Helix traveling wave tubes with resonant loss

To suppress spurious oscillations in a helix-type traveling wave tube (TWT), frequency-sensitive loading is produced by a lossy resonant circuit attached to a dielectric support and coupled to the fields of the interaction circuit. The lossy circuit is resonant near the band-edge frequency. It may be a section of delay line with reflective terminations. In one embodiment, it is a metallized pattern on a dielectric rod used to support the helix.

FIELD OF THE INVENTION 
The invention pertains to broad-band traveling-wave tubes (TWT's), 
particularly tubes using interaction circuits of the helix-derived type. 
In all broad-band TWT's, particularly at high power levels, problems arise 
with instabilities and oscillations at frequencies near the band edges of 
the circuits where the wave group velocity becomes very small and the 
interaction impedance correspondingly large. 
PRIOR ART 
Two basic techniques have been widely used to combat instabilities in 
TWT's. One is to sever the slow-wave interaction circuit, dividing it into 
a plurality of shorter circuits with no wave coupling between them so that 
the gain in any one circuit section is restricted to values below that at 
which oscillation may occur. Severs have serious disadvantages in that 
considerable signal gain is lost by throwing away the circuit wave energy 
and starting a new wave in the following section. Also, limiting the gain 
in the output section involves a compromise with loss of efficiency when 
the output section is too short. 
A second technique very widely used in helix-type TWT's is to provide wave 
attenuation distributed over a length of the circuit, to limit the gain 
and absorb unwanted backward-reflected waves. Such distributed attenuation 
absorbs power at all frequencies across the operating band of the tube. It 
therefore creates problems, particularly in high power tubes, in 
dissipating the absorbed energy, in reducing the gain and in reducing the 
efficiency. 
In high power TWT's using bandpass circuits such as coupled cavities, it 
has been common to provide circuit attenuation which is frequency 
selective so as to be greatest near a band-edge frequency. This has 
sometimes been done by coupling lossy resonant elements such as hollow 
cavities to the interaction circuit cavities. U.S. Pat. No. 3,594,605 
issued July 20, 1971 to C. E. Blinn illustrates resonant cavity loading. 
This technique has not been practical for tubes with helix-type circuits 
because it would be quite difficult to couple such elements to the helix 
which has a low electromagnetic field outside of its sheath. Also, 
helix-type TWT's are generally required to fit inside small bores in 
beam-focusing magnets, so there is no room for a bulky attenuator such as 
a resonant cavity. 
SUMMARY OF THE INVENTION 
An objective of the invention is to provide a helix-type TWT with frequency 
sensitive loss without increasing the tube diameter. 
A further objective is to provide a helix-type TWT in which spurious 
oscillations and instabilities near a band-edge frequency are suppressed. 
A further objective is to provide a stable TWT which is small, light-weight 
and simple to manufacture. 
The above objectives are achieved by including one or more lossy resonant 
circuit elements inside the vacuum envelope of the TWT and coupled to the 
electromagnetic field of the interaction circuit. The lossy circuit is 
attached to a dielectric support which may be one of the dielectric rods 
used to support the helix. In a preferred embodiment, the lossy circuit is 
a section of delay line with reflective terminations, of sufficient length 
to resonate at the desired frequency.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows the well-known .omega.-.beta. or dispersion diagram of a 
slow-wave interaction circuit such as a helix or helix-derived circuit. 
Helix-derived circuits include multiple-conductor helices such as the 
interlaced bifilar helix, the contra-wound helix and its topographical 
equivalent, the ring-and-bar circuit. These circuits have no dc ground 
connection. They propagate frequencies down to zero, (i.e. dc). The 
abscissa in FIG. 1 is .beta.L, that is, the phase shift in radians of the 
transmitted wave per period of the circuit, that is, per pitch of the 
helix. The ordinate is .omega., the transmitted frequency. The 
fundamental, lower branch of the dispersion curve consists of a portion F 
of positive slope indicating a forward wave and a portion B of negative 
slope representing a backward wave. The usual convention concerning 
directions is that increasing phase shifts are taken in the direction of 
the TWT beam propagation. Since the slow-wave circuit propagates 
identically in both directions, the dispersion diagram is symmetric about 
.beta.L=.pi.. If there were no coupling between a forward wave and a 
backward wave, the forward-wave portion F would simply continue as F', 
crossing the backward-wave characteristic B continuing as B'. However, 
there are in fact always some asymmetries which intercouple the waves. 
This causes the two branches to separate instead of intersecting, giving a 
cutoff frequency .omega..sub.c for the fundamental branch at .beta.L=.pi.. 
At cutoff, the wave group velocity becomes zero, shown by the dispersion 
curve becoming horizontal. Since energy is not propagated down the helix, 
its interaction impedance becomes very large for frequencies in the 
neighborhood of cutoff. The resulting strong interaction with the electron 
beam causes instabilities and possibly oscillation near the cutoff 
frequency. Indicated in FIG. 1 are the range of operating frequencies from 
.omega..sub.1 to .omega..sub.2 and the range of higher frequencies from 
.omega..sub.3 to .omega..sub.4 in which instabilities are found. An 
objective of the present invention is to strongly attenuate waves having 
frequencies in the instability range without appreciably attenuating waves 
in the operating range. For this, an attenuating device with a selective 
frequency dependence is required. 
FIG. 2 is a simplified schematic section of a TWT incorporating the present 
invention. A beam of electrons is drawn from thermionic cathode 10 such as 
a conventional barium oxide cathode on a nickel base. Cathode 10 is 
typically of concave spherical shape supported on a base 12 by an 
electrically conducting but thermally isolating support member 13. 
Surrounding cathode 10 is a beam focus electrode 14, also supported on 
base 12. Cathode 10 is heated by radiation from a filamentary heater 15, 
typically tungsten wire insulated with an alumina coating. One leg 16 of 
heater 15 is joined to base 12, and the other leg 18 is brought out 
through the vacuum envelope for external connection via an insulating seal 
20. Base 12 is sealed to the main vacuum envelope 22 by a high voltage 
insulator 24. Inside envelope 22 a projecting anode electrode 26 operated 
at a dc potential positive to cathode 10 draws the electron beam 28 from 
cathode 10, converging it through an aperture 29 in anode 26 and 
projecting it as a cylindrical beam. Beyond anode 26 the beam 28 is 
typically kept focused by an axial magnetic field produced by a solenoid 
or a permanent magnet system (not shown). Beam 28 passes inside a 
slow-wave interaction circuit 30 which is designed to propagate an 
electromagnetic wave at a velocity synchronous with the velocity of the 
electron beam 28. Circuit 30 illustrated in FIG. 2 is the simplest and 
most widely used type--a metallic tape of rectangular cross-section wound 
into a helix. Circuit 30 is supported along its length by a plurality of 
axially extending dielectric rods 32, as of sapphire or alumina ceramic. 
The support may be purely mechanical containment or alternatively rods 32 
may be joined to circuit 30 by bonding glass. Support rods 32 are 
mechanically contained inside a cylindrical portion 34 of the vacuum 
envelope, typically of a non-magnetic metal such as austenitic stainless 
steel. Suport rods 32 may be circular cylinders, suitable for low-power 
TWT's, or in high-power tubes may, as shown in FIG. 3, have a generally 
rectangular cross section with inner and outer surfaces curved to fit the 
helix and the tube envelope for improved thermal conduction. The ends of 
helix 30 are connected to external transmission lines by metallic pins 36, 
40 welded to the ends of helix 30 and extending through vacuum envelope 34 
via insulating dielectric seals 38, 42. In a forward-wave TWT amplifier, 
the input signal would be applied to input terminal 36 and the amplified 
output would be removed through output terminal 40. After leaving helix 
30, electron beam 28 enters a hollow metallic collector 44 and the current 
is removed by an external power supply (not shown). Collector 44 is 
mounted on envelope 34 via a dielectric vacuum seal 46, as of alumina 
ceramic, thereby completing the vacuum envelope. 
On at least one of support rods 32 is affixed the frequency-sensitive lossy 
attenuating member 50 which is the heart of the present invention. In FIG. 
2 the lossy element 50 is illustrated as a meander line formed of a strip 
of resistive conductor bonded to the surface of support rod 32. Flat side 
surfaces on rods 32 (FIG. 3) are well adapted for depositing the 
attenuator 50. Strip 50 may be formed by any of the well-known techniques 
for depositing a metallized pattern on the ceramic. For example, bonding 
metal such as chromium may be sputtered onto the rod through a mask to 
form the desired pattern and then additional metal may be electroplated to 
increase the thickness. Alternatively, a powdered metallizing paint 
comprising molybdenum and manganese powders may be deposited as by a silk 
screened pattern. Alternatively a preformed metallic conductor element 50 
may be affixed as by glazing to the dielectric rod. Meander line 50 is a 
slow-wave circuit. Its electrical length is selected to resonate at the 
frequency to be suppressed as an open-ended transmission line N/2 
electrical wavelengths long, where N is any integer. When N=1 and the 
lossy line is 1/2 wavelength long, it is preferably made with physical 
length not greater than the helix pitch and centered between adjacent 
helix turns so that with .pi. phase shift between turns line 50 is in a 
unidirectional field. An alternative lossy line 51 is shown bridging two 
helix turns. It would preferably be one full wavelength long to be excited 
in full-wave resonance by the antiphased fields of the .pi. mode on the 
helix. The length of the lossy element is selected to provide the desired 
degree of coupling of the electromagnetic fields of the slow-wave 
interaction circuit. 
In FIG. 3 lossy circuit 50 is shown as lying on the surface of a dielectric 
support rod 32. 
FIG. 4 illustrates an alternative embodiment in which the lossy circuit 
element 50' is supported on an independent dielectric support bar 52 which 
in turn is supported inside envelope 34'. The construction shown in FIG. 4 
allows the area of surface for supporting lossy element 50' to be as large 
as desired. 
FIG. 5 shows an alternative embodiment of the resonant lossy element. Here 
a conducting strip 54 is shaped as a resonant ring including a capacitive 
gap 55. 
FIG. 6 illustrates still another embodiment wherein a small metallic helix, 
as of tungsten wire, is affixed to support rod 32"' as by glazing. The 
slow-wave helix circuit 56 is chosen in dimensions to have an open-circuit 
resonance at the frequency to be supressed. That is, it will generally be 
N/2 electrical wavelengths long. 
FIG. 7 shows the transmission and reflection characteristics of a typical 
helix circuit. This particular circuit had a stop-band at around 7.8 GHz. 
A TWT with this output circuit tended to oscillate. 
FIG. 8 shows the characteristics of the same circuit as FIG. 7 with the 
addition of loss circuits resonant at 7.2 GHz and 8.2 GHz. The instability 
frequencies were highly attenuated, and a TWT with this circuit was quite 
stable. 
While the embodiments of the invention described above are intended to be 
illustrative and not limiting, many variations will be obvious to those 
skilled in the art. For example, any of the family of helix-derived 
slow-wave circuits may be used as the interaction circuit. Also, many 
forms of delay line and other resonant circuits may be used as the 
frequency-sensitive loss element, and various means of supporting the loss 
element will become apparent. For best results it is believed that lossy 
elements should be symmetrically disposed with respect to each circuit 
support element so that the loss elements themselves do not give rise to a 
stop band. It is also foreseen that a plurality of loss elements may be 
disposed on each support. The invention is intended to be defined only by 
the following claims and their legal equivalents: