ECR Ion Sources for H" Ion Production:
First Results and Prospects**
A. Girard*, D. Hitz*, G. Melin*, R. Gobin0, R. Ferdinand0, K. Benmeziane0, O.
Delferriere0, J. Sherman**
^Departement de Recherche Fondamentale sur la Matiere Condensee, Service des Basses Temperatures, CEA
Grenoble 17 rue des Martyrs 38054 Grenoble Cedex 9 FRANCE
°Departement d'Astrophysique, de Physique desParticules, de Physique Nucleaire et de I'Instrumentation associee,
Service des Accelerateurs, de Cryogenic et de Magnetisme, CEA Saclay 91191 Gif sur Yvette Cedex FRANCE
** Los Alamos National Laboratory, Los Alamos, N.M. 87 545 USA
Abstract. ECR Ion Sources are well known for their efficient production of Highly Charged Ions [1], and also for the
production of intense proton beams [2]. Recently European laboratories have decided to join their efforts to develop and
improve various plasma techniques for the production of intense beams of H", for a future application to the European
Spallation Source, and possibly for other high power accelerators. Because of its great experience and skill in ECR
plasmas and ion sources, CEA has decided to develop ECR ion sources for negative ion production. At first glance, this
seems to be a real challenge, as negative ion production requires a very low electron temperature, incompatible with
ECR heating. We will show that this contradiction can be solved. In this article we briefly summarize the present status
of Negative Ion Sources (NIS). Then we describe ECR Ion Sources and how they can be of great interest for H"
production. Although this domain is still rather unexplored, some work has already been performed in the field of
negative ion production with ECR plasmas. That work will be shortly summarized. Eventually promising preliminary
results, obtained at CEA Saclay at 2.45 GHz, will be shown.
moreover the pulses should be noiseless and highly
reproducible, and the (normalized rms) emittance of
the source should be less than 0.3 pi mm mrad. Today
no existing source is able to fulfill all the requirements
for ESS. Therefore nine European laboratories have
recently joined their efforts to develop various
techniques for an improved negative ion production.
At CEA it was decided to develop an ECR Ion Source
for Negative Ions. In section 1 the various existing
types of NIS are shortly reviewed. Section 2 deals with
the present status of ECRIS, and the main features of
the plasmas of these sources; we also discuss the
possibility of H" production by an ECR plasma. In
section 3 the first tests on the ECR NIS developed at
CEA Saclay are described, and some prospects for
new developments are given.
INTRODUCTION
Electron Cyclotron Resonance (ECR) plasmas and
ECR Ion Sources (ECRIS) are now widely used in
accelerators: heavy ions are usually produced with
ECRIS delivering multiply charged ions [1]. Moreover
high currents of protons can also be produced in ECR
ion sources [2], although these sources are very
different from the previous type, from the point of
view of the magnetic configuration: ECRIS for highly
charged ions are basically mirror confined plasmas,
while high current proton ECRIS are moderate
confinement plasmas (only an axial magnetic field is
present). All these sources are able to produce both cw
and pulsed beams of high reliability with emittances
meeting the requirements of the accelerators. However
the production of negative ions with ECR plasmas is a
new domain: only few experiments have been
performed and no ECRIS is still used to inject negative
ions into an accelerator. Today there is a great interest
for high current accelerators; in particular the
production of high flux neutron beams for spallation
reactions (ESS), neutrino and muon production for
high-energy particle physics, are challenging new
projects in Europe and in the world. These projects
require intense beams of negative ions (in the range of
60 mA), with a long pulse duration (from 1 to 2 ms);
PRESENT STATUS OF NIS
NIS are usually divided in two main types: surface
sources, and volume sources.
Surface Sources
In surface sources [3] the negative ion production
occurs on electrodes in contact with the discharge
plasma. This plasma is of very small size, and a very
* Work supported by European Commission under contract HPRI-CT-2001-50011
CP642, High Intensity and High Brightness Hadron Beams: 20th ICFA Advanced Beam Dynamics Workshop on
High Intensity and High Brightness Hadron Beams, edited by W. Chou, Y. Mori, D. Neuffer, and J.-F. Ostiguy
© 2002 American Institute of Physics 0-7354-0097-0/02/$ 19.00
282
large power (typically tens of kilowatts) is injected to
sustain it. Hence the power density in the discharge
reaches a very high value, and the resulting currents of
Negative Ions are also very high. Penning SPS are
widely used for accelerators, and further developments
continue in that domain [4] to meet the requirements
for ESS.
ECRIS For Intense Proton Beams
After a pioneer work at Chalk River and Los
Alamos, ECRIS are now widely used for intense
proton beams [2]. As compared to the previous type of
ECR sources, the magnetic configuration is much
simpler: only an axial magnetic field is necessary; the
frequency used is 2.45 GHz, with a resonant magnetic
field of 875 G. The density reached is slightly below
10 12 cm"3, with an electron temperature of the order of
10 to 20 eV (much lower than in confined ECRIS).
The SILHI source at Saclay has now proved to be
efficient and reliable for proton production, and it will
be used for the next generation of high power proton
accelerators.
Volume Sources
In volume sources, the negative ions are produced
directly in the plasma; however small amounts of
cesium added in the discharge lead to a very
significant improvement of the performances for NI
production, which suggests that surface processes are
also of importance in "volume" ion sources. For
accelerator purposes the volume sources are usually of
moderate size (typically 1 liter for the plasma region).
Different techniques are used for the production and
confinement of the plasma in volume sources:
multicusps [5] are widely used for the confinement of
the plasma, but an axial magnetic field may also be
used; the plasma is excited by either filaments [6], or
RF excitation [7,8]. Usually more than ten kilowatts
are injected into such sources, which leads to a lower
power density than in SPS.
Because of these successes, it was decided to
continue to develop ECRIS for negative ions.
ECRIS FOR NEGATIVE IONS
Main Characteristics, Previous Work
In ECRIS the dimension of the plasma is of the
same order of magnitude as the wavelength of the HF
wave. Therefore 2.45 GHz operation requires typically
1 liter of plasma volume. This shows that ECRIS for
Negative Ions are necessary of the "volume" type.
Although only few experiments have been performed
so far, quite encouraging results have been obtained
previously: at Argonne [9], a few mA of H" were
extracted from an ECR with an axial magnetic
confinement. Similarly H" had been extracted from
multicusp ECR ion sources [10,11]. These results
show that it is possible to eliminate the HF-generated
hot electrons, which could destroy the negative ions. A
special magnetic configuration is however necessary.
PRESENT STATUS OF ECRIS
ECRIS For Multiply Charged Ions
ECRIS can produce beams of multiply charged
heavy ions [1]. For that purpose a complex magnetic
configuration is achieved, which confines the plasma
so that atoms can be step-by-step ionized up to high
charges. The plasma is produced by the resonant
interaction of the electrons with the High Frequency
(HF) wave at the electron cyclotron frequency
O)=eB/m; this interaction generates hot electrons
(mean energy up to tens of keV), which are then able
to ionize the atoms to high charges. It was found that
the performances of these sources are related to the
frequency of the wave: the density of the plasma
produced is nearly proportional to the square of the HF
frequency, which leads designers to use higher and
higher frequencies for improved performances. 28
GHz operation has been recently achieved. Because of
the production of the plasma via the HF wave, no
cathode is necessary, which enhances the reliability of
the source. This type of source is now the most
commonly used for heavy ion production in
accelerators.
An ECRIS For Negative Ions at CEA
Saclay
An ECRIS for negative ions has been designed
recently [12]. The starting point was the SILHI
magnetic field configuration. In order to prevent hot
electrons to move to the extraction zone, a magnetic
filter has been designed. A schematic drawing of the
source is shown in figure 1.
As compared to the above reference, the following
changes have been performed: the plasma chamber
source is now biased at HV, while diagnostics are
grounded.
283
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CONCLUSION
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reliability problems for future high power
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FIGURE 1.
1. The
The CEA
CEA Saclay
FIGURE
Saclay negative
negative ion
ion source.
source.
pulsed 90°
90° dipole
dipole analysing
analysing magnet
AA pulsed
magnet has
has been
been
installed behind
behind the
the 66 mm
installed
mm diameter
diameter hole
hole collector,
collector,inin
the diagnostic
diagnostic box.
box. The
The 10
10 mm
the
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central aperture
aperturedipole
dipole
is
made
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entrance
and
is made with non-zero entrance and exit
exit to
to allow
allow
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focusing. Good
Good transmission
vertical
transmission through
through the
the
dipole magnet
magnet is
is predicted
predicted for
dipole
for extreme
extreme particle
particle
trajectories (at the limits of geometrical acceptance).
trajectories (at the limits of geometrical acceptance).
As this magnet is not cooled, it operates in pulsed
As this magnet is not cooled, it operates in pulsed
mode, typically at a repetition rate of 1 Hz. The timing
mode, typically at a repetition rate of 1 Hz. The timing
of the source is tuned to extract a single pulse during
of the source is tuned to extract a single pulse during
the flat-top portion of the magnet current pulse. A
the
flat-topFaraday
portioncup
of isthe
magnet
current
shielded
located
at the
dipolepulse.
exit toA
shielded
Faraday
cup
is
located
at
the
dipole
exitAnto
eliminate plasma effects and secondary electrons.
eliminate
plasma
effects
and
secondary
electrons.
An
ACCT allows to measure the total extracted beam.
ACCT
allows
to
measure
the
total
extracted
beam.
Beam currents could be also measured on extractor
Beam
currents could
be also cup
measured
on extractor
and collectors.
The Faraday
measurements
are
and
collectors.
The
Faraday
cup
measurements
done with an amplifier and its screen is biased byare
a
done
with anvolt
amplifier
its screen
few hundred
tuneableand
power
supply. is biased by a
few hundred volt tuneable power supply.
The dipole was calibrated with positive extracted
+
+
The dipole
positive
extracted
charges
(H*,
H2+was
, H3+calibrated
). Then thewith
source
was negatively
+
charges
(H
,
H
,
H
).
Then
the
source
was
negatively
2
3
biased at -6.4 kV and fed by hydrogen and helium gas.
biased
–6.4 kV
and fed
by hydrogen
andand
helium
gas.
40 mAat total
current
is easily
extracted,
a peak
40
mA total current
is can
easily
extracted,
and a This
peak
characteristic
of H" ions
be clearly
identified.
characteristic
H- ions
can be plasmas.
clearly identified.
This
peak does notofappear
in helium
The presence
peak
in helium
plasmas.
The presence
of H"does
ionsnot
hasappear
therefore
been proved.
However
more
of
H- ions
has thereforehave
beenstill
proved.
precise
measurements
to be However
performed,more
to
give a precise
value ofhave
the H"
current
fromto
precise
measurements
still
to beextracted
performed,
the source.
Nevertheless
this H
result
is very
encouraging
give
a precise
value of the
current
extracted
from
andsource.
demonstrates
(if necessary)
thatisECR
can
the
Nevertheless
this result
very plasmas
encouraging
produce
H- ions. (if necessary) that ECR plasmas can
and
demonstrates
produce H- ions.
REFERENCES
REFERENCES
1. A. Girard and G. Melin, Nucl Instrum. Methods A 382,
1. A. Girard and G. Melin, Nucl. Instrum. Methods A 382,
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9. Modified
D. Spence,
K.R. Lykke
and G.E. McMichael,
Production
of High-current,
high purity cw“Plasma
H+,
D+
and H- Production
beams fromofMicrowave-driven
Modified
High-current, highSources"
purity cwinH+,
Linac96,
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H- beams from Microwave-driven Sources” in
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11. O. Fukusama and M. Matsumori, Rev. Sci. Instrum. 71,
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11.935-938
O. Fukusama
935-938
(2000).
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al, Rev. Sci. Instrum. 73, 983-985 (2002).
12. R. Gobin et al, Rev. Sci. Instrum. 73, 983-985 (2002).
Future Plans
Future
ECR-based NISPlans
needs
This
still many
improvements,
in
particular
for
the
diagnostics
the
This ECR-based NIS needs still ofmany
negative
ion
beam.
Moreover
the
influence
of
the
improvements, in particular for the diagnostics of the
magnetic ion
filterbeam.
has to be
optimized.
negative
Moreover
the influence of the
magnetic
hashand,
to be another
optimized.
On thefilter
other
ECR-based prototype
willOnbe the
developed
at CEA
Grenoble:
a 10 GHz
source
other hand,
another
ECR-based
prototype
will be developed at CEA Grenoble: a 10 GHz source
284