Technical design of an IH-type buncher for KoBRA at RAON

To satisfy the beam requirement of the KoBRA experiment (0.5 nsec in 1σ), rebunching is required in the energy range up to 15 A MeV and for particles with an A/q range from 1 to 7. In cooperation with IBS and Hanmac Corporation, BEVATECH proposed a five-gap IH-buncher cavity which has excellent bunching properties from 5 to 15 MeV/u. Main challenges in this project are the wide energy range and CW operation at RF power levels up to 20 kW. The cavity will offer an effective voltage amplitude of up to 1 MV. To provide the required cooling channels and cooling efficiency, the stems and drifttubes of this cavity will be 3D printed. This paper will discuss the technical design approach and give a status on the development of the buncher cavity.


Introduction
The KoBRA (Korea Broad acceptance Recoil spectrometer and Apparatus) [1] facility being built at Rare Isotope Science Project in Korea will be utilized to produce rare isotope beams by employing multi-nucleon transfer reactions at about 20 MeV/u for studies of nuclear structure.At the target of the KoBRA experiment, beam pulse lengths of 0.5 nsec or less are required.The full drift length between the Linac (SCL3) exit and the KoBRA target is 55 m see Fig. 1.To achieve the desired pulse length at the target, a rebuncher will be positioned about 24 m behind the SCL3 exit to refocus the beam to the KoBRA target.The work performed for this project is done in a collaboration between IBS, Daejeon, Korea and Hanmac Corporation, Seoul, Korea and BEVAT-ECH, Frankfurt, Germany.
Beam input parameters for the simulation of the beam transfer line including the buncher started right in front of the first quadrupole dublet after the second dipole behind the SCL3 exit.As the energy range for SCL3 can vary between 1 and 20 MeV/u, beam simulations for the target ion 40 Ar 9+ were performed (with TraceWin from IRFU, CEA, Saclay, France) at four different energies 15.05 MeV/u, 9.16 MeV/u, 5.12 MeV/u, and 1.83 MeV/u.The energy range for this rebuncher cavity was specified from 15 MeV/u down to 5 MeV/u.The matched envelope for a 1.83 MeV/u beam had to be calculated taking into account that the transversal envelope will be maximum at this energy.This allowed to estimate the minimum rebuncher aperture needed along the position of the rebuncher to avoid beam losses caused to the cavity aperture.As a result from the simulations, a reasonable compromise of 15 mm aperture radius was decided as can be seen in Fig. 2.
A decision to limit the design rf power to 20 kW was made after an IH-type cavity design was achieved providing the high shunt impedance.

Beam dynamics results
Desired beam size and shape follow an approach where the beam sizes in x and in y are almost identical and the transverse beam envelopes on the target are at the beam waist.The beam dynamics simulations have been performed with 10 5 macroparticles in a 6D Gaussian distribution, truncated at 3σ.In Fig. 3, one can see the divergent beam down to the target without bunching resulting in a bunch length of around 6 nsec (FWHM 2.3 nsec) corresponding to a phase spread of ± 170°.With the buncher turned on, a well focused beam of 9.16 MeV/u of Ar 9+ down to the target results in a bunch length of close to 1 nsec (FWHM 0.25 nsec) corresponding to a phase spread of ± 35°, see Fig. 4. A similar bunching efficiency is observed for 15.05 MeV/u and 5.12 MeV/u.

Cavity design
The buncher structure type of choice for this project is an IH-type buncher.The reason for this choice is the high gradients achieved in H-type structures at economical RF power levels due to the high shunt impedance [2].This approach does allow to introduce a cavity in the limited available space of the existing beam line that needs only five gaps to perform the required bunching.Buncher versions with more than five gaps with equidistant and varying period lengths have been investigated.In conclusion, the buncher version with five gaps with equidistant gaps appeared most promising according to the simulation results.To determine the five-gap buncher cavity geometry, RF simulations were performed using CST Studio Suite.A simplified geometry-see Fig. 5-was used in which some elements on the outside of the cavity that have no influence on the RF parameters inside the cavity (e.g., drilled holes, exact flange geometry) were not considered in the simulations.
For the beam dynamics along the cavity, the electric field profile along the beam axis is of decisive importance.Figure 6 shows the corresponding simulated curves.This results in an effective voltage distribution across the individual acceleration gaps as denoted in Table 1.

Production status
The buncher cavity consists of several mechanical parts and ancillary parts which are shown in Fig. 7.The cavity itself will be 614 mm in diameter with a length of 540 mm.It will be made of stainless steel and copperplated from the inside.As the cavity needs a good cooling design for thermal stability, the drift tubes are 3D printed from stainless steel which will be remachined and copperplated later, see Fig. 8.This relatively new method has been well tested at the Institute for Applied Physics, Frankfurt [3] and will allow for novel drift tube arrangements and cavity sizes.During this development, the stems will be simulated in   tube.In the case of the buncher stems, it was achieved that the maximum temperature increase for a stem surface was 50 Kelvin and could be cooled very well.The cavity is supposed to be transported to IBS in Daejeon in Q2 of 2023.
Acknowledgements The authors would like to thank the IBS team at Deajeon for the good collaboration and fruitful discussions that helped to drive the project forward quickly.Designing and building a cavity and all ancilliary systems within roughly 15 months is quite a challenge for all team members and it has worked out very well so far.

CST
Studio Suite to analyze the resulting heat deposition caused by H-field induced currents, see Fig. 9.The stems will be partitioned into multiple sections each requiring individual cooling intensities.After deploying cooling channels inside of the stems, a temperature analysis as one can see from Fig. 10 using, for example, ANSYS running fluid dynamic calculations result in a good model to develop the right cooling geometry for each stem and drift

Fig. 9
Fig. 9 H-field view for cooling channel analysis

Fig. 10
Fig. 10 3D printed drift tubes just after the printing process

Table 1
Distribution of the effective gap voltage U_a across the individual gaps This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material.If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.To view a copy of this licence, visit http:// creat iveco mmons.org/ licen ses/ by/4.0/.