The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Accel-Dec el Strong Focusing Method for a Tandem Accelerat orKenshi SAGARA, Takao NAKASHIMA, Masako NAKAMURA, Haruki URABE,Takanori YAGITA, Hikaru OKUDA, Nobuo IKEDA, and Shunpei MORINOBU
Department of Physics, Kyushu University
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan

Abstract
A new acceleration method for a tandem accelerator wasdesigned and tested to obtain a high-intensity low-energy
beam for an astra-nuclear experiment. The method uses
accel-decel strong focusing. In a normal tandem accelerator
the electric potential uniformly increases to the maximum
voltage V, however, in the new method the potential goesup to V abruptly in a short length (=1/5 of the total length),then goes down to V /4 in the same short length, again goesup to V and down to V/4, and finally goes up to V. The
beam is strongly focused due to alternating focusing anddefocusing actions. The transmission of low-energy 12C
beam was found to be an order of magnitude increased bythis method.

1 Introductio nAstra-nuclear reactions take place at low energy and theircross sections are very small due to the Coulomb barrier.
To measure such reactions we need very high-intensity lowenergy beams together with high-efficiency detectors andparticular targets.
At Kyushu university tandem accelerator laboratory(KUTL), an experiment on 4 He(1 2C, 16 0)y reaction downto E(l 2C) = 2.8 MeV (Ecm = 0.7 MeV ) is planned. This
reaction is a famous key reaction in astrophysics and many
experimental attempts have been made so far, however, noprecise data have been obtained yet. The cross section ofthe reaction is very small as 1 pbarn at Ecm = 0.7 MeV.
First, we use the inverse kinematics to detect 16 0particles in all the c.m. solid angle, instead of detecting yrays in a small solid angle. Therefore the detection
efficiency is very high as about 35 % which is the fractionof 3+ charge state of 16 0. Second, the 4 He gas target has tobe thick enough and windowless because of the low-energy
experiment. A new high-performance windowless gas target
has already been developed at KUTL [1]. The target is now
10 Torr x 3 em thick, and it will be made thicker as 25 Torrx 3 em which is practically the maximum thickness because
the energy spread of the 12C beam and of 16 0 recoil should
be below a certain level.
The last way to increase the counting rate in theexperiment is to increase the beam intensity. To make the4 He(l2C, 16 0)y experiment at Ecm = 0.7 MeV with 10%
statistical accuracy in one month, we need a high-intensity
2.8 MeV 12 C beam of about 10 particle ~-tA. Higher beam
intensity is more preferable.
The beam optics of KUTL 10 MV tandem accelerator isdesigned for the operation above 6 MV. When the terminal

voltage is decreased from 6 MV to 1 MV, the beamtransmission through the tandem accelerator becomes worse
as 1110 or less.
So far in the operation above 6 MV, the 12 C beam
intensity of about 100 nA has been obtained. The beam
intensity is limited by the electric loading due to thesecondary electrons in the accelerator tube.
To get a 12 C beam of 2.8 MeV, the terminal voltage is1.35 MV or 0.9 MV provided that the 12 C beam of 0.1
MeV is injected into the tandem accelerator. At this low
voltage, the loading becomes weak, however, the beamtransmission becomes much worse due to weak focusing.
Therefore to obtain 10 P!!A 12 C beam has been considered
as an impractical hope.
Our new idea of application of accel-decel strongfocusing to the tandem accelerator turns the impracticalhope to a practical plan. The beam transmission in theaccel-decel operation at 1 MV is estimated to be higher than
the transmission in the normal operation at 6 MV. Besidesthe electric loading caused by the 10 P~-tA beam can bestably controlled. The new method is described in thisreport.

2 Strong focusing in tandem accelerator
2.1 Focusing in normal tandem operation
In the normal operation of a tandem accelerator, theelectric potential uniformly increases from the tandem
entrance and the exit to the center terminal. The uniformity
is necessary to suppress the electric discharge and to realize
the maximum acceleration voltage.
The focusing and defocusing actions in the tandem
accelerator is as follows;
1) focusing due to electric field increase at the entrance,2) focusing due to acceleration from the entrance to thecenter terminal,
3) defocusing due to electric field decrease at the entrance ofthe terminal,
4) defocusing during the charge exchange process in theterminal,
5) focusing due to the field increase at the exit of theterminal,
6) focusing due to acceleration from the terminal to thetandem exit,
7) defocusing due to electric field decrease at the exit.
As the whole the beam is focused. All the focusing or
defocusing forces except for 4) are proportional to theelectric field (proportional to the terminal voltage). At low
acceleration voltage, therefore, the focusing becomes weak
and the beam transmission becomes worse.

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center terminal

entrance

exitv

t

V/2

0510Fig. 1.. Upper part: Potential profiles along the beam axis in a ~and em accelerator for the normal
operation (dashe~ curve) and for the accel-decel operation (solid one). Lower part: Schematic viewfor the four shortmg bars to make the accel-decel potential profile. The scale along the beam axis iscommon for both the upper and lower parts.

2.2 Is small tandem a good solution ?

3 Easy switching to accel-decel mode

Is there any way to recover the focusing force at low
voltage acceleration ? For the strong focusing force, strong
electric field is essentially necessary.
The straightforward solution may be install of a shortlength small tandem accelerator. However, it is not apractical solution because (a) a small tandem is not cheap,(b) a new beam transport system is also necessary, (c) newplace for the small tandem and the beam line is necessary,(d) a long time is necessary to design, to install, and toadjust the small tandem and the beam transport system. We
have no extra fund, no place and no long-time.

The upper part of Figure 1 shows the electric potential
profile (solid) for the accel-decel method together with thenormal potential (dashed). The minimum voltage in theaccel-decel method was determined as 1/4 of the maximum
voltage from the estimation of beam acceptance. Much
more accel-decel actions may produce stronger focusing,
however, resulting stronger electric field causes severe
problems of spark discharge. The potential profile in Fig. 1
is a practical solution. In the region from the centerterminal to the exit the electric field is not sensitive to thebeam transmission, and an easily accomplished potential
was chosen. As described in the following sections, theaccel-decel operation was found to be very successful to
produce high-intensity low-energy beam.
Only 4 shorting bars are necessary to realize the acceldecel method, as shown in the lower part of Fig. 2. The
shorting bars disturb the electric field in the tandem
accelerator tank, however, spark discharge does not occur
because the accelerating voltage is low in the accel-decel
operation.
The shorting bars can easily set and easily removed.
Therefore we can switch the tandem accelerator both in thenormal mode and in the accel-decel mode. This means that
we obtained a small tandem accelerator at the cost of only 4
shorting bars. We need no new beam line, no extra space,no long-time construction, no large fund.

2.3 New idea of accel-decel strong focusing
The strong electric field in the low-voltage operation of
the high-voltage tandem accelerator is easily produced if
whole the accelerating sections except for a short part isshorted to be on the same potential. The shorting method,
however, does not increase the total focusing force.
We arrived at a new idea of accel-decel method. In thismethod the electric potential first goes up in a short length(strong focusing), next the potential goes down and up in ashort length like the einzel lens. The down-up process
must has focusing force. If the down-up process is repeateduntil reaching to the center terminal, strong focusing mustbe achieved.

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under construction is used, the transmission will increase
and the energy width will become sharp.

4 Estimation of beam acceptance
KUTL tandem accelerator has a foil stripper with aneffective aperture of 10 mm in diameter and has apertures of28 mm in diameter in all the 40 accelerating tubes. The
beam envelope was calculated by multiplying the successive
beam-transfer 2 x 2 matrices for the accelerating tubes. The
diameter of the envelope was required to be smaller than 10mm at the terminal and smaller than 20 mm everywhere in
the accelerator. The 28-mm-diameter aperture was thought
to have a effective aperture of 20 mm in diameter because of
the aberration near the edge of the aperture. The beam
acceptance of the accelerator was defined as the emittance ofthe beam that satisfied the above requirement. In thefollowing we consider, for simplicity, the acceptance of thebeam only from the tandem entrance to the center terminal,
because the acceptance from the terminal to the exit isusually larger and has no restriction, and the effects of thecharge stripper is hard to estimate.
As the accelerating section is axial symmetrical withrespect to the beam axis, the acceptance was calculated in
one dimension, and the beam transmission was assumed to
be proportional to square of the !-dimensional acceptance.
The estimated beam transmission in the normal
operation at 1 MV was about 1/4 of that at 6 MV. In
practical case, the transmission became about 1110. This isbecause the beam envelope calculated at 1 MV has a waist
at a quadrupole lens in the beam injection line and such a
waist is impossible to make. Hence the practical beamtransmission at 1 MV is 2-3 times smaller than theestimated value.
The estimated beam transmission in accel-decel
operation at 1 MV is about 13 times higher than that in thenormal operation at 1 MV. This implies that the acceldecel transmission at 1 MV is estimated to be about 3 times
higher than the normal transmission at 6 MV. It implies
the accel-decel operation is a very promising method toobtain a high-intensity low-energy beam.
When the beam emittance is smaller than the accelerator
acceptance, the beam transmission becomes 100 %. We
expect nearly 100 % beam transmission in the accel-decel
operation.

Fig. 2 Electric current along the acceleration tube in theaccel-decel operation. Dotted lines show the shorting bars.

5.2 Stability for high intensity beam
In the normal mode the electric current is uniform along
the acceleration tubes, whereas in the accel-decel mode there
are many paths for the electric current as shown in Figure 2.
Besides the electric field in the accel decel mode is 4-5 times
steeper. than that in the normal mode. Hence the current
necessary for the accel-decel operation is 35/2 times higher
than that in the normal operation.
KUTL tandem
accelerator has 3 pellet chains and can bring the electriccharge of about 300 ~-tA which is enough to hold 2.5 MV in
the accel-decel operation.
The high current is another advantage of the accel-decel
operation. For higher-intensity beam acceleration, higher
current is necessary to stabilize the terminal voltage. Since
150-300 ~-tA current is used for the accel-decel operation, theacceleration of 10 P~-tA beam is expected to be stably made.

6 Conclusion
A new accel-decel operation of a tandem accelerator has
been found to be a very promising way to obtain highintensity low-energy beam with a very low cost and a shorttime. A high-energy tandem accelerator is easily changed toa small tandem. The method will ·greatly promote astranuclear experiments.

5 Performance of accel-decel operation
Acknowledgments

5.1 High beam transmission
For a terminal voltage of 1 MV, a 12 C beam of about 1~-tA was accelerated both in the accel-decel method and in thenormal mode. The beam transmission (sum of the wholecharge states) was 46 % and 4.7 % in the accel-decel
method and in the normal method, respectively. The beam
intensity increased about 10 times. The accel-decel method
was proved to be very effective way to obtain a highintensity low-energy beam.
We think that the beam transmission can be increased up
to about 90 %, because the present test was made using acarbon foil stripper of 7 ~-tg/cm2 which was too thick for thelow energy 12c beam. Indeed the accelerated 12 C beam in
the above test had broad energy width. If the gas stripper

The authors wish to thank H. Koga and T. Maeda for
their supports in preparation and adjusting accessories in thetandem accelerator. This work is supported in part by Japan
Ministry for Education, Science and Culture under Grant-inAids No. 10874044 and No. 08404017.

-65-References[1] K. Sagara et al, Nucl. Instr. & Methods A 378 #3(1996) 392