The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Operation of the Superconducting Electron Cooler at the Cooler Ring TARN IITetsumi TANABE,*t lchiroKATAYAMA,*t Shinpei ONO, ·* Katsuhisa CHIDA, t Tamaki W ATAN ABE, tYoshitsugu ARAKAKI, t Toshihiro HONMA, 'I Kouji NODA,'I Youichi HARUYAMA,** Manabu SAITO,**Takeshi ODAGIRI,* and Kazuhiko HOSONdt·center for Nuclear Study, University of Tokyo, Tanashi, Tokyo 188, JAPANtHigh Energy Accelerator Research Organization, Tanashi, Tokyo 188, JAPAN1National Institute of RadiologicalSciences, Anagawa, Chiba 260, JAPAN**Kyoto Prefectural University, Kyoto 606, JAPAN*Tokyo Institute of Technology, Meguro-ku, Tokyo 152, JAPANttHimeji Institute of Technology, Himeji 671-22, JAPANAbstract

2 Adiabatic Expansion of an Electron Beam

A superconducting electron cooler with an expansion factorof I 00 first came into operation at the cooler ring TARN II.
The initial transverse electron temperature of 100 meV wasreduced to the order of 1 meV by changing the electron
guiding field from 3.5 T to 35 mT with a sriperconducting
magnet. The dissociative recombination spectra of heliumhydride ions show a clear change in the resolution from thecorresponding ones measured earlier in TARN II. The liquidhelium-free superconducting magnet realized a compact
design and easy operation.

When an electron moves sufficiently slowly along thevarying axial field (B), the relation EJ. I B = const. holds,
where EJ. is the transverse energy. This is equivalent to theadiabatic invariance of the orbital magnetic moment
associated with the electron motion. The condition that theadiabatic invariants are conserved can be more precisely
described by introducing an adiabaticity parameter ( ~ ) along
the axial coordinate (z) as follows:~= (Ac I B)ldB I dzl,
where A,c is the spiral length of the cyclotron motion, given
by Ac = 2tc~2m,Er 1 I eB, and Err is the longitudinal energy.

1 Introduction

The transition is adiabatic if ~ « 1. If the axial fieldstrength is reduced from the initial value ( B0 ) to a final
value (B), the transverse energy is reduced according toEJ. = (B I B0 )EJ.o• where EJ.o and EJ. are the initial and final
transverse energies, respectively. This equation also holds
for the transverse energy spread of an electron beam, given
by oEJ.. Here, we call B0 I B the expansion factor. The
electron-beam radius ( R) increases from the initial value( R0 ) according to the relation R = Ra~ B0 I B . The
longitudinal energy spread ( 8Er 1 ) consists of two terms: (1)the term resulting from the transformation of the thermalenergy to a frame of reference traveling at the electron
velocity, and (2) the term from the Coulomb field of thesurrounding electrons and random relaxation motion [1]. At
high energies and at the usual electron density, the first term
(typically of the order of tJ.eV) is much smaller than thesecond one (typically -0.1 meV). The increase in thelongitudinal energy spread due to adiabatic expansion is on
the same order as the first term and is negligible at highenergies [2]. Therefore, for the adiabatic expansion, thelongitudinal t~mperature remains almost unchanged
compared with the standard arrangement. The principle of theadiabatic expansion was first implemented at CRYRING [3]in Stockholm in 1993. Since then a lot of electron coolers,including the cooler at TARN II [4], have been converted tosuch a type. However, so far, the expansion factors werelimited to about 10, resulting in a transverse electron
temperature on the order of 10 meV. This is mainly because

When the electron beam is used for atomic-collisionexperiments and for cooling, the longitudinal and transverse
electron temperatures play important roles in somewhat
different ways. In electron-ion collision experiments with arelative collision energy of E,e~, the energy resolution ( oE)
is given byoE = (1 I 2)k7;1 + kTJ. ± ~2E,e~k'I;r ,(1)where T0 and T.t are the longitudinal and transverse
temperatures, respectively, and the energy widths are givenby o£11 = (112)k~ and OEJ. = kTJ.. k1;1 is usually muchsmaller than kTJ., as described later. Thus, kT;1 limits theenergy resolution at high relative energies, whereas kT.tlimits the energy resolution at low relative energies. For
electron cooling, a reduction in either temperature leads to anincrease of the cooling force. In first-generation coolers, thecooling time and the resolution of experiments at low
relative energies had been limited by a transverse electron
temperature of about 0.1 eV, which corresponds to a cathodetemperature of about 1200·K. In this paper we report on thesuperconducting electron cooler with an expansion factor of
100 at TARN II, which realized a fast cooling and extremely
high-resolution experiments. The improvement of theresolution is demonstrated by the experiments on the lowenergy dissociative recombination (DR) of HeW ion(HeW+e~He+H).

-104-The 11th Symposium on Accelerator Science and Technology, HarimaScience Garden City, 19971mFig. 1 Layout of the superconducting electron cooler: (1) electron gun, (2) acceleration tube, (3) normal-conducting solenoid,(4) NEG pump, (5) toroid, (6) beam position monitor, (7) deceleration tube, (8) collector, (9) ion pump, (10) correction coil,(11) steering magnet, (12) refrigerator, (13) superconducting solenoid.
it is technically hard to increase the field strength withnormal-conducting solenoid coils, despite the requirement for
the higher magnetic field in the electron gun region to attainan even higher expansion factor. A superconducting magnet
is inevitably needed to further reduce the electrontemperature.

3 Superconducting Electron Cooler
The cooler at TARN II [51 was converted from the normalconducting type into the superconducting type [6] bychanging the electron gun region, but keeping the remaining
part of the cooler unchanged. In the new cooler, the electron
beam is expanded by a factor of 100 in cross-sectional area
in a gradually decreasing field from 3.5 T to 35 mT.~ Thelayout is shown in Fig. 1.

3.1 Electron Gun
The electron-gun optics consists of a flat cathode with adiameter of 5 mm, a Pierce electrode, an anode and anacceleration column. Electrons are extracted by the anodevoltage, and then further accelerated by an acceleration
column up to 20 kV in a uniform solenoid field. Theperveance of the electron gun is 1 J.LP and the maximum
current is expected to be 1 A at a gun-anode voltage of 10kV.
3.2 Superconducting magnet
The refrigerator-cooled NbTi magnet realizes easy operationand compactness in the present design. The superconducting
magnet shown in Fig. 1 has a 20 em room-temperature
bore, and is approximately 1 min axial length. The magnet
can produce a 3.5 T central field at a coil temperature ofabout 5 K. There is also a small superconducting coilwhich produces a reverse field, which helps the main field todecrease slightly more steeply. In order to make the field in
the gun region more uniform, the outer winding of the main
coil has a notch. Thus, a field uniformity of 1o- 3 wasrealized by the superconducting coil system. There are also
normal conducting. coils, which are used smoothly to join
the superconducting field to the subsequent toroidal field,
while keeping <; as small as possible. Helmholz coils cansteer the electron beam in both the horizontal and verticaldirections. All of the coils are covered by thick mild-steel
return yokes. The main purpose of the return yoke is toprevent any leakage field to the outer region of thesuperconducting coil. The maximum leakage field from thesuperconducting coil is about 20 G on the beam line. The
ramping rate of the magnet is 3.5 T/30 min. The coiltemperature became stabilized at 4.8 K for 3.5 T, although it
increased up to 5.4 K during ramping.

3.3 Electron Trajectories
The electron trajectories were studied using the SLAC ·program [7]. The results of field measurements include
errors; this unsteady field results in a significant increase in
the transverse electron temperature when tracing the electron
trajectories in the variable-field region. By correcting to asmooth change of the magnetic field, the correcttemperatures were obtained. Figure 2 shows a longitudinalmagnetic field on the axis and typical electron trajectories in
the gun region. In this case the maximum value of ~ isabout 0.05 which is small enough to guarantee theadiabaticity condition.

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5040 3020 10

1500

B (T)

RI0lO

I

20 30 40 50 E (eV)

30

1000

500

QLL~~~~~~~~~~~

2500

Z(mm)

Fig. 2 Axial magnetic field and typical electron tr~ectories
in the electron gun region at an energy of 2.5 keV and acurrent of 0.1 A.

2000"0·1400.

ISJ90·-··Ic00

800
1800

.. soo "'3oo

1600

V (V)

s 20too.. Joo-~soo ..... E (meV)o s 20100300

100

20 so100

20 5(b)1500-.;:;: 1000

4 Operation of the Superconducting ElectronCooler500

4.1 Off-line Electron Beam Test
A negative accelerating high voltage of 20 kV was readily
applied to the gun. On the other hand, the voltage between
the gun and the anode was increased up to 6 kV only after along conditioning time, probably due to the ExB drift
motion of electrons under the solenoid field in the space
between concentric anode and Pierce electrode. The measured
gun perveance of 1.1 ~-tP agrees with the calculations. The
tested maximum electron current is 500 rnA, which waslimited by the currently applicable gun-anode voltage.
4.2 Experimental Results

Figure 3 shows a comparison of the 4HeW DR spectra
measured at different expansion factors resulting in different
energy resolutions. Figure 3(a) is the rate as a function of
the electron acceleration voltage over a wide energy range,
which was measured earlier at TARN II using the cooler
with an expansion factor of 1 [8]. The c.m. energy is also
given on the upper horizontal axis. The spectrum is almost
symmetric with respect to the central peak for electron
velocities both slower and faster than the ion-beam velocity.
In this spectrum, the central peak appears to be a single
peak with weak shoulders. On the other hand, Fig. 3(b) is a
close-up view of the corresponding peak, which was alsomeasured at TARN II using the cooler with an expansion
factor of 14 [9]. It was proven that the spectrum consists ofseveral peaks. Figure 3(c) gives the present results measured
at an expansion factor of 100. As can be seen in this figure,the resolution was much improved and the spliUing of theside peaks was newly observed.

5 SummaryThe superconducting electron cooler with an expansion
factor of 100 came into operation for the first time. An
extreme decrease of electron temperatures, probably down tothe order of 1 meV, was observed. The liquid-helium-freesuperconducting magnet realized a compact design and easy
operation.

500

300

500

E (me V)

2000(c)
1500

500

1480 V (V)Fig. 3 Comparison of the 4HeW DR spectra measured at
different expansion factors (approximate resolutions) of (a) 1(-0.1 eV) [2], (b) 14 (-0.01 eV) [10] and (c) 100 (-0.001eV).
Acknowledgments
The authors thank the cyclotron staff for their helpful
cooperation. They are also grateful to Drs. M.Hoshino,T.Yosiyuki and I. Watanabe of Toshiba Corporation for theirefforts in the construction of the superconducting coil aocl
numerical ·calculations of electron trajectories. This work
was performed under Grant-in-Aid for Scientific Research (A)of the Ministry of Education, Science, Sports and Culture.

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