Specification of a new electron cooler for the low energy ion ...

Abstract
Nuclear Instruments and Methods in Physics Research A 532 (2004) 399–402
Specification of a new electron cooler for the low energy
ion accumulator ring, LEIR
Gerard Tranquille*
AB Division, CERN, CH 1211 Geneva 23, Switzerland
Available online 15 September 2004
For the cooling of Pb 54+ ions in the future low-energy ion ring machine a new electron cooling device needs to be
constructed. This new cooler will take advantage of all the recent developments in electron cooling in order to balance
efficient and fast cooling with a sufficiently long ion beam lifetime for beam accumulation. This paper will present the
special features of the device and how their combination will be used to obtain low emittance beams for transfer to the
LHC.
r 2004 Elsevier B.V. All rights reserved.
Keywords: Electron cooling; LEIR; Lead ions; LHC
1. Introduction
To obtain the LHC design luminosity of
2 10 27 cm 2 /s for lead ions, the low energy ion
ring (LEIR) machine [1] will be used to accumulate
and cool the ions at an energy of 4.2 MeV/u.
Machine experiments were made between 1994
and 1997 to determine the optimum parameters
for this new machine and have been the subject of
many publications [2]. One of the critical issues in
the production of dense lead ion beams is the
cooling and stacking process. The electron cooling
device used in the previous tests is now installed on
the Antiproton Decelerator at CERN [3] and
hence a new, state-of-the-art, electron cooling
*PS Division, CERN, CH 1211 Geneva 23, Switzerland. Tel.:
+41-22-767-2554; fax: +41-22-767-9145.
E-mail address: gerard.tranquille@cern.ch (G. Tranquille).
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0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2004.06.072
device will be constructed for LEIR. From the
experience gained in the cooling experiments, the
new cooler will make use of recent developments in
electron cooling technology in order to balance
efficient and fast cooling with long ion beam
lifetimes for accumulation.
2. Electron cooling experiments
The cooling time t can be estimated from the
following simplified relation [4]:
tp y3
ZIe
A
Z 2 b4 g 5
ð1Þ
where b, g are the relativistic factors, A the ion
mass and Z its charge state, I e the electron current,
Z the ratio of the cooler length to the machine
circumference and y the relative angle difference

400
between the ions and the electrons. From the
above relation one sees that a number of machine
and cooler parameters can influence the cooling
time. In our experiments the main points of
investigation were
* The ion beam lifetime as a function of the
electron beam intensity.
* The influence of the machine lattice parameters
on the cooling time.
* The cooling time versus the electron beam
intensity and the cooler length.
* Testing of stacking modes.
* Stack equilibrium emittances and emittance
growth.
2.1. Ion beam lifetime
The lifetime of the lead ion beam in the presence
of an electron beam was measured for a number of
charge states around 53+. An anomalously fast
recombination rate for Pb 53+ was observed in
initial measurements and forced us to switch to a
charge state of 54+. This unusually high rate
points to mechanisms other than radiative or
dielectronic capture and has been observed at
other heavy ion storage rings.
2.2. Influence of the lattice parameters on the
cooling time
Special importance was attached to the dependence
of the cooling time on the optical parameters
of the storage ring. For a given emittance
and momentum spread, the lattice parameters b
and D determine the size and angular distribution
of the ion beam. Cooling times were measured for
a wide range of lattice parameters at the cooler
with protons at 50 MeV and Pb ions at 4.2 MeV/u.
Contrary to what one would expect from theory,
the best cooling was obtained for intermediate b
values. It should however be mentioned that for
these machine settings a non-zero value of the
dispersion function was set in the cooler section. It
is clear that the influence of D is superimposed on
the bh dependence and may even be the dominant
effect.
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G. Tranquille / Nuclear Instruments and Methods in Physics Research A 532 (2004) 399–402
Guided by these observations a series of
measurements were made where only the dispersion
function was varied and the cooling times as a
function of the difference in beam alignment was
recorded. The results showed that with a finite
dispersion in the cooler the cooling times could be
halved and, depending on the sign of the dispersion
function, cooling was faster for positive or
negative offsets in beam position.
2.3. Cooling time versus electron current and cooler
length
In the 1997 experiments the cooler length was
doubled from 1.5 to 3 m. This enabled us to
complement the cooling time measurements as a
function of I e with the variable Z. The results show
that the linear increase in cooling rate, 1/t, asa
function of Ie and Z shows uponly for low electron
currents (o120 mA). Possible explanations include
the effect of the electron beam space charge
increasing the mean transverse velocity of the
beam, misalignment tolerances, which are more
critical for a longer cooler, and electron beam
instabilities observed at high currents due to the
gun design.
2.4. Stacking tests and equilibrium emittances
Cooling and stacking tests were performed with
the lead ion injector Linac cycling at a repetition
rate of 2.5 Hz. The ion beam was injected using a
combined horizontal and longitudinal multi-turn
injection scheme enabling upto 1.5 10 8 ions to
be injected per Linac pulse. After injection into the
ring the beam is cooled in all three planes and
‘dragged’ to the stack momentum leaving space for
the following pulse 400 ms later. This process is
repeated until the losses during the interval
between consecutive pulses balance the number
of particles added per injection. On average
6 10 8
ions were accumulated in about 10
injections with peaks reaching 7 10 8 ions.
The stack emittance was measured as a function
of stack intensity and the cooling was switched off
to record the emittance growth. The results show
that for the required LHC intensity, the emittance,

even 1 s after cooling is switched off, should be
well below the 0.7 mm limit.
The saturation at a stacking factor of about 7,
indicates a lifetime of only 2.6 s. This short lifetime
is probably explained by the increased charge
exchange due to outgassing caused by injection
losses. In the future LEIR machine, special
attention must be paid to the vacuum system and
the injection process.
3. The LEIR electron cooler
The electron cooling device (Fig. 1) planned for
LEIR is conceptually similar to the coolers in
operation at CERN and other laboratories. It is
designed to cool and accumulate pulses of a few
10 7 ions in less than 200 ms.
Taking into account recent improvements [5,6]
tested on various electron coolers during the last
decade, we foresee the use of a high perveance gun
followed by an adiabatic expansion (A) provided
by an additional solenoid. A 90 toroid (B+C)
bends the electrons into the 2.5 m cooling section
(D) where they merge with the circulating ions. A
second toroid (E+F) bends the electrons out of
the cooling section towards the collector (G) where
the electron beam power is recovered.
3.1. The electron gun
The high perveance gun aims at providing an
electron beam with a high density in order to
decrease the cooling rate. For LEIR 600 mA of
electron current will be required for cooling Pb
ions at an electron energy of 2.3 keV. In addition,
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G. Tranquille / Nuclear Instruments and Methods in Physics Research A 532 (2004) 399–402 401
Fig. 1. The new electron cooler for LEIR.
Fig. 2. Electron beam profile measurements made using a
tungsten wire. On the left, the full beam is measured and on the
right, a hollow beam distribution was made by polarizing the
control electrode.
the gun will also have to run at 40 keV with a
current of 3 A. The higher energy might be needed
if it is found necessary to cool the ion beam before
extraction to the PS machine at 72 MeV/u.
However, increasing the electron density induces
first an increase of the recombination rate, which is
detrimental to the ion beam lifetime, and secondly
increases the electron azimuthal drift velocity thus
increasing the cooling time.
To overcome these problems, the new gun has
been designed to deliver electron beams of variable
radial density and the adiabatic expansion will
helpto reduce the transverse temperature of the
electrons after acceleration to the desired energy.
A ‘control’ electrode has been inserted between the
Pierce (or forming) electrode and the grid. The role
of this electrode is to control the radial density of
the electron beam without any increase in the
transverse velocity. The strong magnetic field
(B0=2.3 kG) in which the gun is embedded will
helpto overcome the strong space charge forces
that tend to blow upthe beam. As in almost all
electron cooling devices, the cathode is at negative
potential and the anode is at ground. The potential
difference determines the final energy of the
electrons and the ‘grid’ electrode controls the
intensity of the electron beam. Actual density
measurements of the density distribution using a
tungsten wire are shown in Fig. 2.
3.2. The expansion and toroid region
Electron beam expansion will be necessary in
order to match the electron beam dimension with

402
that of the injected ion beam. Moreover, the
strong longitudinal magnetic field in the gun
would give rise to a large deflection of the ion
beam in the toroid region, which would require
strong dipoles for the correction. The field after
expansion will be 750 G giving an expansion factor
of 3 and an orbit deflection in the toroids of about
15 mrad. A further extension of the expansion
could be to reduce the electron beam diameter
during the cooling process in order to be at the
maximum of the cooling force always.
In the toroidal bend a set of electrostatic plates
will be installed in order to improve the electron
beam collection efficiency. The superimposed
electric and magnetic fields completely compensate
the drift that the electrons usually acquire when
passing through the toroid. Therefore, reflected
and secondary electrons will not be lost as they
oscillate between the gun and collector, but instead
they will have the possibility to be reaccelerated
towards the collector. Initial tests made by INP
Novosibirsk [7] show that the loss rate DI/I can be
reduced to below 10 5 thus improving the vacuum
conditions in the drift section.
3.3. The drift section
The drift section, where the electrons and ions
are merged, will be 2.5 m in length. This is the
maximum space available for cooling after all the
correction elements (dipoles and anti-solenoids)
have been inserted into the straight section where
the cooler will be installed. Two electrostatic pickups,
capable of measuring both the electron beam
and ion beam trajectory in the drift section, will be
placed at the entrance and exit of this section.
Along with 10 electron beam steering coils it will
be possible to perfectly align the two beams for
optimum cooling.
Non-evaporable getter (NEG) strips will be
placed along the whole length of the drift section
between the vacuum chamber and a potential
continuity grid of 140 mm diameter. Activated
during the bake-out, these NEG pumps will ensure
that the vacuum level in the cooling section is as
low as possible during operations with ion beams.
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G. Tranquille / Nuclear Instruments and Methods in Physics Research A 532 (2004) 399–402
3.4. The electron beam collector
Efficient collection of the primary electron beam
is essential if an excessive load on the main highvoltage
power supply is to be avoided. The
collector for the LEIR cooler is designed to collect
electrons with relative current losses less than
10 4 . Combined with the electrostatic bend (see
Section 3.2) these losses can be further reduced by
one order of magnitude.
4. Conclusions
The main parameters of the LEIR electron
cooler have been determined and the construction
phase should soon commence. Building on the
experience of our Pb ion run in 1997 and recent
developments in electron cooling technology, the
new cooler should meet the requirements for ion
beam cooling and accumulation for the LHC. We
will keepa close eye on the cooling tests at IMP
Lanzhou [8], expected in 2004, in order to make
final adjustments such that the new device will be
fully commissioned for the second half of 2005
when the LEIR machine should come online.
References
[1] M. Chanel, Nucl. Instr. and Meth. A, (2004) these
Proceedings.
[2] J. Bosser, et al., Experimental investigation of electron
cooling and stacking of lead ions in a low energy
accumulation ring, Particle Accelerators, 63 171, 1999.
[3] G. Tranquille (on behalf of the AD team), Nucl. Instr. and
Meth. A, (2004) these Proceedings.
[4] J. Bosser, I. Meshkov, G. Tranquille, Magnetised electron
beam cooling time for heavy ions, Internal report CERN/
PS/AR Note 94–11, 1997.
[5] I. Meshkov, A. Sodorin, Nucl. Instr. and Meth. A, (2004)
these Proceedings.
[6] A.V. Ivanov, V.V. Parkhomchuk, B.N. Sukhina, M.A.
Tiunov, The hollow electron beam. The new opportunities
in electron cooling, Proceedings of the Workshopon Beam
Cooling and Related Topics, Bad-Honnef, May 13–18,
2001.
[7] V. Parkhomchuk, private communication.
[8] A. Bubley, Nucl. Instr. and Meth. A, (2004) these
Proceedings.