Feasibility of Folded and Double Dipole Radio Frequency Quadrupole (RFQ) Cavities for Particle Accelerators

The quadrupole and dipole modes frequencies can be very close in a long four-vane RFQ as structure length increases. This requires extra mode stabilizer design to improve field stability, which may add to system complexity and cost. Therefore, we investigate other four-vane RFQ designs which can have potential advantages and different mode spectrum. These designs are realized by modifying the RFQ cut-back scheme to be a folded-dipole (FD) or a double-dipole (DD) RFQs. In this paper, mode spectrums of these structures are further investigated as function of structure length and dipole mode properties are discussed in detail. Comparison of other properties of the three cut-back methods-FD, DD, and the conventional four cut-backs (4C) - are presented as well. Full 3-D simulations have been carried out for mode analysis of these RFQs with some experimental validation.

I. Introduction

The radio frequency quadrupole (RFQ) cavity structures are widely used for focusing, bunching and acceleration of the low-velocity ion particles using RF fields [1], [2]. The 4-vane RFQ cavity structures are commonly used for modern ion accelerators, and usually operate in the 80 to 500 MHz frequency range [3].