Download Experiments on improving the efficiency of the Bevatron Ion Source.

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Thesis and Dissertation Collection
1955
Experiments on improving the efficiency of the
Bevatron Ion Source.
Stone, Troy Edward
Monterey, California: U.S. Naval Postgraduate School
EXPERIMENTS ON IMPROVING THE
EFFICIENCY OF THE BEVATRON ION SOURCE
TROY EDWARD STONE
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EXPERIMENTS ON IMPROVING THE EFFICIENCY
OF THE BEVAXRON ION SOURCE
by
Troy Edward Stone
Lieutenant, United States
Navy
Submitted
in partial fulfillment
of the requirements
for the degree of
MASTER OF SCIENCE
IN
PHYSICS
United States Naval Postgraduate School
Monterey, California
19
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This work
the thesis
is
accepted as fulfilling
requirements for the degree of
MASTER OF SCIENCE
IN
PHYSICS
from
the
United States Naval Postgraduate School
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study has been conducted to determine the focal properties of an
ion source that is typical of those used with the Bevatron,
Beam-intensity
patterns have been obtained for various focusing conditions.
of
School
The effects
space -charge repulsion and lens aberration have been investigated.
Losses through Coulomb scattering on residual gas molecules have been
observed, and a curve showing variation of these losses with variation
of
energy through the focus electrode has been obtained.
Pressure
throughout the accelerating column was found to be an important factor,
and losses occasioned through slight increases
in the
normal operating
pressure have been recorded.
All previous ion sources for the Bevatron have been designed for
presentation of a real image to the subsequent injection system.
A
virtual-image source has been designed, constructed and tested on the
Bevatron.
11
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PREFACE
A
magnitude
machine
beam
of the output
of the
itself during the past
approaching
10
Bevatron.
the output and (or)
one
way
to
increase the
Refinements
in the
year have resulted in a current intensity
protons per pulse, a most significant improvement;
but considerably better performance than this
is
made
continuing and intensive effort is being
improving the efficiency
of attaining
more
is
envisioned.
Increasing
Bevatron"
ion source
of the
work has been con-
beann, and to this end
ducted during the past year by the writer.
This experimentation was conducted under the guidance
B.B. Kinsey, formerly
of the
of
Dr.
Chalk River Atomic Energy Project and
during the past year a visiting scientist at the University of California
Radiation Laboratory.
Frequent recourse has been nnade
edge and experience of Bruce Cork
is
of the
Bevatron
staff.
to the
knowl-
The writer
particularly indebted to them for their enlightenment, assistance,
and encouragement during the course
The writer wishes
to
of this
work.
thank Professor N. L. Oleson of the U.
Naval Postgraduate School for his assistance
in the
S.
preparation of this
paper.
This work was conducted under the auspices
Energy Commission while
the writer
California Radiation Laboratory.
«i
was attached
of the
U.S. Atomic
to the University of
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TABLE OF CONTENTS
Item
Page
Title
Abstract
ii
Preface
iii
List of Illustrations
v
Chapter
I
Introduction
Chapter
II
Focal Properties
Chapter
III
Losses through Space -Charge Repulsion,
Lens Aberration, Coulomb Scattering,
and Residual Gas
Chapter IV
Design and Test
Ion Source
1
of a
Typical Ion Source
of a Virtual
.
.
5
10
-Image
16
Bibliography
20
IV
JHAT
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LIST OF ILLUSTRACTIONS
Page
Figure
\
1.
Block Diagram
2.
Cross Section
3.
Focusing Action
4.
Block Diagram
5.
Original Faraday Cup, Shield with
3/8 -inch Hole
25
6.
Faraday Cup, Long Shield
25
7.
Faraday Cup, Special Shield
25
8.
Mechanical Advantage
9.
Beam-Intensity Cross Sections, Variable-Focus
Electrode Voltage
27
Spreading of Originally Parallel Proton Beami
Due to Space Charge
28
11.
Ion Trap
29
12.
Beam-Intensity Cross Sections, SpaceCharge Effects
30
13.
Spherical Aberration
31
14.
Chromatic Aberration
31
10.
15.
of Ion
of
Source
21
Arc Channber
of
of
2Z
Three Electrodes
Bevatron Injection System
of
2 3
....
Cup Movement
26
Beam-Intensity Cross Sections, Aberration
32
Effects
16.
24
Total Current vs.
Energy, Coulonnb Losses
....
33
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CHAPTER
I
INTRODUCTION
An
ion source provides the flow of ions which are the
particles of an accelerator.
The kinds
of ions that
bombarding
are utilized by pres2
1
ent accelerators are protons (,H ), deuterons (,H ), and alpha parti4
cles (^He ). Lithium ions have been used infrequently. In some cases,
ions of tritium
(,H
3
or light helium
)
(^He
3
)
are desired, but these are
generally obtained by nuclear processes rather than from an ion source.
Sioce the Bevatron
designed for the acceleration of protons, this
is
paper will be confined
to ion
sources that deliver protons.
The original ion source used
as that developed by
in the
Gow and Foster
Bevatron was basically the sanae
Q]
for the Radiation Laboratory,
University of California 32-Mev proton linear accelerator.
With only
minor modifications
use.
this design,
shown
in Fig.
1,
is still in
arc chamber used in this source, shown in detail in Fig.
2,
type generally known as a Penning or Phillips ion gauge C2J
arc chamber was chosen because
it
The
is of the
.
Such an
has one of the most simple geometri-
Aside from simplicity,
cal
forms
its
advantages are low-pressure operation, high efficiency, no filament,
that will
produce a stable discharge.
and small physical size.
cause
the simplest
it is
Axial extraction of the ions
means
of
withdrawal.
is
employed be-
The three focusing elec-
trodes are placed in typical electron-gun fashion, and are referred to
throughout this paper as the probe, the focus, and the accelerating electrodes.
Atonnic hydrogen gas that
source
is
is to
be ionized in the arc chamber of the
obtained by passing hydrogen gas through a palladium leak.
This atomic gas
is
imnnediately introduced into the arc chamber, where
a strong electric field is utilized to strike an arc between anode and
cathode.
An electron released from
coated aluminum,
chamber
is
is
a cathode, which is
accelerated toward the anode.
made
of
un-
As the entire arc
centered in a coil which provides an axial magnetic field
of high intensity,
radial motion of the electron is constrained by this
10
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The constraint
field.
may
be such that the electron
is
accelerated into
the anode, coasts through the field-free region within, loses envirgy
through collisions with the gas, and emerges from the other end with
somewhat less energy than
gained in
it
reflected by the electric field,
its
oscillation
of
35
electron
several hundred volts,
is
within the anode during
by the magnetic
come
it
it
Since the initial energy of the
produces approximately ten ions
These ions are also constrained
its oscillations.
field so that their principal
to one
order
of the
below ionization energy and
falls
taken up by the anode.
is
It is
During
its axial oscillation.
produces several ion pairs, losing energy
it
volts per pair, and finally
subsequently
acceleration.
constrained again by the magnetic
re-enters the anode, and continues
field,
they
is
its initial
motion
Eventually,
is axial.
end of the anode, where they are accelerated into the
Some
corresponding cathode.
accelerated into the forward
of those
cathode pass through the exit hole and become useful proton beam.
A
proton
of
several hundred volts energy incident on the cathode
surface has a finite probability of releasing a secondary electron.
trons so released will have life cycles as described above.
Elec-
No good
data are available on the magnitude of the proton- to-secondary-electron
ratio for
aluminum
between five and
first electron,
Gow and Foster
oxide, but
ten.
If
more
(llj
estimate that
it
lies
ions than this figure are produced by the
the discharge will increase in intensity until limited
by
other mechanisms.
A
fundamental problem within the arc chamber
Let us call
free path for ionization.
it
by the cathode the probability
mean
and d the nnean length
\.,
an electron path from cathode to anode.
is that of the
of
For a single electron emitted
of ionization is
approximately d/.
,
i
where d
is
considerably less than
>.
.
ber were equal to that in the accelerator,
hundred meters.
If this
exceedingly low.
If
we
were
take
If
the
X.
pressure
in the
would be equal
arc cham-
to several
the case, the rate of ionization would be
d
equal to ten centimeters and \.
as
200 meters, we will get approximately one ion for every 2000 electrons.
And since
the exit hole in the cathode through
which ions are drawn
inch in diameter, the rate of removal of icns
only
0.050
low.
Considering probabilities
of
is
is
generally
production and of extraction, we will
no a
t
btt
e
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vd
get relatively few useful ions.
To
the
state
it
simply, the pressure in the injection system and through
Bevatron must be sufficiently low
to
allow the accelerating ions to
travel fronn the source to the target without colliding with residual neu-
On
tral atoms.
have a high probability
the arc
by the cathode naust
the other hand, electrons emitted
chamber
in
of colliding
order
quirements are met by
to give a
(a)
with atomic hydrogen atoms within
These re-
high rate of ionization.
pumping system
utilizing a high-speed
which pernnits relatively high pressure within the arc chamber, and
(b)
forcing the electrons to follow complicated paths before reaching
the anode, as described above.
Pulses
kilovolt
of
current through the arc chamber are provided by a three
power supply
that is capable of three
amperes
of
current during
millisecond pulses, such pulses coming twice per second.
Thus pulses
mixed with ionized molecular hydrogen, are emitted from
of protons,
the exit hole in the cathode with the
The presence
of ionized
same frequency.
molecular hydrogen
is,
of course, undesirable.
Such ions are not acceptable for the Bevatron, which
for the charge and
tons in the ion
mass
stream.
of the proton,
is
synchronized
and they take the place
Several investigators
C3J
have sought
of
pro-
to in-
crease the output of similar sources by introducing impurities that
ionize at low voltage, such as
mercury vapor.
They found
impurities generally give a nnanifold increase of the
magnetic analysis shows that almost
Useful current
is
all the ions
beam
arise
that these
current, but
from
the impurities.
actually decreased.
The proton-to-molecular-hydrogen ratio can be varied within
however.
In general, higher arc currents,
linnits,
stronger magnetic fields,
and lower arc -chamber pressures give the more favorable ratios.
der good operating conditions with the type
of
chambers
Un-
utilized in these
experiments, 75 percent protons are obtained, but under normal operating conditions a
The size
promise.
crease
in
common
figure
is
of the exit hole in the arc
60 percent protons.
chamber
is
necessarily a
conri-
Although the number of protons emitted increases with inaperture size, the increase realized
is
somewhat less than
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directly proportional to the area of the aperture
tA2
And
•
a larger
hole results in higher pressure throughout the subsequent system, which
is
obviously undesirable, and offers a larger object to the focusing sys-
tem, which results
in a
larger image spot.
Larger holes also compli-
plasma emission surface, such control being accom-
cate control of the
plished through adjustment of the probe electrode voltage.
voltage, the
ion.
plasma surface protrudes from
With no probe
the exit hole in tongue fash-
Increasing the probe voltage causes the plasma surface to retreat
back into the hole
under the most favorable emission conditions,
until,
the surface is perpendicular to the axis of the source.
If
the holt is
nnade too large, however, the plasnna can no longer be forced back prop-
erly by probe voltage, the plasma surface
The exit-hole diameter
current decreases.
the best
compromise
kilovolts
from
that has
been found
to be
of the three electrodes is
shown
in Fig.
Pro-
3.
the exit hole are accelerated to approximately
by the probe electrode, decelerated
the focus electrode, and accelerated to
celerating electrode.
beam
perpendicular, and
0.050 inch.
is
The focusing action
tons emerging
is not
45
20
to about eight kilovolts in
50 kilovolts in the ac-
or
With these voltage settings, a real image
is
ob-
tained approximately one foot beyond the accelerating electrode.
The output beam from
Cockcroft-Walton
the ion source goes into a
colunnn, (refer to Fig. 4), which accelerates the protons to
volts.
A turning magnet then deflects
the buncher, a resonator
and decreases
its
width.
to be raised to ten
Mev
the
beam
and sends
20
which increases the height
The beam then enters
and, finally, is turned
500 kilo-
of the
it
into
proton pulse
the linear accelerator
35
by an electrostatic
inflector and injected into the Bevatron proper.
Each
of the
stages subsequent to the ion source constitutes a focusing
system, and the connbination
great complexity.
It is
of these
systems
is a
thus to be expected that the
compound lens
maximum
of
current
reading immediately beyond the ion source or at the end of the Cockcroft-
Walton column will not necessarily result
current output
Indeed, the requirements of the Bevatron are unique,
for the Bevatron.
and the efficacy
of a
settings while
is
it
in the highest
source can only be determined by trial and error
pulsing into the Bevatron.
'X
d
oliTl
CHAPTER
II
FOCAL PROPERTIES OF A TYPICAL ION SOURCE
The purpose
of the
prove the design
some
experimentation herein described was not to im-
of the arc
chamber, whose operation
is
detail in the previous chapter, but rather to study
trolling the
beam
obtained from such a chamber.
for this study is identical to the one
employed
described in
means
of
con-
The source selected
Bevatron with the
in the
exception of the diameter of the focusing electrodes, which are two
inches in the former and
1-1/4 inches in the latter.
Variation
in total
output with variation of the focusing -electrode voltages was of interest
and was investigated, but the main effort was directed toward obtaining
beam-intensity patterns that would show the focal properties of the three
electrode system.
1.
Operating conditions.
The pressure
in the arc
chamber during these experiments was
125 ± 25 microns, and the pressure at the end of the accelerating elec-
trode was about
3
x 10
_5
millimeters
net coils surrounding the arc
of
mercury.
chamber varied from
Current
0.5
1.2
to
depending upon the setting needed for stable operation, and
in a
nnag-
amperes,
this resulted
magnetic field strength within the channber varying from
1000 gauss.
in the
500 to
Voltage across the arc during a pulse was of the order of
300 volts, with the setting of the arc pulser varying from one to two
kilovolts as required for stable operation.
to three
Arc current varied fronn one
amperes, depending upon the requirements
of the
experiment.
High-current arcs over long periods necessitate frequent replenish-
ment
of the oxide layer on the
aluminum cathodes.
running the arc on oxygen for a half hour or so.
the
aluminum cathodes are found
bides of aluminunn, on their
polished.
Complications
what variable, but
within ten percent.
in
of
This
is
done by
And, from time to time,
form compounds, believed to be carennission surfaces and must be removed and
this sort nnake settings for a stable arc someto
most cases results obtained were reproducible
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Measurement techniques.
2.
The
first device
day cup with
making
used for current measurements was a shielded Fara-
3/8 -inch entrance hole (see Fig.
a
It
was found
bias would decrease current readings
15 percent.
still
Such a fluctuation
that electron currents
of the electron
below the
in the readings.
Be-
obtained, the magnitude
currents must be known and their effect minimized.
The purpose
formed on
major role
a
beam current readings can be
fore meaningful
30 percent
current readings showed clearly
of
were playing
Additional positive
farther, at a decreasing rate,
90 volts bias resulting in current readings
no-bias figure.
that
would de-
the cup six volts positive with respect to its shield
crease current readings by as much as
with
5).
of positive bias is to
draw back secondary electrons
the cup and prevent their passing out the entrance hole, being
accelerated back through the source, and causing erroneous beam-
A
current readings on the high side.
tive should be sufficient to
decrease
in
The further decrease
dicates that secondaries
drawn
all
secondaries to the cup, and the
observed current for such a bias
to this action.
ing
reclaim
bias of the order of six volts posi-
to the
formed on
in
is
almost assuredly due
current with larger biases in-
the lip of the entrance hole
cup and causing erroneous readings
In order to provide a long field-free region and
of
secondary electrons formed
Fig.
6
was constructed.
at the
or^
the
Tests on this device showed that multitudinous
Ekraday cup
were obtained for
a
all
to be
So many
shown
holes
makes
it
it
7
in
current readings,
was developed and has been used
for the bulk
The field-free region between the two entrance
unlikely that secondaries formed on the lip of the outer
hole will find their
cup makes
Clearly this was
than before.
in Fig.
of the data collected.
electrons
recorded that negative current readings
To minimize both positive and negative errors
a device
of the shielding
of these
but an axial position of the cup.
much poorer design
the effect
entrance hole, a device shown in
cup when the cup was inclined to the beam.
into the
low side.
minimize
secondary electrons were formed on the inside surfaces
came
were be-
way
to the cup.
A
positive bias of six volts on the
unlikely that secondaries formed on the cup will escape
and gc back through the source.
Readings were found
to
vary less than
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percent for the range of biases up
90 volts.
to
Small permanent magnets placed around the entrance hole
device might further reduce errors.
magnetic
uTsing a
field of several
of his target cup.
Thonemann
CbI]
of this
reports success
hundred gauss parallel
to the surface
secondary electrons, which
In this strong field the
are relatively slow, are immediately sent back to the cup.
3.
Beam-intensity patterns
The measuring device described above was mounted on the end
rod (see Fig.
means
which passes through a vacuum seal and provides a
8),
of positioning the
be pushed into the
from
of a
cup within the vacuuna chamber.
vacuum chamber
The rod can
until the cup lies only
two inches
or pulled out until the cup
the exit of the accelerating electrode,
vacuum -chamber wall and is 15 inches from the accelerating electrode. When in the latter position, the exposed end of the
rod moves 5.6 inches for each inch of movement of the entrance hole
lies next to the
on the cup shield.
Thus by plotting positions
of the free
end
of the rod,
one obtains quite accurate knowledge of the position of the entrance hole
within the chamber.
ings for
beam
To
take advantage of this magnification, all read-
intensity patterns
withdrawn position.
of the individual
The patterns
Total
were made with
beam currents were
the cup at its fully
obtained by integration
currents throughout the observation plane.
that are
shown as Fig.
9
were obtained by leaving
all
settings constant while varying the voltage on the focus electrode.
is
seen to be a sharp focus at five kilovolts with the pattern flattening
and spreading out for voltages above and below this setting.
mum
ampere through an aperture 0.042 inch
A
The mini-
size of the pattern is of the order of one -half inch in diameter,
and for this pattern the central peak carries a current
is
There
peculiar property of these patterns
in
of
O.IZ nnilli-
diameter.
is the definite
noticeable for focus -electrode settings of
6.2
ring effect that
and 7.1 kilovolts.
Similar rings are observed with a thick lens in conventional optics, but
there they are most prominent when the observing plane lies outside
the focal plane of the lens.
The patterns herein obtained show pro-
nounced rings only when the observing plane
lies inside the focal plane
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system.
of the lens
Similar rings were noted
other times when data
They are even more noteworthy
were recorded.
of this type
at
virtual-image source, which
described
is
in
Chapter IV.
Probe electrode length.
4.
It
was believed
the distance
from
the
probe hole
might be greater than necessary and that a shorter
might result
the probe
it
for the
was
to
sharper focus.
drift distance within
To carry
out this experiment,
determine whether displacement
of the
magnet
rear would adversely affect the operation of the arc chamber.
to the
was found
the
necessary
first
in a
electrode
to the focus
that
It
moving the magnet back one and one-«ight inches decreased
beam current by
Runs were also made
25 percent.
effect displacement of the
what
to see
magnet has on proton content
of the
beam.
This was done with a magnetic deflecting device which permits separate
readings of proton and molecular hydrogen currents.
magnet displacement did
It
was found
that
not significantly affect the ratio of protons to
molecular hydrogen.
A probe
5/8 inch shorter than the usual one was constructed and
The arc chamber was moved toward
installed.
ing distance.
The magnet, which
the probe a
lies flush against the
correspond-
back
of the
moved forward, but the effect of this variation of
strength through the arc chamber had been discounted as noted
source, could not be
field
above.
Intensity patterns for this arrangement disclosed a decidedly
poorer focusing ability for the system, with
focus -electrode voltage of
little
The
with less
"pancake" fashion
A
system notably improved
is
indicates that the spreading out of the
mainly due
to
length was installed.
beam
A probe one
it
was decided
to explore
inch longer than the custo-
Intensity patterns taken with conditions
duplicating previous tests with the customary probe indicate that
current decreases slightly and focusing ability
somewhat.
Neither effect
in
space-charge effects.
shorter probe was decidedly detrimental;
the operation of a longer one.
mary
for additional patterns with the shorter
fact that the focusing ability of the
beam current
patterns the rule and
use in sharpening them up.
Lower arc currents were used
probe.
flat
is
of the
pronounced enough
beam
system improves
to establish
whether
the longer probe is preferable;
the likelihood is that either will serve
equally well.
Probe -electrode aperture
5.
The size
of the
size.
probe hole that lies over the arc -chamber exit hole
has customarily been about 0.1270 inch in diameter.
determine the gross effects
to
this hole
by 50 percent.
was tested
by a factor
first,
A probe
from
seemed desirable
and decreasing the size
beam current was down
normal
that realized with the
hole.
Tests with
0.0760 -inch hole were equally unrewarding, the total
beam current in this case being down by a
creases in beam current are great enough
factor of three.
size probe hole as in the neighborhood of
0.1270 inch.
to establish the
These de-
optimum
Probe -tip and exit-plate shapes.
6.
to
form
is
customarily a 90
The probe has customarily been machined
its
entrance hole at the apex.
The probe
hollow cone.
tip of the
Foster
fits
probe spaced about
[^IJ
beam
Tests were made
ventional 90
The exit plate
a
l/l6
inch
from
the exit hole.
Gow and
probe.
to strike the probe.
with the exit plate essentially flat and with the con-
No noteworthy
a hemispherical surface on its end
hemispherically shaped exit plate
seemed
cone with
concentrically into the exit plate with the
alteration in
beam-current out-
put or in focal properties of the system was observed.
sign
90
report that less acute cone angles cause an appreciable
fraction of the
it
of
0.1890 inch diameter
with a hole of
and under optimum conditions
of eight
a probe having a
of enlarging
It
to
be as good in
all
A probe
was developed and used with
of slightly larger radius.
with
a
This de-
respects as the others, and in addition
gave less trouble through sparking between the probe and exit plate.
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1i
CHAPTER
III
LOSSES THROUGH SPACE-CHARGE REPULSION, LENS ABERRATION,
COULOMB SCATTERING, AND RESIDUAL GAS
The desired product
distortions.
It is
beam occur
while
only
the
of this focusing
it is
A
being focused.
is
an innage free of
minimum
further desirable that a
the extent of the object
if
system
attenuation of the
perfect image
of the path is so
be obtained
and image as well as the inclination
image -forming proton rays are exceedingly small,
of the protons is uniform, and
may
if
if
of
the velocity
the proton concentration at all points
small that the mutual repulsion of the protons has
negligible effect.
If
these three conditions are not fulfilled geometrical
aberrations, chromatic aberrations, and space-charge defects arise,
the result of each being to decrease useful
obvious source of image imperfections
is
beam
A further
current.
mechanical misalignment or
defective construction of the electrodes an (or) the exit aperture.
1.
Space-charge repulsion.
The
ability of a focusing
in large
measure upon
system
to obtain a
sharp focus
is
dependent
the effect of space -charge repulsion on the
beam.
Space -charge effects as observed for charged particles have no parallel
A
in light rays.
strong concentration of protons
at
any point has the
effect of reducing the potential or decreasing the speed of the individual
protons.
In a
beam
of given cross section, this concentration is in-
versely proportional to the velocity.
Thus space charge plays a role
primarily in low-velocity, high-current beams.
spreading
Fig.
of the
beam, resembling
Here
it
produces a
the effect of a negative lens (see
10).
Let us consider two identical charged particles traveling together
at the
(1)
same speed.
They exert on each other two different forces;
an electrostatic repulsion
2
'<''){')
10
and
a magnetic attraction, because they
(2)
correspond
to parallel
currents
It
may
be seen that
^A
V
2
e
u
'^'^
=
o
(fl
energy 20 kilovolts,
In the case of protons of
meters per second, so
F
that
/
F./F„
that
x 10
2x10
and we see
,
same energy.
of the
For either electrons or protons
far
3
-5
This would not be the case for electrons
can be neglected.
.
about
is
v is only
from
energy
20
kilovolts, the speed is
the charge density is
x 10"
1.2
8.5 x 10
their charge density would be
7
Coulomb per
hand, protons with the same energy have a
we see
mutual repulsion
For a one-milliampere beam
negligible.
of
the effect of
4.5 x 10
of electrons with an
meters per second and
naeter.
much lower
beam
of the
On
Thus
nneter.
beam
same energy, because
the other
speed, and
Coulomb per
that the space -charge repulsion of a proton
than for an electron
is
is
much
higher
the charge den-
sity is higher and the attractive force is negligible.
A complete treatment
Gibson
in
"The Production
Their results
parallel
beam
may
a
with
problem has been made by Fowler and
of Intense
Beams
of Positive Ions."
be summarized as follows:
centimeters in radius;
repulsion becomes
0.00280A,
of this
2a, 3a,
4a
its
Consider an
D^
.
initially
radius due to space -charge
distances given by 0.00188A,
at
0.00358A, where
U expressed
in volts
and
elementary positive charges and
Fowler and Gibson
I
in anaperes;
M
Z
the molecular
is the
mass
state that, for instance, for a proton
11
number
of
of the ions.
beam
of
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am
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20
kilovolts initially
distances are
0.375
centimeter in radius, the corresponding
24.9, and 31 9 centimeters.
16,8,
While the above rigorous discussion
Spangenberg, and Helm
of
Q7_]
ends
of
"
well be accurate, Field,
have shown that positive -ion neutralization
space charge can be attained even
"ion trap.
may
This trap (see Fig.
11)
at
low pressures by means
of an
consists of suppressor rings at the
electrode tubes with a sufficient positive bias to contain positive
ions within the electrode.
Under these conditions, the investigators re-
port, positive ions are unable to leave the tube, and accumulate in such
number
that the electron space charge,
which would ordinarily depress
the potential in the center of the tube, is almost exactly neutralized.
Their results show, for instance, that an electron current of
amperes, traveling
a pressure of
l/2
the ion trap the
This ion trap
36 centimeters with an energy of
micron, suffered no spread
beam spread
is
in its
130 milli-
5.7 kilovolts at
diameter.
Without
diameter.
to twice its original
equally applicable to proton currents, obviously, and
the principle further raises the question
as described by Fowler and Gibson
ions within the tubes
(76]]
were not taken
whether the limiting currents,
,
have any meaning, for negative
Perhaps there are
into account.
adequate numbers of negative ions within the focusing electrodes of the
ion source to neutralize space -charge effects of the
beam and
lead to
very high limiting currents.
To study space-charge repulsion
patterns were obtained for varying
voltages constant.
effects in the test source, intensity
beam currents
with
all
electrode
Experimental results show that smaller currents
definitely result in a sharper pattern (see Fig.
12).
beam
Increasing
current from 6.1 to 8.7 milliamperes, for instance, cut down the peak
current
at the
center of the pattern by a factor of almost two while the
diameter
of the
apparent
that,
pattern increased by a factor of one -half.
even though the spreading appears
be predicted by the rigorous treatment, there
through space -charge effects in this source.
employ
the "ion trap" to lessen these effects.
12
is
It is
thus
to be less than
would
considerable loss
No attempt was made
to
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2.
Lens aberration.
The primary contributions
to lens aberration for an electrostatic
system are from spherical aberration and chronaatic aberration.
former, sometimes called aperture defect,
The
only geometric aber-
is the
ration which causes unsharpness of the image on the optic axis (refer to
Fig.
This aberration results in a circle
13).
image
ture
a
is the
which
point,
same over
confusion about every
proportional in diameter to the cube of the aper-
is
imaging pencil.
of the
of
In
magnitude the spherical aberration
the entire image.
It is
caused by the fact that rays
passing through the outer parts of the lens are refracted somewhat more
strongly in proportion to the separation from the axis, returning to the
axis at a smaller distance fronm the lens than rays having a smaller in-
clination to the axis.
Chromatic aberration results from
of a lens acts for a shorter
the fact that the deflecting field
period of time on a proton of greater velocity.
Fast protons are thus less affected and focus
the lens than do slow protons.
produced by
as
this action,
The effect
of spherical
A spread
at a
of the
shown by Fig.
greater distance from
image along
the axis is
14.
aberration was investigated by obtaining in-
beam cur-
tensity patterns with identical operating conditions and total
rents for varying beam-input cross section.
the source operated in the usual fashion.
15),
In the first
case (see Fig.
In the second case, a
3/32 -inch-diameter coUimating hole was placed in the probe
from
the
its tip,
beam
as
and this collimator served to reduce the cross section
of
Peak current intensity
at
it
entered the focusing system.
the center of the pattern
coUinnator.
was increased by a factor
The diameter
collimator in as with
it
of the pattern
out.
was
A large portion
of four
current available with
it
in use is
down by
These experimental results indicate
at the
beam is
maximunn beam
of the available
a factor of three.
that spherical aberration con-
tributes considerably to the spreading of the beam.
more pronounced
by using the
half as great with the
screened out by such a collimator, however, and the
effects are
3/4 inch
edges
Since aberration
of a lens field,
it
might be
expected that increasing the diameter of the electrodes would improve
13
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the
performance
of the
source by reducing the spreading
of the
beam.
But increasing the diameter of lenses also reduces their focusing
strength, and thus the electrode diameter
must be
a
compromise between
these two factors.
Chromatic aberration
means was found
is
doubtless present in the over-all problem, but no
for independently investigating its effect.
Such aber-
ration is very likely present in the above space-charge and spherical-
aberration experiments, but we assume that
it
does not play a prominent
role in the effects attributed to them.
Coulomb
3.
scattering.
The residual gas nnolecules within
the ion source constitute a target
material, albeit quite thin, which the protons must pass through.
proton in the
beam should
residual molecules,
suffer
Coulomb scattering
it
may
Thus
it
is not a
a
one of these
loss of energy and change of direction would
its
almost assuredly be great enough for
beam.
off
If
it
to be
eliminated from the useful
question of whether this effect causes losses, as
be in the case of space-charge effects, but rather of how large
such losses are.
Throughout the runs where beam-intensity patterns were recorded
with focus -electrode voltage the only variable,
total
beam current was observed
effect
was not obvious, however,
tested.
manner,
it
was noted
that less
as the focus voltage decreased.
until the virtual
The
-image ion source was
This source operates in an acceleration-acceleration-deceleration
the final or accelerating electrode (using previous terminology)
being used as the variable or "focusing" electrode
A
plot of total
beam
current versus accelerating -electrode voltage for the virtual -image
source showed a much more noteworthy decrease of current with decreasing voltage.
to
Coulomb scattering of charged particles by target materials is known
vary in such a way that a plot of 1/E versus log total bombarding
current
is a
particles.
straight line,
To correlate
creased with losses due
the
arguments
1/E
where E
Coulomb
and log
total
energy
beam current
the loss in
to
is the
of the
as voltage
scattering, a plot
beam
14
bonnbarding
was de-
was made with
current, where
E
is the
energy
riv
ioe
of the accelerating electrode.
This plot
shown as Fig.
is
and
16,
its
close approximation to a straight line confirms that the major contri-
butor to the observed losses
The ratio between
total
is
Coulomb scattering.
beam current
for accelerating-electrode
voltages of ten kilovolts and of five kilovolts
is a
factor of ten.
This
clearly demonstrates that the losses incurred through long drift dis-
tances at low energy are significant.
In the conventional
source the losses from Coulomb scattering are
not so pronounced as those discussed above because the drift distance
at
low energy,
provement
pressure
in
i.
e.
the length of the focus electrode, is not great.
performance
of this
in the focus electrode
source
probable, however,
is
Im-
if
can be reduced, or voltage increased,
or both.
4.
Pressure losses.
The losses through increased pressure
in the focusing
system are
similar in magnitude to and inseparable from Coulomb scattering
losses.
But
it
was
of interest to
observe the gross effects
of
pressure
increase in the systenn, and data were taken for the conventional and
for the virtual -image sources.
An increase
of
pressure from three
system caused a decrease
of
to six
beam current
of
tional source operating at its usual settings.
pressure with the virtual -image source
of
70 percent of the beam.
that low
pressure
is of
x 10
_5
mm
in the^focusing
20 percent for the conven
A
similar increase in
in operation resulted in the loss
These results confirm a fact well known,
prime importance.
15
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CHAPTER
IV
DESIGN AND TEST OF A VIRTUAL-IMAGE ION SOURCE
Experiments conducted during
the
autumn
of
1954
showed
that short-
ening the length of the Cockcroft-Walton column so as to provide a drift
distance between the sovtrce and column of about two feet resulted in a
significant increase in
new arrangement
the exit hole of the arc
of the object in the
feet
from
beam current throughout
the system.
chamber, which
With this
is the
position
composite lens system, was situated more than three
the entrance to the
Cockcroft-Walton column.
This suggested
the possibility of employing a virtual-image ion source, which would
eliminate the drift distance introduced between source and column and
optically provide a virtual
image
of the exit hole at the
same
position,
relative to the column, as the actual exit hole had been with the con-
Such a source would permit the physical position
ventional source.
the exit hole to be at a
minimum
Cockcroft-Walton entrance.
And
distance, about
if it
18 inches,
were possible
from
of
the
to duplicate the
optical characteristics of the conventional source as seen by the
Cock-
croft-Walton, considerable gain might be realized as the result of eliminating two feet of drift distance, for drift distance is, of itself, neces-
These reasons, and
sarily injurious.
the fact that a virtual
-image
source had never been tried on the Bevatron, prompted the attempt to
achieve significant improvement in the useful output
through use
1,
of a
of the ion
source
virtual-image focusing system.
Design.
The arc chamber
was incorporated
that
had been used for the previous experiments
into the virtual-image source (see Fig.
1
),
and the
physical dimensions of the housing of the focusing electrodes were retained.
Thus the new source could be used interchangeably with the
The changes made
to
old.
achieve virtual -image performance were in the
length and diameter of the electrodes.
A
three -inch diameter for the
tubes was adopted with the hope that gains from lower pressure and less
spherical aberration would be greater than any losses occasioned through
u
ix
son
.M
weakening
One inch was added
of the lenses.
to the length of the focus
electrode, and the accelerating electrode was retained at
length by permitting
source than before.
probe
the
tip
it
to
usual
protrude an inch farther from the end of the
The probe length was maintained, but the shape
and exit plate were hemispherical instead
To achieve
its
of
90
of
cones.
a virtual image, the electrodes are energized to provide
an acceleration-acceleration-deceleration action on the protons.
approximation calculations on the focusing systenn,
it
Through
was predicted
that,
with 20 kilovolts on the probe electrode and 40 kilovolts on the focus
electrode, parallel rays would be obtained for an accelerating electrode
voltage of about
At some higher voltage, something less
12 kilovolts.
25 kilovolts, the calculations indicated that a virtual image would
than
be achieved about two feet behind the exit hole.
that
were
to be ennployed,
available to insure
2.
With the power supplies
however, adequate latitude
optimum placement
of the virtual
of settings
was
image.
Test-stand data.
A
series of runs was
made with probe
voltage set at
14 kilovolts
and
Although a crossover focus was expected
focus voltage at 40 kilovolts.
with about eight kilovblts on the accelerating electrode, total beann current
was so diminished by Coulomb scattering
the setting for best crossover
in this
low-energy region that
beam
have been made
was effectively masked and
currents were deceptively low.
If
measurennents could
total
within the accelerating electrode rather than at a distance two feet beyond,
more
indicative patterns and higher currents would undoubtedly
have been achieved.
Settings of ten kilovolts and slightly above on the accelerating electrode
gave
much higher
tering,
total currents,
as would be expected with less scat-
and the patterns were spreading out and flattening
in a
manner
indicative of parallel rays, which would be expected with these settings.
No ready means was
of a virtual
image, and further information on the source could only be
achieved by operating
3.
available on the test stand for determining positions
it
with the Bevatron.
Bevatron runs.
Before new power supplies for use with the virtual -image source were
17
ti&di
jjs us
jodik lo »^«jit>v
a
asw
'ni siorn .bnoy
>um 9VBg
>rfi fan
:9j
srjxfani
breakdown
available, a
source
of the
installation of the virtual source.
It
in the
Bevatron necessitated the
was operated
time with
at this
acceleration-deceleration-acceleration settings, and the best results
were obtained when voltages were such as
The
image.
to give a real
best performance available with this arrangement showed the current
at the
end
of the
Cockcroft-Walton
with the usual source, but total
down by
a factor or
be comparable with that obtained
beam
two or three.
replaced in the Bevatron when
to
it
out of the linear accelerator
Therefore, the usual source was
was
again operative.
Later tests with the virtual-image source operating
showed a
definite
maximum
maximum
from
test information a
minimum
48 kilovolts--
point at
with accelerating voltage at
crossover was expected
this indicates that the first
designed
current through the Cockcroft-Walton with
accelerating voltage at eight kilovolts, a
and a higher
in its
18.5 kilovolts and focus voltage
fashion- -probe voltage
was
maximum was
13 kilovolts,
19 kilovolts.
at about
Since
8 kilovolts,
the crossover or
real-image
minimum a case where the rays were essentially parallel,
larger maximum the desired virtual -image operation. Maxi-
condition, the
and the
mum
current through the Cockcroft-Walton with the accelerating volt-
age in the neighborhood of
is
somewhat
less than the
19 kilovolts
was nine milliamperes, which
12 milliamperes obtained
The maximum available
ditions with the usual source at this position.
current at the input to the linear accelerator was
which compares even less favorably with the
can be obtained with the usual source.
exit of the linear accelerator
the conventional source
the virtual
a factor of
may
6.0
2.2 milliamperes,
milliamperes that
Current measurements
were ridiculously low,
register
under good con-
hoN^ever.
at the
Whereas
300 microamperes at this point,
image source could produce only eight or ten microamperes,
30 down.
The new source was clearly not an improvement.
Reasons why the virtual-image source did not get more current
through the system are obscure, just as the focusing characteristics
of the injection
system are, but indications are
that the
beam was
diffuse, certainly in size and probably also in energy, for
accelerated effectively on through the injection system.
18
it
to
too
be
Smaller tubes
•
is
•0JS
d3 ni
9di
x^m
nj&dli
e»»l
-iHis
iati.
lib
8.-)
Hi
di
IS
yidf
>6
81
or other changes might improve performance somewhat, but
it is
quite likely the systenn prefers a real -image source to any virtual
image source
that
might be designed.
19
BIBLIOGRAPHY
1.
Gow,
J.
Foster,
D. and
J. S.
,
Jr.
A HIGH -INTENSITY PULSED ION
SOURCE,
The Review
Instruments,
pp 606-610,
of Scientific
Vol. 24, No.
8,
August 1953
2.
3
Penning, F. M.
Physica
4,
71 (1937)
Hoyaux, M. and
COMPARATIVE SURVEY OF ION
Dujardin,
GUNS,
I.
Nucleonics Research Laboratories,
Ateliers de Constructions Electriques de Charleroi, Belgium
4
Pierce,
J.
THEORY AND DESIGN OF ELECTRON BEAMS,
R.
D.
5.
Thonemann, P.
6.
Fowler, R.D. and
Gibson, G. E.
7.
Field,
C.
L.M.
Spangenberg, K. and
Helnn, R.
Van Nostrand Company, 1949
Nature 158, 61 (1946)
Phys. Rev. 46, 1075 (1934)
CONTROL OF ELECTRON BEAM
VACUUM
BY IONS,
DISPERSION AT HIGH
Elec.
121,
8.
Comm.
24, No.
1,
pp 108-
March 1947
Zworykin, V. K.
Morton, G. A.
ELECTRON OPTICS AND THE
ELECTRON MICROSCOPE,
Ramberg,
John Wiley and Sons, Inc.
E. G.
J. and
Vance, A. W.
Hillier,
20
,
1948
:>Jeo
•£
T
n
K
noiiT
ffT'^/'n:
bii6
f^n
TWTT
.X
,g
.?
"
1
^^
"
"^
ARC
T\
CHAMBER
ACCELERATING
--
FOCUS
PROBE
i
ELECTRODES'^
-
1
1
1
1
1
30
1
15
1
k V.
k.v.
1
1
^_1_
—
.
_J_-'-
60
!
MU-9546
Fig.
1
Block Diagram of Ion Source.
Diameter for Electrode Tubes,
Lengths of Electrodes:
probe,
focus,
2 in.
2. 5 in.
;
;
accelerating, 4
21
in.
2 in.
;
J
CATHODE
ANODE
V^
MU3409
Fig. 2
Cross Section
22
of
Arc Chamber
n
(
:
3C
U
V,<Vj
V.
j-^
»" ^
v,>v
•*"
MU-9547
Fig.
3
Focusing Action
23
of
Three Electrodes
Fig.
4
Block Diagram
System
of
Bevatron Injection
24
?*^
SHIELD
'i-
FARADAY CUP
3/8'
I
khWIRE
MESH
I
I
0.042
MU-9548
Fig.
5
Original Faraday Cup, Shield with
3/8-inch Hole
Fig.
6
Faraday Cup, Long Shield
Fig.
7
Faraday Cup, Special Shield
15
VACUUM
CHAMBER
SOURCE
AXIS
to
SCREEN
MU-9549
Fig.
8
Mechanical Advantage
26
of
Cup Movement
^
.H
Si'i
"=
0.172"
MU-9550
Fig.
9
Beam-Intensity Cross Sections,
Variable-Focus Electrode Voltage
27
MU-9551
Fig.
10
Spreading
Beam Due
of
to
Originally Parallel Proton
Space Charge
28
9BHHD
TRAP
ELECTRODES
p
^
^
I
-.I.I+
<W^-J
MU-9552
Fig.
11
Ion Trap
29
^
t
^l|H
IM
ki
6.1
m.a.
8.7 m.a.
1"= 0,172"
MU-9553
Fig.
IZ
Beam-Intensity Cross Sections,
Space-Charge Effects
30
.i
^^^
LENS
MAGE PLANE
OBJECT
FAST PROTONS
I
ginw PRhTON f;
MU-£554
Fig.
13
Spherical Aberration
Fig.
14
Chromatic Aberration
31
.-^.
103180
!
eWOTOfiq T2A1
-.
wnp
with
l"=
3/32"
col
I
i
motor
0.172"
MU-9555
Fig.
15
Beam-Intensity Cross Sections,
Aberration Effects
32
I
\ij}
/'i^
—
1000-
liJ
100
3
O
UJ
CO
<
O
CD
O
10
20
10
.2
i/e
X
30
_3
10^
MU-9556
Fig.
16
Total Current Versus Energy, Coulomb
Losses
(E is energy of Accelerating Electrode)
33
—
JL 19
ST
INTERLIB
2847
Stone
on
^P^^'^^the efficiency
+.<=
JH
A^i
Stone
Experiments on imnro-^nn^
efficiency of the bevatron
ion source.
+''»
thesS765
Experiments on improving the efficiency
3 2768 002 02042 2
DUDLEY KNOX LIBRARY
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