SLOW-WAVE CIRCUIT FOR A TRAVELING WAVE TUBE

CIRCUIT A ONDES LENTES POUR TUBE A ONDES PROGRESSIVES

English Abstract

-11-

Slow-Wave Circuit For A Traveling Wave Tube

Abstract

A coupled-cavity slow-wave circuit for a
millimeter-wave TWT is formed by forming cavities
through a metallic bar or half-cavities in a pair
of comb-shaped bars. The ends of the cavities are
covered by cover members, one of which has a
longitudinal groove to form "in line" coupling
apertures between cavities.

Claims

-8-
WHAT IS CLAIMED IS:

1. A coupled-cavity slow-wave circuit comprising:
a first elongated metallic cavity bar defining
an axis, a first electron beam passageway parallel to
said axis, a first array of cavity openings extending
through said bar in a direction perpendicular to said
axis, said cavity openings spaced axially, said bar
having two smooth regular surfaces on generally
opposite sides, each of said cavity openings defining
openings in said surfaces,
two metallic cover members having smooth, regular
surfaces covering said smooth surfaces of said bar,
at least one of said cover members having a uni-
form axial first channel aligned with said cavity
openings and narrower than said cavity openings,
said cover members being bonded to said bar to
at least partially cover said cavity openings to form
hollow cavities and to complete a vacuum envelope
surrounding said cavities.

2. The circuit of claim 1 wherein said bond is a
sintered connection.

3. The circuit of claim 1 further including a
second cavity bar which is the mirror image of
said first cavity bar and defines a complementary
second electron beam passageway and a complemen-
tary second array of cavity openings, said cavity
openings and said beam-passageways being grooves
in said cavity bars, said bars being aligned on
the mirror plane such that said grooves align to
form cavities, and said electron beam passageways
align to form an electron beam path centered on
said axis, said cover members covering both of said

-9-
bars, and said axial first channel covering only
part of said cavities of both said arrays.

4. The circuit of claim 1 wherein at least one of
said cover members defines a channel shape comple-
mentary to the shape of said bar such as to fit
tightly about said cavity bar and bond to the
other of said cover members.

5. The circuit of claim 2 wherein said bar has a
plane of symmetry containing said axis and
perpendicular to said smooth surfaces and wherein
said first and second cavity bars define respective
complementary first and second arrays of vanes,
said cavity opening grooves being defined by said
arrays of vanes, the vanes of said first array
being bonded to the vanes of said second array on
said plane of symmetry.

6. The circuit of claim 5 wherein said cover
members both define channel shapes complementary to
the shape of said bar so as to fit tightly about
said bar, and in which both said shapes are generally
symmetrical to said plane of symmetry.

7. The circuit of claim 3 wherein said cavity
bars have a second plane of symmetry containing
said axis and parallel to said smooth surfaces,
and wherein each of said cover members respectively
defined complementary first and second smooth flat
mating faces, said faces being positioned to bond
on said second plane of symmetry.

8. A coupled-cavity slow-wave circuit comprising:
a first elongated metallic cavity bar defining


-10-

an axis, a first electron beam passageway parallel
to said axis, a first array of cavity openings
extending through said bar in a direction perpen-
dicular to said axis, said cavity openings spaced
axially, said bar having two smooth regular surfaces
on generally opposite sides, each of said cavity
openings defining openings in said surfaces,
a uniform axial first channel defined in one of
said smooth surfaces of said bar, aligned with
said cavity openings and narrower than said cavity
openings,
and two metallic cover members having smooth,
regular surfaces covering said smooth surfaces of
said bar,
said cover members being bonded to said bar to
at least partially cover said cavity openings to
form hollow cavities and to complete a vacuum
envelope surrounding said cavities.

9. A coupled-cavity slow-wave circuit comprising:
a first one-piece metallic comb having a first pair of
opposed limiting surfaces lying on a pair of parallel side planes,
an axis midway between said side planes, said comb further
including a generally rectangular elongated backing member extend-
ing along the direction of said axis and perpendicular to said side
planes, an array of identical vanes of generally rectangular shape,
each vane extending at right angles from said backing member and
evenly spaced along said axis, said vanes being of equal length
and each terminating in an end forming a rectangular tip, said
rectangular tips of said vanes lying in a symmetry plane contain-
ing said axis and perpendicular to said side planes, a groove
being formed on each said rectangular tip of said vanes centered
on said axis, said vanes, backing member and opposed limiting
surfaces defining therebetween an array of slots;
a second one-piece metallic comb having vanes, a
backing member, a groove, slots, and opposed limiting surfaces
which are the mirror image of said vanes, backing member, groove,
slots and opposed limiting surfaces of said first comb as mirrored
in said symmetry plane,
said first and second combs being aligned on said symmetry
plane such that said vane grooves align to form an enclosed elec-
tron beam passageway, said opposed limiting surfaces of said
second comb lie in said side planes of said first comb, and said
slots align to form an array of openings extending through said
pair of limiting surfaces;
a pair of metallic cover members having flat surfaces,
said flat surfaces covering and in electrical contact with
said opposed limiting surfaces of said combs;
at least one of said cover members having a uniform
axial channel in said flat surface;
said cover members being bonded to said combs in
electrical contact therewith to partially cover and short circuit
said openings to form an array of coupled cavities and to form
part of a vacuum envelope for said circuit.
10. The coupled-cavity slow wave circuit of claim 9
wherein at least one of said cover members has, in addition to
said flat surface, sides extending beyond said flat surface
perpendicular to said flat surface to fit around said backing member
of said combs and form a cover around said combs.

11


11. A coupled-cavity slow-wave circuit comprising:
a first one-piece metallic comb having a first pair of
opposed limiting surfaces lying on a pair of parallel side
planes, an axis midway between said side planes, said comb further
including a generally rectangular elongated backing member extending
along the direction of said axis perpendicular to said side planes,
an array of identical vanes of generally rectangular shape, each
said vane extending at right angles from said backing member and
spaced along said axis, said vanes being of equal length and
each terminating in an end forming a rectangular tip, said
rectangular tips of said vanes lying in a symmetry plane containing
said axis and perpendicular to said side planes,a first and a second
groove being formed on each said rectangular tip of said vanes,
said first groove being centered on said axis and said second
groove being centered on a line parallel to said axis, said vanes,
backing member and opposed limiting surfaces defining therebetween
an array of slots;
a second one-piece metallic comb having vanes, a backing
member, a first and a second groove, slots and opposed limiting
surfaces which are the mirror image of said vanes, backing member,
grooves, slots and opposed limiting surfaces of said first comb
as mirrored in said symmetry plane;
said first and second combs being aligned on said
symmetry plane such that said first vane grooves align to form an
enclosed electron beam passageway, said opposed limiting surfaces
of said second comb lie in said side planes of said first comb,
said slots align to form an array of openings extending through
said pair of limiting surfaces, and said second vane grooves align
to form an axially extending channel communicating with each of
said openings; and
a pair of metallic cover members having flat surfaces,
said flat surfaces covering and in electrical contact with said
opposed limiting surfaces of said combs, said cover members
being bonded to said combs in electrical contact therewith to
cover and short circuit said openings to form an array of
cavities coupled by said channel and to form part of a vacuum
envelope for said circuit.
12. A coupled-cavity slow-wave circuit as in claim 11,
in which said cover members have in addition planar surfaces
perpendicular to said flat surfaces formed so that said cover
members together fit tightly about said backing members.

12

Description

7~




_scription

Slow-Wave Circuit For ~ Traveling _ave Tube

Field Of The Invention
The invention pertains to traveling wave tubes
for operation at very high frequencies such as
millimeter wavelengths, with relatively high power
output. At these frequencies the slow-wave circuits
become very small. In making and assembling them,
dimensional tolerance errors can lead to severe
troubles, particularly if they are cumulative~ Also,
the tiny assemblies have problems of inadequate -
thermal and electrical conductivity.

Brief Description Of The Drawings
FIG. lA is a schematic section of a prior-art
coupled-cavity slow wave circuit.
- FIG. lB is an axial section of the circuit of
FIG. lA.
FIG. 2 is a perspective view of an improved
prior-art circuit.
FIG. 3 is an exploded perspective view of a
slow-wave circuit embodying the invention.

76~



FIG. ~ is a cross-section perpendicular to the
beam axis of a circuit similar to that of FIG. 3.
FIG. 5 is an axial section of the circuit of
FIG. 4-

Prior Art
For high power, traveling wave tubes (TWT's)have generally used a slow-wave circuit of the
"folded waveguide" or "coupled cavity" type. The
coupled-cavity slow-wave circuit has been widely
used in high-power ~FWTs of moderate bandwidth. At
low frequencies, such as below 20 GHz, a typical
construction of such a circuit is illustrated by
FIGS. 1. The interaction cavities 10 are formed by
spacer rings 12 as of copper, stacked alternating
with end plates 14, also copper. The assembly is
bonded together by brazing at joints 16 with a
silver-copper or gold~copper alloy to form a
vacuum tight envelope. Each plate 14 has an axial
aperture 18 for passage of an electron beam (not
shown) which interacts with the axial component of
the rf electrie field in the cavities. Aperture
18 is often lengthened axially by protruding lips
20 whieh confine the electrie field to a shorter
axial gap 22, thereby raising the interaction
impedanee and beam coupling factor of the cavity.
Ad]acent eavities 10 are mutually coupled by a
coupling slot 24 in each end plate 14, located
near the outer edge of cavity 10 where the rf
magnetie field is highest, thus providing coupling
by mutual inductance. Alternate coupling slots 24
are staggered on opposite sides of cavities 10.
This provides the "folded waveguide'l characteristie
whieh provides a large interaction bandwidth.
With this type of coupling, the fundamental eireuit

~12~9~


wave is a backward wave. The tube is operated in
the first space-harmonic wave mode, which is a
forward wave so that near-synchronous interaction
with a constant-velocity electron beam can be
achieved over a relatively wide band of frequencies.
The prior-art circuit of FIGS. 1 is satisfactory
at low frequenciesO However, when built for
frequencies such as 20 GHz and higher, it develops
serious difficulties. The many parts are tiny and
costly to machine accurately. The axial spacing
is subject to cumulative errors in stacking. When
the stacking errors are in the periodic spacing of
elements 14, they deteriorate the bandpass charac--
teristic and impedance of the circuit. When there
are errors of alignment on the axis, they can
cause beam interception with consequent power loss
or tube failure.
Also, the brazed joints 16 can cause two kinds
of trouble. If the braze alloy does not flow
completely, there is a crack which can present a
high resistance to the circulating cavity current
which must cross the crack. On the other hand, if
the braze alloy flows out on the cavity inside
surface, the high electrical resistance of common
braze alloys increases the attenuation of the
circuit. rf the alloy forms a fillet across the
corner, the cavity volume is decreased, thereby
-~ detuning the cavity resonance and impairing
circuit impedance and bandwidth. Thus, if said
joints cannot be avoided altogether, at least one
should reduce their number and length and locate
them where circulating current crossing them is
small.
FIG. 2 is a schematic perspective view of a
coupled-cavity circuit suitable for high frequency

7~9


TWT's which eliminates some of the mechanical
problems oE the circuit of FIG. 1. This circuit is
described in U.S. Patent No. 3,711,943 issued
January 23, 1~73 to Bertram G. James. The cavities
are formed by inserting metallic plates 30 into
slots 32 milled into a metallic channel 34. Each
plate 30 has a central hole 36 for passing the
electron beam and a coupling slot 38 for electro-
magnetic coupling between adjacent cavities 40.
Coupling slots 38 are all aligned on the same side
of plates 30, the so-called "in line" configuration.
This configuration gives a somewhat different wave-
transmission characteristic from the "staggered"
slots of FIG. 1. Plates 30 are brazed to channel
34 and the vacuum envelope is completed by brazing
on a metallic cover-plate (not shown).
The circuit of FIG. 2 has the advantage that
the periodic spacing of activities 40 is determined
by the positions of slots 32 which may be accurately
machined. Thus cumulative errors due to stacking
parts as in FIG. 1 are greatly reduced. Some
problems remain, however. A large number of joints
must be brazed vacuum-tight. Also the braze alloy
may form fillets at the corners of cavities 40,
changing their-volume and resonant frequencies.
Also braze alloys have high electrical resistance
so the microwave surface currents create unwanted
~- power loss.

Summary Of The Invention
An object of the invention is to provide a TWT
slow-wave circuit suitable for millimeter waves
having improved mechanical accuracy.
A further object is to provide a circuit having
lower electrical losses.

Z7i~


A further object is to provide a circuit which
is easy to fabricate.
These objects are fulfilled by a circuit
comprising at least one comb-like member fabricated
from a single piece of metal which is captured
within a pair of channel members which are sealed
together ~o form the vacuum envelope. In-line
coupling is provided by one or more additional
groove in one of the channel members.

Description Of The Preferred Embodiments
In FIG. 3, the cavities 50 are fonned by a
periodic array of openings or slots 51 between the
complementary opposed vanes 52 of a pair o unitary
comb elements 54. Slots 51 are machined into comb
54 and thus may be spaced with great accuracy and
without cumulative error inherent in an axially
stacked set of parts as in FIG. 1. Slots 51 may
have rectangular bottoms as in FIG. 5, or may have
the slightly more efficient rounded bottoms of
FI~. 3. The two combs 54 are axially aligned so
that teeth 52 meet precisely. In each comb a
semicircular groove 58 is machined in the end of
vanes 52 (preferably before cavity slots 51 are
machined). Upon assembly of the combs, a line of
holes 56 at the center of cavities 50 is then
formedO These holes define a series of closed
passageways which together define the electron
beam pathway. Combs 54 are of oxygen-free high-
conductivity copper. Slots 51 may be formed by
conventional cutting or by electrical discharge
machining. The ends of vanes 52 are joined to
their opposite counterparts before or during final
assembly of the circuit, as described below.
Cavities 50 are made symmetrical with respect to

lZ~2~;Ç~



the plane of the tips of vanes 52 so that in opera-
tion no rf current or heat flow crosses that plane.
Thus a perfect contact is not necessary.
The cavities 50 are completed by enclosing
comb struc-tures 54 within a pair of cover or
envelope members 60, 62. Member 60 has a relatively
wide channel 64 cut to complement the shape o-f combs
54. Upon assembly, member 60 will then fit tightly
over combs 54. Member 62 has a similar wide channel
64' and in addition a narrower central groove or
channel 66 which leaves spaces 68 between combs 54
and envelope channel 66. Lined-up spaces 68 form
the inter-cavity coupling irises which make the
array of cavities into a propagating band-pass slow-
wave circuit.
In assembling the circuit, cover members 60,62 are brought together to tightly enclose combs 54
and are joined together as by brazing or sintering
to form the vacuum envelope. In the same operation,
members 60, 62 are joined as by sintering or brazing
to combs 54 to form the end walls of cavities 50.
These walls also serve to conduct heat efficiently
from combs 54. The joining plane 70 of channels
60, 62 is preferably a plane of symmetry about the
axis, so that no rf cavity current flows across
the joint. Preferably the channels 64 and 64' also
are of complementary shape with respect to each
other such that they are generally symmetrical
with respect to the plane of the tips of vanes 52.
The various joints in the structure are formed by
brazing as with silver-copper eutectic or a gold-
copper alloy. Alternatively the joining surfaces
may be electroplated with gold or silver to form
the alloy at exactly the right places when heated.
A preferred method for very high frequencies is to

i9



sinter the copper parts together under externally
applied pressure at a temperature somewhat below
the melting point. With this method there is no
high-resistivity alloy at all. A compromise method
is to plate the contact surfaces with gold and
sinter together at a temperature below the melting
point of gold (there is no gold-copper eutectic).
With this method there can be no liquid alloy to
flow out to undesired areas.
Many other embodiments will be obvious to
those skilled in the art. The pair of combs 54 may
be replaced by a unitary slab or bar with complete
cavity holed drilled through it and the beam hole
drilled through the entire slab. (Drilling a
long, straight hole is very difficult, however.)
The cover members 60, 62 may not necessarily define
symmetrical channels; one member could be a flat
slab (but the symmetrical arrangement is better as
described above). For greater coupling, a second
coupling groove similar to groove 66 may also be
cut in cover member 60. Also the axial coupling
groove or grooves need not be defined in the cover
members, but instead could be defined in combs 54
or the alternate unitary cavity bar. Such an
embodiment would have the advantage of allowing
both cover members to be identical in configuration,
and also provide superior cavity coupling in certain
applications, since the rf pathway between adjacent
cavities would be shorter. The embodiments described
above are exemplary and not limiting. The scope
of the invention is to be limited only by the
following claims and their legal equivalents.