An electron beam device which extracts energy from an electron beam before the electrons of the beam are captured by a collector apparatus is described. The device produces refocusing of a spent electron beam by minimizing tranverse electron velocities in the beam where the electrons, having a multiplicity of axial velocities, are sorted at high efficiency by collector electrodes

Kosmahl, H. G.

English

NASA Technical Reports Server

NATIONAL AERONAUTICS AND SPACE ADMlNIStRATlON
WASHINGTON, D.C. 20546
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REPLY TO
ATTN OF: GP 'NOV 5 1973
TO: KSI/Scientif ic & Technical Information Division
Attention » Mias Winnie M. Morgan
FROM: GP/Office of Assistant General Counsel for
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United States Patent
Kosmahl
[in 3,764,850
[45] Oct. 9, 1973
[54] ELECTRON BEAM CONTROLLER
[75] Inventor: Henry G. Kosmahl, Olmsted Falls,
Ohio
[73] Assignee: The United States of America as
represented by the Administrator
of the National Aeronautics and
Space Administration
122] Filed: June 27, 1972
[21] Appl. No.: 266,832
[52] U.S. Cl 315/5.38, 315/5.35
[51] Int. Cl HOlj 23/02
[58] Field of Search 315/5.35, 5.38
[56] References Cited
UNITED STATES PATENTS
3,450,930 6/1969
3,297,907 1/1967
2,853,641 9/1958
Lien 315/5.38 X
La Rue et al 315/5.38 X
Webber .'^ . 315/5.38
3,273,006 9/1966
3,175,120 3/1965
2,305,884 12/1942
Osepchuk 315/5.38 X
Wendt 315/5.38
Litton 315/5.35
Primary Examiner—Rudolph V. Rolinec
Assistant Examiner—Saxfield Chatmon, Jr.
Attorney—J. R. Manning et al.
[57] ABSTRACT
A magnet applies a magnetic field of predetermined
intensity and shape to a spent electron beam over an
axial distance having a predetermined relationship to
the frequency of operation of a utilization device
which has extracted energy from the electron beam
and at a predetermined axial distance after the beam
has left the magnetic field of the utilization device and
entered the beam expansion region and beam stabili-
zation region. The stabilizing magnetic field is termi-
nated abruptly before the electron beam enters a col-
lector apparatus.
12 Claims, 2 Drawing Figures
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PATENTED OCT 91973 3,764,850
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ELECTRON BEAM CONTROLLER is applicable to solenoid, permanent magnet or perma-
__ _ _. _ nent periodic magnet focused tubes.ORIGIN OF THE INVENTION ,„
 s
P
ummary th
g
e inventjon produces refocusing of a
The invention described herein was made by an em- spent electron beam by minimizing transverse elctron
ployee of the United States Government and may be 5 velocities in the beam whereby the electrons having a
manufactured and used by or for the Government of multiplicity of axial velocities are sorted at high effi-.
the United STates for governmental purposes without ciency by collector electrodes,
the payment of any royalties thereon or therefor.
 BRIEp DESCR,pT,ON Qf THE DRAWINGS
BACKGROUND OF THE INVENTION ,0 p,G , ,g ,he upper .ha,f of , longjtudinal cross.
This invention relates to electron beam devices and sectional view of a microwave oscillator tube embody-
is directed more particularly to apparatus wherein en- ing the invention.
ergy is extracted from an electron beam before the FIG. 2 is a graph whose abcissa is proportional in
electrons of the beam are captured by a collector appa- length to the length of the tube of FIG. 1 and which is
ratus. IS a plot of magnetic Field strength versus axial length of
Modern space communication system require the the tube.
highest possible efficiency from transmitting devices DFSrRlPTiON OF THF PRFFFRRFD
not only to conserve the primary power but also to min- EMBODIMENT
imize the heat dissipation which may occur in space hMBOJJIM
due to poor efficiencies. Aside from high internal effi- 20 Referring now to FIG. 1, it will be seen that a micro-
ciencies, called electronic efficiencies, depressed-type wave tube embodying the invention may be divided
collectors offer a solution for achieving high overall ef- generally into an interaction region 10, a refocusing re-
ficiency in microwave tubes. It can be shown, however, gion 11, a beam stabilization region 12 and a collector
that the realization of energy recovery of such collec- region 13. The foregoing four regions are defined by
tors depends strictly and critically upon the degree of 25 vertical lines 14, IS, 16, 17 and 18.
disorder in the spent beam. Theoretically, 100 percent An electron beam 19 will, in accordance with the in-
efficiency should be possible in a well-designed collec- slant invention, have the general shape as shown in
tor operating in relationship to a point source electron FIG. 1 wherein the spent beam from which energy has
beam having only an axial velocity spread. been extracted in region 10 has a relatively constant di-
In actual practice, however, spent beams have a very 30 ameter as indicated at 20 in the interaction region 10
complex velocity spectrum including radial and azi- of the tube, increasing radius as indicated at 21 in refo-
muthal velocities and radial position distribution in ad- cusing region 11, stabilization of the radial velocities as
dition to the spe'ctrum of axial velocities. Furthermore, shown at 22 by limiting of the velocities and of the ra-
at high microwave frequencies and at kw power ranges, dius to a maximum average fixed value as indicated at
current densities in the electron beam may be as great 35 23 in the collector region 13. Focusing and refocusing,
as 20 to 1200 amperes per square centimeter. It is as used herein, indicate that the transverse velocities of
known that such beams explode radially after the fo- electrons comprising an electron beam are reduced,
cusing fields have been removed and, accordingly, are Consequently, although an electron beam may be ex-
not suitable for injection into many collectors, and par- panding, a limiting or decreasing of the amount of ex-
ticularly in a depressed collector. Consequently, it is 40 pansion will reduce the transverse velocity components
apparent that a substantial dilution of current densities of electrons in the beam. This causes the axial velocity
down to a few A/cm1 must take place before the elec- of the electrons to increase, as may be shown by the
tron beam is injected into certain type collectors if a well-known adiabatic expansion,
high efficiency is to be achieved. In addition to diluting The interaction region 10 may include a solenoid
the current densities, transverse velocity components comprising a current carrying winding 24 and a pole
should be eliminated or minimized. piece 25 which produce a magnetic field of constant
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to provide a method carrying windings 27, a rear pole piece 28 and a front
and apparatus for minimizing the transverse velocity pole piece 29. The second solenoid produces a cons-
components in a spent electron beam. tant magnetic field in stabilization region 12 as indi-
It is another object of the invention to provide for a cated at 30 in FIG. 2 and of much lower intensity than
spent electron beam apparatus for refocusing the the magnetic field of the first solenoid,
beam, which apparatus is easily adaptable to a micro- In stabilization region 12 the transverse velocity corn-
wave electron tube. ponents of the beam are minimum. Advantageously,
Still another object of the invention is to provide a the groups having slower axial velocities drift preferen-
method and apparatus for refocusing a spent electron tially to the edge of the beam thus providing a natural
beam utilizing negligible power or no power at all. velocity-position sorting which reduces the sorting
Yet another object of the invention is to provide a . which must be done subsequently in the collector re-
method for refocusing a spent electron beam for injec- gion 13. This presorting increases the collector effi-
tion into a collector at high efficiency without any com- ciency because less energy is expended by the collector
plex monitoring or programming steps being required. in sorting.
A still further object of the invention is to provide ap- It will be seen that the magnetic field in the refocus-
paratus for refocusing a spent electron beam with a ,, ing region 11 of FIG. 1 decreases smoothly but not nee-
minimum of parts. essarily linearly, as indicated at 31 in FIG. 2, from its
It is an additional object of the invention to provide intensity as produced by the first solenoid to its lower
a method for refocusing a spent electron beam which intensity as produced by the second solenoid. The ratio
3,764,850
of the magnetic field as given by 26 to that indicated by
30 in FIG. 2 is determined by the degree of the required
dilution of the current density in the spent electron
beam. Since the average beam radius is inversely pro-
portional to the local magnetic field, the dilution is ap- 5
proximately given by the factor (Bj/Bs)2, where Br and
Bs designate the values of the magnetic fields in the
tube region 10 and the stabilization region 12, respec-
tively. Because the current density Jr in the tube region
is known from the tube design, and the current density 10
Jc at the entrance into the collector should not exceed
the amount of approximately 10A/cms, we have for the
ratio
(BS/BT)2 = Jc/Jr . A pole piece 32 of smaller diameter ]5
than the pole piece 29 may be disposed on the front of
the pole piece 29 so as to produce a smooth, gradual
change of the magnetic field to its stabilizing value.
This smooth change is shown at 33 in FIG. 2.
In the collector region 13 the electrons comprising 20
electron beam 19 are all collected and, as shown in 38
in FIG. 2, there is no magnetic field in this region ex-
cept for a residual amount as shown at 39 after the
magnetic field beings an abrupt decrease towards 0 as
at 40. The collector mechanism of the collector region 25
13 may comprise apertured dished plates 34, 35, 36
and 37. However, it will be clear to those skilled in the
art, that other suitable collectors of the prior art such
as those used for microwave electron tubes may be
used. To apply an increasingly negative voltage to each 30
of the collectors 34 through 37 in a direction away
from the electron source, the collectors are connected
to a voltage divider 41 which is energized by suitable
d-c source 42. The positive side of the d-c source 42 is
grounded as at 43 as are the pole pieces 25, 28, 29 and 35
32. Numerals 44 through 49 indicate some of the paths
which may be followed by various electrons in the elec-
tron beam as they travel toward the collector plates.
Referring again to FIG. 2, L,, the distance between
vertical dash lines 15 and 16, and L2, the distance be-
tween the vertical dash lines 16 and 17, represent the
length of the refocusing region and the stabilization re-
gion, respectively. In accordance with the invention, L,
has a length from about one cyclotron wavelength to
about three cyclotron wavelengths while L2 has a
length from about one-half cyclotron wavelength to
about 1 cyclotron wavelength, where a cyclotron wave-
length is defined as Ac = tir[Tfl(elm) B], where H, elm
and B designate the average electron velocity, the elec-
tronic charge to mass ratio equal to 1.76X10 inch
Coul/Kg and the local magnetic field B, respectively. It
will be understood by those skilled in the art of electron
beam dynamics in microwave amplifiers, that the opti-
mum lengths L, and L, can be determined exactly only
through complex complications such as to produce
least transverse velocity at the entrance into the collec-
tor. However, the present invention gives the magni-
tude of the required fields the range of suitable lengths
L, and L, and the method and shape of the refocusing
 6Q
fields.
It will, of course, be understood by those skilled in
the art that permanent magnets may be used in place
of the solenoids described in FIG. 1.
If the tube is of a type utilizing permanent magnets
 65
for focusing in the interaction region to minimize
weight and power consumption, then the magnets for
the stabilizing region will preferably be also of the per-
45
manent type. In this case one single permanent magnet
can be constructed such as to produce the required
profile of the refocusing fields by a proper placement
of pole pieces 25, 29 and 28 and proper shaping of the
magnet itself.
What is claimed is:
1. A method of minimizing transverse velocity com-
ponents in an expanding spent electron beam compris-
ing the steps of:
restricting the expansion of said beam within prede-
termined limits for an axial distance of from about
one cyclotron wavelength to about three cyclotron
wavelengths to reduce the radial velocity of elec-
trons in the beam;
then stabilizing said beam for an axial distance of
from about one-half cyclotron wavelength to one
cyclotron wavelengths; and
collecting said electron beam.
2. A method of conditioning a spent electron beam
after it emerges from a first magnetic field of a utiliza-
tion device comprising the steps of:
subjecting said electron beam to a gradually decreas-
ing magnetic field which tapers to a stabilizing in-
tensity, said decreasing magnetic field having an
axial length of from about one cyclotron wave-
length to about three cyclotron wavelengths, sub-
jecting said beam to said stabilizing intensity for an
axial length of from about one-half wavelength to
one cyclotron wavelength after which it becomes
zero; and
collecting the electrons comprising said electron
beam downstream of the point at which the stabi-
lizing intensity drops to zero.
3. The method of claim 2 wherein said stabilizing in-
tensity is from about V4 to 1/10 as great as the intensity
of said first magnetic field.
4. Apparatus for focusing an electron beam compris-
ing:
means for limiting the expansion of said electron
beam within a predetermined limit for an axial dis-
tance from about one cyclotron wavelength to
about three cyclotron wavelengths:
means for substantially eliminating further expansion
of said electron beam for an axial distance of from
about one-half cyclotron wavelength to one cyclo-
tron wavelength; and
means for collecting the electrons of which the beam
is formed, each of said means being positioned in
order as named in a direction away from said
source of electrons.
5. An improved microwave tube comprising means
for producing a beam of electrons;
means for extracting energy from said beam;
first magnetic means for focusing said beam;
second magnetic means disposed downstream of said
first magnetic means to provide a constant mag-
netic field of lower intensity than that of said first
magnetic means;
the fields of said first and second magnetic means
merging to provide a refocusing field which de-
creases gradually in a downstream direction from
the intensity produced by the first magnetic means
to the intensity produced by the second magnetic
means; and
means for collecting the electrons of said beam
downstream of said second magnetic means.
3,764,850
6. The apparatus of claim 5 wherein said first and
second magnetic means are solenoids.
7. The apparatus of claim 6 wherein said second sole-
noid includes a front pole piece including an upstream
extending annular step portion of smaller diameter
than said pole piece.
8. The apparatus of claim 5 wherein said first and
second magnetic means are permanent magnets.
9. The apparatus of claim 5 wherein said collector is
of the depressed type.
10. The apparatus of claim 5 wherein said second
magnetic means includes auxiliary magnetic means dis-
posed at the downstream surface of said second mag-
10
netic means to provide a smooth gradual change as the
decreasing magnetic field between said first and second
magnetic means merges into said lower intensity field
produced by said second magnetic means.
11. The apparatus of claim 5 wherein said second
magnetic means produces a magnetic field having an
intensity 1/10 to % as great as the intensity produced
by said first magnetic means.
12. The apparatus of claim 5 and including means for
abruptly terminating the field produced by said second
magnetic means upstream of said collecting means.
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Electron beam controller

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