Design and transmission characteristics of high pass, mechanical velocity filter for fast neutral molecular or atomic beam

Manista, E. J.

Sheldon, J. W.

Tischler, R. F.

English

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N A S A TECHNICAL
MORAN D U
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NASA TM X,2332
A MECHANICAL ¥1LGCIIT ''
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Ettgene J*> Maimta, John W, Sheldon,
' * ^'* **"""V, Thchler
Lewh Research Center
, Ohio 4435
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NATfO«*l JllliiABTICS AMD SPACI AfiilSISTIATIOK • WASfliBSfiM, B. C. * Jltf 1971
https://ntrs.nasa.gov/search.jsp?R=19710021269 2020-03-23T15:24:32+00:00Z
1. Report No. 2. Government Accession No.
NASATMX-2332
4. Title and Subtitle
A HIGH- PASS, MECHANICAL VELOCITY FILTER FOR FAST
NEUTRAL MOLECULAR BEAMS
7. Author(s)
Eugene J. Manista, John W. Sheldon, and Richard F. Tischler
9. Performing Organization Name and Address
Lewis Research Center
National Aeronautics and Space Administration
Cleveland, Ohio 44135
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546
3. Recipient's Catalog No.
5. Report Date
July 1971
6. Performing Organization Code
8. Performing Organization Report No.
" E-6333
10. Work Unit No.
120-27
11. Contract or Grant No.
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
16. Abstract
The transmission characteristics of a mechanical velocity filter are analyzed, and a design is
described in which mean velocity of transmitted beam divided by rotor speed is equal to
o5. 99x10 centimeters per revolution. Preliminary results for operation of the velocity filter
with a fast cesium atom beam are presented.
17. Key Words (Suggested by Author(s)) 18. Distribution Statement
Atomic-molecular beams Unclassified - unlimited
Velocity filter
19. Security Classif. (of this report) 20. Security Classif. (of this page)
Unclassified Unclassified
21. No. of Pages 22. Price*
11 $3.00
For sale by the National Technical I nformation Service, Springfield, Virginia 22151
A HIGH-PASS, MECHANICAL VELOCITY FILTER FOR FAST
NEUTRAL MOLECULAR BEAMS
by Eugene J. Manista, John W. Sheldon *
and Richard F. Tischler
Lewis Research Center
SUMMARY
The transmission characteristics of a mechanical velocity filter are analyzed for
time-independent particle beams. Results are presented for a monochromatic beam of
particles of velocity v and for a rectangular velocity distribution centered at v with
a full width of 2Av. The problem arising from a slight misalinement of the filter axis
and the beam is also discussed.
A design is described in which the ratio of the cutoff velocity to the rotor speed is3
equal to 5. 99x10 centimeters per revolution. Results of the operation of the filter in
analyzing the velocity distribution of a fast cesium atom beam are presented.
INTRODUCTION
The use of rotating helical slots to select velocities from atomic and molecular
beams has become a standard technique. In practice the rotor is constructed of slotted
disks placed in careful angular alinement with axial spacings chosen to provide a unique
helical path through the slot configuration (Hostettler and Bernstein, ref. 1; Kinsey,
ref. 2). Velocity resolutions of 3 to 5 percent are typical, and molecular speeds up to
6x10 centimeters per second have been measured when the rotor speed is extended to
40 000 rpm (Trujillo, Rol, and Rothe, ref. 3). The need for measuring considerably
higher velocities in some molecular beam studies is anticipated, as for example in the
analysis of the neutral products arising in ion, molecule reactions. This report de-
scribes the design and preliminary tests of a mechanical velocity filter which is capable
of yielding information about a molecular velocity distribution centered about a high
mean velocity without resorting to high rotor speeds. With the filter a molecular
* Professor of Engineering Science, Florida State University, Tallahassee, Florida.
cvelocity of 1. 5x10 centimeters per second can be measured with a rotor speed of
15 000 rpm.
TRANSMISSION CHARACTERISTICS
The filter described is essentially a modified slotted disk velocity selector of the
Hostettler and Bernstein type with zero pitch; that is, all pass channels are alined
parallel to the axis of rotation. Hence, for open shutter times that are long compared
to the transit time down the axis of the rotor, the channels operate as a standard beam
chopper. However, as the open shutter time approaches the axial transit time, the
probability of transmitting a particle of velocity v depends both on the time of its
admittance relative to the opening of the channel and on the angular velocity of the rotor.
The transmission of a finite beam where the width is comparable to the pass channel
slot width can be easily constructed from the results for a narrow line beam. Thus,
we consider the case of a line beam of unit intensity and velocity distribution I(v) inci-
dent on the structure shown in figure 1.
For a narrow, time-independent source of particles alined with the axis of the dis-
criminator the time-averaged transmission T is defined by
- 2;:Ro / • z 1 + z 2 7 r r°T = 2ffR" / 1 Z I P(v,t)I(v)dvdt (1)
where P(v, t) is the probability that at time t a particle with velocity v will be trans-
mitted. The time axis origin has been chosen to be coincident with the opening of the
pass channel slot. The remaining symbols are defined in figure 1.
A beam particle will be transmitted through the velocity filter only if the axial
transit time is less than or equal to the time remaining in the shutter . This condition
is given by
vmin- f o r O s t r s - (2a)
min
 l L _
v = (2b)
m ,
v .mm
m
where v is the minimum velocity capable of transmission at a rotor speed of n. The
axial-spacing -of-the-six.disks. shown -in-figure. l.is-chosen-to.interrupt_particle_trajec-
tories with pitch greater than li/L, (velocity less than v ).
The form of P(v,t) is shown in figure 2. The limiting values of P(v,t) are
P(v,t) = 0 for li/2nR& < t < (l^ + l2)/2nRQ, and all v, since the tooth of width l^
blocks the beam; P(v,t) = 1 for 0 ^ t s l^/2nR^ and v — °°, since the rotor operates
as a standard beam chopper; and P(v, t) = 1 for only a fraction of the total open shutter
time of l*/2irRn for velocities v- v m and t > 0. This last condition is given by
for v a vm (3)
The time -averaged transmission T then becomes
/
l^/2wRSl /•<*>
/ P(v,t)I(v)dvdt (4)
^
 u
 •V/ l - (2 j rROt /2
v , t, and P(v, t) are used to interchange the order of the double integration in the
since P(v,t) = 0 for I j/23rRJ2 < t < (Zj + ZgX^jrRJZ. When the relations among v -n,
m' t? and P^ '^ are(v,t) plane, they yield
y2irRn)[l-(vm/v)]XZ
= _2IRn_ J
ix + Z2 -t)
dt / I(v)dv (5a)
T = i— / (l - -B I(v) dv for v > vm (5b)
T = 0 for v < vm (5c)
To illustrate the operation of the filter, we consider the transmission of a beam of
particles with velocity v and also the transmission of a beam of particles with a mean
velocity of v and a full width of 2Av.
The result for particles of unit intensity and velocity v is obtainable at once from
equation (5), since I(v) = 6 (v - v ).
T =
T - 0
|1.J»J f o r v o & v m
o,
for vo< vm
(6)
Equation (6) shows that, as the rotor speed is increased, the transmission falls linearly
towards cutoff at v = v .
The transmission of a unit intensity rectangular beam of full width 2Av centered
at VQ is given by
T =
T =
T = 0
2Av
2Av
- Avj
2Av m
for VQ - v ^ v > 0
for VQ + Av > vm > vo - v
for vm > VQ + Av
(7)
Equation (7) shows that this transmission decreases linearly with increasing v for
0 ^ v ^ v - Av. The linearly extrapolated intercept v at T = 0 is given bym mo
2Av
mo (8)
In
- Av/
n
where the approximation is for (Av/v ) « 1. Hence, an extrapolation of the experi-
mental transmission curve to zero transmission will yield the mean velocity of the beam
particles. Analysis of the region of the transmission curve near cutoff yields informa-
tion about the velocity spread in the incident beam.
If the beam axis is misalined with respect to the rotor axis by a few degrees,
extrapolation of the linear portion of the transmission curve does not give v «* v .
It can easily be shown that, if the misalinement is by an angle a (taken as positive in
the direction of rotation), extrapolation yields
Essentially the open shutter time for a velocity is increased or decreased by L tan a
depending upon the sign of a. Therefore, if transmission curves are recorded for both
directions of rotation (at constant beam velocity distribution conditions) and each is
extrapolated to zero transmission, the larger value of vmo is the result of rotation in
the direction of misalinement. When the beam and rotor axes are in perfect alinement,
the transmission and the extrapolated intercepts are independent of the direction of
rotation. This property can be used to aline the beam with the rotor axis.
EXPERIMENTAL TEST OF THE FILTER
The filter disks were machined for use in a velocity selector of the Hostettler and
Bernstein design. The rotor assembly, mounting-frame, and drive-motor, as well as
other details of the overall facility have been described elsewhere (Manista and Sheldon,
ref. 4). The rotor dimensions inserted in equation (2b) give the relation v /JZ =o in
5. 99x10 centimeters per revolution. The angular speed of the rotor is monitored by a
light pulse tachometer system.
Preliminary tests of the filter have been conducted by operating it in conjunction
with a fast cesium atom source designed after that of Rubin, Dittner, and Bederson
(ref. 5). A schematic of the apparatus is shown in figure 3. Cesium vapor from the
charge-exchange chamber is ionized upon striking the surface of the tantalum cathode,
which is heated by an electron gun. Ions are extracted and accelerated toward the grid
by the 90-volt potential difference between the target and the grid. The overall ion
acceleration is determined by the voltage between the cathode and the grounded charge-
exchange chamber. Ions which fail to be neutralized in the chamber are removed from
the beam by 600 volts applied across parallel plates along the beam path. The neutral
beam is detected by a surface ionization technique.
Typical operating conditions are cesium reservoir temperature, 95° C; charge-
exchange chamber temperature, 200° C; and background pressure in the vacuum en-
velope, 3xlO~7 torr.
In the present apparatus, alinement of the fast beam, the rotor axis, and the de-
tector is facilitated during operation by the adjustment of the collimating slit and
detector with push rods which pass through the vacuum chamber wall. Extrapolated
intercepts were obtained from experimental transmission curves for both directions of
rotation and several detector positions. The beam was considered alined when the dif-
ference between the extrapolated intercepts for both directions of rotation was a few rps.
The collimating slit was then centered at this detector position.
After alinement the velocity filter was used to determine the mean velocity of the
fast neutral cesium atoms for various ion accelerating voltages applied to the source.
Typical data are presented in figure 4. The transmission of thermal cesium atoms
from the charge-exchange chamber was negligible for rotor speeds above 20 rps. The
c
linear extrapolation (dashed line) to zero transmission indicates v = 1.18X10 centi-
meters per second. The solid curve is a plot of equation (7) for this value of v and
AV = 0. 3x10 centimeters per second. The velocity v + Av corresponds to an energy
of 152 electron volts, which compares favorably with the applied acceleration voltage of
150 volts.
CONCLUDING REMARKS
The transmission characteristics of the velocity filter were derived for a beam of
particles of velocity VQ and for a beam of particles with a rectangular velocity distri-
bution centered at v with a full width of 2Av. The filter was used to analyze a fast
cesium atom beam produced by the charge-exchange mechanism. Good agreement was
found between the maximum expected atom energy and the maximum ion energy.
Lewis Research Center,
National Aeronautics and Space Administration,
Cleveland, Ohio, May 13, 1971,
120-27.
REFERENCES
1. Hostettler, Hans U.; and Bernstein, Richard B.: Improved Slotted Disk Type
Velocity Selector for Molecular Beams. Rev. Sci. Inst., vol. 31, no. 8, Aug.
1960, pp. 872-877.
2. Kinsey, James L.: Design of Slotted Disk Velocity Selectors. Rev. Sci. Inst., vol.
37, no. 1, Jan. 1966, pp. 61-64.
3. Trujillo, S. M.; Rol, P. K.; and Rothe, Erhard W.: Slotted-Disk Velocity Selector
of Extended Range. Rev. Sci. Inst., vol. 33, no. 8, Aug. 1962, pp. 841-843.
4. Manista, Eugene J.; and Sheldon, John W.: Preliminary Experiments with a
Velocity-Selected Atomic-Beam Apparatus. NASA TN D-2557, 1964.
5. Rubin, K.; Dittner, P.; and Bederson, B.: Alkali "Triode" Ion and Fast Neutral
Source.—Rev—Sci-Inst.-,.-vol.-3.5.,_no._12,_Dec.._19.6.4,_pp._1.72.0-1.7.21
-1-12X1-
2 X
X
Figure 1. - Velocity discriminator design. Spacing unit A, 0.833
centimeter; disk thickness d, 0.16 centimeter; slot width, I,,
0.081 centimeter; tooth width lp 0.091 centimeter; mean rotor
radius R, 7.6 centimeters; overall length L, 10.16 centimeters;
number of slits per disk, 278.
-For vm <v < «>
-For v<v ,m
ZrRQ
Time, t
Figured - Probability function P(v, t) for 0<t<( f j + y 27ifi° and various velocities v.
Electron gun
/-Cathode
Grid
^Charge exchange chamber
5.0
17.5
t
!cm
1cm
H
(1
"^ —
1—
L
H — — '
~~~^Collima
^-- Cesium reservoir
— Source slit, 0 .20x5.0 mm
^Velocity discriminator
-—Charge particle extraction
ting slit, 0.60x5.0 mm
ion ization detector
"^Tungsten filament, 0.055 mm diam
Figure 3. - Fast cesium beam apparatus.
£•
I 4
•e
Rotation-
CD Clockwise
D Counterclockwise
20 60 100 140 180
Rotor speed, rps
Figure 4. - Velocity discriminator transmission.
220 260
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A high pass, mechanical velocity filter for fast neutral molecular beams

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