25 years of technical advances in RFQ accelerators

The radio frequency quadrupole (RFQ) accelerator began as "The ion linear accelerator with space-uniform strong focusing" conceived by I.M. Kapchinskii and V.A. Teplyakov [1970]. In 1979, R.H. Stokes, K.R. Crandall, J.E. Stovall and D.A. Swenson gave this concept the name RFQ. Shortly after Valentine's Day in 1980 a telegram was sent to I.M. Kapchinskii. It stated, "The RFQ is alive and well at the Los Alamos Scientific Laboratory". Thus begins a very informative story of the early history of the development of the RFQ. By 1983, at least 15 laboratories throughout the world were working on various RFQ designs. H. Klein wrote an excellent review of a number of different RFQ structures [1983]. In the early years, there were many types of geometry considered for the RFQ, but only a few types have survived. The two cavity geometries now used in almost all RFQs are the 4-vane and 4-rod structures. The 4-vane structure is the most popular because its operating frequency range (80 to /spl sim/500 MHz) is suitable for light ions. Heavy ions require low frequencies (below 200 MHz). Because the 4-rod structure has smaller transverse dimensions than a 4-vane RFQ at the same frequency, the 4-rod RFQ is often preferred for these applications. This paper will describe how the RFQ accelerates and focuses the beam. The paper also discusses some of the important technical advances in designing and building RFQs.

1 Rfq Description

The RFQ has 4 electrodes with alternating RF voltage impressed on them. The dominant characteristic of the electric field is that of a quadrupole shown in Figure 1. A beam of ions traveling down the axis of an RFQ, with a cross section similar to that shown in Figure 1, sees alternating focusing and defocusing electric quadrupole fields. Because the fields oscillate at the frequency of the RF, and are spatially continuous along the axis of the RFQ, the focusing force does not depend on the velocity of the ions. By modulating the radius of the pole tips, a longitudinal electric field can be obtained with the same energy ion beam while bunching and accelerating. Figure 2 defines some of the typical parameters that describe the geometry of the RFQ pole tips. Beta “” is the velocity of the ions in units of “c,” the speed of light, and “” is the free space wavelength of the RF Frequency. The modulation factor is “m” and “a” is the minimum distance from the pole tip to the RFQ axis. The gap voltage between adjacent vanes is “V”. It is apparent from looking at Figure 2 that the voltage on the axis is tending toward +V/2 at the position indicated with “a”. Not shown in figure 2 are the horizontal vanes that have their minimum distance from the axis of “a” where the vertical vanes are “ma” from the axis. Figure 3 shows how the horizontal vanes are offset from the vertical vanes by one cell length. The voltage on axis at the position marked “ma” is tending toward -V/2. Thus there is a longitudinal electric field on axis that peaks half way between “a” and “ma” in Figure 2. The “unit cell” in the RFQ is defined in Figure 2 by .