Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 47 (2014)

Conceptual design of SC linac for RIBF-upgrade plan
K. Yamada,∗1 K. Suda,∗1 N. Sakamoto,∗1 and O. Kamigaito∗1An upgrade plan for the RIKEN RI-Beam Factory is under discussion, with the objective of significantly increasing the uranium beam intensity. Diﬃculty in the present acceleration scheme mainly stems
from the two-stage charge stripping located at 11 and50 MeV/u, respectively, which yields a maximum total stripping eﬃciency of 5%. In the upgrade plan, theﬁxed-frequency Ring Cyclotron will be replaced by anew cyclotron1) that will be designed to accept U35+
ions without charge stripping at 11 MeV/u, and theRIKEN ring cyclotron will be replaced by a new linac,
mainly consisting of superconducting (SC) cavities, toimprove the transmission of the high current beam. To
evaluate the feasibility of the new linac, we started adesign study of the SC linac in ﬁscal year 20132).
A layout plan of the new linac is shown in Fig. 1.
The present injector, RILAC2, will be used at the lowenergy end. We will add a short room-temperature(RT) section to RILAC2, which will boost the beamenergy from 0.68 to 1.4 MeV/u. The main part isthe succeeding SC section working in the energy range
from 1.4 to 11 MeV/u.
Additional
RT-cavities

Rebuncher

1.4 MeV/udinal acceptance large enough to capture the output
beam from the RT section. Finally, we chose a conﬁguration that consists of 14 cryomodules, each of whichcontains four QWRs operating at 73 MHz, and a RT
quadrupole doublet placed in each space between thecryomodules. Quadrupoles with an aperture diameterof 50 mm and a ﬁeld gradient of less than 20 T/mwould be easier for us to make and operate comparedto the SC solenoid.
The SC QWRs were designed using CST Microwave
Studio 2013. The RF surface resistance is assumed tobe 25 nΩ on the safe side, where the BCS resistanceis negligibly small. The curently used parameters ofthe SC section are listed in Table 1. The deﬁnitionof the eﬀective length for the determination of Eacc isselected to be βgeom λ.
Table 1. Design parameters of the SC section.
Frequency [MHz]Duty [%]
Mass-to-charge ratio (m/q)
Input energy [MeV/u]
Output energy [MeV/u]
Number of cavities
Number of cryomodules
Number of quadrupole magnets
Total length [m]Cavity inner diameter [mm]Cavity height [mm]Gap length g [mm]Gap voltage Vgap [kV]βgeom of cavity
Beam aperture a [mm]Synchronous phase φs for βgeom [◦ ]Operating temperature T [K]G = Q0 × Rs [Ω ]Ra /Q0 [Ω]Rs = RBCS + Rres [nΩ]Q0Shunt impedance Ra [Ω]Rf power loss P [W]
Eacc [MV/m]
Epeak /Eacc
Bpeak /Eacc [mT/(MV/m)]

RILAC2
RFQ

28-GHz SC-ECRIS

SC-Linac, 14 cryomodules, L=43 m

11 MeV/uFig. 1. Layout plan of new linac injector.

The beam energy at the border of the RT and SC
sections was chosen so that the SC section could becovered by a single structure of a quater-wavelength
resonator (QWR) with two acceleration gaps. Because
a broad range of velocity had to be covered, the gap
length and cavity diameter of the SC section were optimized to minimize the number of QWRs in the section. The energy gain of each gap was calculated basedon hard-edge approximation. The gap voltage was assumed to be 800 kV, and the synchronous phase waschosen to be -25◦. After several iterations, we determined a length d = 160 mm, which is the length between the gap centers, and a total cavity number of56. The gap length was decided to be 60 mm.
The modular conﬁguration of the SC section wasoptimized based on ﬁrst-order approximation for thetransverse and longitudinal motions. Some conﬁgurations were checked to determine whether a semiperiodic envelope could be obtained with moderatestrength focusing elements, while keeping the longitu∗1RIKEN Nishina Center73100∼71.4
11.0561428
43φ3001103
60
8000.078
φ40
-25
4.5
22.6
718
25
9.0×108
6.5×1011
4.0
4.5
6.0
9.5

Further study is under way on the SC QWR, including the mechanical considerations, tuner design,and coupler design. We are also going to start thermal and mechanical studies of cryostats based on theinitial design shown above.
References1) J. Ohnishi et al.: Proc. of Cyclotrons2013, MOPPT022(2014).
2) K. Yamada et al.: Proc. of SRF2013, MOP021, 137(2014).

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