New Radio-Frequency Quadrupole design with symmetric direct transversal fields for efficient compact particle accelerators

Author: Victor Etxebarria, professor, Dept. Electricidad y Electrónica. Fac. Ciencia y Tecnología, Universidad del País Vasco-EHU

A Radio-Frequency Quadrupole (RFQ) is a resonant cavity with a cylindrical symmetry divided in four lobes resembling a clover-like geometry and four vanes to focus and accelerate charged particles. This structure can accept a continuous flow of low-energy massive particles (such as protons or heavier ions) and accelerate them from the keV to the MeV range. This structure was conceived and published in 1970 by I.M. Kapchinskii and V.A. Teplyakov in the Institute for Theoretical and Experimental Physics of Moscow, and ten years later, R.H. Stokes, K.R. Crandall, J.E. Stovall and D.A. Swenson sent a telegram to Kapchinskii stating that “The RFQ is alive and well at the Los Alamos Scientific Laboratory”, which was the first operational experimental RFQ.

This geometry enhances the excitation of the TE210 quadrupolar resonant mode in the cavity, with the quadrupole symmetry providing a continuous, alternating quadrupole along the RFQ’s axis. Due to the shape of the section, the magnetic field is dominant at the lobes, and the electric field dominates in the central channel, where the particles will travel and accelerate (Figure 1). Simultaneous with the acceleration, the RF field confines the particles and bunches the continuous beam into a collection of macroscopic packets. This provides pure electrical focusing that is not dependent on the particle velocity and is well suited for massive particles, far from c even at tens of MeVs.

The TE210 quadrupolar mode has to be excited at the operating frequency of the RFQ but avoiding undesired modes on the cavity, as is the case of dipolar modes such as TE110. A common way to couple the RF power in proton RFQs has been the use of loop-couplers inserted into the mid-section of the lobes. This technique using magnetic field in the cavity has been demonstrated in the past to be reliable but this introduces a significant perturbation into the lobe that can be even more noticeable when dealing with higher frequency resonators and their corresponding compact structures. For our 750 MHz compact RFQ, we propose here [1] a double symmetrical RF injection scheme using direct transition from the coaxial line to the RFQ by connecting the inner coaxial conductor into the RFQ vane body, as shown in Figure 2

As a result, the insertion of metallic parts into the lobe itself is avoided, resulting in minimized lobe field perturbations (Figure 3). Note that, in this way, the transversal electrical fields are directly excited through the vanes. Also, to an important degree, our method, by combining two symmetric couplers connected to opposed vanes at a given transversal plane of the RFQ, allow us also to excite the quadrupolar mode while avoiding the undesirable dipolar modes.

Our design ¹ for the RFQ cold model is measured to verify its performance. The set-up for its characterization in IZPILab-Beam Laboratory is shown in Figure 4.

The measured transmission coefficient as a function of frequency is plotted in Figure 5. The remarkable results are, as displayed in the Figure, that the first two modes to appear (dipolar modes TE110) are quasi-degenerated. Afterwards, it comes the predominant mode (the quadrupolar TE210 mode). Finally, the harmonics of those three modes are observed. Note finally that the dipolar mode is basically removed using our design, as desired for an efficient compact RFQ.

This new generation of compact hadron accelerators working at higher frequencies, particularly for medicine, must be enhanced to avoid perturbation in a single clean quadrupolar resonance for appropriate acceleration and beam focusing. This proposed innovative approach which delivers the power via electric coupling and symmetry in RF coupling, highly improves the resonance with respect to the classical perturbing loops in RFQ lobes. Moreover, this new design using two coaxial-like entrances from opposite sides of the RFQ whose relative phase can be adjusted, allow us to maximize the excitation amplitude of the desired TE210 mode. Simultaneously this removes the relative amplitude of the TE110 dipolar modes with respect to the main quadrupolar resonance.