Design and test of the triple-harmonic buncher for the NSCL reaccelerator

A unique triple-harmonic buncher operating at the fundamental frequency of 80.5 MHz upstream the Radio Frequency Quadrupole (RFQ) linac has been designed, manufactured and tested at the National Superconducting Cyclotron Laboratory (NSCL) to meet the requirement of a small output longitudinal beam emittance from the reaccelerator. The buncher consists of two coaxial resonators with a single gridded gap. One cavity provides both the fundamental and the third harmonic simultaneously with λ/4 and 3λ/4 modes respectively, while the other provides the second harmonic in λ/4 mode. This buncher combines the advantages of using high quality factor resonator and only a pair of grids. Details on design considerations, electromagnetic simulations, and primary test results are presented. INTRODUCTION The National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University is developing a facility named ReA3 to demonstrate the technical feasibility and performance characteristics for stopping and reaccelerating rare-isotope beams, as an important step towards a next-generation rare-isotope facility in the United States [1]. Beams of rare isotopes will be produced and separated in-flight at the NSCL Coupled Cyclotron Facility and subsequently stopped by a novel gas stopper, breeded by a state-of-the-art electron beam ion trap based charge-breeder, and reaccelerated by a modern linear accelerator. The linac consists of a low energy beam transport line, a cw radio-frequency quadrupole, a quarter wave resonator based superconducting linac, and a high energy beam transport line [2]. ReA3 will delivery various exotic beams with charge-to-mass ratios (Q/A) of 0.2 – 0.4 and variable energies of about 0.3 to 3 MeV/u. Nuclear experimental programs require a beam on target with an energy spread of ~1 keV/u and a bunch length of ~1 ns simultaneously. Therefore, a longitudinal beam emittance of less than 0.3 π·ns·keV/u from ReA3 is demanded. Since the intensities of the rare-isotope beams will be low, the scheme of using an external multiharmonic buncher upstream of the RFQ has been adopted to produce a small longitudinal emittance beam from RFQ with high bunching efficiency [3-6]. A unique triple harmonic buncher using two high quality factor resonators with one pair of grids has been designed, fabricated and tested at NSCL for this application. DESIGN CONSIDERATIONS The buncher is designed to operate with three harmonics, a fundamental frequency of 80.5 MHz and two additional harmonics of 161 and 241.5 MHz, respectively. The fundamental frequency, same as that of the downstream RFQ and superconducting cavities, is mainly determined by the small longitudinal beam emittance requirement. Since beams from the charge breeder will have a larger intrinsic energy spread (e.g. ΔE ~ ±25eV/u for Q/A = 0.25), the beam micro-bunch frequency should not be lower than ~80 MHz, otherwise the longitudinal emittance of the bunched beam will be too large to achieve the required time and energy resolution on target. Considering the higher bunching efficiency and lower output longitudinal emittance needed, a total of three harmonics are chosen for the operation of the buncher [6]. A high quality factor resonator was proposed, which needs a lower power amplifier to drive it. This buncher consists of two coaxial cavities, as shown in Fig. 1. One cavity provides both the fundamental and the third harmonics. The other cavity provides the second harmonic with a λ/4 mode. The dual frequency cavity will operate simultaneously at the λ/4 and 3λ/4 modes, as was done at PIAVE in Legnaro [5]. The buncher bunches beams with a nominal relativistic velocity β = 0.00507 (beam energy of 12keV/u), so the βλ is small especially for harmonics. For example, βλ = 9.4 mm for second harmonic. On the other hand, the beam diameter is about 30mm at the buncher position in order to match it into RFQ. Therefore gridded electrode tubes are necessary to achieve uniform field distributions and thus satisfactory transit time factors. We proposed to design the buncher in such a way that all three harmonics are applied in one single gridded gap. This configuration with only one pair of grids minimizes the beam losses on grids and makes the buncher longitudinally more compact as well. Figure 1: Triple harmonic buncher with two coaxial resonators and a single gridded gap: design drawing (top), photograph (bottom). ________________________________________ *Work supported by Michigan State University. zhao@nscl.msu.edu THP069 Proceedings of LINAC08, Victoria, BC, Canada

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