I -2

MULTIPLY CHARGED HEAVY ION SOURCES
Fumio Fuk:uzawa
Department of Nuclear Engineering, Kyoto University
Abstract
Formation processes of multiply charged heavy ions in the electron-atomcollisions are reviewed and recent developement of various ion sources arebriefly summarized.
1.

Introduction

Multiply charged heavy ion sources are necessary to construct economicallyRequireheavy ion accelerators such as a cyclotron and a linear accelerator.
ments on the modern heavy ion sources are; production of all atomic species,
high intensity, good emittance and lon lifetime.
For application to atomic
spectroscopy of highly charged ions, extraction of ions as a beam is not necessarily required.
Recent progress in heavy ion sources can be seen in the papers reported
at two Gatlinburg Conferences on heavy ion sources 1 • 2).
Excellent review
given by D.J.Clark3) is also frequently referred for the present paper.
2.

Formation of Multiply Charged Ions
2.1

Ionization Processes

There are two different processes for the formation of multiply charged
ions by electron-atom impacts; single collision ionization and multiple collision ionization.
In the single collision, only one impact can cause highly charged ions.
The cross sections of the simultaneous ejection of many electrons are verysmall, and main process for the production of multiply charged ions in the
single collision is the production of the inner shell vacancy followed bysuccessive Auger or shake off processes.
The times required for theseprocesses to complete are about lo-15 sec.
In the multiple collision, only one electron is removed by one impact andmultiply charged ions are formed through step by step ionization.
Ejectionsof more than one electrons in one impact do not change the overall featuresof the charge state distribution, but may have important contribution in
determination of the population of extremely high charge state.
In the heavy ion sources, single collision ionization and multiple collision ionization occur simultaneously.
Although determination of the relative
contribution of each process is difficult, there is some general argument 4).
In the ion source with high energy electrons and low current density, only
single collision process is to be considered for explaning high charge state.
While, that with low energy electrons and high current density, multiple ones
may be dominant.
2.1

Charge Fraction Ratio in the Plasma

Charge state distribution of ions in the ion source is determined by7

Expressions for charge
the balance between production and destruction rates.
fractions are different according as what processes are taken as production
and destruction processes.
In the "corona rnodel"S), as representative, collisional ionization(Az+ + e ~ A(z+l)+ + 2e) and radiative recombination (A(z+l)+ + e-+ Az+ + hv)
are considered to be balanced,
neNzSz = neNz+laz+l(1)zwhere Nz' Nz+l : ion density of charge state z+, (z+l)+
electron density
necollisional ionization coefficient for z+ -+ (z+l)+
Szaz+l : radiative recombination coefficient for (z+l)+ -+ z+.
Sand a are functions of electron temperature T and ionization potential Xz·
By using suitable expressions for S and a, one 5btains
Nz/Nz+l = az+l/S(2)
Nz/Nz+l = 7.87xlo- 9xz 2 ( Xz/kTe) 3/ 4exp(xz/kTe)'
where ionization potential Xz is in eV and electron temperature Te is in °K.
This model is valis in an electron density domain 6 )(3)n e < 3 x10 13Xz 3 (kT e ) 112
This means that the corona model is applicable
where kTe and Xz are in eV.
laser plasma type.
includingsourcesionpractical
to allIn deriving above expressions, it is assumed that changes in the population
densities caused by variations in the electron temperature or density take place
very slowly in comparison with the intrinsic rate of relaxation of the atomic
The intrinsic relaxation timeT canprocesses (steady state corona model).
be obtained approximately from the growth equations in the neighbourhood ofsteady state, that is,(4)
dN z/dt = n eNz- 1sz- 1 - n eNza z(5)
Nz-l + Nz = constant.
Solving these eqs., one obtains the following relation,(6)
ne T = 1/(S z- 1 +a).
z
-12 2 -1 andIn considering the steady state values one may put a z "' 10 ern sec
Sz- 1 "' a z , and obtains the relaxation time T to an order of magnitude,(7)'"' 1012 ;n (sec).
eFor most ion sources, ion confinement times are shorter or about the same as
the values ofT, so that it is necessary to use a time-dependent corona modeland to solve set of differential equations.
From eqs. (2) and (7), it is clear that electron temperature kT andne'i (Ti : ion confinement time) are good measures chracterizing theeabilities
In Fig. 1 are shown the operatingof production of multiply charged ions.
ranges of several type of heavy ion sources 3).
2.3
8Stripping of Projectiles

I -2

In the formation of multiply charged ions in the plasma, only relatibe
It is possible to use
motion between electron and atom (ion) is concerned.
the reverse process for production of multiply charged ions, that is, highvelocity projectile ions passing through electron cloud at rest.
It seems in the first that if the projectile velocity is same as theelectron velocity in the plasma case the charge state distribution of the
projectile ions after passing through electron cloud is same as that in theIn Fig. 2 are shoen the average charge
corresponding plasma ion source.
calculated with corona model and projectile data using T (eV) = 363 E/A QMeV/e6)
Comparison should be made between corona model and gas target data.
nuc)
Qg , because solid target data may contains complex phenomena such as density
effect, excitat1on pile up and surface effect.
Very good agreement can be obtained if the corona model calculation is
Therefore, the relation between
made with four times lower temperature.
electron temperature and corresponding projectile energy should be(8)Te(eV) = 90 E/A (MeV/nuc).
This lowering of temperature is due to the fact that the corona model charge
distribution is largely determined by the high energy tail of the Maxwellian.
For fast projectile, the time spent in the charge changing foil is soFor
short that equilibrium charge state distribution cannot be attained.
example, 10 MeV/ A projectile spents 10-12 sec in the foil of 10 mg/cm2 thick.
While, electron density in the foil of n = 10 22 cm- 3 leads to T = 10-lO sec.
Therefore projectile ions should be in ngn-equilibrium charge state distribution.
3.

Various Ion Sources for Production of Multiply Charged Ions

Characteristics of multiply ion production in some typical ion sources
are compared in Table I 3).
3.1

PIG

PIG type are standard and most widely used heavy ion sources for many
Plasma characteristics are; arc voltage of 300-2000 volts andaccelerators.
When solid materials arearc current of 1-20 amps, de or pulsed operation.
used for heavy ions, either oven is connected to the anode or the block isplaced tangentially in the bore of anode.
3.2

DuoplasmatronThis type of source for high beam current can be used also for the producCharge state is lower than that of PIG, buttion of multiply charged ions.
UNILAC prefers a duoexcellent in emittance and lifetime characteristics.
plasmatron than a PIG for light ions.
3.3

ECR (Electron Cyclotron Resonance)

Most advanced ion sources of this type are MAFIOS and TRIPLEMAFIOS at
Output current and charge state from MAFIOS is similar to thoseGrenoble.
In TRIPLEMAFIOS, average charge increases about two times, but atof PIG.
These sources work well more than one day without
100 timee lower intensity.
Direction of developement of ECR sources is to use superconductmaintenance.
ing coil to reduce present megawatt power and to make the source more compact.
93.4

EBIS (Electron Beam.Ion Source)

EBIS is expected to produce very highly charged ions with sufficient
intensity, and is veveloped at Dubna, Orsay, Frankfurt, Texas A&M, Kiessen andFully
This source is hopeful for synchrotron injector.
Nagoya University.
stripped ions are produced now up to Ne. Without large accelerator, EBIS has
been used for charge exchange collision experiment on highly charged ions.
Technical problems on EBIS are;·high precission in construction, superconducting
solenoid, good vacuum (l0-10 Torr).
3.5LaserLaser plasma also produces very higlliy charged ions, but repetition rate
This source is very usefulis too small to use for accelerator ion source.
for atomic spectroscopy of highly charged ions in the same way as vacuum sparkand exploding wire.
3.6

Tokamak, Electron ~ing Accelerator, Linear Electron Beams

These machine do not aim the production of multiply charged ions, but canproduce them effeciently because of large nT values.
3.7 Stripping of Fast Projectiles

As mentioned in 2.3, foil or gas stripping of fast projectiles can producehighly charged ions.
Expected charge states from various ion sources stated above are summarized
in Fig.3.
In Figs. 4-8, are shown some of the multiply charged ion sources developed
The hot cathode PIG source with a sputtering electrode (Fig. 4)
in Japan.
and that with oven (Fig. 5) are developed for the IPCR cyclotron and linac.
The hot cathode PIG source for N and Ne ions (Ping. 6) was installed in the INS
SF cyclotron. A compact ECR source (Ping. 7) is developed for Cockcroft-Walton
The EBIS (Fig. 8) is build also at Nagoya
Accelerator at Nagoya University.
University for study of atomic processes.
References
1. International Conference on Multiply-Charged Ion Sources and Accelerating
Systems, IEEE Trans. Nucl. Sci. NS-19 No2 (1972)
2. International Conference on Heavy Ion Sources, IEEE Trans. Nucl. Sci. NS-23
No2 (1976)
3. D.J.Clark, IEEE Trans. Nucl. Sci. NS-24 No3 1064(1977)
4. A.Septier, article in ref.l p22
5. R.W.P.McWhirter, Plasma Diagnostic Techniques (Acad Press) Chap 5 (1965)
6. D.J.Nagel, Beam Foil Spectroscopy, vol 2 p961. ed, I.Sellin andD.Pegg,(Plenum Press) (1976)

10I -2......q105

SOURCENEONXENONPEAKOUTPUTALL q's
PART/SEC

DUTY
FACTOR

1015_1017.02-1.0

10 15-1017

.03-1

PIG

2

DUOPLASMATRON

1ECR-1 STAGE

21016.3-1.058

10 14.3-1.0

8-9

24

a.1o 10 -1o 11

10- 4

3:0z0(l)3 STAGESw

EBIS-b.to 13 -to 14

Duoplosmotronqweighted by part/sec.
a. Average output over long times, assu;uing
10 pulses/sec.
b. Output during 100 "sec, 10 9 -10 10 part/pulse.

Table I. Ions and intensities.
( D.J.Clark, ref. 3 )Fig. 1. Plasma parameters.
( D.J.Clark, ref. 3000N"'00Q00
Q-g aJ ()"'>cOo-0~"'c::lss:(!)

a:

Q ..!!
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.E·.C

"'o-o;

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N0

Fig. 2. Average charge states
for iodine.
D.J.Nagel, ref. 6 )Fig. 3. Charge states available
from ion sources.
( D.J.Clark, ref. 3 )11--~~1...

Fig. 4.

Fig. 5.

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Fig. 7.

CO~IPENSATION

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