II-8. Accelerator

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Charge stripper ring for RIKEN RI beam factory†
H. Imao∗1
The use of charge strippers is almost inevitable forthe eﬃcient acceleration of particularly heavy ions suchas uranium in heavy-ion accelerator complexes. At
the RIKEN RI Beam Factory (RIBF), the total charge
stripping eﬃciency of two strippers, He gas1,2) and rotating graphite sheet disk strippers,3) used for uranium
acceleration is less than 5%, which creates a seriousbottleneck for potential intensity upgrades in the nearfuture. We have proposed the use of charge stripper
rings (CSRs)4,5) as a cost-eﬀective method to achieve a10-fold increase in the intensity of the 238 U beams atRIBF.
Figure 1 shows a design view of the CSR for theﬁrst stripper (CSR1), which is a compact isometricring with the same design circumferences of 37.1953 m(15 times the distance interval of the beam bunchesfrom the RIKEN Ring Cyclotron (RRC) at a frequency of 18.25 MHz) for all circulating uranium beamswith eight diﬀerent charge states from 59+ to 66+.
CSR1 consists of gas strippers (He and nitrogen strippers), eight main bending magnets (BM1-8), two acceleration cavities, a re-buncher, four charge-dependent
quadrupole stations, injection magnets (IBM and injector quadrupole triplets), extractor bending magnets(EBM1 and EBM2), steerers for closed-orbit distortion
corrections, and diagnostic boxes involving beam diagnostics and vacuum pumps. In the CSR1, beams otherthan the selected U64+ beams reenter the stripper witha ring after recovering the energy lost in the stripper.
The U35+ beams are injected simultaneously into thecharge stripper ring using the charge exchange injection method. The recycling cycles are repeated, andonly the U64+ beams are continuously extracted, using a magnetic deﬂection channel. The isometric ringis used to hold the bunch structure of beams to matchthe acceptance of the latter-stage cyclotrons.
The CSR1 lattice is a mirror-symmetric reversed lattice with the symmetry plane at the center of thestripper section. The high-density quadrupole stations
equipped with a quadrupole doublet or triplet for all
charge states will be used to control the optics for all
charge states. The design of a compact quadrupole
magnet is a key issue. We have already ﬁnished the calculations and are preparing to manufacture “hourglasslike” quadrupole magnets6) as shown in Fig. 1.
We conducted some calculations for the key design issues of CSR1. A realistic lattice for 8 charge states circulating in CSR1 was derived. Possible sources of emittance growth with a new multi-stage stripper scheme
were also investigated. We also performed calculations
for the beam transport of U35+ from RRC to CSR1 andthen those of U64+ from CSR1 to fRC. The eﬀective
charge stripping eﬃciency of CSR1 in the present calculation was approximately 60%, and further detailed
calculations and optimizations were conducted.
References

1) H. Okuno et al., Phys. Rev. ST Accel. Beams 14, 033503(2011).
2) H. Imao et al., Phys. Rev. ST Accel. Beams 15, 123501(2012).
3) H. Hasebe et al., EPJ Web Conf. 229, 01004 (2020).
4) H. Imao et al., Proc. CYC2016, Zurich, Switzerland,
2016, pp. 155–159.
5) H. Imao et al., Proc. IPAC2018, Vancouver, Canada,2018, pp. 41–46.
6) Patent application filed by RIKEN and HITACHI Engineering Co., Ltd. with the application number JP2020056540 in Japan.

Fig. 1. Design view of the CSR1 (left) and “hourglass-like” quadrupole magnet (right).
†∗1

Condensed from the article in J. Instrm. 15, P12036 (2020)
RIKEN Nishina Center- 77 -1