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NATIONAL BUREAU OF STANDARDS -1963
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LEGAL NOTICE
This report was prepared as an account of
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United States, nor the Commission, nor any
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A. Makes any warranty or representa -
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or that the use of any information, appa-
ratus, method, or process disclosed in this
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rights; or
B. Assumes any liabilities with respect
to the use of, or for damages resulting from
the use of any information, apparatus,
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that such employee or contractor of the
Commission, or employee of such contractor
prepares, disseminates, or provides access
to, any information pursuant to his employ-
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his employment with such contractor.
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APR 27 1966 Brip
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14
DEVELOPMENTS IN GRIDDED ION LENSES
- LEGAL NOTICE -
The report w prepared u rocount of Government sponsored work. Ketther the Uutted
to , moto Conteston, nor y para trabalho commitustaa:
A. Med u
t or reputation, expruund or implied, we respect to the accu-
mcy, completo, or wodne of the bodoration contatood to dois report, ar dhur dhe ne
of my beformation, eppuretu, odhod, or procese decloond to the report may not inten
petuated on me or
D. ALL Hamu wa mpact to the wood, or for de su ruolttas from the
of any bubormados, appunto, method, or proces diaclound to this reporte
Asind ta the above, para ottaa bell of the Contactoo" tacladus my
ploys or contractor of the Constanta, ar aployw of red contractor, to the extrot that
med waployee of contractor d the Commiuston, ar aployu o much contractor properu,
Het ton, or provides accoto, wy tormation par to a sployees or contract
tal de conducted, or e ampio, non ta rich costructor.
J. W. Johnson
Oak Ridge National Laboratory
Oak Ridge, Tennessee
WIU
Rru L-1134

ARE ON FILE IN THE RE1}EIVING SECTION,
2 PUBLIC IS APPROVED. PROHEDURES
TRT ALEARANGE OBTAINED. RELEASE TO
Abstract For most applications, the use of 5 MV Van de Graaff, there was not sufficient :
grids in focusing elements can improve beam :' space for the installation of the larger pulser. ...
current densities per unit solid angle by a
Also, test bench data indicated that the compact : :
factor of 10 or more. The coupled problems of
pulser lens could handle a larger peak current
.high grid transparency and long grid life have
without adversely affecting the pulse duration,
limited their usefulness in accelerator systems. |
The development of machined grids with a 10 fold
Machined Grids
increase in life expectance will be described.
These grids are for the Einzel lens of the ORNL
It was concluded that the grids would have
Nanosecond Pulser in which wire mesh grids had
to be made with a depth (parallel to the ion beam
an average life to failure of 75 hrs. The grids axis to width ratio greater than one if the grid
were machined by electrical discharge (Elox)
life were to be increased without a corresponding
techniques in a sheet of tungsten or molybdenum. decrease in grid transparency. At the suggestion
Transparencies as high as 84% have been obtained. of R. M. Farnham of ORNL Fabrication Department,
The sheets can be pre-shaped to compensate for
mechining deep grids by electrical discharge
residual spherical aberrations in the lens,
techniques was attempted. The method used con-
Shaped grids have shown further increases of
sisted of cutting or to be more accurate burning
beam brilliance of 30-40%. For fast pulsing op- ' through an 0.015 in, thick sheet of tungsten
erating in which there is no space charge
with a brass tool cut to form the negative of
neutralization, beam brilliance figures of l amp the desired gridded area. The tool was gridded
per sq cm per steradian have been obtained with with an 0.006 in, thick slitting saw to & depth :
beam currents up to 3 mA.
of at least 5 times the thickness of the sheet to
be cut. The time required to make the tool was
Introduction
reduced considerably by continuously cooling the
tool with liquid nitrogen. This kept the thin
In most electrostatic lens-systems for ion-
sections of the tool from softening from the
beams, except for those systems with very large heat of cutting and from bending out of position.
magnifications, the use of grids in the focusing Also, ice, frozen out of the air, acted as support
elements can improve beam current densities per as it filled up the slits behind the saw. Figure
unit solid angle by a factor of 10 or more,
3 shows the developmental cutting tool and grid
According to Liebmannt and subsequent measure-
which proved the technique practical,
ments made at Oak Ridge the grids greatly re-
The electrical discharge machine used was a
effectively removing the divergent parts of the model M-500 with PS-28 power supply made by the
field pattern in the lens geps. The mild di-
Elox Corp. of Michigan, Royal Oak, Michigan. The
vergent nature of the individual apertures of
power supply was run at 200 Kc.
the grids makes them impractical to use in lens-
systems having very large magnifications i.e.,
Because of the wear on the cutting tool which
electron microscopes. Figure 1 illustrates the may be seen in Fig. 3, it was necessary to use two
reduction in focus diameter which can be ob-
cutting tools pei' grid, one for the roughing cut
tained by using grids. The traces were made by and the other for finishing. Careful re-alignment
slowly sweeping the beam across two closely
of the finishing cutter was, of course, recessary.
spaced apertures at the Lens focus. The beam
diameter as determined for the sharper traces
Grids with a transparency of up to 84% have
(grids in lens) was about 1/3 the diameter ob-
been made; however, 80% seems to be a reasonable
tained from the broader traces (grids removed)
average value for transparency which can be rou-
Itinely obtained with the present techniques and
The main disadvantage to using grids is that equipment.
they are gradually sputtered away. In the
Manosecond Pulser? installed in the ORNL 3 MV
Grid Life Test
Van de Graaff, average grid life to failure has
been 250 hrs. The development of a compact
A compact pulser was set-up and run on a
pulser, which has an overall length and lens dia test bench to lire test the machined grids. The
meter of approximately 50% that of the pulser
ion source, a Duo-Plasmatron, and the pulser were
used in the 3 MV Van de Graaff, accentuated the operated at the same conditions in which 0.0015 in.
sputtering problem; for the same beam current, . W wire grids had failed after 70 to 80 hrs. The
the current density on the grids was 4 times
total beam current from the source was about 2.6
greater. The result was to reduce the time be-
mA, and the pulser was run at 2 Mc. The test was
tween grid replacements to an unacceptable
concluded after 200 hrs. when it was found that
interval of 75 hrs. As may be seen in Fig. 2,
there had been no noticable decrease in the depth
- the Duo-Plasmatron Pulser Terminal on the ORNL
of the grids. The only evidence of sputtering
.
.
-.
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-
. -
-
..

hun
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..
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-
.6 x 9 PRINT SURFACE
.
.
-
-..
-
...
..
..
.
...
...
was a slight rounding of the surface of the first. i. Further increases in transparency of machined
grid facing the beam. This sputtering may be . grids should be possible with improved machining
seen in Fig. 4 which is an enlargement of the
techniques and equipment. Personnel from the
center of the grid area. The grid mesh is 40 x 40 ORNL Fabrication Department think that the newer
per in, and the webs width averages 0.0037 in.
models of the electrical discharge machines will ·
This greatly reduced sputtering rate probably was give better surface finishes and therefore better
due to a lower grid temperature since the machined dimensional control. This should allow for a re-
grids have much more heat transfer area than the
duction in grid width without endangering the
wire grids.
mechanical inbegrity of the mesh.
..
Curved Grids
Optimizing the shape of curved grids is
being approached empirically. The unknown effect
The development of practical grids machined of the rf sweep voltage on the focus and the
in sheet material led, naturally, to attempts to distortion of the lens fields by secondary
make further improvements in beam quality by
electrons makes analytical methods impractical in
shaping the gridded surface. The only shaped sur-. this application.
face tested to date was spherical with a radius
equal to 2 times the lens diameter. The convex
Acknowledgments
surface of the grids face the center element of
the Einzel lens as may be seen in Fig. 5. The
I gratefully acknowledge the contribution
lens diameter is 2 in, and the overall length from to this experiment of C. R. Rickard, J. L. Neil,
image to object is 7.1 in. The diameter of the
Sr., and C. M. Berge who were instrumental in
gridded area of the machined grids, also shown in development of the grid machining techniques. I .
Fig. 5, can be limited to 1 in. because the beam also wish to thank C, D. Moak for his helpful and
diameter is only 0.75 in. at that point.
encouraging discussions during the experiment and
the preparation of the manuscript.
The curved grids were made by forming an
0.010 in, thick sheet of molybdenum to the de-
References
sired radius and then cutting the grid in the same
ranner as previously described.
Research sponsored by the U.S. Atomic Energy
Commission under contract wich the Union Carbide
Since our major interest is in fast beam-
Corporation.
. pulse work, the flat and curved grids were com-
pared on a basis of results produced during pulsed 1. G. Liebmann, Proc. Phys. Soc. (London). 62B,
operation on a test bench. The beam pulses were
213 (1949).
collected on an ultrahigh frequency Faraday cup
located immediately below the sweep aperture of 2. C, D. Moak, W. M. Good, R. F. King. J. W.
the pulser. From the trace displayed on a sam
Johnson, H. E. Banta, J. Judish, and W. H.
pling oscilloscope (Tektronix 661) the beam ::
Dupreez, Rev. Sci. Instr. 35, 672 (1964).
current and shape were determined. The traces
.
F. W. Johnson, and R. F. King, Rev. Sci.
Instr. 30, 694 (1959).
Igrids with no change in the ion source or
extractor geometry or operating conditions. The
peaks are E, H, and Hž beams, left to right,
respectively, as separated by transit time effects
during fast sweeping. As may be seen in the table
in Fig. 6, the beam brilliance and current density,
as determined from H peaks, for the curved grids
are 37% higher than that of the flat grids. The
higher H peak current from the curveå grid set-
up was due to higher transmission of the curved
grids and was normalized out for the determination
of beam quality. The significant information from
· trace was obtained from the fiull width at half
diameter:
Discussion
Under normal operating condition in a Van de
Graaff accelerator the life of machined grids is.
expected to exceed 1000 hrs. This type grid
recently was installed in the lens-pulser in the
ORNL 5 MV Van de Graaff; therefore the grid life
in this machine should now exceed the operating
life of other items in the terminal. The 12-16
hr loss of operating time every 75 hrs to replace
men-
----
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.
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m
anggagaya
00:41 - AEC - OFFICIAL
Figure Captions
Fig. 1 - Effect of Einzel Lens Grids on Focus.
Fig. 2 - Duo-Plasmatron Pulser Terminal for ORNL 5 MV Van de Graaff.
Fig. 3 -- Developmental Cutting Tool and Grid.
Fig. 4 - Enlargement of Machined Grids.
Fig. 5 - Compace Lens-Pulser and Grids.
Fig. 6 - Beam Quality; Flat vs. Curved Einzel Lens Grids.
UNCLASSIFIED
PHOTO P-62219

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Effects of Einzel Lens Grids on Focus.

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ORNL-DWG 65-1974
.
DUO PLASMATRON
ION SOURCE
U
-EXTRACTOR
ELECTRODE
l111113
-UPPER DEFLECTOR
PLATES
4.05 R
3.26
..
WIRE MESH TI
0.26
-
1.20
2.00 DIA —
- EINZEL
LENS
UP-
TO
2
INCHES
000
0.26
FLAT
MACHINED
3.85
-LOWER DEFLECTOR
PLATES
CA
2
-
SWEEP APERTURE
DIMENSIONS IN INCHES
.
.
.
.
.
.
'N
CURVED
MACHINED
GRID TYPES
LENS-PULSER
Compact Lens - Pulser and Grids.
+



ORNL-DWG 65-1973
usik, plavi
M
GRID
SHAPE
CURRENT DENSITY BEAM BRILLIANCE
1 |
AOC
amp
amp
FLAT | 0.043 cm
CLEVED 0.059 cm
amp
0.77 cm. Sterodian
1.06 cm. Sterodian
059
amp
cm2.steradian
(r=2D)
cm2
-
-
-
--
--
-
--
-
-
.
Ī
S
.
Beam Quality; Flat vs Curved Grid Einzel Lens.
END
DATE FILMED
7 / 2 /65






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