30 YEARS OF HIGH-INTENSITY NEGATIVE ION SOURCES FOR
ACCELERATORS
Vadim Dudnikov
Fermi National Accelerator Laboratory
*
, Batavia, IL 60510, US
Work supported by the U.S. Department of Energy under contract
No.DE-AC02-76CH03000. Dudnikov@fnal.gov
Abstract
Thirty years ago, July 1, 1971, significant enhancement
of negative ion emission from a gas discharge following
an admixture of cesium was observed for the first time.
This observation become the basis for the development of
Surface Plasma Sources (SPS) for efficient production of
negative ions from the interaction of plasma particles with
electrodes on which adsorbed cesium reduced the surface
work-function. The emission current density of negative
ions increased rapidly from j~ 10 mA/cm
2
to 3.7 A/cm
2
with a flat cathode and up to 8 A/cm
2
with an optimized
geometrical focusing in the long pulse SPS, and to 0.3
A/cm
2
for DC SPS, recently increased up to 0.7 A/cm
2
.
Discovery of charge-exchange cooling helped decrease
the negative ion temperature T below 1 eV, and increase
brightness by many orders to a level compatible with the
best proton sources, B= j/T> 1 A/cm
2
eV. The
combination of the SPS with charge-exchange injection
improved large accelerators operation and has permitted
beam accumulation up to space-charge limit and
overcome this limit several times. The early SPS for
accelerators have been in operation without modification
for ~25 years. Advanced version of the SPS for
accelerators will be described. Features of negative ion
beam formation, transportation, space-charge
neutralization- overneutralization, and instability damping
will be considered. Practical aspects of SPS operation and
high brightness beam production will be discussed.
1INTRODUCTION
One practical result of development of high brightness
negative ion source is accepting of the charge-exchange
injection in circular accelerators for a routine operation.
Now negative ion sources are “Sources of life” for
gigantic accelerators complexes as FNAL, BNL, KEK,..
and an efficiency and reliability of these sources
operation are determined a productivity of these big
collaborations. Many results of the high energy physics
were discovered with using of negative ion sources.
Development of high brightness H
-
sources was
stimulated by first success of high current proton beam
accumulation with using a charge-exchange injection [1]
and supported by interest of “Star War” [2]. A recent
circumference was a reason of difficulties and long delay
of publications, but nonofficial communication was
relative fast. Until 1971 a main attention was concentrated
on the charge-exchange ion sources, because was no hope
to extract from the plasma directly more than 5 mA of H
-
.
At July 1, 1971 at the Institute of Nuclear Physic (INP),
Novosibirsk, by author almost occasionally was observed
very short in time enhancement of negative ion emission
from the magnetron (planotron) plasma source following
by introducing a cesium admixture to the gas discharge.
Fortunately this brief observation, considered in review
[3], was not lost and was developed and understood as a
new method of negative ion production in interaction of
plasma with a surface and a basis for development of a
Surface Plasma Sources (SPS). In the patent application
[4] was considered “ a method of negative ion production
in gas discharges, distinguished by adding to the discharge
along with a working substance an admixture of substance
with a low ionization potential as cesium, for example, for
enhance a negative ion formation”. Further development
of SPS was conducted by cooperation Belchenko, Dimov,
Dudnikov (BDD). The first high brightness SPS for
accelerator has been developed by author [5]. The
Semiplanatron SPS with an efficient geometrical focusing
has been developed by author at 1977 [6]. Further R&D
of high brightness SPS was conducted in cooperation with
Derevyankin. Very fast development and adaptation of
SPS has been start in many USA laboratories, in Europe
and in Japan, and International Symposiums for
Production and Neutralization of Negative Ions and
Beams has been established [7]. Now the Surface Plasma
Method of negative ion production and SPS considered in
many books (recently in book [8]). Good review of SPS
for accelerators presented in reports of J. Peters [9-11].
Development of high current SPS (tens of Ams) for
thermonuclear plasma heating is in progress and used in
experiments [12]. Production of polarized negative ions
by charge- exchange with a slow negative ion in SPS has
been proposed by author and has been realized with a
good success [13]. This development has permeate to use
a charge exchange injection for accumulation of high
intense beams of polarized ions in circular accelerators. In
R&D of heavy negative ion SPS for technology
application a good success has been reached [14].
2FEATURESOFSPS
The efficiency of negative ion formation depends very
much on the catalytic property of the surface, mainly the
work-function. For enhanced negative ion formation in
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Proceedings of the 2001 Particle Accelerator Conference, Chicago
the SPS a mixture of substances with a low ionization
energy, such as alkaline or alkaline earth elements or
compounds, are used. Most efficient is the addition of
cesium. Still the surface work-function and catalytic
properties of the surface for negative ion formation
depends very much on many parameters such as surface-
cesium concentration, admixtures of other compounds,
such as oxides, halides, nitrides, and surface temperature.
Small changes in the surface condition dramatically
change the efficiency of negative ion formation. It is a
fine art and some magic to optimize the surface and
plasma condition for high efficiency of negative ion
formation. This condition is a strong reason for the
variation in efficiency of negative ion production although
conditions look very similar. Small changes in the surface
condition can increase or decrease the intensity of a
negative ion beam by large factors. Often the intensity of
H
-
and D
-
beams from a 1x10 mm emission slit as a
function of discharge current I
d
could vary from 200 mA
to 10 mA for the same discharge current. A stronger
variation can has a beam brightness. An efficient ion
temperature can have a variations from a
Figure 1: The discharge voltage and level of noise vs.
magnetic field in SPS with Penning geometry.
part of eV to some keV. It is easier to have stable
operation with relatively low beam parameters such as
intensity I ~30-50 mA, emission current density J ~0.5-1
A/cm
2
, transverse ion temperature Ti ~5-10 eV. Present
experience permits better optimization for long stable
production of high-brightness high-intensity beams of
negative ions ( I ~0.1-0.15 A, B ~J/Ti > 1 A/cm
2
eV,
lifetime N>10
8
-10
9
pulses). Highest brightness could be
reached only with noiseless operation. The level of
discharge noise (hash) is depend of many parameters. For
stable discharge a surface properties should be in the
stable conditions and frequency of electron scattering by
plasma particles should be higher than Larmour
frequency. A discharge noise could be suppressed by
decrease of magnetic field as shown in Fig. 1 and by
increase a gas or Cs density. Admixture of heavy gas
could be useful for noise suppression, but it increase a
sputtering. Examples of discharge voltages with a
different level of noise are shown in Fig. 2. A transition
from the noise discharge (a) to noiseless one (c) increases
a beam brightness at order of magnitude.
Figure 2: The examples of discharge voltages for different
conditions in SPS. (a) a discharge with noise; (b)a
discharge with RF generation: (c) noiseless discharge.
Vertical scale is 100 V/div; Horizontal scale is 0.2 ms/div.
3NEGATIVEIONSOURCESFORACELERATORS
The first versions of the Surface-Plasma Sources (SPS)
developed for charge-exchange injection of protons have
an operating intensity I ~50 mA with pulse lengths of
0.05-1 msec, noisy discharges and a repetition rate up to
50 Hz [6-9]. H
-
beam parameters of these SPS was
sufficient for normal operation of large proton accelerator
complexes during the past 25 years without significant
modernization of ion sources. Now, new accelerator
projects need an increase of the ion beam intensity and
brightness. Some upgrading of existing SPS could achieve
the necessary increase of intensity, duty factor and beam
quality without degradation of reliability and availability
of the achieved satisfaction level.
The Fermilab Magnetron SPS has been operational since
1978 [15]. The peak current of the H
-
ion beam at the exit
of the 750 keV
accelerator column is I
b
=65 mA with an
extraction voltage Uex= 20 kV, and I
b
~70 mA with Uex
= 25 kV with a beam pulse length T= 0.075 msec at 15
Hz. The pulse length could be increased with a new arc
discharge pulser and adjusted parameters. It is useful for
stable operation to have a discharge power supply as a
current source with a high impedance (Z= 5-10 Ohm, now
Z=1 Ohm) and corresponding higher voltage.
Optimization of the discharge electrode configuration
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Proceedings of the 2001 Particle Accelerator Conference, Chicago
should help to increase the intensity above I
b
= 0.1 A
without increasing the discharge power above acceptable
levels. Gas delivery optimization should allow a longer
pulse and higher intensity without an increase of the gas
loading.
An optimized extraction system with a suppression
electrode should improve the beam intensity, beam
quality and beam space-charge neutralization with a low
gas pressure. A suppression of the positive ion extraction
to the accelerating gap should suppress cathode and anode
sputtering by accelerated positive ions - a main reason for
the short ion source lifetime. Improved cathode and anode
cooling is necessary for increased discharge pulse length
and intensity. The Semiplanotron version of the SPS is
good for operation at higher duty factor.
From previous experience it is possible to have reliable
operation of a SPS with parameters: peak current after
extraction (bending magnet) I
b
~0.12-0.15 A with pulse
duration of T ~0.25 msec, repetition rate F= 100 Hz
normalized emittance ε(90%) =1π mm mrad. SPS with
these parameters was tested with a relatively long run
[16].
Lifetime of SPS determined by electrode sputtering and
flakes formation. It is dominate a cathode or anode
sputtering by back accelerated positive ions. Suppression
of positive ion is important for increase lifetime.
Optimized cesium film recycling (deposition-desorbtion)
could be used for shielding of electrodes from the
sputtering and can reduce the sputtering to a very low
level. Cesium in the SPS acts as an oil in an engine,
increasing the operational lifetime. “Cold Start” of a
discharge without cesium for a few minutes could be
more destructive than many hours of low voltage
operation. Emission current density of H
-
up to J ~ 1
A/cm
2
has been observed in discharges without cesium. A
fingerprint with a trace of Na or K could increase the
efficiency of H
-
production significantly. But the power
density in a discharge without cesium is very high and the
sputtering rate is much higher. Electron emission from ion
source without cesium is very high.
Recently, new version of RF SPS has been developed in
DESY[9-11], SPS with pulsed arc discharge in Frankfurt
University and in KEK for high intense Proton Driver.
4LOWENERGYBEAM TRANSPORT
The ion beam from a compact SPS has a very high current
density (j~1-3 A/cm
2
) and perveance. For transport of
these beams it is necessary to use a deep space-charge
neutralization (compensation) or very strong continuous
focusing by electrostatic forces as in the RFQ.
Partial compensation of space charge with magnetic
focusing and nosy operation will create a strong variation
of focusing and lead to an increase of emittance by ellipse
rotation. Still, this mode of transport is used in almost all
injectors, and until recently it was acceptable. Space
charge compensation by ions has some difference from
the compensation by electrons. Ion oscillation in the
potential of the beam is more coherent and can be a
reason for very strong and fast beam-ion instability.
Beam-ion instabilities have been observed recently in the
electron beam of the Advanced Light Source (LBL) with
increased residual gas density. In low energy negative ion
beams this instability has been observed many years ago
(1976). A development of this instability along H
-
beam,
coherent oscillations of positive ions in the beam potential
excite quadruple and dipole oscillation of the H
-
beam,
and developed a decompensation and emittance growth.
To eliminate this problem many versions of an
electrostatic focusing-transport LEBT has been proposed.
Now under development and testing is an ELEBT for
SNS. Transport of a H
-
beam of energy 65 keV with
intensity up to 40 mA under development [16].
The beam-ion instability could be damped by over-
neutralization of the beam, changing the sign of the beam
potential with an increase of the ion density in the beam.
With increased an ion and electron density a stable beam
transport could be reached with additional focusing by
reversed space charge. This solution could be used for a
short transport with acceptable levels of ion loss by
stripping. This solution is convenient because it is
possible to locate a second (spare) ion source in front of
one RFQ. Ion beam pulses from this ion source could be
long enough for reaching a deep over-neutralization.
A good solution could be a short LEBT with a fast beam
over-neutralization by streams of noiseless plasma from a
separate plasma source. With magnetic focusing, beams
from 2 SPS could be steered to the entry of the RFQ.
Close-coupled systems has been tested in ion
implantation.
5REFERENCES
[1] G.Budker, G. Dimov, V. Dudnikov, in Proc. Internat.
Symposium on Electron and Positron Storage Ring,
France, Sakley,1966, rep. VIII, 6.1 (1966)
[2] C. Robinson, Aviation Week&Space Tech.,p.42, oct.,
1978. Rev. Mod. Phys., 59(3), Part II,1987.
[3] V. Dudnikov, Rev. Sci. Instrum., 63(4),2660 (1992).
[4] V.Dudnikov, The Method for Negative Ion Production,
SU patent, C1.H013/04, No 411542, Appl. 3/3/72.
[5] V. Dudnikov, Proc. 4
th
All-UnionConf.OnCharged
Part. Accel.,Moscow,1974’ V.1, p.323.
[6] V. Dudnikov, Yu. Belchenko, Preprint, INP 78-95,
Novosibirsk,1978.
[7] C.W. Schmidt, C. Curtis, IEEE Trans. Nucl. Sci. NS-
26,4120 (1979).
[8] H. Zhang, Ion Sources, Springer,1999
[9] J.Peters, LINAC’ 98, Chicago, 1998.
[10] J.Peters, Rev. Sci. Instrum., 71(2),1069 (2000).
[11] J.Peters, EPAC’2000.
[12] Y.Okumura, et al., Rev. Sci. Instrum.,71(2),1219
(2000).
[13] A. Belov, V. Derinchuk, This Conference.
[14] J. Ishikawa, Rev.Sci.Instrum., 67(3) 1410 (1996).
[15] C. W. Schmidt, Prod. Neutralizat. of Negative Ions
and Beams, 8
th
Internat. Symp. AIP 1-56396-773-5.1998.
[16] G. Dimov, V. Dudnikov, G. Derevyankin, IEEE
Trans. Nucl. Sci. NS-24,1545 (1977).
[17] R. Keller et al., This conference.
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