQ structure has any mechanical imperfections in its vanes, local field distributions around the beam axis could be different from the design field profiles. The distorted field may affect beam quality as well as the operational stability and reliability of the RFQ system. In order to understand changes of field distribution in the RFQ against localized mechanical imperfections, a study with systematic 3D simulations has been carried out using a model developed for the RFQ in Spallation Neutron Source (SNS) of Oak Ridge National Laboratory. Field information at various locations such as around the beam-axis, bead-pull positions, and pickup probe positions are investigated with the vane tip perturbations. Design considerations for proper field retuning with perturbation are also presented and discussed to help future RFQ designs.

I. Introduction

Radio frequency quadrupole (RFQ) structures are used in front-end acceleration of protons or heavy ions in linear accelerators (LINAC) [1]–[4]. The distance between the vane tips is the most sensitive part of the RFQ structure [5], [6]. After manufacturing, an RFQ is tuned to obtain the needed field distribution and frequency of the quadrupole mode. In this tuning process, bead-pull measurements [7], [8] are usually performed at the four RFQ quadrants near vane gaps as indicated in point A in Fig. 1(b). Because of the narrow gap length between the vanes (point B in Fig. 1(b)), it is hard to pull a bead on the beam-axis. Slug tuners (Fig. 1(a)) are used to fine tune the measured field at point A to the design field, and this tuned field is used as a reference field to infer the on-axis field at point B. Magnetic pickup probes which measure magnetic fields near the cavity wall (point C or C' of quadrant in Fig. 1) are often calibrated with this reference field. Fig. 1.

(a) PISL and Tuner locations in an RFQ (b) field observation positions of an RFQ (a) Bead-pull measurement position (b) Beam-axis position (c) Pickup probes position 1 (c') Pickup probes position 2 .