Ion Sources and Beams

In this chapter we discuss ion sources, many of which owe their origins
to the experimental necessity of producing ion beams of gaseous, rare, or
refractory elements.
Some of these sources, like the von Ardenne, Zinn, and Penning sources,
originated with atomic and nuclear physics research, while others, like
the Kaufman and surface ionization sources, were developed in the
1960’s for space propulsion applications.
Other ion sources had their origins in ion implantation for
microelectronic applications.
In this chapter we briefly survey some of the more widely used and most
useful ion sources; a more complete discussion of ion sources and beam
optics may be found in several of the monographs listed at the end of this
chapter, including Brown (1989), Dearnaley et a1 (1973), Forrester
(1988), Humphries (1990), and Wilson and Brewer (1973).

CHARACTERISTICS OF ION SOURCES

 The variety of electron sources is limited by the relatively
few physical processes which produce free electrons.
 These include thermionic, photoelectric, secondary
electron, and field emission; and extraction from a hollow
cathode plasma.
 Ion sources, on the contrary, exist in a profusion of forms
because of the requirements of a wide variety of industrial
and research applications; because of the necessity of
generating ions from solids, liquids, or gases; and because
of the many ways of generating the DC glow discharge or
arc plasmas from which the ions are extracted..

A generalized ion source is shown in figure
6.1.
Since the purpose of any ion source is to
produce a more or less unidirectional,
monoenergetic ion beam, such sources are
operated in vacuum systems at pressures
below 1 m Torr, low enough that the ion
mean free paths are longer than the distance
between the source and its target.

Figure 6.1 Schematic drawing of a generalized ion source.

The ionization source must have a
provision for introducing and metering the
material from which the ions are formed.
 This is usually accomplished with
consumable electrodes or rods for solid
materials; fine capillaries for liquids; and
conventional gas flow and metering
systems for gases.

 In all cases, the material to be
ionized must be supplied at a rate
equal to the flow carried away by
the beam, plus any losses in the
source.
Once introduced to the ion source,
the material must be ionized.

This is usually done in a low pressure DC
glow discharge plasma, from which the
desired ionic species is extracted through a
grid.
 The discharge plasma can be generated by
a wide variety of processes which include
magnetized or unmagnetized operation, and
a wide range of (sub- atmospheric) neutral
gas pressures.

 The mechanism for generating the plasma
discharge is the principal distinguishing
characteristic among ion sources, and has
been used to organize the material later in
this chapter.
 An infrequently used ionization mechanism
is surface contact ionization, in which atoms
of the material to be ionized come into
contact with a hot, refractory metal.

If the ionization potential of the atom is less
than the work function of the refractory
metal, the incident atom will be ionized on
the surface.
This requirement is met only for alkali
metals, such as sodium, potassium,
rubidium, thallium, and cesium.
Refractory metals used in such sources
include tungsten and molybdenum.

After ions are generated in a glow discharge plasma and
extracted, it is usually desired to form them into a
unidirectional, monoenergetic beam.
 This is done with a set of accelerating grids, indicated on
figure 6.1.

These grids may be designed for near
space charge limited flow at the Child
law limit, or for lower current density
beams.
The ion optics of the accelerating grids
is designed to reduce the beam
divergence angle to its required value.

Upon exiting the ion source in figure 6.1, the
beam will diverge with an angle θ, the value
of which will depend on the design of the
extraction and accelerating grids.
In some applications, the ion beam is
neutralized by electron emission from a
hollow cathode beam neutralizer.
6.1.3 Ion Beam and Neutralization

This prevents a spacecraft from
charging up in space
applications, possible plasma
instabilities, or beam ‘blow-up’
due to mutual repulsion of ions
in the beam.

FIGURES OF MERIT OF ION SOURCE

Perveance
Perveance is a notion used in the description of
charged particle beams.
The value of perveance indicates how significant the
space charge effect is on the beam’s motion.
The term is used primarily for electron beams, in
which motion is often dominated by the space charge.
The perveance of an ion or electron source with
fixed cross-sectional beam area is given by equation
(5.18) 𝑷 =
𝑰𝒃
𝑽𝟎
𝟑/𝟐
=
𝑱𝝅𝑫𝟐
𝟒𝑽𝟎
𝟑/𝟐
(6.1) , (5.18)

where D is the initial beam diameter
on figure 6.1, and J the current density.
The perveance of an ion source with
fixed cross-sectional area is a function
only of the charge state and mass of the
ions, and the source geometry.

The geometric factors include the thickness
and spacing of the extractor and
accelerating grids, the diameter of the
extractor holes, and the ratio of open area to
total cross-sectional area of the grids.
The perveance increases, for example, as the
ratio of open area to webbing area of the
grids increases.

An ideal ion source with planar accelerating
grids separated by the distance d, with no
webbing between the extractor holes in the
grids, and an extraction voltage Vo across
the accelerating gap d would operate at the
space-charge limited Child law current
density given by
𝑱𝑪 =
𝟒𝜺𝟎
𝟗
𝟐𝒆𝒁
𝑴
𝑽𝟎
𝟑/𝟐
𝒅𝟐 (A /𝒎𝟐
) (6.2)

where M is the ion mass, and 2 its
charge state.
 Such an ideal ion source would
operate at the maximum perveance,
(6.3)
The closer a source is to this value,
the better its design.

Source Energy Cost
The power required to produce an
ion beam is composed of the beam
power P𝒃 = 𝐈𝐛𝐕𝟎 (6.4)
and the power required to operate the
source, 𝑷𝒔.
The source power may include the
following contributions:

1. 𝒑𝒎 - power to electromagnets,
2. 𝒑𝒅 - power to maintain the source
discharge,
3. 𝒑𝒇 - power to supply filaments or cathodes,
4. 𝒑𝒐 - other power drains to maintain the
source.
The source power may be therefore be written
(6.5)

Where 𝑰𝒃 is the beam current, and V* is the 𝒆𝑽
per beam ion required by the source to produce
the ion.
For ion sources, one can make the following
observations about V* :
1. The absolute minimum value of V* is the
ionization potential of the gas used.
2. For space-related ion engines prior to 1975,
170<V*<250 eV/ion.

3. For current state-of-the-art in ion engine
technology, 90<V*<190eV/ion.
4. In fusion and non-optimized industrial ion
sources, V* is usually much greater than these
values.
5. From the gas discharge physics of Chapter 8,
the best one should hope for is (ղ𝒎𝒊𝒏)−𝟏, in volts
per ion-electron pair.
This value is V* = 50 eV/ion for a mercury plasma.

Efficiency of Ion Sources
The total power required by the
source is given by
(6.6)

The electrical efficiency
of the source, ղ𝑬, is the
ratio of the beam to the
total power, given by
(6.7)

Figure 6.2: The electrical efficiency ղ𝑬 as a function of
V*(eV/ion) and the accelerating potential V𝟎.

For space-related ion engines, values of ղ𝑬 > 0.95
have been achieved.
 Equation (6.7) can be written as
(6.8)
The electrical efficiency is plotted as a function of the
ratio V*/V𝟎 on figure 6.2.
 By making a calorimetric measurement of the beam
power, and the total electrical energy into the source,
ղ𝑬 can be calculated, and V* can be obtained from
figure 6.2.

Another important figure of merit for ion
sources is how fully the source uses the input
neutral gas flow to produce ions.
 If too little of the gas is ionized, the source
becomes a heavy load on the vacuum system.
The gas (propellant) utilization efficiency ղ𝐠 is
defined as
(6.9)

In space propulsion applications, values of ղ𝒈 >0.90 are
routinely achieved.
In such applications, the conservation of propellent mass is
imperative on long space missions, but many industrial
applications involving radioactive, precious, or rare atomic
species can benefit from a high gas utilization efficiency,
aside from the desirability of reducing the gas load on the
vacuum system.
Some recently developed electrode-less, high density
plasma sources also have a high mass utilization efficiency,
because of their high ionization fraction.

A comprehensive figure of
merit for an ion source is the
overall efficiency ղ𝒐 ,
(6.10)
Values of ղ𝒐 > 0.85 have been achieved for
Kaufman ion engines; MPD and other
plasmadynamic sources used industrially rarely

ION SOURCE PERFORMANCE PARAMETERS

In this section, we will discuss performance
parameters which are particularly important in
applications of ion sources.
 Some of these are specializations to ion beams of the
electron beam parameters discussed in Chapter 5,
between equations (5.23) and (5.27).

Required Gas Flow Rate
Ion sources must be provided
with ionizable neutral gas, at a
flow rate implied from equation
(6.9), which is
(6.11)

At background pressures below 1mTorr, (0.13Pa), this
represents a significant pumping load on most industrial
vacuum systems.
For example, an ion source producing 1 A of singly-charged
ions from a source with gas utilization efficiency ղ𝒈 =0.50
will require a gas flow of 1.25× 𝟏𝟎𝟏𝟗
atom/s.
At a pressure of p=𝟏𝟎−𝟒
Torr of argon, this would equal the
flux of room temperature argon atoms impinging on a
circular opening in the vacuum system wall 18.5 cm in
diameter-the size of a moderately large diffusion or turbo-
molecular pump.

Ion Flux in Beam
For an ion beam with total
current I and diameter D, the
flux in the beam may be written
(6.12)

If the source is space-charge
limited, we can write the
beam flux in the form
(3.145)
(6.13)

Note that 𝜒 is a function of ion
mass.
This ion flux must be provided by
the discharge plasma which is part
of the ion source.

Beam Number Density
If the source is other than
space-charge limited, the beam
number density may be
obtained from
(6.14)

where 𝒗𝒃 is the velocity of the beam ions,
obtained from conservation of energy in
equation (3.156).
 Substituting equation (3.156) into equation
(6.14) and solving for the beam number
density yields
(3.156)
(6.15)

If the source is space-charge limited, the
number density at the exit and in the beam is
given by equation (3.150)
(6.16)
This density is independent of the mass of the
(3.150)

Electrode Lifetime Limits
Important to the lifetime of ion sources is the reduction of
particle bombardment of the accelerating electrodes of a
kind shown in figures 6.3 and 6.4.
Figure 6.3 shows ion impingement due to an
inappropriately designed electrode geometry; figure 6.4
illustrates the defocusing effect of operating at other than
the designed extraction voltage.
 Figure 6.4(a) shows non-intersecting trajectories at the
design point.

Figure 6.3 Illustration of direct ion impact on an
accelerating grid due to improper design of the grid.

a b
Figure 6.4 The defocusing of an ion beam as the result of off-optimum
operating conditions. (a) Properly optimized ion trajectories at the operating
voltage. All ion trajectories, including those at the edge of the beam, pass
through the gap in the accelerating electrode. (b) Off-optimum operating
voltage, which results in ion trajectories that intersect the accelerating
electrode.

figure 6.4(b) shows ion impact on an
electrode due to the defocusing resulting from
a lower than designed extraction voltage.
 Eliminating ion impact from these or any
other causes reduces parasitic power drains,
reduces electrode erosion by ion sputtering,
increases the electrode lifetime, and improves
mass utilization and electrical efficiency.

One therefore must design ion sources to
eliminate direct ion impact on the electrodes.
 Not quite so obvious is the fact that one
must also eliminate the possibility of charge-
exchange neutral impact with the electrodes,
where the charge- exchange neutrals can
originate anywhere along the ion
trajectories.

In figure 6.5, above the centerline, is shown an
optimum ion trajectory, for which neither ions
nor charge-exchange neutrals formed along the
ion trajectory will intersect the electrodes.
Below the centerline is shown an over-focused
trajectory, some charge-exchange neutrals of
which will hit the electrodes as a result of the
straight-line trajectory followed by such neutrals
after charge exchange occurs.

Figure 6.5 Illustration of an optimum ion trajectory, above the centerline, for
a three-electrode ion accelerating grid system; and below the centerline,
illustration of an over-focused ion trajectory, which allows charge-exchange
neutrals formed along the early portions of the ion trajectory to impact the
accelerating grid, thus reducing its lifetime.

 Ion and charge-exchange neutral impact
occur with high enough energy that significant
sputtering of the electrode material can occur,
shortening the lifetime of the electrodes.
With properly developed ion optics, common
in the field of space propulsion, electrodes can
be made to operate for tens of thousands of
hours without significant sputtering-induced
erosion.

Power Deposition Limits
The power deposited on electrodes by radiation, and by
parasitic ion, electron, and neutral fluxes must be removed
either by radiation or with a flowing coolant.
 In many ion sources, the accelerating electrodes are made
of sheet metal, and cannot be cooled except by radiation.
If the parasitic power flux to an electrode surface is
∅𝒑watts per square meter, this can be set equal to the
radiated power, from the Stefan-Boltzmann radiation law,

𝐰𝐡𝐞𝐫𝐞 𝛆 𝐢𝐬 𝐭𝐡𝐞 𝐞𝐦𝐢𝐬𝐬𝐢𝐯𝐢𝐭𝐲, 𝐚𝐧𝐝 𝛔 𝐢𝐬 𝐭𝐡𝐞 𝐒𝐭𝐞𝐟𝐚𝐧 −
𝐁𝐨𝐥𝐭𝐳𝐦𝐚𝐧𝐧 𝐜𝐨𝐧𝐬𝐭𝐚𝐧𝐭, 𝛔 = 𝟓. 𝟔𝟕𝟏 × 𝟏𝟎−𝟖
𝐖/𝐦𝟐
− 𝐤𝟒
(6.17)
The maximum power which uncooled sheet
metal electrodes can radiate is found at the
melting temperature of the electrode material,
𝑻𝒎 and at 𝜺𝒎 the emissivity of the electrode
material at its melting point.

For
aluminum,(∅𝒑)𝒎𝒂𝒙 ≈0.82W/cm𝟐
,
a relatively low value, indicating
the importance of reducing to an
absolute minimum the incident
power flux on uncooled electrode
surfaces.

If active cooling with a flowing coolant is
necessary, heat fluxes as high as 5kW/cm2
can be accommodated with the most
advanced aerospace-related methods.
 Such high fluxes, however, shorten the
lifetime of materials so cooled, by thermal
stress-induced cracking, leading to an
unacceptable failure mode for equipment
which must be operated in a vacuum system.

Design Optimization
The geometry of an ion source must be optimized to
assure unobstructed ion trajectories through the
electrode system at or near space-charge limited
current conditions.
In figure 6.6(a), Coupland et al (1973) have optimized
the electrode thicknesses t, the distance between
electrodes d, and the electrode radii r, as well as the
accelerating potential Vo and the decelerating
potential −V𝒂, to obtain the non-intersecting ion

Figure 6.6 Optimization of ion trajectories in an accel-decel three-
electrode source. (a) A schematic illustration of the parameters which
must be optimized to ensure proper focusing of the ion trajectories:
the electrode thicknesses, t; electrode separations, d; and electrode

These trajectories also have the characteristic that
their tangents (the trajectory of charge-exchange
neutrals) also do not intersect the electrodes.
The presence of the inset in the positive extraction
electrode was found by these authors to facilitate
the design of optimum, non- intersecting ion
trajectories.
The equipotentials of a three-electrode
configuration studied by Cooper et a1 (1972) are
shown in figure 6.6(b).

6.6 (b): Focused ion trajectories and
equipotentials from an axisymmetric, three-
electrode geometry. From Cooper et al (1972).

This is an accel- decel configuration with V=V𝟎 on
the source, V=-0.l V𝟎 on the accelerating electrode,
and V = 0 on the final electrode, to ensure that the
beam is at ground potential when it leaves the source.
The geometry of this source has been optimized so
that ion trajectories and their tangents do not
intersect the electrodes.
The equipotentials between the source electrode on
the left and the accel electrode in the middle are
indicated by the dotted lines.

This source was developed by Harold L Kaufman (1961,
1963) and colleagues at the NASA Lewis Research Center
for space applications over a period of several decades
following 1958.
 Also known as the Kuufmun ion engine or the electron
bombardment ion source, this source achieved very high
mass utilization and electrical efficiencies, higher than 0.9
and 0.95 respectively.
It also demonstrated extremely long lifetimes, exceeding ten
thousand hours in actual orbital applications as an
electrostatic thruster.

Basic Configuration
A characteristic Kaufman source is shown in figure
6.7, incorporating many of the advances in ion engine
technology of the 1960’s and 1970’s.
 In this axisymmetric source, ions are produced in a
discharge chamber by a DC glow discharge operating
below 1mTorr.
The discharge chamber is permeated by a weak
magnetic field, between 5 and 12 mT, enough to
magnetize the electrons but not the ions.

Figure 6.7 The Kaufman ion source, also known as the electron
bombardment ion source. This space-charge limited source was
developed for space propulsion applications.

The glow discharge is maintained by a few hundred
volts across the gap from the hollow cathode ionizer
to the axisymmetric anode at the outer circumference
of the discharge chamber.
The Kaufman ion source uses a hollow cathode to
ionize the working gas, and as an electron emitter.
Since neutralization of the ion beam is required in
space applications, a second hollow cathode
neutralizer emits electrons into the beam to provide
neutralization.

In space applications, the Kaufman
source shown in figure 6.7 was
optimized for operation with mercury
and xenon gas.
These engines had lifetimes greater
than ten thousand hours, and current
densities between 80 and 90% of the
Child law current.

Alternative Configuration
During the decade of the 1960’s, many configurations of the
Kaufman source were tested, some of which may have
unique advantages for certain gases or specialized
industrial applications.
On figure 6.8 are shown various cathode configurations,
including the hairpin filament in figure 6.8(a), an
incandescent refractory emitter, shown in figure 6.8(b), and
the hollow cathode in figure 6.8(c), which is the preferred
arrangement for space and many other applications.

Figure 6.8 Various cathode configurations for the
Kaufman ion source.
Early sources of this type, and sources for which
lifetime considerations are not important, use:
(a) one or more hairpin filaments as the electron
source, with independent neutral gas feed into the
discharge chamber, or
(b) the button cathode configuration with independent
gas feed and a heater coil which raises a button
electron emitter to incandescence, from which the
electrons are emitted.
(c) The hollow cathode, which acts as an electron
source, and ionizes the gas flow before it reaches
the discharge chamber.

On figure 6.9 are shown some of the gas feed
arrangements which have been tested.
 Figure 6.9(a) shows a reverse feed of neutral gas into
the discharge chamber.
 Figure 6.9(b) shows the ‘shower head’ feed of
neutral gas from many small holes in a plenum at the
rear of the discharge chamber.
In the hollow cathode shown in figure 6.9(c), the gas
flows through the hollow cathode and is ionized in the
process.

Figure 6.9 Gas feed mechanisms for the Kaufman
ion source.
(a) Reverse feed of neutral gas, in which gas is
injected toward the rear of the discharge
chamber.
(b) Rear feed, in which the gas flow is distributed
by a plenum at the rear of the chamber, and
escapes into the discharge chamber through a
large number of small holes.
(c) The hollow cathode ionizer, in which the
neutral gas flows through the hollow cathode,
where it is ionized and contributes to the
electron emitting plasma formed in the
neutralizer.

 All of these gas feed configurations are capable of
producing a high overall gas utilization efficiency,
probably because at the low pressures at which
Kaufman sources are operated, the gas distributes
itself evenly in the chamber regardless of how it is
injected.
The magnetic field configurations which have been
used to improve the electrical efficiency of the
discharge chamber of Kaufman ion sources include
the multipolar configuation shown in figure 6.10(a),
viewed along the axis.

Figure 6.10 Magnetic field
configurations for the Kaufman ion
source.
(a) Cross sectional view of the
multipolar magnetic configuration,
in which small permanent magnets
are placed axially along the
outside surface of the discharge
chamber, providing a multipolar
cusp.
(b) Axial cross section of a discharge
chamber with a uniform magnetic
field through the discharge
chamber.
(c) Axial cross section of the divergent field configuration, which leads to a more uniform radial
distribution of plasma.
(d) Axial cross section of an over-divergent magnetic field configuration, in which the efficiency of the discharge
chamber is impaired by direct current flow along field lines from the cathode to the cylindrical anode.

 This is a relatively recent development, in
which permanent magnets with opposite
polarities are placed around the circumference
of the discharge chamber.
These multipolar magnets create a magnetic
field-free plasma in the center of the discharge
chamber, the transport of which to the anode is
retarded by the increasing magnetic field
(magnetic mirror) as the plasma approaches the
anode.

The least useful axisymmetric magnetic field configuration
is the uniform field along the axis of the discharge chamber
shown in figure 6.10(b).
Here, the plasma tends to be constrained to the field lines
in contact with the hollow cathode surface.
The most efficient of the axisymmetric magnetic field
configurations is that shown in figure 6.10(c), in which the
magnetic field diverges from the hollow cathode ionizer to
the accelerating grid, with the magnetic field line
connecting the edge of the hollow cathode just touching the
end of the discharge chamber.

This arrangement spreads the plasma
relatively uniformly over the surface of
the accelerating grid.
 Finally, the over-divergent field,
shown in figure 6.10(d), is less efficient,
because of the direct transport of ions
along the magnetic field lines from the
cathode to the anode.

The Kaufman ion source developed for space
applications had accelerating voltages ranging
from 1 to 10 kV, total beam currents ranging
from 0.05 to 10 A, current densities that ranged
from 0.70 to 0.95 of the space-charge limited
values, and beam diameters that ranged from 10
cm to 1.5 m (Kaufman 1965, Poeschel et al
1979).

The lifetime of sources of this type which
were intended for space applications could
exceed 10000 hours.
 The Kaufman ion source was by far the
most extensively developed of any ion source
during the period from 1958 to the present
and has been widely applied industrially to
ion beam sputtering and ion milling of
surfaces.

Penning Configuration
• A Penning ion source similar to that developed
by Meyerand and Brown is shown schematically
in figure 6.11.
• The Penning ion source consists of a
magnetoelectrically confined plasma discharge,
in which gross confinement of electrons is
provided by an axial magnetic field and an axial
electrostatic potential well.

Figure 6.11 The Penning ion source, with a cylindrical anode ring at the
center and two cathodes at either end. A small hole on the axis of one
cathode allows a beam of ions to escape.

• Electrons are electrostatically trapped axially, and
magnetically trapped radially until they collide and
ionize, enabling the plasma to be maintained at
background neutral gas pressures as low as Torr (1.33Pa).
• The electron-neutral ionizations avalanche until the
ionization and loss processes reach a steady state.
• Starting electrons for the Penning plasma can be provided
by volume ionization from background radiation,
secondary emission from cold cathodes, a hot filament as
shown in figure 6.12, or a hollow cathode.

•Although the axial
electrostatic potential
distribution is a potential well
for electrons, it repels ions
produced between the anode
and cathode.

• Penning ion sources can be operated at gas
pressures between Torr (1.33 x Pa) and 100
mTorr, (13.3 Pa), magnetic inductions
from 0.01 to 3T, anode voltages from 100
to 50000 V, currents from 0 to 20 A,
electron kinetic temperatures from 2 to
15eV, and the ion energies produced can
range from less than 1eV to several keV.

Ion Heating in Penning Sources
• It is an interesting property of the Penning discharge as
an ion source that it can produce a beam of ions with
kilovolt kinetic energies, far higher than the kinetic
temperature of the background electrons (Meyerand and
Brown 1959, Roth 1966, 1973a, b).
• The high ion energies associated with Penning sources
result from the circumstance that Penning discharges can
have radial electric fields in excess of 1 kV/cm between
the plasma and the anode.

•This leads to magnetoelectric heating by
E/B drift in the azimuthal direction, with a
velocity
The kinetic energy corresponding to this
drift velocity is given by

• Thus, in Penning discharges, the particle energy is
proportional to mass, and this is why ions in low
pressure Penning discharges are often more
energetic than the electrons.
• At pressures below 30 pTorr, and at magnetic fields
above 0.2 T, Penning discharges become very
turbulent, and can provide a Maxwellian
distribution of ions with kinetic temperatures up to
several keV (Roth 1973a, b).

Figure 6.12 A Penning ion source in the duo PIGatron configuration, in which electrons are
emitted from a filament or a hollow cathode on the left-hand surface, and generate a Penning
plasma between the cathode and anticathode. In the duo PIGatron, the working gas is injected
radially into the anode cylinder and ions are extracted through holes in the anti-cathode, which
may be replaced by an accelerating grid.

BEAM-PLASMA ION SOURCES
• The beam-plasma ion sources have in common the
use of an electron beam or linear DC arc discharge
between a cathode, usually a heated filament, and an
anode.
• The interaction of the electron beam with a gas of the
species of interest produces a plasma, from which the
ions are extracted.
• Separate electrodes then accelerate the ions to the
desired energy.

Capillary Arc Source
• The capillary arc source shown in figure 6.15 was
developed for high energy physics applications,
where it is sometimes desirable to generate an ion
beam from radioactive, rare or precious solid
materials.
• In this source, an arc is drawn between a cathode
and an anode through a small capillary opening on
the left.

Von Ardenne Source
• A characteristic von Ardenne source is shown in figure
6.18.
• This source is typically an axisymmetric geometry, is
relatively small in physical size, and operates much like
a hollow cathode electron source, but with extraction
electrodes which suppress electron emission and instead
extract ions from the dense, concentrated plasma which
is located between the filament and the intermediate
electrode.

• A strong magnetic field is created by using iron for the
accelerating electrodes, and shaping the magnetic field to
achieve the desired ion optics.
• Like the hollow cathode electron source, the von Ardenne ion
source requires an axial gas flow, a keeper electrode for stable
operation, and usually has accelerating electrodes to produce a
monoenergetic axial beam of the desired ions.
• The gas flowing through the source consists of the atomic
species which one wishes to ionize to form the beam.
• Because some versions of this source use a hot filament for its
operation, the von Ardenne source is subject to chemical
attack.

•Some modified versions of the
von Ardenne source use a
hollow cathode instead of a
filament as an electron source,
and or dispense with the axial
magnetic field.

Capillaritron Ion Source
• A related ion source is the capilluritron ion source shown in
figure 6.19.
This source resembles the von Ardenne ion source, but is
usually much smaller in size.
• The capillaritron ion source, unlike the von Ardenne source,
has no externally heated cathode, and a keeper electrode is
used instead to stabilize its operation.
• It operates in the steady state, requires an axial gas flow, and is
good for the production of low currents at the level of
milliamps and below.

Figure 6.19 The capillaritron ion source.
This ion source resembles the Lidsky hollow cathode, in that it has a gas
flow passing through a region of dense plasma in the opening of a small
capillary nozzle.
A high voltage is maintained between a keeper electrode and the
capillary nozzle, thus extracting ions from the plasma region

Zinn Ion Source
• The Zinn ion source (Zinn 1937) shown in figure 6.20 is also similar to
the von Ardenne ion source, but instead draws an arc between a ring
cathode and an anode.
• The arc terminates on a coaxial anode, after making a right angle turn
through an annular space.
• Ions formed in the dense plasma between the end of the anode and the
intermediate electrode may be accelerated by an extraction electrode,
to form an ion beam.
• Ions can be formed from a working gas flowing coaxially, or by
vaporization or sputtering of the solid anode material.

Figure 6.20 The Zinn ion source. In this source, an arc is struck between the axial
anode on the left, and radially through a small opening to a ring cathode. Ions are
extracted from the plasma formed in this region and accelerated by the extraction
electrode at the right to form an ion beam

Electron Heated Vaporization Source
• In some applications it is desired to form an ion
beam from solid or even refractory materials.
• One option is to use the electron heated
vaporization source shown in figure 6.21.
• The material of which it is desired to make an
ion beam is incorporated in the consumable
axial electrode.

Figure 6.21 The electron heated vaporization source operates in a manner
similar to that of the Zinn source. A consumable axial electrode is used to
draw an arc to an annular filament. As the material of the electrode vaporizes,
it forms ions which are extracted to form an ion beam

• It operates in some respects like the Zinn source,
but relies on heating the axial anode to
incandescence, thus vaporizing the electrode
material and making it available for ionization by
the intense plasma that forms between the anode
and the annular filament cathode.
• Such sources usually operate above 1000 K.
• To reduce power requirements, heat shields are
usually employed around the source as shown in

Hollow Cathode von Ardenne Source
• A smaller version of the von Ardenne source,
utilizing a hollow cathode electron source
instead of hot filaments, is shown in figure
6.22.
• This configuration can be made small in size,
and is useful when only small currents of
extremely rare or radioactive materials are
required.

Hollow Cathode von Ardenne Source
Figure 6.22 The
magnetized
hollow cathode
ion source is
similar to the
von Ardenne
source, with the
addition of a
hollow cathode
to improve the
efficiency of ion
extraction.

Figure 6.24 The ‘canal ray’ source. This primitive ion source
consists of a DC glow discharge, in which a small hole is opened
in the cathode, allowing ions to escape and be accelerated to
form an ion beam.

Figure 6.25 The pulsed spark ion source. In this source, a consumable high voltage
electrode is fed in axially, and a spark drawn between this electrode and a refractory
aperture electrode. The pulsed arcs produce a dense cloud of plasma, ions from
which are accelerated by the external electrode.

Figure 6.26 The field emission ion source with gas feed, in which an emitter at a
high potential maintains a very strong electric field at the tip. Gas flowing past this
tip becomes ionized, and is then accelerated by the electric field through a hole in
the cathode, to form an ion beam.

Figure 6.28 The porous tungsten surface ionization source. In this source, an
incandescent plate of porous tungsten is permeated by an akali metal vapor which is
fed from the rear. Upon leaving the front surface of the ionizer plate, alkali metal
atoms are ionized, then accelerated to form a space-charge limited beam.

Ion Sources and Beams

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