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

Development of polarized 3He ion source at RCNP
M. Tanakaa* , T. Yamagatab, K. Yoneharac, Y. Arimotod, and N. Shimakurae,a Kobe Tokiwa College, Ohtani-cho 2-6-2, Nagata, Kobe 653-0838, Japan
Department of Physics, Konan University, Okamoto 8-g-1, Higashinada, Kobe, 658-8501, Japanc Department of Physic, University of Michigan, Ann Arbor, Michigan 4810g, USA.
d Japan Synchrotron Radiation Research Institute (JASRI), Kouto 1-1-1, Mikazuki-cho, Sayo, Hyoto 61g-51g8, Japane Department of Chemistry, Niigata University, Ikarashi Nino-cho 8050, Nngata 950-2181, Japanb

A long history is presented on the polarized 3 He ion source
being developed at RCNP for nuclear physics research at anintermediate energy region. A particular emphasis was placedon how we solved serious problems encountered in each phaseof the development by employing delicate nature of atomicphysics.

INTRODUCTION

Over the decade ago a development of a polarized
3He ion source started at RCNP, Osaka University primarily for nuclear physics research at an intermediate energy region. We started constructing a device based on a•J!
.
polarized electron capture reaction
wra £as t 3He2+ Ion
incident on a polarized alkali vapor at first. Though thiswas a start of painful development, we could finally obtain many fruitful results. In this paper, we will present
why we should overcome many difficulties and how wecould overcome them.

OPPIS (PHASE 1]

We initiated to develop a polarized 3He ion source
based on a polarized electron capture by a fast ion [1]later named, "OPPIS" (Optical Pumping Polarized IonSource). The reason why we chose this was because
the OPPIS revealed great success in producing a polarized proton beam with high intensity and high polarization [2,3] relative to other methods such as an AB(Atomic Beam) or a direct 3He pumping method [4].
The 3He+ OPPIS ion source applied to the 3He polarization uses an electron capture between an incident
fast 3He2+ ion and a polarized alkali atom optically
pumped [5-9]. After completion of a bench test device,
we measured the 3He+ nuclear polarization by varying
a hepp impact energy for optimization of the 3He OPPIS. The result showed a gentle decrease with an incident
3He2+ energy. This behavior was reasonably reproducedby the theory based on the semi-classical impact parameter method [10,11]. Observed absolute values of the3He nuclear polarization were, on the other hand, greatly
reduced relative to the alkali atomic polarization. Thiswas in striking contrast with the proton OPPIS, where almost no proton depolarization was observed with respect
to the alkali polarization [3]. The origin of this reduction

was understood in terms of an insufficient LS decoupling
field for a polarized 3He+ ion; the LS decoupling field
needed for hydrogen is only about 2 T, whereas that for
3He+ ion was estimated to be over 30 T [12].
In addition to the above discouraging result, we met
another serious problem; an extracted beam intensityfrom the 3He OPPIS [13] was greatly reduced. Thiswas simply understood in terms of a transversal emittance growth due to fringing fields of the solenoidal coilat both ends [14].

EPPIS (PHASE 2]

This urged us to discover an alternating idea whichuses multiple electron capture and stripping collisions
between an incident 3He+ ion and a polarized Rb vapor [15]. Since this method uses multiple charg.e char_rging .collisions between a he atom and 3He+ Ion withalmost no contribution from a 3He2+ ion, the LS decoupling could be accomplished with a practically av~ilable
magnetic field( ...... a few T). This eventually results m theexpectation that most of the alkali polarization would beconverted to the 3He nuclear polarization. We named
this novel method an "electron pumping" from analogyof the optical pumping. After an enormous effort, wesucceeded in experimentally proving validity of the electron pumping [16]. Later, we named this polarized source"EPPIS" (Electron Pumping Polarized Ion Source).
Another principal superiority of the EPPIS over theOPPIS is a suppression of emittance growth since a
3 He charge state is the same, i.e., +1 when the ion is incident on and emerging out of the solenoid coil. Nevertheless there may be additional emittance growth inducedby the multiple stripping and capture collisions underthe strong magnetic field. This may further influence aspatial distribution of the 3 He nuclear polarization. Tosee these effects more closely, we carried out the Monte
Carlo simulations. We could find that no sizable emittance growth occurs thanks to the fact that a penetrating
3He spends a substantial time with a 3He atomic state
free from a magnetic field [17]. On the other hand, concerning the polarization distribution of the 3 H~+ be~m,a hole having a less polarization the surroundmg regwn
was predicted, which was later named a "polarizationhole" [18]. These findings would be beneficial not only
to design a practical ion source but also to study plasma- 40-

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

Though they could not theoretically reproduce the experimental result at first, they finall succeeded in reproduction by a sophisticated theory as described in the following:

physics and probably astrophysics.

0.7 , . . - - - - - - - - - - - ,0.6_..-..,!

0.5

Fast component

Two states calc .

...... /Eight states calc.-"'Slow component
0"3 J·7o ---:-JO'-:.l---,J0'-:.2---,.Jl0 .3

t (ms)
FIG. 1. Rb polarization (arbitrary unit) is observed as afunction of time. Pumping laser is switched off at t = 10.07
ms, where an external magnetic field is 4T, and Rb cell temperature is 125 °C.
In spite of great success in the EPPIS, we met a somewhat serious problem. The EPPIS requires a polarized
Rb vapor with thickness higher than 1014 cm- 2 , an orderof magnitude higher than that required for the OPPIS ion
source [15,16]. However, fabrication of highly polarized
Rb vapor with such a high thickness is not easy. In addition, no basic study has been systematically done on thissubject. In our work, we studied the relaxation mechanism by a time differential measurement with a chopped
pumping laser [19]. One of the remarkable results was anobvious presence of a fast decaying component in addition to a well established slow component caused by thewall relaxation, diffusion, effusion and so on as shown in
Fig. 1. This new component was understood in terms offormation of a sheath with higher polarization due to aradiation trapping predominated at high vapor thickness.
We found that the radiation trapping and absorption of
the pumping light make it difficult to attain a highlypolarized Rb vapor homogeneously distributed, which isindispensable to the EPPIS.
SEPIS [PHASE 3]

This urged us to devise a new polarization principle
which does not use a thick Rb vapor. A new idea whichmeets the above requirement was proposed after a detailed analysis of our measurement of the electron pumping [21]. In their work, the spin exchange cross section
could be deduced from the observed electron pumping
effect (a closed circle with an error bar in Fig. 2-(a)).

. ..

.······... ·w· 2~··

Target excitation
l->4

...
10- 1

1

10Impact Energy ( keV/amu )FIG. 2. (a) Calculated spin exchange cross sections plotted
as a function of impact energy of 3 He+ ion, where the dashedcurve is those calculated with the two states, and closed triangles and solid curve are those calculated with the eightstates. (b) Calculated cross sections for the charge transfer(open triangles and a dot-dashed curve) and target excitation(open diamonds and a dotted curve) channels. The detail isexplained in ref. [20].
The atomic collision theory based on the semi-classicalclose-coupling method allowing inclusion of the charge
exchange and target excitation processes unexpectedly
predicted an anomalously large spin-exchange cross section between a 3 He+ ion and a polarized Rb atom inparticular at a 3 He+ impact energy lower than a few
100 eV /amu (a solid curve in Fig. 2-(a)) [20]. This is in
striking contrast with the behavior of the hydrogen spinexchange cross section. Since our new method uses anenhanced spin-exchange cross section at a low 3 He+ incident energy, we named it SEPIS (Spin Exchange Polarized Ion Source). Since the SEPIS requires neither an
un-practically large magnetic field nor an extremely thickRb vapor, the 3 He ion SEPIS will hopefully be one of
the most practical polarized 3 He ion sources in the nextgeneration.

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

CONCLUSION AND FUTURE PROSPECT

After a long journey we have reached the concept ofthe SEPIS. We believe that the SEPIS is one of the mostpromising polarized 3 He ion source ever proposed. Anapplication of the SEPIS should not be restricted in nuclear physics regime but must be extended to particle
physics such as RHIC or DESY. In Fig. 4, a schematic
view of a practical polarized 3 He ion source based on theSEPIS is illustrated.

FIG. 3. A schematic view of the SEPIS ion source

Since the SEPIS ion source requires neither an unpractically large magnetic field nor an extremely thickRb vapor, the 3 He ion SEPIS will hopefully be one of
the most practical polarized 3 He ion sources in the nextgeneration.

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