Note: Descriptions are shown in the official language in which they were submitted.
BAC~GROUND OF THE INVENTION
10 . The present invention relates in general to anisotropic
shell loading of high power helix traveling wave tubes and,
more particularly, to such loading elements which are readiiy
physically realizable for high power applic.ations, i.e., cw -.
power outputs in the range of ten watts to several kilowatts. ..
DESCRIPTION OF THE PRIOR ART
Heretofore, it has been proposed to anisotropically shell
load the helix of a traveling wave tube by arranging an array
of fine wires extending lengthwise of the helix and surrounding
the helix in spaced relation, such wires being disposed between
2~: the helix and a surrounding electrically conductive barrel
structure. Theory predicted that such anisotropic shell loading
of the helix would greatly improve the operating bandwidth over .-
which relatively high gain.and efficiency could be obtained by
reducing the positive dispersion of the helix structure. The
problem with this theoretical approach was that there was no
practical way proposed for supporting the array of conductive
wires around the helix.~
In another prior art tube it was proposed to simulate the
array of conductive wires by an array of electrically conductive
3a . vanes projecting toward thc helix from a surrounding barrel
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structure, such vanes extending lengtllwise of the h~lix. While
such an arrangement provides some degree of anisotrlpic shell
loading, it was less than entirely satisfactory because at
relatively high frequencies, i.e., in the microwave range of
' S-band an~d above, the vanes became very small and only a
relatively small number of such vanes could be accommodated
around the helix such number being for example 12 to 16. This
number of vanes did not provide enough loading.
It was also proposed ln the prior art to anisotropically
lQ: shell load a helix of a low power traveling wavè tube by
extruding the inside wall of the glass envelope of the tube with
a plurality of flutes projecting inwardly for supporting the
helix in spaced relation to a relatively heavy glass wall. The
relatively heavy glass wall served to anisotropically load the .
helix for improving the bandwidth over which relatively high
gain and efficiency could be obtained. This structure turned
out to be practical at low po~ers but could not be extended to
high power because glass envelopes are not suitable for high
power applications due to their re~atively poor thermal
2Q: conductivity. ~ore specifically, due to the poor thermal
conductivity of the helix support structure at high power
applications, i.e., over ten watts cw, the helix intercepts
substantial power which produces heating thereof. Because the
heat cannot be conducted from the helix the helix reaches
excessive operating temperatures and results in failure of the
helix and therefore failure of the tube~
SUMMARY OF THE PRESENT INVENTION
The principal object of the present invention is the
provision of a high power helix traveling wave tube having
increased bandwidth over which relatively high efficiency and
gain are achieved.
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. ,In one feature of the present invention, anisot~opic shell
loading of the helix structure is obtained by use o~ a plurality
of arcuate quartz sectors surrounding the helix in spaced
relation therefrom, such quartz sectors supporting an arr,ay of
longitudinally directed conductive elements on the surface ~ :
thereof facing the helix.for adding a negative dispersion loading ~
effect. to the otherwise positive dispersion characteristic of the . ,.
traveling wave tube. ..
In another feature of the present invention, anisotropic
shell loading of a helix lS provided'by means of a plurality ,~
of alumina ceramic arcuate sectors disposed surrounding the-
helix in spaced relation t~erefrom and in between the helix and
the barrel of the traveling ~ave tube, whereby a negative .
dispersion component is added to the otherwise positive
daspersion characteristic of the traveling wave tube.
More particularly, there is provided in a high power ..
traveling wave tube means for producing a stream of electrons;
a helix radio fr.equency slow wave interaction.circuit d;sp~s~d
:along the path of said stream of electrons i~ radio frequency
energy exchanging relation therewith for cumulative stream- j
field interaction with the stream to produce a growing radio
frequency wave on said circuit;
an evacuated envelope structure having a metallic portion
surrounding said interaction circuit.;
dielectric support means selected from the group consisting ~'
of beryllia and boron nitride circumferentially spaced apart
around said helix slow wave circuit and extending along said
circuit for supporting said helix from said envelope in ~`
electrically insulative and heat exchanging ,relation therewith; .~ ~.
and
anisotropic loading means surrounding said helix radio
frequency interaction circuit and being interposed between said
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envelope and said helix for adding a negative dispersion effect
t,o the normal positive dispersion characteristic of said helix
slow wave circuit, there~y obtaining a less positive or net
negative dispersion characteristic, said loading means comprising
a plurality of elongated arcuate loading portions of alumina
extending along the length of said helix slow wave circuit, said
loading portions being circumferentially spaced apart around and
radially spaced from said helix circuit.
There is also provided in a high power traveling wave tube ~ :~
means for producing a stream of electrons;
a helix radio frequency slow wave interaction circuit
disposed along the path of said stream of electrons in radio -
frequency energy exchanging relation therewith for cumulative -~
stream-field interaction with the stream to produce a growing ' .
radio frequency wave on s,aid circuit;
an evacuated envelope structure having a metallic portion
surrounding said interaction-circuit;
dielec~ric support means selected from th,e group consisting ¦ '.
of ~eryllia and boron ni~ride circumferentially spaced apart :~
around said helix slow wave circuit and extending along said
circuit for supporting said helix from said envelope in ,. ~ -
electric;ally insulative and heat e*change relation therewith; and
'anisotropic loading means surTounding said helix radio
.frequency interaction circuit and bei-ng interposed between said
envelope and said helix for adding a negative dispersion effect
to the normal positive dispersion characteristic of said helix
slow wave circuit, thereby obtaining a less positive or more
negative dispersion characteristic, said loading means comprising
a plurality of elongated arcuate dielectTic suppor* sectors
extending along the length of said helix slow wave circuit, said
suppo,rt sectors including an array o:E elongated longitudinally
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directed circumferentially spaced electric conductors formed on
the inner face thereof facing said helix slow wave circuit, said
array of conductors~surrounding `said helix and being supported
from the inner face of said arcuate dielectric sectors.
Other features and advantages of the present invention will
become apparent upon a perusal of the following specification
taken in connection with the accompanying drawings wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
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~ig. 1 is a longitudinal sectional schematic line diagram
o'f a traveling wave tube of the prior art,
Fig. 2 is a transverse sectional view of a physical
realization of the structure of Fig. 1 taken along line 2-2 in
the direction of the arrows,
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Fig. 3 is a plot of phase velocity versus frequency showing
the dispersive characteristics for the prior art and for the ' -
anisotropically loaded helix of the present invention, ' ~-
Fig. 4 is a transverse sectional view similar to that of
Fig. 2 depicting a dispe~sion correcting structur~ of the prior art.
Fig,. 5 is a view similar to that of Fig. 4 depicting an
alternative embodimcnt of the prior art,
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Fig. 6 is a view similar to that of Fig. S depi~ting an
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alternative embodiment of the prior art,
Fig. 7 is a plot of interaction efficiency and gain per
inch as a function'of,the velocity synchronism parameter (b),
Fig. 8 is a plot of velocity synchronism parameter (b) as a
function of fre~uency for two values of microperveance and
j depicting characteristics of the prior art and that of the
present invention,,
Fig. 9 is a sectional view similar to that of Fig. 2
10' depicting the anisotropically shell loaded helix of the present '
invention, '
Fig. 10 is a view of a portion of the structure of Fig. 9
taken along line 10 10 in the direction of the arrow's,,and '- -
,Fig. 11 is a vie~ similar to that of Fig. 9 depicting an
alternative embodiment of the present invention. '
DESCRIPTION OF THE PREFERRED EMBODIMENTS . -:
Referring now to Fig. 1 there is shown the typical traveling '-''''
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wave tube 1 of the prior art. The traveling wave tube li~clu~s' -~' '
an elongated evacuated envelope 2 having an electron gunoss~0blY ~ ~ -
20: 3 disposed at one end for forming_and projecting a beam of
electrons 4 over an elongated beam path't'o a beam collector
structure 5 disposed at the terminal end of the beam path and at
the other end of the tube 1. A helix slow wave circuit 6 is
disposed along the beam path intermediate the electron gun 3 and
the beam collector 5 for cumuiative electromagnetic interaction
with the beam to produce an amplified output signal. More
particularly, RF input energy to be amplified is fed onto the
helix at the up~tream end thereof via an input terminal 7. The
microwave energy propagates along the helix in synchronism with ` ~ ,
the electrons of the beam for cumulative electromagnetic '
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interaction to produce a ~rowing eLectromagnctic waye on the
circuit 6 which is extracted from the circuit at the downstream
end via an output terminal 8 and thence fed to a suita~le
utilization device or load, not shown.
Referring now to Fig. 2 there is shown the typical-high ,
po~er prior art helix support structure. More particularly,
the helix 6 is supported from the ~inside wall of thermally
and electricall~ conductive barrel structure 9, as of copper,
which also forms the vacu~m envelope of the tube via the
lQ: intermediary of three electrically insulative thermally
conductive refractory rods 11 as of beryllia ceramic or boron
nitride. The support rods 11, in one embodiment of the prior
art, are captured in an interference fit between the helix 6 and
the barrel 9 to provide a good thermally conductive path from
the helix to the barrel 9.
Referring now to Fig. 3 there is shown the dispersion curve
12 for the prior art tube of Figs. 1 and 2. As can be seen from
Fig. 3 the helix traveling wave tube has a positive dispersion
characteristic over an octave of bandwidth from f to 2f . The
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basic principle behind traveling wave tube interaction is that
the electron beam travels at approximately the same velocity
as the microwave signal on the helix so that interaction is
continuous along the ~ength of the tube. If this synchronism
condition is not exactly satisfied, the tube has poor gain and
poor efficiency if it is expected to operate over octave band-
widths.
Referring now to Fig. 8 there is shown a plot of velocity -
synchronism parameter ~b) as a function of frequency for two
values of microperveance for the sama voltage of the electron
beam. As can be seen by solid curves 13 and 14, the velocity
synchronism parameter (b) ~ -
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varics widcly over the octave of bandwidtll, therefore the prior
art tube with a positive dispersion characteristic,~as shown by
curve 12 of Fig. 3, has relative~y poor efficiency and gain over
the octave of bandwidth.
Referring now to Fig. 7 there is shown the plot of inter-
action efficiency in percent and gain per inch versus the
synchronism parameter ~b) showing that maximum gain is obtained
for a synchronism parameter ~b) value of approximately 1 and the
tube has relatively high efficiency for that value. However,
the gain falls off on either side of the value of 1 for the
sync~ronism parameter~
It had been proposed în the prior art, as shown in Fig.-4, .
to anisotropically shell load the helix circuit by disposing
an array of longitudinally directed ~ires 15 around the helix 6
intermediate the helix and the barrel 9. In an optimum design, -
there would be an infinite number af the very fine wires 15. The
- ~ires 15 serve to load the helix in such a manner as to introduce
a negative dispersion characteristic to the otherwise positive
dispersion characteristic of the helix so that either a flat or
2Q: negativè dispersion characteristic could be obtained by the
proper loading as indicated by dotted lines 16 and 17 of Fig. 3.
The anisotropic loading shell 15, as approximated by the
multitude of longitudinal wires, is a boundary surrounding the
helix which can conduct in the axial direction but not in the
- circumferential direction. The theoretical effect of the
anisotropic shell on phase velocity is shown by curves 16 and
17 in Fig. 3 and this loading also serves to decrease the
interaction impedance generally uniformly over that obtained
by the unloaded circuit over wide bandwidths. If the loading is
3~: sufficiently great the anisotropic shell shows anomalous or
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negativc dispersion as shown by the curve 17. The exact amount
of reduction of the dispersion of the helix depends!on how close
the anisotropic loading shell is brought to the helix. If just
the right ratio of shell diameter to helix diameter is chosen,
the dispersion can be completely eliminated as shown by curve 16.
However, even better performance can be obtained with the
negative dispersion shown by curve 17, which can be obtained
by using a diffe.rent ratio of anisotropic shell diameter to
helix diameter.
Although it has been known that an array of tiny wires
running axially of the helix, as shown in Fig. 4, could be
employed for obtaining the desired negative dispersion, this
idea has not been used for traveling wave tubes because of the
impracticality of fabricating such an ~anisotropic loading shell.
Attempts have been made to solve this fabrication problem by
using an array of metallic vanes 21, as shown in Fig. 5. However,
in a practical embodiment, the maximum number of vanes 21 that
can be accommodated around the helix, due to its relatively small
diameter at microwave frequencies, is between nine and twelve.
2Q As'a result, the true anisotropic shell propertles are not
achieved. Secondly, even though only a few vanes 21 are used,
- they are extremely difficult to fabricate because they must be
thin-and must be maintained straight throughout the length of
the tube.
In relatively low power traveling wave tubes of the type
utillzing a glass envelope, as shown in Fig. 6, anisotropic
loading has been obtained by fluting the glass envelope
~ ~tr~cture 22 with inwardlr directed projections 23 serving to
support the helix 6 within the fluted glass barrel 22. The gap
3Q between the helix and the glass tube reduces the dispersion of
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diameter to helix diameter, negative dispersion can be achievcd.
However, the glass ~envelope structure of Fig. 6 has the
disadvantage that the thermal conductivity of the glas's is
r'elative'ly low so that heat is not removed from the helix via th~
helix support structure. As a consequence, the fluted glass
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!envelope is useful only for relatively low power applications, ~`
i.e., cw power outputs less than 10 watts. The glass envelope 2Z
was surrounded by a thin metallic shield structure 24. The glas~
lQ: served as an anisotropic loading structure between the helix and - ''
the shield.
Referring now to Figs. 9 and 10 there is shown an
anisotropic loading structure 26 of the-present invention. The
anisotropic loading structure 26 comprises a plurality of
arcuate sectors of quartz 26 having an array of electrically
conductive stripes 27 formed on the inner arcuate surface of the
quartz members 26, as by photoetching. In a typical example,
13 line segments 27 are photoetched onto the inner surface of '
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each of the quartz segments 26. The lines 27 are ten mils wide
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20:an~ the spacing between each line is ten mils. Therefore, a -~ '
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total of thirty-nine conductive lines 27 are used around the
circumference of the helix. The quartz sectors are held to the
inside wall of the bore in the envelope 9 via a plurality of
metallic clips 28 w}lich grip the sector 26 at end relieved
shoulder portions 29 provided at both ends of the arcuate
sectors 26.
The elec~rically conductive lines 27 are fabricated by ~'
sputteri~ a thin layer of molybdenum onto the inner surface of
the quartz sectors 26. The molybdenum coating is then copper ~
3~ platedO The copper plated moly~dcnum layer is then photoetched ~ ' ~///- ' . ' .
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to providc thc fino lino pattcrn. I'he anisotropic loading shell
structure 26 of Figs. 9 and 10, as expected, reduce~ interaction
impedance over the operating band and also provides negativc
dispersion. The amount of negative dispersion that can be
obtained for this structure is the same as would be predicted
for the ideal anisotropic structure as shown in Fig. 4. In such
a s'tructure and for the structure of Fig. 9 an optimum negative
dispersion is obtained when the ratio of the diameter of the
conductive array to the mean diameter of the helix is approximate-
10' ly 1.34, and preferably within the range of 1'.3 to 1.4. Curves
31 and 32 show the loading effect on the velocity synchronism
parameter (b) of the array of wires 27. From Fig. 8 it is seen
that the velocity synchronism parameter (b) is much more nearly
uniform over the octave bandwidth, the~reby obtainlng uniform gain
and efficiency over an octave of bandwidth.
Referring now to Fig. 11 there is shown a second embodimentof the present invention for obtaining anisotropic shell loading
of the helix 6. In this case, the anisotropic shell loading
comprises three arcuate sectors 34 of alumina ceramic having a
2a: dielectric constant of 9.6. These dielectric loading members 34
have no conductive lines printed thereon? as utilized in'the
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, embodimen't of Figs. 9 and 10. Therefo're, they are more eas'ily '
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fabricated. ' ' '
The resultant phase velocity for the helix circuit of Fig. 11
is almost constant with frequency over an octave of bandwidth
and the interaction impedance is not reduced as much as found in
the array of conductive lines on the quartz substrat0 as
emp10yed in the embodiment of Figs. 9 and 10.
In a preferred embodiment, the dielectric loading sectors
3Q have an inside diameter to mean helix diameter ratio falling
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within the range of 1.3 to 1.4, where the ratio of t~ie insidc .. - .
diameter of the barrel 9 to the mean diameter of the helix 6
falls within the range of 2,0 to 3Ø With the alumina ceramic
loading sectors 34, an octave bandwidth was obtained between .
-4db points. Preferably, in the embodiments of Figs. 9-11,
the loading means should be axially co-extensive with the
helix along at least 90 percent of the length of the helix.
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