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

STORAGE OF BIO-MOLECULAR IONS IN AN ELECTROSTA TIC
STORAGE RING
T. Tanabe, KEK-High Energy Accelerator Research Organization, Oho, Tsukuba 305-0801, Japan
K. Noda, National Institute of Radiological Sciences, Anagawa, Chiba 263-8555, JapanI. Watanabe, Toshiba Corporation, Keihin Product Operations, Yokohama 230-0045, Japan

Abstract
The cooler ring TARN IT, which had been used to study
the recombination of atomic and molecular ions withelectrons since the late 1980s, was shut down in 1999
along with the closing of the Tanashi branch of KEK. To
replace TARN II, a new electrostatic storage ring wasbuilt and a beam was first stored in May 2000 at the
Tsukuba site of KEK. The ring has a circumference of 8m and can store light-to-heavy ions with an Elq of up to
30 keV. Light ions are produced with an electron
cyclotron resonance ion source, while bio-molecular ionsare produced with an electrospray ion source. The
measured lie-lifetimes of stored single-charge ionsinjected from the ECR ion source are from 11 to 50 s. On
the other hand, ions from the electrospray ion source have
lifetimes from 12 to 20 s. In order to study electron-ion
collisions, a compact electron beam target has been
designed, which will be installed in a straight section of
the ring.

1 INTRODUCTIO N
Since the late 1980s, many storage rings with electron
coolers have been operational, some of which have beenused for studying atomic physics. For atomic physics
using these cooler rings, many new phenomena weredissociative
theconcerningespeciallyfound,
recombination of molecular ions with electrons [1]. The
object of the present research is being directed to heavier
molecular ions. One of the difficulties in studying lowcharge and heavy molecular ions with the usual magnetic
storage rings is the increasing magnetic rigidity with ion
mass ( m ), because the magnetic rigidity is proportional to~mE I q . Here, E and q are the ion energy and charge,
respectively. An electrostatic storage ring is more suitable
for such ions because the electrostatic rigidity is
proportional to E I q and is independent of the ion mass at
a given energy. There are also many advantages ofelectrostatic storage rings, such as compactness, ease of
construction and good reproducibility. The ring is
expected to be very useful for the research of atomic andmolecular science in the future. Pioneering work for theelectrostatic storage ring was performed by S.P.M~ller(the ring ELISA in Aarhus [2]), where sphericaldeflectors were used to balance the horizontal and verticalfocusing forces in the deflectors. This focusing resulted invery narrow waists in the middle of the deflectors.

However, it was observed that the beam lifetime
decreased greatly with increasing beam intensity [3]. In
order to avoid such a difficulty, we designed a ring using
cylindrical deflectors instead of spherical deflectors [4].
The lattices using the cylindrical deflectors allow both
solutions, with and without waists, in the middle of thedeflectors.
It is very attractive to expand the research object fromlight molecular ions mainly studied so far to heavy
macro-molecular ions. In order to study the feasibility ofthe storage of bio-molecular ions in the ring, anelecrospray ion source accompanied by an ion trap wasconstructed.
An electron-beam target is indispensable for studying
electron-ion collisions. In magnetic storage rings, electron
coolers have been used to compress the phase space
occupied by beams and at the same time as an electron
target for electron-ion collision research. On the other
hand, in the electrostatic ring, the electron cooling device
can function mainly as an electron-beam target except
light-ion beams, because the ion velocity is notnecessarily high enough to cool the heavy-ion beams in
the longitudinal directions. An electron-beam target
which is a compact electron cooler has been designed.
In this paper, we report on the design of the ring and theresults of beam tests for the light ions and bio-molecularions produced by an electron cyclotron resonance (ECR)
ion source and an electrospray ion (ESI) source,
respectively.

2 LAYOUT OF THE EQUIPMENT
The layout of the ion-sources, the beam-injection
system and the storage ring is shown in Fig. 1. Lightatomic and molecular ions are produced in a compact
ECR ion source [5] with an acceleration voltage of 20 kV,
and are injected into a beam-analyzing dipole magnet. A
momentum-analyzed beam is then injected into the ring
through a matching section consisting of an electrostatic
quadrupole triplet. On the other hand, bio-molecular ionswith an energy of 30 keVIq are produced by the ESI
source and injected into the ring after being bunched andmass-analyzed. The mass analyzer consists of a sphericaldeflector, a sextupole magnet and a dipole magnet which
also serves to analyze the momentum of ion-beams fromthe ECR source. The ECR and ESI sources can be usedalternatively without breaking the vacuum in the merging
section.

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

Figure 1, Entire layout of the ion sources, the mass
spectrometer and the electrostatic storage ring.

3 ELECTROSPRAY ION SOURCE
A schematic drawing of the ESI source and an ion trap
is shown in Fig. 2. The ESI source (JEOL, model API15,
Japan) was installed on a high-voltage terminal. A sample
solution at flow rates of about 5 J.lllm enters the ESI
source chamber through a stainless-steel capillary. The
capillary is electrically biased at a few kilovolts relative toa counter electrode. The resulting field at the capillary tip
charges the surface of the emerging liquid, dispersing it
by Coulomb forces into a fine spray of charge droplets.
Driven by the electric field, the droplets move towards thecounter electrode. A flow of nitrogen gas at a flow rate ofabout 5 lim assists to nebulize the solution and hasten
evaporation of the solvent from each droplet, decreasing
its diameter as it drifts towards the orifice of the counterelectrode. Consequently, the charge density on the droplet
surface increases until the Coulomb repulsion exceeds thesurface tension, resulting in producing charged daughter
droplets and ions by Coulomb explosion. The desolvatingplate, heated at a temperature of about 200 °C, helps todry the solvent. A syringe pump controls the flow of thesolution, typically a water-methanol mixture, containing
the sample.

TMP

order to independently apply a DC voltage superimposedon the RF voltage to each electrode. This makes it
possible to form an axial confinement field during
accumulation, and also to form an axial gradient field
during ejection, in collaboration with the entrance andexit electrodes. The ions trapped into the potential well
are thermalized through collisions with He gas fed into
the trap at a flow rate of about 0.1 cc/m. After
accumulation for a period of a few seconds, ions are
released by abruptly changing each DC bias with aswitching time of about 0.4 JlS and transferred to anotherquadrupole RF ion-guide with a drift length of 14 em. The
thus-obtained bunched beam is then accelerated up to 30
keY/charge by an accelerator tube. The trap and guides
are operated at a frequency of 1 MHz and a voltage of 60500 VPP'
The ion source is evacuated by a 500 lim rotary pump
and a 210 /Is turbo-molecular pump. Furthermore, two
500 /Is turbo molecular pumps are added to evacuate theion guide and trap region. The typical vacuum at thesecond ion guide is about lxl0- 6 Torr. The entire system
is installed on a high voltage terminal and controlled byfiber optical links.
The ion trap and the beam bunching system
dramatically increased the pulse beam current. A pictureof the ESI source is shown in Fig. 3.

TMP

Figure 2. Schematic drawing of the electrospray ion
source, ion guides and an ion trap.
Ions created by the ESI source are then conveyed to theentrance of a linear octupole RF ion-trap having an
inscribed radius of 4 mm and a length of 15 em by way of
a quadruple RF ion-guide, as shown in Fig. 2. The iontrap accumulates ions initially, and then ejects the trappedions quickly in order to transfer them to the storage ringas a bunch. Trap electrodes composed of 8 rods of 3 mm
in diameter are electrically isolated into five pieces, in-156-

Figure 3 . Photograph of the electrospray ion source.

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

4 MASS SPECTROMETER
In order to mass-analyze macro-molecular ions fromthe ESI source, a mass spectrometer was designed whileaiming at a mass resolution of 5000. The spectrometer iscomposed of a spherical deflector with a bending radiusof 119 em and a homogeneous field dipole magnet with abending radius of 120 em and an entrance and exit angleof 26.5°, as shown in Fig. 1. This constitutes a doublefocusing mass-analyzing system where the final image isindependent of both the velocity and the direction spreadamong the ions emerging from the object slit. A sextupole
magnet was installed in front of the dipole magnet, whichhelps to compensate for the second-order aberration. The
magnet was also used to momentum-analyze ions fromthe ECR source.
The vacuum pressure at the ion source is high, while it
has to be extremely low in the storage ring. A highvacuum was attained by a differential pumping system.
The typical vacuum pressure in the ion source is lx10-6
Torr, while the vacuum in the ring is 3x10- 11 Torr withbeam loading.

5 ELECTROSTATIC STORAGE RING
The layout and a picture of the ring are shown in Figs. 4
and 5, respectively and the main parameters are listed inTable 1. A race-track lattice consists of two 160°
deflectors, four 10° deflectors, which allow beam
injection and extraction, and four quadrupole doublets.
For the 160° deflectors, a cylindrical shape has been
adopted. In this ring, there are four types of stable regionswith and without waists of the beam envelope in the
middle of the deflectors.
The type of beam injection is single-tum injection.
During injection, the high voltage for the first 10°
deflector is turned off, and after filling the beam in thering it is turned on. The rising and falling times of thehigh voltage power supplies used in the injection areabout 0.1 ~s. which is sufficiently short compared withthe circulation time of ions (for example, 17 ~s for 20keV o+)

Figure 5 _Photograph of the electrostatic storage ring.

Table 1. Design parameters for the electrostatic storagering
Maximum energy
30 keV/charge
Circumference
8.136 m
Revolution time
14 ~s (0+, 30 keY)
160° cylindrical deflectors
Central radius
250mm
Gap
30mm
Nominal voltage±3.6kV10° parallel plate deflectors
Gap
60mm
Dt
Ion Beam+V·STNeutral Beam Length
92mm
Nominal voltage±2.6kV
Quadrupoles._E=--e&-!Dz
Inscribed radius
25mm
Length100mm
N~tralBeam V-ST
detector
Nominal voltage+0.75 kVlm
RF
Figure 4. Layout of the electrostatic storage ring. D 1
Drift tube length
203mm
and D2 indicate 10° and 160° deflectors, QF and QD the
Voltage<lOYhorizontally focusing and defocusing electrostatic
quadrupoles, PM-H and PM-V the horizontal and vertical
The beam can be bunched with an RF electrodeposition monitors, RF the drift-tube RF system and V-ST
consisting of a 20-cm long drift tube which is driven
the vertical steerer.
directly by an RF signal generator. The bunched beam isused for monitoring the intensity of the circulating beam
current and for measuring the beam position._~~~:~~-157-

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

Several kinds of beam diagnostic elements are installedin the straight sections, as shown in Fig. 4. The injectedbeam profile is observed destructively with a viewer. The
injected beam current is accurately measured by a
Faraday cup equipped with an electron suppressor. Four
horizontal and vertical electrostatic position monitors
provide position information concerning bunched beams
nondestructively. A beam scraper defines the maximum
beam size both horizontally and vertically, which can also
be used for the beam position and profile measurements.
A vacuum pumping system is composed of four ion
pumps and six titanium sublimation pumps. The entire
ring is bakeable at a temperature of less than 300 °C. The
vacuum pressure of the ring is on the order of 10-11 Torr.
A neutral beam produced by collisions with the residual
gas is measured by a micro-channel plate with a phosphor
anode (32 mm in diameter) installed in a vacuum
extension 1.25-m downstream of the 10° deflector, as
shown in Fig. 4. The count rate of the neutral beam is
proportional to the number of stored ions at a constantresidual-gas pressure, which gives exact information
about the beam lifetime. Furthermore, a projection of thecirculating beam can be observed from the neutral beamprofile on the phosphor screen. The position and size ofthe neutral beam profile are useful for adjusting the ringparameters.

6 STORAGE OF ATOMIC AND
MOLECULAR IONSThe 20-keV beams from the ECR ion source weremomentum-analyzed (typical momentum resolution:
ap/p-10' 3) and injected into the ring after being chopped.
The injected beam size observed by the viewer is about 5
mm in diameter.
The change in the number of stored atomic ions withtime is expressed by N = N0 exp( -t I "C), where "C is the
lifetime. If the ring is operated at a stable working point,
the finite lifetime of ions in the ring is mainly determinedby interactions with the residual gas. First, ion beams are
multiply scattered by the Coulomb field of the residualgas. The ions of which direction of motion exceed an
acceptance angle of the ring are lost. The 1/e lifetime,( "C MS ), for losses due to multiple scattering is proportional

multiple scattering, and those due to capture and strippingbyl/ "C = 1/ "CMs + 1/ "Ccap + 11 "Cstrip •We studied the characteristics of a stored beam at aworking point in the four stable regions for 20-keV o+.
The lifetimes depend greatly on the working points. The
difference in the lifetimes has been deduced to come fromthe difference in the multiple-scattering lifetimes, which
depend on the maximum acceptance angle. In fact theworking point giving the longest lifetime has the largest
acceptance angle among the four stable regions.
The dependence of the coasting-beam lifetime on theinjected beam current was measured for a 20-keV o+beam. At low currents, beams decay almost exponentiallywith a lifetime of about 12 s, while at higher currents, thebeams decay faster initially. The lifetime does not depend
much on the intensity at a current of less than about 100
nA, which corresponds to 1 x 107 injected particles. This
current is tolerable for practical use, like in atomic
collision experiments. A rapid decrease in the beam
intensity observed at high currents, which does not obey
an exponential decay law, seems to be caused by intrabeam scattering, which is the scattering between beam
particles. In the ELISA ring in Aarhus, sphericaldeflectors were recently replaced by cylindrical deflectors,
and a similar lifetime dependence on beam intensity has
been reported [7].
So far, we have stored atomic and molecular ions of N+,
0+, Ne+, Ar+, Xe+, N2+, NO+, 0 2+, and H20+ at 20 keV.
These ions are stored at the same voltages for the
deflectors and quadrupoles. The decays of the beam
intensity for atomic ions are shown in Fig. 6.
to• ~--.-..........--,---.--.~..,.......,,..--.-..--T',--...........--.-...-,-.-....-.--.--:j

to'

to'

to e~ax• where E>maxis a maximum acceptance angle [6].
Second, ions which change their charge state by capturing
or stripping electrons deviate from the central orbit andare lost. The lifetimes by capture, ( "Ccap), and stripping,

0

30

20

40

50

Time (s)Figure 6. Time dependence of the neutral beam
production rate for 20-ke V atomic ions at an injectedcurrent of 10-20 nA. The vacuum pressure was about
3x10- 11 Torr. The decay curves are arbitrarily scaled inmagnitude to facilitate a comparison [4].

( "C strip), can be expressed by the cross sections ( G cap andG strip ) as "Ccap =11 pvacap and "Cstrip = 11 PVCJ strip •respectively. The cross sections for the residual gas
interactions of atomic ions are almost energy-independent
at these low energies. Therefore, the lifetimes increase
with a decrease in the ion velocity. Unlike in the case ofmultiple scattering, the lifetimes due to the chargechanging process do not depend on the acceptance angle.
The entire lifetime ( -r) is related to the lifetimes due to

10

The lifetimes increase with the mass numbers (A). Thisis partly due to the fact that the lifetimes ( 't'MS , 't' cap and. ) are all proportional to .fA for a fixed energy,"CStrip
although they also depend on the cross sections.

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

7 STORAGE OF RIO-MOLECUL AR IONS
The ions from the ESI source were accumulated in thetrap for a period of a few seconds. The ions were thenejected in a bunch and accelerated to an energy of 20
keY/charge. After being mass-analyzed, the ions were
injected into the ring. Figure 7 shows the neutral particle
production rate as a function of the storage time for the
singly and multiply charged positive ions of the arginine
dimer, reserpine, angiotensin, insulin chain B and insulin
from bovine pancreas. As can be seen in the figure, thebeams have lifetimes of 12 s- 20 s, which are surprisingly
long because large bio-molecules have usually been
assumed to be fragile, resulting in a short surviving timein collisions with residual gas. The cross sections for thecollision with residual gas deduced from the beam
lifetime are on the order of w- 15 cm2 •

!o'

1 lo'

.

parameters are: interaction length, 20 em; electron beam
diameter at the interaction region, 20 mm; maximum
energy, 100 eV; maximum current, 2 rnA; maximum
solenoid field at the gun, 1 kG and maximum solenoid
field at the interaction region, 100 G. This will be
installed in one of the straight sections of the ring (see Fig.
4). Steering of the ion beam at the toroidal field iscorrected by a pair of electrostatic horizontal steerers at
the entrance and the exit of the electron beam device.

9 CONCLUSION
It was shown that an electrostatic storage ring can beused to store atomic and molecular ions, including biomolecular ions, at sufficiently long lifetimes withmoderate intensities. The lifetimes are long enough tocool vibrationally excited ions for those molecules with
dipole moments. The ring can store ion beams
iridependent of their masses. Therefore, it is especiallysuitable for studying macromolecules, like bio-molecularions. If we introduce electrons into the ring, they can open
up a way to conduct research on atomic collisions. Thus,
the ring is very attractive for studying chemistry andbiology as well as physics.

$

~

~

10 ACKNOWLEDG MENTS
10'

10'

10'
0

10

w

w

~

50

T (s)

Figure 7. Time dependence of the neutral beam
production rates for 20-keV/charge bio-molecular ions:(a) arginine dimer (charge, 1+; M.W.,-331), (b) reserpine(1 +; -609), (c) angiotensin (1 +; -1298), (d) insulin chain
B (3+, -3499) and (e) insulin from bovine pancreas (5+;-5738). The 1/e lifetimes are indicated in the figure. The
vacuum pressure was about 4xl o- 11 Torr. The decay
curves are arbitrarily scaled in magnitude to facilitate acomparison.

8 DESIGN OF AN ELECTRON BEAMTARGET
The electron beam target can also function as a beamcooler as well as an electron target for light ions [8]. The
structure is almost the same as the adiabatic-expansiontype electron cooling device [9]. Electrons are emitted
from a thermo-cathode with a diameter of 5 mm (3.5 mm)
and guided in a uniform solenoid field after accelerationand expansion by a factor of 16 (32). The main design

The authors thank R.Arakawa for valuable discussions
on the design of the ESI source, E.Syresin for importantsuggestions on the design of the electron beam target,
M.Wada for discussions on the design of the ion trap,
K.Chida for vacuum designing, and M.Yoshizawa for
analyzer controlling. This work was performed under a
Grant-in-Aid for Scientific Research (A) from the
Ministry of Education, Culture, Sports, Science andTechnology.

11 REFERENCES[1] T.Tanabe et al., Phys. Rev. Lett. 70 (1993) 422.
[2] S.P.M~Z~ller, Nucl. Instr. and Meth. A 394 (1997) 281.
[3] S.P.M~Z~ller, U.V. Pedersen, Proceedings of the 1999
Particle Accelerator Conference, New York, 1999, p.
2295.
[4] T.Tanabe, K.Chida, K.Noda, !.Watanabe, Nucl. Instr.
and Meth. (2001), in press.
[5] M.Sekiguchi, H.Matsushita, IJonoshita, M.Kabasawa,
Rev. Sci. Instrum. 69 (1998) 837.
[6] D.Habs et al., Nucl. lnstr. and Meth. B43 (1989) 390.
[7] S.P.M~Z~ller, P.Bowe, J.S.Nielsen, U.V.Pedersen,
Proceedings of the 7th European Particle Accelerator
Conference, EPAC2000, Vienna, 2000, p. 788.
[8] E.Syresin, private communications.
[9] T.Tanabe et al., Nucl. lnstr. and Meth. A441 (2000)
326.

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