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Патент USA US3066247

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Nov. 27, 1962
J. E. NEVXNS, JR
3,066,237
SLOW-WAVE STRUCTURE
Filed Dec. 15, 1958
4 Sheets-Sheet 1
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Nov. 27, 1962
J. E. NEVINS, _JR
3,066,237
SLOW-WAVE STRUCTURE
Filed Dec. 15, 1958
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Nov. 27, 1962
3,066,237
J. E. NEVINS, JR
SLOW-WAVE STRUCTURE
4 Sheets-Sheet 3
Filed Dec. 15, 1958
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Nov. 27, 1962
J. E. NEVINS, JR
3,066,237
SLOW-WAVE STRUCTURE
Filed Dec. 15, 1958
4 Sheets-Sheet 4
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United States Patent 0 ’ ice
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Patented Nov. 27, 1962
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3,066,237
vidual ferrules which are supported by the discs and which
are concentric with the beam axis. The remainder of the
slow-wave structure is completed by conductive spacer
SLOW-WAVE STRUCTURE
John E. Nevins, (In, Los Angeles, Calif, assignor to
rings between separate adjacent pairs of discs, by a highly
Hughes Aircraft Company, Culver City, Calif., a cor
poration of Delaware
CI conductive surface on the interior faces of the ferrules,
discs and rings, and by coupling holes in the webbed
portions of the discs between the ferrules and the radially
separated spacer rings. A variation in the periodicity of
Filed Dec. 15, 1958, Ser. No. 781,420
10 Claims. (Cl. 315-35)
This invention relates to traveling-wave tubes, and par
ticularly to improved forms of slow-wave structures for
traveling-wave tubes.
the traveling wave with respect to the electron stream is
10 achieved within this structure without affecting the rela
‘It is well known that in the operation of traveling-Wave
tubes the interaction between the electron stream and the
traveling wave causes a diminution in the axial velocity of
the electron stream. The consequence of this fact is that
the relative axial velocity of the traveling wave and the
electron stream become appreciably different toward the
collector end of the traveling-wave device. Accordingly,
there is a reduction in the effective interaction between
the stream and the traveling Wave, and a consequent re
duction in the efliciency of the device.
A number of different techniques have been suggested
for modifying the slow-wave structure so as to achieve
improved interaction along the length of the traveling
wave tube. Thus, with the helix type of slow-wave struc
ture there has been provided a gradual decrease in the
pitch of the helix along the path of the electron stream to
cause a decrease in the axial velocity of the traveling
wave to correspond to the similar decrease in the axial
velocity of the electron stream. It is, however, difficult to
fabricate helices of uniform pitch with the desired ac
curacy and rigidity, and even more di?icult to fabricate
the desired varying pitch helix. It is also exceedingly
troublesome and complex to design a tapered pitch slow
tive positions of the successive interaction cells. In this
arrangement, the axial spacings between successive fer
rules is kept substantially unchanged. The position of the
ferrule with respect to its associated disc, however, is
successively shifted along the length of the tube, because
the periodicity of the ferrules is different from that of
the cells; that is, the length of the ferrule plus a gap
is different from the length of a cell. Also, the ferrule
length may vary slightly as a function of distance along
the tube. Thus, as the traveling wave is propagated down
this slow-wave structure, the point within each interaction
cell where the interaction between the electron stream
and the traveling wave occurs is successively shifted. The
distance the traveling wave moves within the given inter—
action cell remains constant, so that the actual periodicity
remains the same. The action of the slow-wave circuit
is greatly bene?ted because, as seen by the traveling-wave
energy, it is purely periodic even though it appears to be
tapered to the decelerating electron stream.
The conductive discs and ferrules may be nonmagnetic
if the traveling-wave tube is focused by an external mag
net. On the other hand, they may be of a magnetic
material and actually be the individual magnets or magnet
pole pieces if the tube is periodically focused with mag
wave structure which has proper correction and compen
nets which are integral with the slow-wave structure.
sation along the structure for the parameters of the struc
The novel features of this invention, as well as the in
ture, for example, its impedance, which are unavoidably
vention itself, may be better understood when considered
altered in the tapering scheme.
in the light of the following description taken in conjunc
The advent of the folded waveguide and various modi
tion with the accompanying drawings in which like refer—
?ed folded waveguide slow-wave structures has resulted 40 ence numerals refer to like parts and in which:
in the provision of a number of modi?cations intended to
FIG. 1 is an overall view, partly in longitudinal section
achieve the desired maximum interaction of the traveling
and‘ partly broken away, of a traveling-wave tube which
wave and the electron stream. The modi?cations em
may be constructed with a tapered slow-wave structure in
ployed have usually been analogous to the change in the
with the present invention;
pitch of the helix. These modi?cations have in most in 45 accordance
FIG. 2 is a detailed longitudinal sectional‘ view of a
stances involved considerable modi?cation of the slow
portion of the tube illustrated in FIG. 1;
wave structure. In many instances, the predetermined
FIG. 3 is an exploded view of a set of typical elements
change in the axial velocity of the traveling wave causes
included
in the‘ structure of an embodiment of the present
a number of problems such that although tapering of the
‘
tortuous path to be traversed by a traveling wave may 50 mvention;
FIG. 4 is a simpli?ed schematic type view showing a
provide increased interaction, it may also tend to decrease
tapered slow-wave structure constructed in accordance
the stability and bandwidth of the traveling-wave device.
with the present invention; and
‘It is therefore an object of this invention to provide
FIG. 5 is a longitudinal sectional view of a practical
an improved traveling-wave device which has a maxi
embodiment
of a conventionally focused high power
mum of interaction between the traveling Wave and the
traveling-wave tube utilizing a tapered slow-Wave struc
electron stream passing therealong.
It is another object ‘of this invention to provide an
improved slow-wave structure which has desirable varia—
tions in the phase relationship between the traveling wave
and associated electron stream, but which is easy to fabri
cate and install.
ture constructed in accordance with the present invention.
Referring to the drawings and their description, 3.
number of features are shown for completeness of de
scription of an operable traveling-wave tube according
to the present invention, which features are not claimed
in the present application but are claimed and described
A further object of this invention is to provide an im
more fully in applications assigned to the assignee of
proved slow-wave arrangement for the selective control
the present application, for example: Periodically Fo
of axial slow-wave velocity and which also can provide
cused
Traveling-Wave Tube, by D. J. Bates, H. R. John
regularly and equally spaced interaction cells or cavities. 65
son, and O. T. Purl, Serial No. 764,884, ?led October
These and other objects of this invention are achieved,
2, 1958, now Patent No. 2,985,792; Severed Periodically
in one exempli?cation, by a slow-wave structure which
Focused Traveling-Wave Tube, by D. J. Bates and O.
employs a plurality of conductive discs which set off
T. Purl, Serial No. 764,883, ?led October 2, 1958, now
individual interaction elements de?ning cells or cavities
or interaction cavity resonators spaced along and normal 70 Patent No. 2,985,791; Periodically Focused Traveling
Wave Tube With Tapered Phase Velocity, by D. J. Bates,
to the axis of an electron stream. The inner periphery
Serial_No. 764,885, ?led October 2, 1958, now Patent
of the discs, adjacent the stream, is terminated by indi
f
3,066,237
9
o
No. 2,956,200; and Self-Aligning Traveling-Wave Tube
and Method, by Eugene J. Flannery and Ted Leonard,
_
j
v
,
‘
4.
tube 48 further thermally insulates the inner intensively
heated members of the electron gun 28 and also provides
Serial No. 764,886, ?led October 2, 1958, now Patent
electrical insulation between the cathode-beam focusing
No. 2,957,102.
assembly and the higher potential accelerating anode 52.
Referring with more particularity to FIG. 1, there is 5 Substantially encasing the electron gun 28 and secured
shown a traveling-wave tube 12 utilizing a plurality of
to the central or radio frequency structure of the travel
annular disc-shaped focusing magnets 14. In the example
ing-wave tube 12 is a hollow cylinder 50, which may be
of this ?gure, these are permanent magnets and are dia
Kovar, to which is sealed the ceramic cylinder 48, thus
metrically split, as discussed later in connection with the
completing the vacuum envelope about the right-hand
description of FIG. 3, to permit their being easily slipped 10 end of the traveling-wave tube 12.
between assembled adjacent ones of a series of ferro
At the left-hand end of the tube 12, as viewed in FIG.
1, there is shown a cooled collector electrode 60 which
has a comically-shaped inner surface 62 for collecting the
and magnets 14 form both a slow-wave structure and
electrons from the high power electron stream and dis
envelope 18.
15 sipating their kinetic energy over a large surface. The
Coupled to the right hand or input end of the slow
collector electrode is supported within the end of a water
wave structure 18 is an input waveguide transducer 20
jacket cylinder 64 which is in turn supported by an end
which includes an impedance step transformer 22. A
plate 66. A water chamber 68 is thus formed between
?ange 24 is provided for coupling the assembled travel
the outer surface of the collector electrode 62 and the
ing-wave tube 12 to an external waveguide or other 20 inner cylindrical surface of water jacket 64. A water
microwave transmission line (not shown). The con
input tube 70 supplies cool water to this chamber and a
struction of the ?ange 24 includes a microwave window
water output tube 72 exhausts the heated water out of
(not shown) transparent to radio frequency energy but
the water chamber 68. Thus, considerable power may
capable of maintaining a pressure differential for main~
be dissipated without destruction of the collector elec
taining a vacuum within the traveling-wave tube 12. At 25 trode. Although water has been speci?ed, obviously,
the output end of the tube 12, shown in the drawing
other liquids or gases may be used as coolants.
as the left-hand end, an output transducer 26 is provided
The end plate 66 is sealed to a supporting cylinder 74,
which is substantially similar to the input impedance
which may be 0E Kovar, and which is in turn sealed
transducer 20.
to a ceramic insulating cylinder 76. This ceramic insulat
An electron gun 28 is disposed at the right-hand end,
ing cylinder 76 is sealed at its opposite end to another
as shown in the drawing, of the traveling-Wave tube 12
Kovar supporting cylinder 78, which is in turn supported
and comprises a cathode 30 which is heated by a ?la
and sealed to the slow-wave structure end disc 80. The
ment 32. The cathode ‘30 has a small central opening
collector 62, the end plate 66, the supporting cylinders 74
34 to aid in the axial alignment of the gun assembly
and 78 and the ceramic insulating cylinder 76 are all
with the remainder of the traveling-wave tube 12. The 35 coaxially supported in alignment with the axis of the
cathode 30 is secured about its periphery by a cylin
traveling-wave tube 12.
drical shielding member 36 which is constructed in a
For vacuum pumping or out-gassing the traveling-wave
manner to fold cylindrically, symmetrically back upon
tube 12, a double-ended pumping tube 86 is connected to
itself to form a double cylindrical shield and an extended
both of the input and output waveguide transducers
thermal path from the cathode 30 to its outer support 40 20 and 26. Out-gassing during bake-out of the entire
ing means. Such support and an electrical, highly con
traveling-wave tube 12 is thus achieved as rapidly as
ductive path to the cathode is thus achieved while pro
possible. After the out-gassing procedure, the tube 86
magnetic pole pieces 16, which are shown in more
detail in the later ?gures. The system of pole pieces 16
viding considerable thermal insulation for the cathode
and ?lament due to the extended or tortuous path for
heat conduction, as well as because of the multiple cylin
is separated from the vacuum pumping system by pinching off the tube at the tip 88.
The traveling-wave tube of the present invention may
be severed into a number of amplifying sections 90, 92,.
drical shielding against radiant heat which is provided
by the cylinders shown. For additional details of this
94, 96 and ‘98. Each of the amplifying segments or sec*
type of gun construction, see the patent to J. A. Dallons,
tions is isolated from the others by an isolator or termina
No. 2,817,039, entitled “Cathode Support,” issued De
tion section 100, 102, 104 or 106. The structure of these
cember 17, 1957, and assigned to the assignee of the
50 isolating sections will be discussed in detail in connecpresent invention.
tion with FIGS. 2 and 4. It su?ices at this point to dc
A focusing electrode 38 supports the cylindrical shield
scribe
their function generally as providing a substan
ing member 36. The focusing electrode 38 is generally
tially complete radio frequency isolation between adjacent
maintained at the same potential as that of the cathode
sections of the slow-wave structure 18 while at the same
30 and is shaped to focus the electron stream emitted
time
allowing the electron stream to pass straight through‘
55
by the cathode in a well-collimated, high perveance beam
the
entire
length of the traveling-wave tube 12. Each
of electrons which traverses the slow-wave structure 18
amplifying section thus provides an optimum gain while;
and electro-magnetically interacts with microwave en
providing freedom from oscillations due to regeneration.
ergy being propagated therealong. The electron gun
The
loss in gain due to each of these isolation sections is;
con?guration is in accordance generally with the teach
ings in the Patent No. 2,811,667, by G. R. Brewer, which 60 of the order of a few decibels and is a low price to pay
for the large overall gain and power handling capabilities;
issued October 29, 1957, entitled “Electron Gun,” which
of a traveling-wave tube constructed in accordance with
is assigned to the assignee of the present invention, and
the present invention. It should be noted that although.
to which reference may be made for a more detailed ex
the isolation sections provide substantially complete:
planation. The focusing electrode 38 is in turn sup
ported ‘by a hollow cylindrical support 40 which ex 65 radio frequency isolation between adjacent amplifying,
sections, the electron stream is modulated at the output
tends from the periphery of the focusing electrode to the
right-hand end of the traveling-wave tube 12. Its open
ing is' hermetically sealed with a metal-to-ceramic seal
of each amplifying section. The stream thus modulated,
as it enters the subsequent amplifying section launches a;
new wave therein which is further ampli?ed by the inter
42 by means of a sealing ?ange 44 made of a material
having a low coefficient of thermal expansion, such as 70 action between the new traveling wave and the elec-.
tron stream. Thus there is provided unidirectional couKovar. The right-hand extremity of the cylindrical sup
pling through the electron stream between adjacent ampli~
port 40 is supported by an annular ?ange member 46,
fying sections.
which also may be Kovar, and which is sealed in turn
Referring with more particularity to FIG. 2, there is
to a hollow ceramic supporting tube 48. The ceramic
75 shown a detailed sectional view of a portion of the travel~
.
,
5
ing-wave tube of FIG. 1. The ferromagnetic pole pieces
16 are shown to extend radially inwardly to approximately
the perimeter of the axial electron stream. Disposed
contiguously about the electron stream in each case
is a short drift tube 110. The drift tube 110 is in the form
of a cylinder or ferrule extending axially along the strea
and supported by the pole piece 16.
‘
Adjacent ones of the drift tubes 110 are separated by
122 and which are substantially ?lled with the lossy
attenuating material.
The two cavities 131! are substantially isolated from
each other by a short circuiting vane, shown in a later
?gure, and are isolated from interaction with the electron
stream by means of a central portion of the special spacer
which has the form of a ring having substantially the
tween the electron stream and miscrowave energy travers
same radial dimensions as the drift tubes 110 and which
extends between two of the drift tubes 110 as shown in
a manner to substantially shield the electron stream from
the slow-wave structure in the region of the isolator sec
tion 1%.
ing the slow-wave structure.
At a radial distance outwardly from the drift tubes
vidual microwave cavities or interaction cells 118 are
a gap 112 which functions as a magnetic gap to provide
a focusing lens for the electron stream and also as an
electromagnetic interaction gap to provide interaction be
110 each of the pole pieces 16 has a short cylindrical ,
extension 114 protruding from its surface. The exten
sion 114 provides an annular shoulder concentric about
Along the length of slow-wave structure 18 the indi
coupled to the electron stream by means of the gaps 112
between adjacent ones of the drift tubes 110'. In accord
ance with the present invention, the position of the indi
vidual coupling gaps 112 in each cell with respect to the
axial center of the end walls of that cell may be varied
the axis of the tube for aligning the assembly of the
component elements of the slow-wave structure 18. Dis
posed radially within the extension 114 is a conductive, 20 in a manner to provide a taper of the slow-wave struc
ture. For example, in each section of the traveling-wave
nonmagnetic circuit spacer 116 which has the form of an
tube 12, or particularly in the output section, it may be
annular ring having an outer diameter substantially equal
advantageous, as discussed above, to correct for the
to the inner diameter of the cylindrical extension 114.
deceleration of the electron stream as it gives up energy
The axial length of the spacer 116 determines the axial
to the traveling waves traversing the slow-wave structure.
length of the microwave cavities 118 which are inter
As also discussed above, this deceleration is normally in
connected along the length of the slow-wave structure 18.
herent and results in a loss of synchronism between the
It is thus seen that the slow-wave structure may be
assembled and self-aligned by stacking alternately the
pole pieces 16 and the spacers 116. Each spacer 116
has two annular channels 120 in which, during the stack
ing procedure, a sealing material, such as a brazing alloy,
is placed. When the slow-wave structure 18 is as—
sembled, it may be placed in an oven within a pro
tective non-oxidizing atmosphere and heated so that
the brazing alloy in the channel 120 melts and fuses
or brazes the adjacent members of the slow-wave struc
ture 18 together to form a vacuiun-tight envelope. The
modulated electron stream and the traveling waves which
in turn results in a decrease in efficiency of the tube.
One way to provide such tapering is to actually change
the periodicity of the slow-wave structure so that, for
example, the microwave cavities 118 are closer together
so that the electron stream, though it is decelerating, will
continue to interact with successive ones of the interac
tion cells with a constant periodicity.
However, such
changing of the geometric parameters of the interaction
cells gives rise to a great many problems which makes
spacers 116 are fabricated of a nonmagnetic material,
such as copper, thus providing a highly conductive cavity
the process extremely complicated since altering the
geometric parameters of the cells affects the electric
For interconnecting adjacent interaction cells, a cou
The drift tubes 110 may be shortened slightly and are
wall while not magnetically shorting out the focusing gaps 40 parameters, such as the impedance of the circuit. In ac
cordance with the present invention, the geometric param
112. The entire interior surfaces of the cavities are pref
eters of the individual interaction cells 118 which affect
erably plated with a highly conductive material, such as
the electromagnetic properties thereof are not altered.
a thin silver or gold plating 121.
pling hole ‘122 is provided in each of the ferromagnetic 45 shifted upstream to the left progressively along the tube
so that the electron stream may experience interaction
pole pieces 16, the more detailed shape and orientation
through the gaps 112 at a substantially constant periodic
of which will be ‘described in connection with the descrip
ity, even though the stream is decelerated. In other
tion of FIG. 3 below. Also disposed between adjacent
words, the axial placement of the coupling gap 112 with
pole pieces 16 are the focusing magnets 14 which are
in each of the interaction cells 118 does not affect the
annular in shape and ?t angularly or azimuthally sym'
electrical properties of the interaction cell, but the place
metrically about the cylindrical shoulder extensions 114.
ment of the gaps does affect the point at which the elec
The magnets 14 may be diametrically split to facilitate
tron stream interacts with the particular interaction cell.
their being applied to the slow-wave structure 18 after
in FIG. 2 it may be seen that the drift tubes 110‘ in
it has been otherwise assembled. The axial length of the
the section of the slow-wave structure 18 disposed to the
magnets 14- is substantially equal to the axial spacing be
left of the isolator section 101} have been shifted to the
tween adjacent pole pieces 16, ‘and their radial extent is
left, that is, upstream so that in the last cell of that sec
approximately equal to or may be, as shown, greater than
tion, viz., that adjacent the isolator section 100, the cou
that of the pole pieces 16. To provide the focusing lenses
pling space 112 is all the way to the left in its respective
in the gaps 112, adjacent ones of the magnets 14 are
stacked with opposite polarity, thus causing a reversal (50 interaction cell. It may also be seen that in the ampli
?er section to the right of the isolator section 160- the
of the magnetic ?eld at each successive lens along the
drift tube 110 in the ?rst interaction cell is disposed all
tube.
the ‘way to the right of that cell, and that in subsequent
Referring to a typical isolator section 1120, there is
interaction cells it is shifted progressively to the left as
shown a substantial continuity of the pole piece-magnet~
shown. Other practical embodiments and a more sche
spacer assembly. However, the- pole pieces 124 at either
matic and simpli?ed version of the invention is described
end of the isolator section and the spacer 126 are some
in connection with FIGS. 4- and 5.
what modi?ed with respect to pole piece 16 and spacer
Referring to FIG. 3, one set of the plurality of pole
116, respectively, which will be shown with greater clarity
in FIG. 3. It is sufficient here to point out that attenuat
pieces, magnets, spacers and drift tubes or ferrules is
buttons 12% which extend from within a coupling hole
individual elements of the slow-wave structure 18 are
ing material, which may be in the form of lossy ceramic 70 shown for purposes of describing more clearly how the
122 through the special spacer 126 and partially into the
wall of the pole piece 124 opposite the coupling hole.
fabricated and assembled. A typical pole piece ‘16 is
The spacer 126 forms a pair of modi?ed cavities 130‘
shown twice in the ?gure, once in plan and once in side
elevation. A typical magnet 14 and a typical spacer 116
which lie opposite respective ones of the coupling holes
are shown in side elevation only.
'
3,066,237
Referring‘ to the side elevation view of the pole piece
16, it may be observed that the orientation thereof is
concentric about the electron stream. Substantially im
mediately surrounding the electron stream and supported
by the pole piece 16 is the short ferrule or drift tube 110
which extends axially along the electron stream. As
indicated in FIGS. 2, 4 and 5, the axial position of the
drift tube 110 with respect to the pole piece 16 may vary
from pole
slow-wave
concerning
is deferred
8
successively alternating spacer rings 116, 117 or 126 and
pole piece discs 16 joined together, as by brazing, in
clude the fact that a combined slow-wave structure may
be provided which is hermetically sealed and extremely
rugged. At the same time, this structure does not require
special aligning rods or other aligning devices. It is
very precisely positioned, so that focusing of the electron
stream may be accomplished with members which extend
piece to pole piece successively along the
to the very edge of the electron stream, thereby increas
structure 18. Again, additional discussion 10 ing the e?’iciency of the tube. The construction of the
the axial placement of the drift tubes 110
device from separate ceramic or metallic shapes of in
to the description of FIGS. 4 and 5 below.
herently strong con?guration means that problems of
The pole piece apart from the drift tube 110 extends
tube deterioration or destruction due to heating or ex
treme environmental conditions are minimized. The
centrically about the drift tube 110 and radially separated 15 shoulders 114 in the discs 16 are concentric with the
radially outwardly therefrom as shown. Positioned con
therefrom are the cylindrical shoulder extensions 114
desired electron beam path. Therefore, when the outer
periphery of the rings 116, 117 and 126 registers with
the shoulders 114, all the members are accurately posi~
The outer diameter of the cylindrical extension 114
tioned and concentric. Furthermore, when the brazing
supports the focusing magnet 14 coaxially about the elec 20 material
is fused, the result is a rugged air-tight envelope.
tron stream while the inner diameter of the extension
In the operation of the travelingewave tube 12, micro
which extend axially outwardly from either face of the
pole piece 16.
114 rests against the outer periphery of the spacer 116.
The inner diameter of the spacer 116 determines the
outer radial dimension of the interaction cell which is
formed between adjacent ones of the pole pieces 16.
Before assembly, a sealing material is placed in the
channels 120 which are continuous annular grooves in
the end surfaces of the spacers 116. As indicated previ~
ously, in connection with the description of FIG. 1, the
magnets 14 may be diametrically split into an upper
‘half 14a and a lower half 14b to facilitate their insertion
or replacement after the tube is otherwise assembled.
An off-center coupling hole 122 is provided through
each of the pole pieces 16 to provide the transfer of radio
frequency energy from cell to cell along the slow-wave
structure 18.
The size, shape and orientation of the coupling hole
wave energy traverses from right to left along the slow
wave structure, being ampli?ed ?rst in section 98 due to
its interaction with the electron stream. Near the out
put of this amplifying section, the traveling wave has
grown and has caused considerable density modulation
in the electron stream. At the ?rst isolator section, sec
tion 106 in the drawing, the radio frequency energy in
the slow-wave structure 18 is substantially completely
absorbed. However, the modulated electron stream
passes on into the next ampli?er section, section 96,
where it launches a new traveling wave in that section.
The new traveling wave grows and is ampli?ed by the
electron stream until reaching its output end at the iso
lator section 104. The electron stream is further modu
lated and the rf energy in the slow-wave structure is
again completely absorbed. This procedure is repeated
122 may be more clearly seen in the plan view thereof
until the highly modulated electron stream enters the out
at the left hand end of FIG. 3. The drift tube 110 is
put ampli?er section 90 through the isolator section 100
shown as having an inner radius r1 slightly larger than 4.0 and launches a high energy traveling wave upon the out
the radius of the electron stream and having an outer
put section 90 of the slow-wave structure 18. The out~
radius r2 which substantially de?nes the inner radius of
put of this ?nal section is fed into the output waveguide
the interaction cell. The kidney-shaped coupling hole
through the transducer 26;
122 may be formed by an end mill having a diameter
The isolator sections 100, 102, 104 and 106 each repre
extending from r3 to r4. The end mill is pressed through 4.5 sent a loss of a few decibels of ampli?cation. However,
the thickness of the pole piece 16 centered upon the arc
overall they vastly increase the amount of power ampli
of a circle 132. The end mill, or preferably the work,
?cation or gain which may be achieved in a single travel
may then be swung along this are keeping its center
ing-wave tube. The isolation sections isolate adjacent
on the circle 132. The work is rotated through an arc
amplifying sections, thereby to preclude instability and
of angle a where a may be any angle between zero de 50 oscillations due to re?ections and to too great an ampli
grees and, for example, approximately 60°. Thus,
?cation in a single traveling-wave tube section.
the kidney-shaped coupling hole 122 lies between a radius
Having considered the nature of the construction of
r3 and 1'4 and has circular ends of diameter r4—r3.
this novel slow-wave structure and traveling-wave tube,
Disposed radially outwardly from the coupling hole
the arrangement by which maximum beam and traveling
122 is a cylindrical shoulder extension 114, the inner
wave interaction is provided may be explained with ref
radius of which is designated rs and is substantially equal
erence to FIG. 4. FIG. 4 is a simpli?ed schematic repre
to the outer radius of the spacer 116. The inner radius
sentation of a slow-wave structure 140 which utilizes
r5 of the spacer 116 determines the outer dimension of
tapering in accordance with the present invention. The
the radio frequency interaction cell. The outer radius
general form of intercoupled cavity slow-wave structures
of the extension 114, designated as r7, is substantially 60 is illustrated by a number of conductive walls or ?ns 142
equal to the inner radius of the magnet 14. The outer
equally spaced to form interconnected cavities 143 along
radius of the pole piece 16 is designated rs, and the outer
the path 144 of an electron stream. The electron stream
radius of the magnet 14 is designated r9.
is assumed to travel from left to right in the drawing,
For angular alignment purposes during assembly, one
which is the same direction 146 as the axial movement
or more sets of holes 134 are provided through the pole 65 of the traveling wave.
The electron stream moves be
pieces 16 to hold them in a predetermined angular posi
tween an electron gun 148 at the left-hand end and a
tion with respect to each other. A reference notch 136
collector structure 150 at the right-hand end.
may be provided on the periphery of each of the pole
A separate ferrule or drift tube 152 lies within and
pieces 16 in order that one may always know from an
observation of the outer surface of the assembled tube 70 may be supported by each of the cavity walls or ?ns 142
and concentrically encompasses the electron stream. Each
what the angular orientation of each pole piece is. In
of
the cavity walls 142 is apertured at some point 154
the example described here, the notch is always provided
spaced radially apart from the respective drift tube 152
opposite the center of the kidney-shaped coupling hole
to provide energy coupling to the adjacent cavity. With
122.
this
arrangement, it may be seen that the geometrical di~
The advantages of the slow-wave structure formed of 75
mensions of the separate cavities 143 are the same. That
It)
is, the characteristic length of each cavity 143 as de?ned
by the separation between adjacent walls is constant. The
distance between adjacent ferrules 152 which de?nes the
gap 156 of each cavity is maintained constant also. The
length of each of the drift tubes 152 may, however, vary
.
easy to manufacture and which has important operative
advantages but which does not increase the complexity of
the slow-wave structure. Maximum interaction between
the traveling wave and an electron stream can be achieved
by creating an independence between the desired con
stant periodicity of the slow-wave circuit and the tapered
“periodicity” of the stream interaction gaps without the
tive cavity in order to facilitate the shifting of the drift
introduction of phase instabilities or traveling wave deg
tubes.
radation.
The distance which is varied in this structure, however,
What is claimed is:
is a distance 158 between the centers of the adjacent gaps. 10
l. A slow-wave structure for providing interaction be
This may also be visualized as a shifting of each gap with
tween an electromagnetic wave being propagated thereby
respect to the cavity. Note that the shifting is in the
and a stream of charged particles being projected along
‘direction toward the electron gun 148 so that in effect the
a predetermined path comprising: a series of electro
ferrules or drift tubes are shifted upstream with respect
magnetic“ elements each de?ning an interaction cavity
to the electron stream. The maximum amount of taper
disposed in sequence along said path, each of said interac
ing of this type which can be done is determined by the
tion cavities being electromagnetically exposed to said
length of the drift tubes and by the gap spacings with
stream at‘ an axial position within said interaction cavity,
respect to the cavities, as well as by the degree of taper
said axial position with respect to the axial center of each
or the amount of incremental shifting which is desired
of said interaction cavities being shifted progressively
between cavities.
upstream along the length of said slow-wave structure in
slightly without electromagnetically affecting its respec
The operation of the general intercoupled cavity struc
ture 140 illustrated in FIG. 4 provides maximum inter
action between the electron stream and the traveling
wave.
The traveling wave is isolated, in a sense, frorri
a manner to effectively taper the interaction along said
slow-wave structure while not otherwise affecting the
physical parameters of said interaction cavities.
2. A slow-wave structure having a changing apparent
the electron stream over an appreciable portion of its 25 periodicity to the electron stream and an actually con
travel through each of the cavities 143. Interaction oc
curs between the traveling wave and the stream only at
each of the gaps 156 in the cavities. The interaction re
sults in the charged particle stream giving up some of its
energy to the traveling wave because of the slightly great
er average axial velocity of the electron stream.
stant periodicity, said slow-wave structure comprising: a
plurality of elements each de?ning an interaction cavity
positioned along the axis of the electron stream, said
interaction cavities being de?ned by a plurality of regu
larly spaced planar Web members exending radially out
This
wardly from a point adjacent the electron stream and
electromagnetic coupling in turn slows down the electron
ring member disposed between and interconnecting adja
beam. It is important to note that if the traveling wave
cent web members, said ring members being radially
slow-wave structure is periodic throughout, it will have 35 spaced apart from said electron stream; and a plurality
greatest stability and be easiest to control. This constant
of drift tube segments, each encompassing the electron
periodicity is maintained, in the present invention, even
stream and concentric therewith, and each coupled to a
though the interaction point of the gaps 156 is adjusted
different web member, the relative axial position of said
along the length of the slow-wave structure to compensate
rift tubes with respect to said regularly spaced web
for the decrease in the axial velocity of the electron 40 members being shifted along the length of the slow-wave
stream.
structure.
As the beam gives up energy, the time of passage from
3. A slow-wave structure for providing interaction be
gap to gap increases, if the gaps remain equally spaced.
tween an electromagnetic Wave being propagated there
The tapering or incremental shifting here of the gaps 156
along and a stream of charged particles traversing a pre
relative to the cavities 143, however, compensates for 4.5 determined path, said structure comprising: a series of
this slowing down. In effect, the electrical length of each
planar conductive disc members disposed transversely to
cavity as seen by the stream is kept equal to the phase
and concentrically about said stream, said members being
substantially equally spaced and being the axial termini
shift per cavity.
Whereas, the arrangement of FIGS. 1 through 3 illus
of adjacent electromagnetic elements each de?ning an in
trates the operation of the invention with periodic perma 50 teraction cavity, and a like series of conductive ferrules
nent magnet focusing, non-periodic and electromagnet
having an axial length greater than the axial thickness of
focusing may also be employed, as is illustrated in FIG. 5.
said planar disc members, individual ones of Which are
As shown therein, a traveling-wave tube 160 having an
supported in a predetermined axial position by and with
electron gun end 162, a collector end 164 and an inter
mediate slow-wave structure 166 may have an electro
magnet 168 encompassing the slow~wave structure 166.
The slow-wave structure 166 is again of the intercoupled
cavity type, and an input 170 and output 172 for the
traveling wave are coupled to the extremities of the slow
wave structure 166. As described in conjunction with
previous arrangements, the intercoupled cavities 174 in
clude drift tubes 176 having ?xed gap spacings 178 rela
tive to each other and concentric with the electron stream.
The necessary focusing of the electron stream for this
respect to respective and individual ones of said disc
55
members, said ferrules being axially spaced to provide
interaction coupling between said stream and respective
ones of said interaction cavities, said conductive ferrules
along said path being progressively shifted upstream with
respect to its respective cavity to provide a slow-wave
structure which appears to the electron stream to be
tapered whereby the time between interaction with suc
cessive ones of said interaction cavities as experienced
by the decelerating stream of particles remain substan
tially constant throughout the length of said structure.
arrangement is provided because the permeability of the 65 4. A high power periodically focused traveling-wave
tube having a tapered slow-Wave structure comprising:
intercoupled cavity structure is not su?icient to provide
means for producing an axial electron stream along the
shunting of the magnetic focusing ?eld created by the
length of said tube; a plurality of radio frequency ele
electromagnet. The gaps 178 between the drift tubes 176,
ments each de?ning an interaction cavity intercoupled
are, however, successively shifted upstream, in accord
ance with the previous description, to keep the periodicity 70 along the length of said tube, each comprising a ferromag
netic drift tube disposed contiguously about said electron
of the traveling wave structure constant while utilizing
stream; a ferromagnetic pole piece forming an end wall
the optimum, substantially equally time spaced, interac
of each cavity and a nonmagnetic conductive short hollow
tion points in or along the electron stream.
cylindrical spacer disposed concentrically about said elec
Thus there has been described an improved slow-wave
structure for traveling-Wave tubes which is extremely
' tron stream between adjacent ones of said pole pieces; an
‘3,066,237
11 ‘
annular focusing magnet having an axial length substan
tially equal to that of said spacer and disposed concentri
cally thereabout, said pole piece extending radially from
said drift tube to approximately the radial extremity of
said magnet and being relieved forming a coupling hole
therethrough between said drift tube and said spacer,
said spacer being hermetically bonded along its end to
said pole pieces whereby along the length of said slow
wave structure a vacuum envelope is provided, said drift
12
ture of a different ferromagnetic disc and each being con
centric with the electron stream, said ferrules being of
like length and having like axial spacing, the axial rela
tionship along the electron stream of successive ferrules
with respect to the associated magnetic disc being suc
cessively shifted along the length of the traveling-wave
tube; a plurality of conductive spacer elements, each
positioned between a different adjacent pair of ferromag
netic discs and spaced radially apart from the ferrules
tube having an axial length substantially greater than the
axial thickness of said pole piece, the drift tubes associated 10 thereof; and highly conductive surfacing disposed in the
with said cavities toward the output end of said tube being
axially shifted with respect to said pole piece toward the
input end of said tube whereby said electron stream
traverses a progressively shorter distance between interac
tion cavities, thus effectively tapering the traveling-wave
tube in a manner whereby said electron stream, while
giving up a portion of its energy thereby slowing down,
continues to deliver energy to the radio frequency travel
ing waves traversing said slow-wave structure.
interior portions of said spacer rings and on said ferrules
and the interior portions of said ferromagnetic discs.
8. A traveling-wave tube slow-wave structure compris
ing: a plurality of radio frequency interaction cavities
disposed successively along the path of the electron stream
of the traveling-wave tube, each of said interaction cavities
having the general form of a pair of axially gapped, sepa~
rated drift tubes and contiguously encompassing the elec
tron stream; a pair of conductive supporting discs extend
5. A slow-wave structure for providing electromag 20 ing radially outwardly from each of the drift tubes and a
spacer ring between the adjacent discs and disposed sub
netic energy being propagated by said slow-wave struc
stantially concentric with the drift tubes, said interaction
ture and a stream of charged particles traveling in a
cavities being arranged to have different apparent, as seen
given direction along a given path, said slow-wave struc
ture comprising: a series of conductive, substantially N) CH ,by the electron stream, and actual, as seen by the travel
planar members disposed sequentially along said path
perpendicularly thereto, and a series of conductive ferrules
supported in axial registry contiguously about said path
by respective ones of said conductive planar members,
ing wave, periodicities in a selected pattern, at least some
of said cavities having a successive variation in the axial
position of the drift tubes therein with respect to the sup
porting discs, the gap between the drift tubes remaining
the same while the position of the spacing axially with
mining axially a series of interaction cavities, adjacent 30 respect to the discs is successively shifted within the
cavity from a point adjacent one disc to a point adjacent
ones of said conductive ferrules being spaced from each
the relatively opposite disc.
'
other to provide interaction coupling between said stream
9.
A
traveling-wave
tube
comprising:
an electron gun
and said interaction cavities, said conductive ferrules
positioned at one end thereof; means for projecting an
being shifted upstream with respect to their respective
electron stream of predetermined average diameter along
supporting planar conductive members in a manner such
adjacent ones of said planar conductive members deter
that the coupling spacing between successive adjacent
the longitudinal axis of said traveling-wave tube; input
means including an input microwave transmission means
for impressing upon the input end of said tube a micro~
not otherwise affecting the geometric parameters of said 40 wave signal to be ampli?ed; output means including an
output microwave transmission means for extracting an
interaction cavities.
ampli?ed signal from said traveling-wave tube; a
6. A slow-‘wave structure for propagating an electro
plurality of electrically conductive magnetic disc mem
magnetic wave in energy exchange relation with a stream
bers axially positioned along the electron stream path
of charged particles projected along an axial path com~
and being spaced apart by a substantially constant axial
prising: a series of electromagnetic elements each de
ones of said ferrules is progressively shifted upstream,
thereby effectively to taper said slow-wave structure while
?ning an interaction cavity disposed in sequence along
said path, each of said cavities being axially terminated
by a conductive Wall disposed transversely to said path
at opposite axial ends of said cavity, and each of said
distance, each of said discs comprising an inner axially
extended drift tube portion defining a central aperture
for passage of the electron stream, adjacent ones of
said drift tubes ‘being axially separated at a predeter~
mined point between adjacent ones of said discs to per
cavities being radially determined on its outer extremity
by ‘a conductive wall extending between said axial end 50 mit interaction between said microwave signals and said
electron stream, said predetermined point being pro
walls substantially orthogonal thereto and being termi
gressively varied with respect to its axial position between
nated on its inner extremity by a surface comprising a
said discs along said traveling-wave tube to provide a
conductive ferrule supported by one of said end walls
tapered slow-Wave structure whereby as the electron
and extending into said cavity in a manner whereby a
series of ferrules corresponding one to one with said 55 stream is decelerated it continues to interact to a maxi
conductive walls are disposed contiguously about said
path with a gap between adjacent ones of said ferrules
to provide electromagnetic coupling between said stream
of charged particles and said cavities, the relative axial
position of successive ones of said ferrules with respect
to its respective supporting conductive wall being shifted
toward the upstream direction of said stream of charged
particles to thereby provide a tapering for said slow-wave
structure.
7. A slow-wave structure for traveling-wave tubes of
the character having a longitudinal axis along which an
electron stream is projected, said slow-wave structure
comprising: a plurality of planar ferromagnetic discs,
said discs being positioned at successive individual points
along the electron stream axis of said traveling-wave
tube, and substantially normal thereto and concentric
therewith, each of said discs having a central apertured
portion about the electron stream; ferromagnetic ferrule
members, each of said ferrules de?ning the inner aper
mum degree with the microwave energy traversing the
length of said traveling-wave tube, each of said disc
members also including an inner web portion extending
radially outwardly from said drift tube and having an
axial intercoupling' aperture therethrough; a plurality of
conductive nonmagnetic annular cylindrical spacers, each
concentric with said longitudinal axis and hermetically
sealed between adjacent ones of said disc members at
a radial position such that the inner surfaces of said
spacer de?ne the outer cylindrical surfaces of the radio
frequency interaction cells between adjacent ones of
said disc members; highly electrically conductive sur~
facing disposed over the outer surfaces of said drift
tubes and over the inner web portion of said disc mem
bers for providing a continuing surface of high electrical
conductivity along the surfaces de?ning the slow-wave
structure of said traveling-wave tube; a plurality of split
annular permanent magnets concentric with the longi
tudinal axis of said tube and at least coextensive with the
75 outer portions of said disc members radially beyond the
3,066,237
13
outer surfaces of said spacers, said split annular magnets
having an axial length a‘ong said tube substantially equal
to that of the spacer between respective ones of said
disc members and substantially registering with the ‘outer
extremity, each of
split annular magnets being posi
tioned between a different successive pair of said disc
members and being magnetized substantially parallel to
said longitudinal axis to form magnetic poles of opposite
cavity, the pole pieces having coupling holes there
through between said drift tube and said second cylin
drical extension, whereby there is formed an interaction
cavity determined by a pair of said pole pieces separated
by said spacer, the inner surface thereof de?ning the
outer cylindrical surface of said cavity, and said drift
tubes extending axially from each of said pole pieces
defining the inner cylindrical surface of said cavity, said
drift tubes being separated by a gap disposed at a pre
polarity on the axial extremities of adjacent respective
ones of said drift tubes, thus providing periodic focusing 10 determined position Within said cavities for coupling
radio frequency energy in said interaction cavities with
lenses for the electron stream passing con-tiguously there
said electron stream, as well as a magnetic focusing lens
within.
disposed contiguously about said electron stream, said
10. A periodically focused traveling-wave tube in~
predetermined
position of said gaps being shifted pro
eluding a plurality of annular focusing magnets and
disc-like ferromagnetic pole pieces arranged alternately
in sequence along the length of said tube, each said pole
piece extending radially outwardly to approximately the
gressively upstream along the length of said traveling
wave tube toward its output end to provide an effec
tively tapered slow-Wave structure while maintaining the
actual periodicity of said interaction cells as well as their
radial extremities of said magnets ‘and extending in
electrical parameters substantially constant along (the
wardly to a point radially contiguous to the electron
stream of said tube and having a short axially extending 20 length of said traveling-Wave tube.
cylindrical extension protruding from the plane of said
References Cited in the ?le of this patent
disc-like pole piece at the inner region thereof about said
UNITED STATES PATENTS
electron stream to form a drift tube therefor, said
traveling-wave tube also including nonmagnetic conduc
2,636,948
Pierce _______________ __ Apr. 28, 1953
tive annular spacer ring members having inner and
2,841,738
Pierce ________________ __ July 1, 1958
outer diameters between that of said drift tube and the
outer diameters of said pole piece individually disposed
between a pair of adjacent pole pieces for maintaining
critical axial spacing therebetween, each said spacer ring
member having an inner diameter equal to a predeter 30
mined diameter of a desired radio frequency interaction
2,847,607
2,922,920
2,956,200
Pierce _______________ __ Aug. 12, 1958
Convert ______________ __ Jan. 26, 1960
Bates ________________ __ Oct. 11, 1960
753,999
Great Britain __________ __ Aug. 1, 1956
FOREIGN PATENTS
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