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

ELECTRON COOLING EXPERIMENTS AT IDMAC
K. Noda•l, S. Shibuya"\ T. Furukawabl, M. Muramatsu"\ T. Honma•l, T. Fukushimac\ H. Izumiyac\ H.
Ogaw c)' T. Iwashimac\ A. Nodad\ F. Hichamd\ T. Shiraid\ T. Nagafuchi•l, K. Maeda•),H. Uchiyamab)' E. Takada•J and S. Yamada•)a) National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba, 263-8555 Japanb)Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522 Japanc) Accelerator Engineering Corporation, 2-13-1 Konakadai, lnage-ku, Chiba, 263-0043 Japand) Institute for Chemical Research, Kyo to University, Gokanosho, Uji, Kyo to, 611-0011 Japane) Toshiba Corporation, 1-1-1 Shibaura, Tokyo, 105-8001 Japan

Abstract
Electron-cooling experiments have been being carriedout at HIMAC in order to provide high-quality and highintensity beam for medical and other applications. In thetransverse cooling-experiments, a cooling time and an
equilibrium emittance at horizontal direction weremeasured and an intrabeam scattering was also observed.
Beam intensity was increased by one order through thecool-stacking method. Beam bunch was compressed fromaround 400 ns to 40 ns by the electron cooling.

electron energies range from 3 to 30 ke V, whichcorrespond to ion energies from 6 to 55 MeV/n. The
electron-beam size at the cooling section is designed to be
100 rum in diameter so as to cover the ion beam withhorizontal emittance of 264n mm·mrad. The effective
length of the cooling section is limited to 1.0 m, because
the available length of the straight section is only 4.0 m.
Thus the fraction of the cooling length relative to the ring
circumference of 129.6 m is 0.8%. Figure 1 shows theelectron cooler of RIMAC.

1 INTRODUCTION
Clinical trials of heavy ion therapy in the HIMAC(Heavy Ion Medical Accelerator in Chiba) [1] has beensuccessfully progressed since June 1994.
One of the objectives of HIMAC is to develop newtechnologies in heavy-ion therapy and related basic andapplied research. For the purpose, it is very important toimprove beam property and enhance capability ofhandling it. The electron-cooling method can provide
high-intensity or high-quality beams by cool stacking andby its strong phase-space compression. The aim of ourstudy is to apply those techniques of accelerator physics
to medical and other fields. These techniques will lead tothe following: (1) an increase in the intensities of
positron-emitter beams for the ion range measurements
and of heavier ions, such as Fe and Ni, (2) microbeam
probe for the cellular radiation-response, and (3) shortbunched beams for time-resolving measurements. As thefirst step, an electron cooler was designed andconstructed [2], and was installed at HIMAC-synchrotron.
After the preliminary test [3], the transverse-coolingmeasurement, the cool stacking and the short-bunchedbeam production have been carried out. The paper reports
the experimental results.

2 E~ECTRON COOLER
In the electron cooler, an adiabatic expansion factor isdesigned from 1 to 10 for faster transverse cooling. TheFig. 1: Fish-eye view of the electron cooler at RIMA C.

3 COOLING EXPERIMENTS
3.1 Experimental conditions
The experimental conditions are summarised at Table 1.
In the cooling experiments, fully stripped argon-ions withthe energy of 6.0 MeV/n has been used in DC operationof the ring. In order to observe cooling phenomena easily,
the horizontal emittance is adjusted to about 60 from 264
1t·mm·mrad by decreasing the time-width of the injection
beam to half compared with the normal operation, whilethe vertical one of atound 10 1t·mm·mrad is kept constant.
After precious COD-correction around the ring, the ion
beam was aligned with electron one in a cooling section
by the local-bump method [4]. The alignment accuracy-100-The 13th SympOIYium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

was verified by using a pair of position monitors in thecentral solenoid. A revolution frequency of circulating
ions was measured by observing its higher-harmonics
component from a Schottky monitor in order to determine
electron energy roughly. The electron energy wasdetermined precisely by changing the energy so as tomaximise a bunch signal in applying rf-field with anamplitude of ±IOV. The bucket height corresponding to
the rf-amplitude is almost consistent with a linear regime
that is proportional to a gradient of a velocity profile.

6.0±0.7, which is slightly larger than the expected valueof 5 from the analytical result.
-llngproflle(lnJeotlon). . '-- ----i-~l--------l4030Table 1. The conditions of the cooling experimentElectron energy
Electron current
Expansion factor
Electron beam diameter
Field strength at gun section
Field strength at cooling section

3.465 keV
200mA
3.364mm0.167 T
0.05T

Argon-ion energy
Initial momentum spread
Tune (Qx/Qy)
f3x!f3y in cooling section
Dispersion in cooling section Dx
Transition energy, 'Y,
Phase slip factor, TJ
Revolution frequency, f0Intensity

6.00MeV/n
1·10·3 at FWHM
3.68/2.88
9.9m/10.7m
2.2m
3.70.930.2614MHz
106 - 109 ppp--------~------------~·----f----------L-50

1..

60

80

70

-llngproflle(3.5Hc)::[ imm'm·m: ' O ' ' ' ' · : ·~··---~----

80 E.
60

~·········---~----··---- _, ________ - ·- - --------- -------······l

·- --

---~

~t~ ··'3 ..l50

4030

70

60

80

Fig. 2: Upper figure: Horizontal beam-profile with
FWHM of around 30 mm just after multitum injection.
Lower: that with FWHM of 4 mm after cooling of 3.5 sec.

......::j::..;:·::::·::!:

I..

3.2 Transverse cooling·c(1) Cooling-time measurement

First, the position and the angle of the electron-beamaxis was preciously adjusted to minimise the ion-beamsize after cooling for 2 sec by using Helmholtz coil inboth the gun solenoid and the central one. Next, thecooling time in the horizontal direction was measured byusing a non-destructive profile monitor (MCP monitor)[5]. The horizontal beam-profile just after the multiturninjection and that after the cooling for 3.5 sec are shownin figure 2. The cooling time is estimated at around 0.5 s
in relatively small beam-size, as can be seen in figure 3.
Considering the spatial resolution of the MCP monitor of
0.5 mm at FWHM and the beta function in the monitor
position, further, the equilibrium emittance can beestimated less than O.l1t mm·mrad.
(2) Intrabeam scattering
When the electron beam was switched off after coolingthe ion-beam sufficiently, the ion-beam size was growing
up due to the intrabeam scattering (IBS), as shown infigure 4. According to the analysis described in the ref.
[6], the beam size after switching the electron off isrepresented as follows; o(t) = ['Y· D0 ·t+a/t'~', 0'0 is an
initial beam-size, D0 is a constant which depends on theion current. As the result of the fitting, 'Y is estimated at

1=IL:

i.J-..1-.l.-L....L..I-.i.-.L.~ •00.5.i.

---s-$$

2

2.5

3

3.5

' • ~...L...I-1-.!.-L..

1.5

EiJ
...L...l........o..

4Time(sec)Fig. 3: Compressing horizontal beam-size (FWHM) as afunction of time.

e0!

'l____ _jj

1::&!

.=.

.......... -----~~.

----;-----j.!!

"'~---·------- --~-~~'---'--~-'---'--..l__i__.___.__. ._.,l_j

= 1.5
01015

2025

Time I'J"om EC off(sec)Fig. 4: The beam-size growth due to IBS after switching
the electron beam off. Circles are measured ion-beam size.
Solid line indicates the fitting result by the analyticalformula.
(3) Cool stacking
Beam intensity was increased through cool stacking,as shown in figure 5. The beam was injected to the ring atevery 3.3 sec by the multitum-injection method. The-101-The 13th Symposium on Accelerator Science- and Technology, Suita, Osaka, Japan, October 2001

intensity gain by the cool stacking was around 10, which
depends on a cooling rate and a beam lifetime.
'iO'-S200t

ISOl..
~~
J= 100 r··········:··i "~m•ot .... , ... ,; .... ; ...
0200

100300;,.
400soo600RF Voltaxe (Vp·p)

Fig. 7: The rf-voltage dependency of the bunch width.
Circles and triangles indicate bunch width in thesinusoidal rf-field and in sawtooth-wave one, respectively.

Fig. 5: Intensity increased by cool stacking. Horizontal
scale is 10 sec/div. Vertical one around 6·107 ppp/div.

3.3 Short-bunched beam
A short-bunched beam can be obtained by the coolingand applying rf-field [7]. A sawtooth-wave rf-field issuitable for production of a short bunched beamcompared with a sinusoidal one, because a filamentation
is not occurred. As can be seen in figu~ 6, a shortbunched beam with 40 ns at FWHM was obtained inusing the sawtooth-wave rf-field and the cooling, whilethat with 80 ns in the sinusoidal rf-field. As figure 7
shows the rf-voltage dependency of the bunch width, thebunch width is decreasing as increasing the rf-voltage. In
this experiment, the beam was injected and cooled down
while the rf-field continuously applied. Further the bunch
width was measured by an electrostatic position-monitor
and the sawtooth-wave rf-field was applied by a superwide-band cavity [8].
As cooling the ion beam down with the rf-field in thebeam-intensity higher than around 108 ppp, several peaks
were observed in one bunch and the bunch width wasconsiderably widened to around 400 ns at FWHM. At thesame time, the beam position was oscillated vertically andthe beam loss was begun.
. , . Stopped:4771 A<qulsltlons

56:452mY251mY•=4SUMMARY

The cooling experiments have been carried out since
the end of March 2000. As the results of the experiments,
the horizontal beam-emittance was cooled down less than
0.11t mm·mrad and the cooling time was estimated at 0.5
sec. The cool stacking was also observed with theintensity gain of the one order without adjusting theparameters of the bump-magnets for the multitum
injection. The short bunch with the width of 40 ns wasobtained, while the bunch width was widened to 400 ns
and instability in the vertical direction was observed inhigh beam-intensity.

5 ACKNOWLEGMENT
The authors would like to express their thanks to Dr.
Soga and members of the Department of Accelerator
Physics and Engineering at NIRS for continuousencouragement. They are indebted to Prof. T. Tanabe at
KEK for useful discussion concerning the coolingexperiments. This work has been performed as a part of
Research Project with Heavy-Ion at NIRS-HIMAC

REFERENCES[1] E. Takada et al., Proc. 7'h EPAC, Vienna, 2000,p.2557-2559.
[2] K. Noda, etal., NIM A441 (2000) 159-166.
[3] K. Noda et al., Proc. 7tli EPAC, Vienna, 2000,p.1259-1261.
[4] T. Furukawa et al., in these proceedings.
[5] T. Honma et al., in these proceedings.
[6] M. Beutlespacher et al., NIM A441 (2000) 110-115.
[7] K. Noda et al., Proc. Workshop on S-ring, NIRS,
1997, HIMAC-report-017, p.1-33 (in Japanese).
K. Ohtomo et al., Proc. 7th EPAC, Vienna, 2000,
p.1540-1542.
[8] H. Tamura et al., Proc. 12th SAST, Wako, 1999,
p.248-250.

Fig. 6: Bunch shape in cooling and applying sawtoothwave rf-field. Horizontal scale is 50 ns/div.

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