The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

Liquid Film Stripper for RIKEN RI Beam Factory
H. Akiyoshi, T. Chiba, H. Hasebe, M. Kase and Y. Yano
Cyclotron Center, RIKEN
2-1 Hirosawa, Wako, Saitama 351-0198, JapanNTERFERENCE COLOR

Abstract

liNG DISC

A charge stripper system using a thin oil film was developed for high intensity heavy ion beams of RIKEN RI
beam factory. A stable free-standing film of silicone oilwith fairly large area was made by a rather simple system.
The design, performance, and preliminary results of low
energy beam test are described.

Figure I: Conceptual diagram of liquid film charge stripper.

1 INTRODUCTION
A charge stripper plays an essential part in heavy ion accelerators. Commonly used methods are foil stripping andgas stripping. Gas stripping has the advantages of causing
less energy straggling and less multiple scattering, and being able to withstand intense heavy ion beams, but has thedisadvantage of producing lower equilibrium charge states
than foil stripping. Foil stripping has the advantage of producing higher equilibrium charge states but has the disadvantage that it breaks down with short lifetimes under bombardment by high-intensity heavy ion beams according tostructural changes induced by radiation damage and possibly beam heating [1]. With higher intensity and high-mass
beams of RIKEN RI beam factory (RIB F), the foil lifetime
is expected to drop markedly to as low as one minute. Substantial progress has been made in developing carbon foil
production techniques, which has lengthened the foil lifetime more than 10 times as compared with foils fabricatedby conventional methods [2, 3]. Cramer et al. suggested adifferent approach to solving the stripper foil lifetime problem, which uses a thin free-standing oil film spun from theedge of a razor-sharp-edged rotating disc touching the surface of an oil reservoir [4]. Because the density of the oilfilm is roughly a half of that of a carbon foil, we can expect
higher equilibrium charge states. Leemann et al. developed
a liquid film stripper at the Lawrence Berkeley Laboratory
SuperHILAC based on the same concept [5]. Another liquid film stripper system based on the same concept wasdeveloped for high intensity heavy ion beams of RIB F.

icone oil with kinematic viscosity of 50 eSt was used assuggested in Refs. [4, 5]. The reservoir is suspended byslide rods so that the depth to which the disc is immersedin oil is adjustable over a range from zero to about 3 em. A'scraper' is mounted on a separate positioning mechanism
so that it can be moved ±2.5 em in horizontal direction
and ±4 em in vertical direction. The oil reservoir and the'scraper' are also placed in the vacuum chamber.
At first a disc such as those used in Refs. [4, 5] wasadopted, and observed the three types of films described inRef. [4]. When the disc was immersed very deeply in theoil and rotated at a very low frequency, a thick film of large
area was produced. This film exhibited no distinguishable
optical interference pattern. The next type of films showed·strong interference colors and was of small area. The third
type of films was observed to flicker in and out of existence, and showed a large area comparable to the first typeof films.

2 PRODUCTION OF LIQUID FILM
Figure 1 shows a conceptual diagram of the liquid film production apparatus. A disc with a diameter of 10 em in a
vacuum chamber is rotated by an AC servo motor which isplaced outside of the vacuum chamber through a ferrofluid
sealed rotary m<?tion feedthrough. The lower edge of thedisc is immersef in a reservoir of silicone oil. SH200 sil-Figure 2: Photograph of liquid film.
Films with strong interference colors and a large area ofmore than 50 cm 2 were successfully produced by changing,,-331-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

the shapes of the rotating disc and the 'scraper'. Figure 2
shows a photograph of the liquid film. A slightly slanting
dim white line at the upper part of the photograph shows
the upper edge of the liquid film. Higher rotating frequencies of the disc generally produced larger areas with thethinnest bands, as described in Ref. [5]. The 'scraper'
seemed not to play an essential part in producing the liquid film, but play an important part in prevention from destructive turbulence of oil in the reservoir. The stabilityof the film became worse when the rotating frequency wasincreased. One possible source of this instability is disc vibration. The displacement in the normal direction against
the disc was measured using an inductive displacement sensor, and found that it was as large as about 40 11m when thedisc was rotated at 1800 rpm.

lium ions which passed through the liquid film is somewhat
lower than 500 ke V because of the energy loss in the liquidfilm. Therefore, roughly speaking, 4 He+ and 4 He2+ ionswith two energies go out of the stripper system.
The helium ions emerged from the stripper were injected
into a parallel plate electrostatic deflector, and split according to the energy and the charge states. A beam stopper wasplaced behind the electrostatic deflector. The beam stopper was divided into nine blocks in each two degree, andthe current of incident helium ions on each block was leadout separately. The currents were measured by varying thevoltage of the deflector.

!?/~filml+3 BEAMTEST
A low energy beam test of the liquid film stripper was performed by the Cockcroft-Walton injector ofRIKEN heavy
ion linac (RILAC). 4 He+ ions were accelerated to 500 keV
and focused on a liquid film. 4 He+ ions undergo electron
loss and capture processes while passing through the liquid film. 4 He+ and 4 He2+ ions which emerged from thestripper were separated by a parallel plate electrostatic deflector, and the currents were measured by a beam stopper
which was divided into nine blocks.

LIQUID FILM

BEAM STOPPER>

12Figure 4: Sorted helium ions by electrostatic deflector according to their energy and charge states. The horizontal
axis indicates the deflection angle, and the vertical axis indicates the voltage of the deflector.

Figure 3: Schematic layout of experimental setup.

3.2
3.1

Experimental Procedure

Figure 3 shows a schematic layout of the experimental
setup. 4 He+ ions at 500 keV were focused on the thinnest
area of a liquid film by observing the fluorescent image ofthe beam on a quartz plate in front of the liquid film. Thedistribution of thickness of the liquid film was observed by
an optical interference pattern of visible rays. The shape ofthe beam spot was nearly spherical, and the diameter was
about 6 mm. A blue fluorescent image of the beam wasalso observed on the liquid film itself when a high intensity
beam was bombarded. It is probable that a certain numbersof 4 He+ ions go without passing through the liquid film
because the beam was focused nearly on the upper edgeof the film. When beam intensity was increased, the upperedge of the film was broken like a waterfall into which a log
was thrust. It increases the numbers of helium ions which
go without passing through the liquid film. Some of thehelium ions are likely to be charge exchanged by oil mist
or other residual gases without passing through the liquidfilm. The energy of these helium ions is 500 keV as sameas that of incident helium ions, while the energy of the he-Preliminary Result

Figure 4 shows the differences of the deflector voltages according to the energy and the charge states of the helium
ions coming to each block of the beam stopper. The datapoints roughly form four lines. These lines are supposed
to be corresponding to, from the upper line to the lowerline, the 4 He+ ions which did not pass through the liquid film (first line), the 4 He+ ions which passed throughthe liquid film (second line), the 4 He2+ ions which did notpass through the liquid film (third line), and the 4 He2+ ionswhich passed through the liquid film (fourth line). Therefore one can estimate the thickness of the liquid film at thebeam position from the difference between the first and second lines. The interpretation of the lines was checked byobserving the data points of 80 11glcm 2 carbon foil [6] lay
closely on the second and fourth lines which had been fitted to the data points of the liquid film produced by 1600rpm disc rotation.
The thickness of the liquid film varies as a function of
the rotating frequency of the disc. Derived thicknesses areplotted on Fig. 5 as a function of the rotating frequency ofthe disc. When the disc is rotated at 1500-1800 rpm, thethickness of the liquid film at the thinnest area is suitable-332-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October2001

175more than 50 em 2 was made. The thickness of the liquidfilm is suitable to be used as a stripper for the ions with energies of about 4-6 MeV/u when the disc is rotated at 15001800 rpm. Further modification should be made in order tobe used for a high intensity heavy ion beam ofRIBF. We arepresently preparing a higher energy beam test at RILAC inorder to measure the equilibrium charge state distribution
and obtain more practical information about the durability
against high intensity heavy ion beams .

•150"E'3.

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75

5 REFERENCES

•~~
1200

1400

1600
f1800

(rpm)

Figure 5: Thickness of the liquid film as a function of rotating frequency of the disc.
to be used as a charge stripper for the ions with energies ofabout4-6MeV/u [1].
101

[I] E. Baron, IEEE Trans. Nucl. Sci. NS-26, 2411 (1979).
[2] I. Sugai, M. Oyaizu, H. Kawakami, C. Ohmori, T. Hattori,K. Kawasaki, M.J. Borden, and R.J. Macek, Nucl. Instrum.
Methods A 362, 70 ( 1995).
[3] H. Hasebe et al., in this proceedings.
[4] J.G. Cramer, D.F. Burch, R. Rodenberg and P.B. Cramer,Nucl. lnstrum. Methods 185, 29 (1981 ).
[5] B.T. Leemann, P. Merrill, H.K. Syversrud, R. Wada, R.B.
Yourd, IEEE Trans. Nucl. Sci. NS-28, 2794 ( 1981 ).
[6] ACF-Metals, The Arizona Carbon Foil Co., Inc.

10010_,
0~w-2

w-310-41020

Beam Intensity (Jl..A)

Figure 6: The current of 4 He2+ ions which passed throughthe liquid film divided by the current of 4 He+ ions which
did not pass through the liquid film as a function of beam
intensity. The disc was rotated at 1500 rpm.
Figure 6 shows ratios of the current of the 4 He2+ ionswhich passed through the liquid film to the current of the4 He+ ions which did not pass through the liquid film inthe case of 1500 rpm disc rotation. As beam intensity wasincreased, the upper edge of the film was broken. It drastically decreases the ratio as is seen in Fig. 6. The limit
value of beam intensity is estimated to be about 10 pA,
which corresponds to a power density of 8 W/cm 2 . If a
5.85 MeV/u 136 Xe beam has the same beam spot size, thislimit value corresponds to the beam current of 300 pnA. In
the case of0.9 MeV/u 238 U, the limit value corresponds to200pnA.

4 SUMMARYA liquid film stripper using free-standing film of silicone
oil was developed. A liquid film with fairly large area of-333-