INITIAL ELECTRON COOLING STUDIES IN THE TARN IT
T. Tanabe~ K. Chida, T. Honma, T. Hattori', M. Kanazawa 2 , T. Katayama,A. Mizobtichi, M. Nakai 3 , A. Noda, K. Noda 2 , M. Sekiguchi, F. Soga,
N. Ueda, $. Watanabe, T. Watanabe, M. Yoshizawa and A. Ando 4
Institute for Nuclear Study, University of Tokyo, Tanashi, Tokyo, Japan'Tokyo Institute of Technology, Meguro, Tokyo, Japan
2 National Jnstitute"of Radiological Sciences,Anagawa, Chiba, Japan
3 NKK Co, M~runouchi, Tokyo, Japan
4 Research Center' for Nuclear Physics, Osaka University, Mihogaoka, Ibaraki,
JapanAbstract
Proton beam with energy of 20 MeV has been cooledin the TARN IT ~ith electron beam current of 0.3 A for
the first time. After a brief jummary of the electron
system and the storage ring, the results of electron
cooling measurements are described.
Introduction
Strong phase-space compression of ion beams
achieved by electron cooling will offer new possibilities for the eKperiments in nuclear, atomic andplasma physics. The TARN IT 1 is one of many cooler rings
under construction in the world and its main aim is in
the study of accelerator technology. The initial commissioning of the ring began in December 1988. On the otherhand, the electron cooling device produced its firstelectron beam at the end of 1988 and its properties werestudied before it was installed in the TARN IT . Details
concerning the design of the cooling system and theresults of electron beam test have been presented elsewhere.2-7 The cooling device has then been installed in
the ring in early summer of 1989.
In this paper, we shall report the results of thefirst cooling eKperiments performed with 20 MeV protons
on 30 September 1989, during 1.5 days of beam time.

long cooling section. Horizontal and vertical steering
magnets on each side of the electron cooler permitted
the correction of the deflection of proton beam in thetoroid and also steering of the beam in the electron
cooling device. Details of the correction was describedin ref. 8. A pair of electrostatic position monitors installed at the cooling section allowed the position
measurements of both electron and proton beams in thehorizontal and vertical directions. The other position
pickups distributed aroun~ the ring offered thetransverse position and lifetime information of bunchedbeams. Density distribution of the protons in the longitudinal direction was measured by using a travellingwave type signal pickup. 9 In order to observe theneutral hydrogen beam which is created in the cooling
section when cooling electrons are captured by circulating protons, a detection system was mounted at the exitof a thin window (100 fl. m in thickness) downstream of
the cooling section. It consists of a large solid state
detector with aperture of 4 em which has a thickness of
1 mm. In order to obtain better magnetic field regulation for the dipole and quadrupole magnets of the TARN
IT ring during cooling, usual thyristor-regulated power
Table 1 Electron cooler parameters for the cooling experimentElectron cooling system
The basic parameters of the electron cooler during
the experiments are listed in Table 1. The control ofthe electron cooler mainly consists of setting the acceleration voltage and of fine tuning the electron beam
position and direction in the overlap region with theproton beam. The vacuum pressure was about 5 X 10-• 0Torr before turning the proton and electron beams on.
During continuous cooling operation at 20 MeV, the pressure was about 1 X 10-" Torr on the gauge situated about
1 m upstream of the cooler. Collection efficiency ofelectron beam is typically better than 99.95 %. The highvoltage power supply (HVPS) which accelerates electrons
was controlled by a 16 bit DAC and the output voltage
was stable to within about 0.5 V. The ripple componentof the HVPS is less than 0.5 V. The output voltage at
the high voltage terminal had, however, a 50 Hz ripple
component of about 3.5 VPP' which was mainly induced by
the coupling of the isolation transformer to the highvoltage terminal. The stability and the ripple of thegun-anode high voltage power supply which defines theoutput electron current ~Rere about 5X 10- 4 and l0- 3 ,respectively.
Storage ring and its main parameters
Protons accelerated to 20 MeV in the SF cyclotron
were injected into the ring in multi-turns. Number ofinjected particles was about 10 7 • The electron cooler issituated in one of the six straight sections. The
electron beam merges with the proton beam in the 1.5-m

Acceleration voltage
Typical beam energy on axis
Corresponding velocityGun perveance
Typical beam current
Beam radiusElectron density
Solenoid field strength
Length of interaction region

Table 2cooling

TARN IT parameters relevant to the electron

Nominal proton beam energy
Corresponding velocity
Initial momentum spread, b. p/p
Ring circumference
Fraction of orbit occupied by electron
Nominal revolution frequency
RF frequency
Horizontal {3 in cooling section
Vertical {3 in cooling section
Momentum dispersion in cooling section
Transition energy, Y ,'f}=l/y2-l/y,2

Horizontal betatron tune
Vertical betatron tune

-201-11.168 kV
11.035 keV0.20 c
1.1 fl.Av-3/2
0.3Aem2.5
1.5Xl0 7600
1.5

20.26 MeV
0. 20 c-2Xl0- 3
77.76 m0.019
0.789 MHz1.578 MHz
m
10.2m
3.7

4.7

1.88
0.676
1. 71
1. 80msupplies eKcitiog these magnets ~ere replaced by theother· trar)sistor-regulaled po~-er· supplies. Typical field
regulation including ripple components ~as about 0.01 %.
Experiments cere started ~ith relatively poor vacuum
condition, primarily because beam pipes are still un·baked. The vacuum pressures ~ere about 10~ 9 Torr at thecooling section and about lo-• Torr at the opposite sec·
Lion in the ring during operatio;,. The TAWi II 11achineparaneters relevant to this eKperiment are suwnarized inTable 2.

ii)

Initial cooling studies~easurenents of the cooling •ere nade with 20 MeV
proton beam. Much of our data on cooling is still
preliminary as they have not been critically analyzed orreproduced due to limited bean tine.
Before switching-on the electron beaw, the proton
orbit deformation due to the toroidal field and thedipole field in the cooler was corrected by eKciting
horizontal and vertical steering magnets. The correctionoas optimized by observing the proton beam lifetime. Thelifeti~e at the solenoid field of 600 G was about 60 Xof the one without solenoid field. The short lifetime
with solenoid field on seems to be due to the coupling
between horizontal and vertical motions induced by thesolenoid field since the difference of tune values between both directions is small as listed in Table 2.
The positions of the electron and proton beans atthe cooling section •ere then measured. The accuracy ofthe position measurements for protons was not enough incomparison with that of the electron beam because of thelow and fluctuated beam intensity of the proton beam. So
it is not certain that the proton and electron beam axesin the cooling section ~ere properly aligned.
ilith 0.3 amp of electron beam on (but detuned inenergy) proton beam lifetime vas reduced by a factor ofabout 3 even after the correction of the tune shifts due
to electron space-charge. The reason of the short
lifetine with electron bean on is not •ell knovn, but it
appears to be caused by stray electrons bound ln thesolenoid field and ions trapped in the electron spacecharge cloud. The lifetime is improved by sveeping thesecharged particles. The increase of lifetime by a factorof 2 was actually observed by applying voltages in thetransverse directions to the split drift electrodes. The
situation will also be improved after attaining better
vacuum.
If the electron cooling acts effectively, the os·
cillation damping of protons introduced by electron
cooling causes beam contraction in the transverse andlongitudinal directions and suppresses the expansion due
to multiple scattering. With the electron and proton
beams '•'ell matched in velocity (to about 4 parts in10000), increase of lifetime of bunched beam was imnediately observed. A typical mean life of about 20 sec
was observed for the cooled beam, which contrasted withthe lifetime of about 2 sec for the uncooled bean, as
sho1·1n in Fig. 1. After the beam l·'as injected in a stationary rf bucket with height even lo~er than the energy
width of injected beam, the decrease of momentum spreadcaused by the cooling increased the beaw density in thebucket and consequently increased the signal anplitudeof the beau pickup as can be seen in Fig. 1, a). Afterreaching equilibriun, the signal then decreased
gradually due to the scattering of the bean ~ith theresidual gas. This lifetime increase for the proton beam
~as the first effect observed in experiments on electroncooling. The lifetime increased vith decreasing rf voltage. It vas also sensitive to the beam axes alignment.
~ith the change of the steering field of 0.5 G in thecooler, the increase of bean lifetine by a factor of 2
was observed. Data shovn in Fig. 1 are the results vhich
have not yet been optimized. The absolute value of theacceleration voltage of electrons under the conli11g condition ~as consistent ~ith the one expected rteglecti;lgb)

Fig. 1 Evolution of bunch signal as a function of timeobserved by position pickups (sun signal) with electronbeams a) tuned and b) detuned to proton beams invelocity. Spikes represent injection points. Horizontal
scale: 2 sec/div and vertical scale: 5 dB/div.

the electron-beam space-charge conpensation, within anerror of 10- 3 •Tine evolution of cooling process was observed byneasuring the bunched-beam frequency spectrun. Figure 2sh.O.'·'S frequency spectra of the 38th harmonic of thebunch signal from the longitudinal beam monitor. The
decrease of the momentum spread, caused by the cooling,
reduced the bunch length and increased the signalanplitude. The frequency •··idth '·'hich reflects monentum
Yidth decreased rapidly with time after beam injection.
The time dependence of the frequency spread is alsoplotted in Fig. 3. It gives an estinate of the coolingtime at about 1.4 sec for bunched bean. The final ,~jdthis narrower than the initial one by a factor of 10.
These results bring an outlook on the cooling process,although the accurate value of the cooled-beam momentun
spread has to be measured by observing the coasting-bean
Schottky signal and this has not yet been tested.
Increase of neutral beam production rate under thecooling condition was observed. The rate was about 10
counts/s. Neutral beam was also detected even in the uncooled condition ~ithout electron beam. This may be
produced by protons grazed the chamber ~all due to thepoor alignment of the circulating beau and also by interaction of protons ccith the residual gas. In order todo quantitative analysis related to the deduction ofelectron temperature, the background has to beelinlnated as ~ell as the accurate mcasurcnents ofneutral particles and of number of stored particles in
the ring.
Periodic change of bunched beam density cith a period
of about 3 sec ~as observed at a hlgher harmonic of the-202-

,....
::E:r~

IJ:,

0seclo-s,....

.....
.....

-r-1.4 sec......
<l•~......

....

0.3 sec

10-00.6 sec•4246810

Time <sec>Fig. 3 Time dependence of the frequency spread of 38thharmonic of beam signal.

longitudinal frequency spectrum. This seems to be related to some instability as observed in the LEAR 1 0 •Transverse side-bands observation is necessary for thereasonable understanding of this phenomenon.
Systematic cooling studies have just begun and effort
will continue to obtain better cooling conditions and toclearly observe subtle effects.

1. 2 sec

Acknowledgements
1. 9 sec

The authors thank Messrs. S. Okawa and K. Ornata for thepreparation of the neutral beam measuring system and Mr.
M. Fujita for his advice concerning the design ofvarious electronic system. They also wish to thank Dr.
I. Sugai and his collaborato rs for their aid inlaborious work. The research was supported by a Grant-in
Aid for Scientific Research from the Ministry of Education, Science and Culture.

3.5 sec

References

5.1 sec

Fig. 2 Time evolution of frequency spectrum of bunched
proton beam after injection with cooling. Horizontalscale: 15 kHz/div, vertical scale: 5 dB/div and center
frequency: 29.985 MHz.

1. T. Katayama et al., presented at this symposium(1989).
2. T. Tanabe et al.,Proc. of Workshop on Electron
Cooling and Related Applications, Karlsruhe, KfK
3846 (1984), p. 127.
3. T. Tanabe et al., IEEE Trans. NS-32, 2412 (1985).
4. T. Tanabe et al., INS report INS-T-454 (1986).
5. T. Tanabe, Proc. of 13th Int. Conf. on HighEnergy Accelerators, Novosibirsk, (1986) vol. I,
p. 216.
6. T. Tanabe et al., Nucl. Instrum. A278, 241 (1989).
7. T. Tanabe et al., Proc. of Particle Accelerator
Conference, Chicago, (1989) (to be published).
8. T. Tanabe, Proc. of 14th Int. Conf. on HighEnergy Accelerators, KEK, Tsukuba,(1989) (to bepublished in Particle Accelerators)
9. H. Yonehara et al., INS report INS-NUMA-49 (1983).
10. H. Poth et al., z. fuer Physik A 332,171 (1988).

-203-