The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

INTENSE BEAM PRODUCTION FROM RIKEN ECRISS
Y. Higurashi*, T. Nakagawa, M. Kidera, M. Kase and Y. Yano RIKEN, Wako, JapanT. Aihara SHI accelerator Service Ltd, Kita-shinagawa-ku, Tokyo 141-0001,JapanAbstract
We have constructed several high performance ECRISs
in RIKEN and produced intense beam of heavy ions.( e.g.,2mA of Ar8+, 0.6mA of Kr13+, 0.3mA of Xe20+) During
the improvement of their performance, we found that theseveral key parameters play essential role to increase thebeam intensity ( plasma electrode position, magnetic field
configuration, property of the chamber wall material.
position of the biased disk and so on).

ions. During the increase of the beam intensity, we haverecognized that it is important for increasing the beam
intensity not only to increase the magnetic field strength,
but also to optimize magnetic field configuration[7].
Applying these methods, the beam intensities fromRIKEN ECRISs have been dramatically increased. For
example, the beam intensity of Ar 8+ has been increasingfrom several 10 eµA to 2 emA during the past decade.

RIKEN 18GHZ ECRISINTRODUCTION
ECR ion sources have been widely used as the external
ion sources for heavy ion accelerators. Especially,
ECRISs became one of the key devices for production ofradioisotope beam (RI beam). Since middle of 1990s,
RIKEN has undertaken construction of new accelerator
facility so-called RI Beam Factory.[1] In this project, theproduction of intense heavy ion beams is an importanttask to produce the intense RI beam. For this reason, atRIKEN, several high performance ECRISs have been
constructed ( RIKEN 10 GHz ECRIS[2], 18 GHz ECRIS[3]and liquid He–free Super conducting ECRIS[4]) in thelast decade.

A detailed description of the RIKEN 18 GHz ECR ion
source and performances are described in ref. [3]. The
mirror ratio has a nominal value of 3.0 with the maximumof mirror magnetic field ~1.4 T. The field strength at thesurface of magnets (Br) is ~ 1.4 T. To increase the beam
intensity, we used the negatively biased electrodeinstalled in plasma chamber and the aluminum cylinder tocover the inner wall of plasma chamber [3]. For
production of highly charged Ar ions ( Ar11+, Ar12+ ), weused the Al2O3 plating method. [6] Furthermore, theposition of plasma electrode and magnetic field
configuration play essential roles to improve theperformance.

Figure 1: Schematic drawing of the plasma chamber
around the plasma electrode position.
Since the first beam was extracted from RIKEN 18
GHz ECRIS in 1995, several methods have been applied
to increase the beam intensity of highly charged heavyions( e.g., optimization of plasma electrode position [5]and Al2O3 plating[6]). The solenoid coils of liquid Hefree SC ECRIS is cooled by small refrigerator instead ofliquid He. This feature allows us to avoid the complicatedoperation and bulkiness of ordinary super conducting
magnets using liquid He.[4] After one year ofconstruction, we have successfully extracted the firstbeams of highly charged heavy ions, such as Xe and Kr_________________________________________*

higurashi@riken.jp

Figure 2: Beam intensity of Ar ions as afunction ofplasma electrode position.
As the plasma electrode makes the part of the boundaryof ECR plasma, it is natural to think that the position ofplasma electrode affects the condition of plasma. It also
affects the beam extraction conditions. Then, the beam
intensity may be increased finding the most suitable
position of the plasma electrode. Based on thisassumption, we tried to optimize the plasma electrodeposition to increase the beam intensity of highly charged
heavy ions. We have chosen different points (see Fig. 1)
and measured the beam intensity of Ar, Kr and Xe ions.
Figure 2 shows the beam intensity of Ar ions as a function

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003of the plasma electrode position. The beam intensity
gradually increases when moving the plasma electrodetoward the ECR zone and then decreases as shown in thisfigure. It seems that an optimum electrode position exists
to maximize the beam intensity. The optimum plasma
electrode position moves toward the ECR zone when thecharge state becomes lower. The same tendency can beseen in case of Kr and Xe ions.
Figure 3 shows the typical beam intensities produced
from gaseous elements at the extraction voltage of13~17kV and RF power of 700 W. We obtained 0.3emAof Xe20+, 0.6 mA of Kr13+ and 2mA of Ar8+ at only 700 Wof RF power.

dependent on the volume of the plasma chamber and themirror ratio of mirror magnetic field. It may conclude thatthe different confinement time of ions in plasma in thetwo ion sources brings the different results of the beam
intensity.

Figure 4: Beam intensities of Xe ions from SHIVA(closed circles) and CAPRICE (open circles) and Kr ion
from RAMSES (closed squares) and RIKEN 18GHzECRIS.
Figure 3: Charge distribution of Ar, Kr and Xe ionsproduced from RIKEN 18GHz ECRIS.

LIQUID HE⎯FREE SC-ECRIS
The detailed design of the ion source was described inref.[4]. The RF injection side coils generate a maximum
field of 3T. The beam extraction side coils generates amaximum field of 2T. The solenoid coils are cooled bysmall refrigerator. The radial magnetic field strengths on
the surface of plasma chamber is 1.2T. The operational
microwave frequency is 18 GHz. It should be stressedthat the plasma chamber volume (1.5 L) is much smaller
than those of other SC-ECRISs.
To investigate the effect of magnetic field strength andplasma chamber volume, we compared the beamintensities from the RIKEN 18 GHz and liquid He-freeSC-ECRIS. Figure 4 shows the beam intensities of Krions. Open and closed squares are the beam intensities ofKr ions from RIKEN 18 GHz and liquid He-free SCECRIS.
The obtained beam intensities of Kr20+,27+ from the SCECRIS were 40 and 5 eµA at 12 kV of the extraction
voltage and 600 W of the microwave power. In thisexperiment, the typical magnetic filed strengths of Binj,
Bmin and Bext of the SC-ECRIS were 2.0, 0.5 and 1.2T,
respectively. The beam intensities of highly charged Krions (25+, 27+) extracted from the SC-ECRIS are muchhigher than those from RIKEN 18 GHz ECRIS. Thoughboth ECRISs have the almost same chamber diameter(~7.2 cm in diameter) and the RF power was also almost
same, there are two essential differences between twosources, i.e., the plasma chamber length and its magnetic
field strength. The ion confinement time is strongly

It is well known that the magnetic field strength andshape at the beam extraction side and injection side
influence the charge distribution and beam intensity.[8]Furthermore it is natural to think that Bmin should
influence to the beam intensity of heavy ions from theECRIS[9]. Because the magnetic field configuration ( orgradient of magnetic field ) affects the plasma
confinement and the effectiveness of the electron heating
at resonance zone.

Figure 5: Beam intensities of Kr 15+,12+ as a function ofBmin.
For studying the effect of Bmin on the beam intensity
produced from the SC-ECRIS, we changed Bmin from0.25T to 0.6 T without changing Bext and Binj.[7] Figure 5
shows the beam intensity of Kr 20+,15+ as a function of Bmin.
In this experiment, Bext and Binj were fixed to 1.15 and 1.9T, respectively. The injected microwave power was 600

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

W. The extraction voltage was 12 kV. The gas pressure,the position of electrode and its negative bias voltage
were changed to maximize the beam intensity. The typicalgas pressure at Bmin=0.5 T was 3x10-7 Torr for Kr15+ ions.
We did not use the gas mixing method for simplifying theexperiment. The beam intensity gradually increased withincreasing Bmin up to ~0.49T and then gradually decreased.
Above 0.6 T of Bmin, the extracted beam became unstable.
The value of Bmin (optimum Bmin) for maximizing thebeam intensity of Kr15+ was almost same as that for Kr20+ions.
Figure 6 shows the summary of the experiment(optimum Bmin for multi charged O, Ar, Kr and Xe ions).
We measured the optimum Bmin at several conditions( Bext = 0.9~1.25T, Binj =1.72 ~1.9T ) for various heavyions.

Xe20+, the Bmin is slightly higher than the optimum value.
It means that if we can set the optimum value for Bmin
without changing Bext, we may increase the beam.

Figure 6: Optimum Bmin for various charge state of heavy
ions.

Figure 7: Bmin and Bext of RIKEN 18GHz ECRIS for
production of various charge state of heavy ions.

In these experiments, we observed that the existence of
optimum value for Bmin to maximize the beam intensity
and it is not dependent on the charge state of heavy ions.
The optimum value for Bmin was 0.47~0.49T for
production of various multi-charged heavy ions as shownin Fig.6.
RIKEN 18 GHz ECR ion source has only two sets ofsolenoid coils for producing the mirror magnetic. It meansthat we cannot control Bmin and Bext independently. Figure
7 shows the magnetic field strength of Bext and Bmin for
maximizing the beam intensity of various heavy ions. For
most of the experiments, we kept the Binj ~1.40T, which isthe maximum magnetic field strength of RIKEN 18 GHzECRIS. The dashed line is the optimum Bmin obtained
from the experiment using RAMSES. For production ofO5+, the Bmin is significantly lower than the optimum Bmin ,which is expected from the experiment with RAMSES.
This is likely one of the reasons why we obtained only
1.5mA of O5+. For production of heavier ions, such as[1] Y. Yano, Proceedings of the 15th International
Conference on Cyclotron and Their Application (IOP,Bristol, 1999)p696..
[2] T. Nakagawa, Jpn. J. Appl. Physics 30(1991)L930.
[3] T. Nakagawa and Y. Yano, Rev. Sci. Instrum.
71(2000)637 and references therein[4] T. Nakagawa et al, Rev. Sci. Instrum. 73(2002)513
and references therein.
[5] Y. Higurashi et al., Jpn. J. Appl. Phys. 42(2003)3656[6] M. Kidera et al, Rev. Sci. Instrum. 73(2002)611.
[7] H. Arai, et al, Nucl. Instrum. Methods A491(2002)[8] S. Gammino et al, Rev. Sci. Instrum. 70(1999)3577.
[9] D. Hitz et al., Rev. Sci. Instrum. 71(2000)839.

REFERENCES