Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 47 (2014)

Recent development of RIKEN 28-GHz superconducting electron cyclotron
resonance ion source†
Y. Higurashi,*1 J. Ohnishi,*1 K. Ozeki,*1 M. Kidera,*1 and T. Nakagawa＊1
Over the past several years, we have endeavored toimprove the performance of the RIKEN superconducting
electron cyclotron resonance ion source using several
methods.1,2) For the production of U vapor, we employed thesputtering method, although the beam intensity in thismethod is assumed to be weaker than that in the oventechnique. We also used an aluminum (Al) chamber insteadof a stainless steel (SS) one. It is possible to observe theso-called “wall-coating effect.”3) Using these methods, wesuccessfully produced ∼180 eμA of U35+ and ∼230 eμA ofU33+ at the injected radio frequency (RF) power of ∼4 kW(28 GHz). Very recently, with the aim of further increasing
the beam intensity of U35+, we have the development ofhigh-temperature oven and have successfully produced ahighly charged U ion beam.
In this paper, we present a detailed report on the effect of
the Al chamber on the beam intensity of highly charged U
ion beams. We also report the effects of the two-frequency
injection method on the U ion beam intensity.
For this experiment, the maximum mirror magnetic fieldstrength at the RF injection side (Binj), minimum strength ofthe mirror magnetic field (Bmin)4), maximum mirror
magnetic field strength at the beam extraction side (Bext),
and minimum magnetic field strength at the surface of theplasma chamber (Br) were fixed at 3.2, 0.65, 1.8, and 1.85 T,
respectively. The microwave frequency generated by thegyrotron was 28 GHz. The diameters and lengths of both
plasma chambers (Al and SS) were 150 and 575 mm,respectively. The typical sputtering voltage wasapproximately −5.5 kV. We used oxygen as the ionized gas.
The gas pressure was (4–5) × 10−5 Pa. The extraction
voltage was fixed at 22 kV in these experiments. Figure 1
shows the charge state distributions of the highly charged U
ion beams. The open and closed circles denoted the resultsin the cases where SS and Al chambers, respectively wereused. The injected RF power was 2 kW for both cases. Theion source was tuned to produce U35+. As shown in Fig. 1,the intensity of the highly charged U ion beam producedwith the Al chamber was higher than that produced with theSS chamber. For example, the intensity of the U35+ beam
produced with the Al chamber was 110 eμA, which wasalmost twice the value (60 eμA) obtained with the SSchamber.
Ever since enhancement of the beam intensity of thehighly charged heavy ions was achieved by injecting power
at two frequencies simultaneously,5) this mechanism has
been investigated and used at several laboratories toincrease the beam intensity. At RIKEN too, we employed†

Condensed from the article in Rev. Sci. Instrum 85, 02A953 (2014)
RIKEN Nishina Center＊1Fig. 1. Charge state distribution of the U ions with the Alchamber (closed circles) and SS chamber (open circles).
this method to increase the beam intensity. Figure 2 shows
the beam intensity of U35+ as a function of Bmin. The
opencircles represent the beam intensity of the U ions undera single frequency operation (28 GHz [1.5 kW]). At thelower Bmin, we added an RF power of 500 W (18 GHz). The
closed circles denote the results obtained with 28 GHz (1.5kW) + 18 GHz (500 W). The beam intensity at a Bmin of0.57 T (18 + 28 GHz) was slightly higher than that at a Bminof 0.66 T (28 GHz). On comparing results with those at Bminof 0.66 T with a 2 kW injection (28 GHz), we did not findany beam enhancement in this experiment. However, asshown in Fig. 2, the X-ray heat load with a Bmin of 0.57 T islower than that with a Bmin of 0.66 T; this is mainly due tothe magnetic field gradient effect. As we obtained nearly thesame beam intensity with a lower X-ray heat load, thisresult indicates that the two-frequency injection could be
advantageous for our SC-ECRIS.

Fig. 2. Beam intensity of U35+ with two frequencies (18 +28 GHz) for several Bmin values and with a single frequency(28 GHz) for Bmin = 0.66 T (upper panel). X-ray heat load in
the cryostat with two frequencies (18 + 28 GHz) (closedcircles) and a single frequency (28 GHz) (closed squares)
for several Bmin values (lower panel).
References
1) Y. Higurashi, et al.:Rev. Sci.Instrum. 83, 02A308 (2012).
2) T. Nakagawa et al.: Rev. Sci. Instrum. 81, 02A320 (2010).
3) T. Nakagawa: Jpn. J. Appl. Phys. 30, L930 (1991).
4) G. D. Alton and D. N. Smithe, Rev. Sci. Instrum. 65(1994) 775
5) Z. Q. Xie and C. Lyneis: in Proceedings of the 12th
International Workshop on ECRIS, Wako, INS-J-182(University of Tokyo, 1995), p. 24.

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