Patent Publication Number: US-6987360-B1

Title: Backward wave coupler for sub-millimeter waves in a traveling wave tube

Description:
This invention was made with United States government support under Grant NAS3-01014 from National Aeronautics and Space Administration. The United States Government has certain rights in this invention. 

   FIELD OF THE INVENTION 
   The present invention is related to coupling structures for microwave traveling wave tubes. More particularly, it is related to a structure for coupling traveling waves into and out of a traveling wave tube, including the class of traveling wave tubes operating in the sub-millimeter wavelength region. 
   BACKGROUND OF THE INVENTION 
   A Traveling-Wave Tube (TWT) may act as an amplifier or an oscillator for Radio Frequencies (RF). This is accomplished through the interaction of an electron beam and an RF circuit known as a slow wave structure, where the RF wave velocity as it travels down the circuit is much less than that of light in a vacuum. As the electron beam travels down this interaction region, an energy exchange takes place between the electrons and the RF circuit wave. When a traveling wave tube is configured as an amplifier, RF energy is applied to an input port, and the interaction between the RF and the electron beam produces power gain, and the amplified signal is removed from an output port. When a traveling wave tube as an oscillator, at some frequency there is sufficient internal RF coupling through the gain element at a particular frequency to enable oscillation at that frequency. Backward wave devices have the property that this oscillation frequency can be controlled by the voltage applied between the cathode and anode of the electron gun. 
     FIG. 1  shows the three basic components to any TWT or linear beam device. A TWT includes an electron gun which has a thermionic or field emission cathode  108 , a slow wave circuit shown as input coupler  116 , output couplers shown as backward wave couplers  118  and  120 , and a collector shown as  112 . The electron gun emits electrons and the application of a high differential voltage optionally combined with a magnetic focusing circuit (not shown), the electrons travel down electron beam  114  tunnel terminating in collector  112 . The voltage applied to the cathode may range in value from several hundred to several hundreds of thousands of volts. The slow wave structure  116  which is shown generically coupled to electron beam  114  may couple RF energy into the electron beam  114 , or it may provide a source of oscillation coupled to electron beam  114 , or it may act as an amplifier whereby it includes an input port (not shown) and has the characteristic of a bandpass filter for RF waves in the region of interest. Over a particular band of frequencies, which can range as high as two or more octaves, the slow wave structures  118  and  120  may provide a frequency transfer function for the RF energy traveling through them. There are numerous types of slow wave structures including helical, coupled-cavity, and ring-and-bar circuits. The frequency at which the device operates is determined by the geometry and size of the slow wave structures  116 ,  118 , and  120 . In a backward wave device, the slow wave structures  118  and  120  cause RF energy in the circuit to counter-propagate, or propagate toward the electron gun to an output port, as will be explained later. After the RF energy has been coupled into and extracted from the electron beam using slow wave structures such as backward wave couplers  118  and  120 , the beam enters a region known as the collector  112 , which collects the spent beam. There are many collector configurations used in linear beam devices. Some of these include single-stage grounded collectors and multiple stage collectors. The driving concept behind the selection of collector used is efficiency and power supply considerations. 
   A backward wave device, whether it be an amplifier or an oscillator, is a type of traveling wave device which includes a slow wave structure which causes the phase velocity of a forward moving wave to have a negative value, so that it travels in a direction counter-propagating (opposite the direction of) the electron beam  114 . 
     FIG. 2  shows a ω-β curve for an electron beam interacting with a slow wave structure such as backward wave coupler  120  of  FIG. 1 , where the x axis  105  is the wave number, which for corrugated structures are normalized to k*d, where:
         k is the wave number, or 1/λ, and λ is the wavelength of interest;   d is the depth  123  of the corrugations shown in  FIG. 1 ;   and the period of pitch p  121  of  FIG. 1  is constant. The y axis of the graph shows the upper cutoff frequency, for a structure, where   f cutoff  is proportional to 1/d*c   where   d=depth of corrugation, as before,   c=velocity of light.
 
Curve  102  is the electron beam line, the slope of which indicates the electron beam velocity as electrons leave the cathode and travel down the beam tunnel, and the slope of this line  102  increases with larger voltage applied by cathode  108  in  FIG. 1 . The functional characteristics of a slow wave structure having a fixed pitch p  121  from  FIG. 1  and varying depth d  123  from  FIG. 1  is shown as curve  106   a ,  106   b , and  106   c , which for corrugation structures is governed by the parameters p  121  and d  123  both from  FIG. 1 . Smaller values of d yield a higher cutoff frequency, and larger values of d result in a lower cutoff frequency. Operation of the RF slow wave structure with a large cathode electron acceleration voltage results in an intersection point between the electron beam line  102  and the slow wave structure curve  106   a ,  106   b , or  106   c  in the region 0 to n, and the device operates as a forward wave device. A reduction of the cathode electron acceleration voltage results in a lower slope of the electron beam line  102 , and the electron beam line  102  intersects the RF slow wave structure characteristic curve at point  104 . Operating point  104  is shown in the region from n to 2n known as the backward wave region, and the RF waves are counter-propagating with the electron beam, where the RF is propagating in a direction opposite the direction of the electron beam. For a given slow wave structure geometry, as the electron beam voltage is slightly increased, curve  102  has a greater slope, and intersection point  104  supports at a higher operating frequency F 1   101 . For given operating point  104 , traveling waves can be supported up to a frequency F 1   101  where the corrugation depth d=80u, as shown in the present example. If the traveling waves experience a change in corrugation depth to 100u as shown in characteristic curve  106   c , the slow wave structure will no longer support traveling waves at this frequency, and the waves will be reflected in the region of the discontinuous interface where the depth d in increased. The curves  106   a ,  106   b , and  106   c  are normalized to wave number in the x axis and show the relationship between corrugation depth and the maximum RF frequency the slow structure can support. The curves of  FIG. 2  are ordinarily computed using numerical techniques for a specific structure. In the present example, curves of  FIG. 2  were calculated for the case where the corrugation pitch p=50u and the width of the individual structures is 20u for a variety of depths d  123  (from  FIG. 1 ) ranging from 40u to 100u. These curves, in conjunction with the electron beam line  102  enable the design of reflecting structures for use in forward or backward wave regions. One of the problems with devices that operate in backward wave regions is the inefficiency of coupling between the slow wave structure and the output waveguide.
       
     FIG. 3  shows a backward wave structure from the unpublished design of a Russian-designed microwave tube available commercially in Russia. An electron beam  135  travels from a beam tunnel entrance  130  through a beam shaper  132  to a beam tunnel exit  138 , and beam shaper  132  is at the same height as corrugations  136  having a depth d in accordance with the characteristics of  FIGS. 1 and 2 . Additionally, the beam shaper includes a series of slots parallel to the electron beam  135  axis which cause the electron beam  135  to travel over and around the corrugations which are perpendicular to the electron beam  135 . This dual corrugation produces pin structures known as pintles  136  which have a depth d and pitch p perpendicular to the axis of the electron beam  135 . These pintles  136  include longitudinal slots which allow the electron beam to surround the pintles  136 , and therefore interact with the them in an enhanced manner. Section z—z through the beam shaper  132  of  FIG. 3  is shown as  FIG. 3   a  showing the slots in the beam shaper  132  and the electron beam  135  forming around these slots. These slots continue in the pintles  136  shown in section view a—a in  FIG. 3   a  with electron beam  135 . The cross section through pintles  136  of section b—b is shown in  FIG. 3   b , which effectively shows a top view of the pintles  136  and also pintles  134  from the sloping region of  FIG. 3 . The pintles  136  are physically small and not well thermally coupled to substrate  131  in  FIG. 3 , and an imperfectly aligned electron beam  135  directly impinging on these pintles would cause them to overheat and melt. By machining the beam shaper  132  to the same height as the pintles  136 , and including slots in beam shaper  132  which continue through pintles  134  and  136 , the shaper  132  is able to very closely couple the electron beam  135  with the pintles, tightly coupling the tops and sides of the pintles  136  with the electron beam  135  as shown in  FIG. 3   a . The pintles are therefore shielded from overheating due to direct exposure to a misaligned electron beam by the beam shaper  132 , which conducts excess heat into the slow wave structure body  131  from  FIG. 3 . The operation of the backward wave coupler of  FIG. 3  includes the reflection of RF energy carried in the beam by sloping structure  134 , whereby reflected wave energy is coupled into the output aperture  140 . In the unpublished RF device of  FIG. 3 , the output port  140  is placed between a row of pintles in the sloped region  134 . Fabrication of the device shown in  FIG. 3  for use in sub-millimeter wavelengths is very difficult, as the features are on the order of 10s of microns, and the sloping section  134  must be completed prior to the pintle fabrication. The best method for pintle feature manufacturing is electro-discharge machining, which is best done using substantially planar surfaces, as opposed to the sloping surface  134 . 
   In prior art devices such as in U.S. Pat. No. 4,263,566 by Guenard and shown in  FIG. 1  structures  118  and  120 , the slow wave structures are corrugated in one dimension only such that the cross section of  FIG. 1  is correct for any section through the slow wave structure. Similarly, the slow wave structure described in U.S. Pat. No. 4,149,107 by Guenard comprises 1-dimensional slots as shown. In the Russian device of  FIG. 3 , the corrugations perpendicular to the electron beam are supplemented by slots parallel to the electron beam which produce structures referred to as pintles, which are a plurality of pins spaced on regular intervals, typically 10–20 pintles per wavelength, in accordance with the desired frequency performance as described in  FIG. 2 . While backward wave devices enable operation over a wide range of frequencies tunable by changing the electron beam voltage, backward wave devices suffer from inefficient coupling of RF energy to the output port and the use of pintles increases the efficiency of this coupling. 
   OBJECTS OF THE INVENTION 
   A first object of the invention is a slow wave structure for reflecting RF energy either co-propagating with (traveling in the same direction) an electron beam or counter-propagating with (traveling in the opposite direction) an electron beam. 
   A second object of the invention is a slow wave structure having a reflector, said reflector causing RF energy counter-propagating in an electron beam to co-propagate to an output port which is spaced a half wavelength from the reflector. 
   A third object of the invention is a slow wave structure comprising a plurality of pins placed in a substrate, the depth of said pins changing a half wavelength from an output port. 
   A fourth object of the invention is a slow wave structure comprising a plurality of pins forming a substantially planar surface, said plurality of pins located on a substrate, the depth of said pins undergoing a step change a half wavelength from an output port. 
   A fifth object of the invention is a slow wave structure comprising a plurality of pins forming a substantially planar surface, said pins located on a substrate, the depth of said pins undergoing a plurality of step changes, each said step change being a distance of half a wavelength from an output port. 
   A fifth object of the invention is a slow wave structure for an electron beam having an axis, said slow wave structure having, in sequence, a electron beam entrance, an optional beam shaper, a reflection region, a half wave region, an RF output port, a gain region, and an electron beam exit, the slow wave structure having a substrate which includes a plurality of corrugations perpendicular to said axis, said corrugations having a first depth in a region from said beam exit to a half wavelength past the RF output port, and a second depth thereafter, the pins having a substrate end and an unsupported end which is substantially parallel to said electron beam. 
   A sixth object of the invention is a slow wave structure for an electron beam having an axis, said slow wave structure having a substrate, said substrate having corrugations, said corrugations having one end forming a substantially planar surface, said slow wave structure including, in sequence, an electron beam entrance, a beam shaper having a surface substantially planar with said corrugations, a reflection region having said corrugations at a first depth, a half wavelength region having corrugations at a second depth, an RF output port located a half wavelength from said corrugations changing from said first depth to said second depth, a gain region having corrugations at said second depth, and a electron beam exit. 
   A seventh object of the invention is a slow wave structure for an electron beam having an axis, said slow wave structure including, in sequence, an electron beam entrance, a beam shaper having a plurality of slots parallel to said electron beam axis, a plurality of pins having a first depth below said beam shaper and attached to said substrate, a plurality of pins having a second depth below said beam shaper and attached to said substrate, an RF port located a half wavelength from the change from said pin first depth to said pin second depth, a plurality of pins having said second depth and attached to said substrate, and a an electron beam exit. 
   SUMMARY OF THE INVENTION 
   A slow wave structure for a backward wave traveling wave tube comprises a substrate having a plurality of pins, known as pintles. The pintles are elongate cantilever structures interacting with an electron beam traveling in a beam tunnel. The pintles have one end mounted to/and perpendicular to the substrate, and an opposing cantilever end. The pintles are small in comparison to the physical wavelength of the electromagnetic wave counter-propagating with the electron beam. The cantilever end of the pintles forms a substantially planar surface in the region of the electron beam, and the substrate supporting the pintles and located below the electron beam includes an exit aperture and at least one step change located a half wavelength from the exit aperture on the electron beam entrance side of the beam tunnel. In backwards wave mode, Radio frequency (RF) energy counter-propagating with the electron beam is reflected by the change in height of the pintles, and is coupled into the output port which is located half a wavelength away from the step change in pintle height. For broadband devices, there may be a plurality of step changes for a plurality of wavelengths, each step change located a half wavelength at some frequency of operation from the exit aperture. The slow wave structure may also include a beam shaper, comprising a ramp perpendicular to the electron beam axis, positioned near the electron beam entrance, and having a plurality of slots parallel to the electron beam axis, such that the slots and pintles form common channels for the electron beam. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a section view of a prior art traveling wave tube. 
       FIG. 2  is an ω-β graph showing the maximum operating frequency of a microwave tube as a function of electron voltage versus pin depth. 
       FIG. 3  shows a section view of a prior art backwards wave coupler. 
       FIG. 3   a  is a section view through section a—a of  FIG. 3 . 
       FIG. 3   b  is a section view through section b—b of  FIG. 3 . 
       FIG. 4  shows a section view of a backward wave coupler according to the present invention. 
       FIG. 5   a  shows the detail of the pintles near the waveguide of  FIG. 4 . 
       FIGS. 5   b  and  5   c  show a section view of the pintles in the reflection region, the beam shaper region, and the half wave region of  FIG. 4 . 
       FIG. 6  shows a section view of a backward wave coupler according to the present invention. 
       FIG. 7  shows a traveling wave device configured as an oscillator. 
       FIG. 8  shows a traveling wave device configured as an amplifier. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 4  shows the side view of a backward wave coupler  150  for a traveling wave tube, which is defined in a coordinate system y and z axis as shown, and an x axis (shown in  FIGS. 5   b  and  5   c ) perpendicular to the y and z axis. An electron beam  152  is emitted from a cathode (not shown) and enters beam tunnel entrance  162 , where it travels over beam shaper  153 . This beam shaper  153  may have a plurality of slots parallel to the axis of electron beam  152  and over and around a plurality of pintles  154 , which comprise corrugations having a pitch p, a width w, a depth d 1  as shown in  FIG. 5   a , and may also include slots substantially aligned with the slots of beam shaper  153 , and parallel to the axis of electron beam  152 . The electron beam  152  may include counter-propagating RF at a wavelength λ, and the pintles  154  are spaced at less than 0.1λ in the z and optionally x directions. The pintle surface plane  166  is planar with a surface of the beam shaper  153  and a z-x plane below the electron beam  152 . The pintles may follow the shape of the electron beam  152  to enable maximal coupling between the pintles and the RF carried in the electron beam  152 . The pintles  154  are cut to a depth  168  in a half wavelength region having a distance of a multiple of a half wavelength (λ/2)  157  on the beam tunnel entrance  162  side of output aperture  158 . The half wavelength distance  157  may also be any integer multiples of wavelength such as (n+1)λ/2 where n is an integer&gt;0. In the reflection region beyond the half wavelength separation distance  157 , the pintles change depth  170  while maintaining the same pintle surface plane  166  as the pintles  154  and beam shaper  153 . This change in substrate  151  to depth  170  causes RF energy counter-propagating with the electron beam  152  to reflect and co-propagate towards the exit aperture  158 , where the counter-propagating RF energy and reflected co-propagating RF energy add in phase to a maximum level in the region of output aperture  158 , and couple out. The pintles  154  are shown having a regular period leading up to the output aperture in gain section  161  and following the output aperture  158  in half wave section  157  and reflection section  159 . It has been found that removing one or more rows of pintles in the region of the output aperture  158  increases the coupling of reflected RF into output aperture  158 . This is shown in  FIG. 5   a , which is a detailed view of  FIG. 4  showing the removed rows of pintles  155  in phantom outline with the RF output aperture  158  centered in the resulting gap between pintles  154 . 
   Increased interaction between the RF counter-propagating in the electron beam  152  and the corrugations  154  occurs when slots parallel to the electron beam axis are cut into the beam shaper  153  and corrugations  154 , resulting in a slotted beam shaper  153  and pintle structures  154 . When slots parallel to the electron beam  152  axis are added to enhance coupling between the counter-propagating RF and corrugations  154 ,  FIG. 5   b  section e—e shows the resulting slotted beam shaper  153 .  FIG. 5   b  also shows section c—c through  FIG. 4  in the x-y plane, showing electron beam  152 , pintles  160  at uniform height  166  and a second depth  168 , and  FIG. 5   c  shows the same view through section d—d of  FIG. 4  where the pintles  160  are cut to a first depth  170  in the reflection region  159  of substrate  151  from  FIG. 4 . 
   The structure of  FIG. 4  can be used as an input port in the forward wave mode by coupling power into input port  158 , which co-propagates through gain section  161 . It is also possible to use the slow wave structure of  FIG. 4  as an output port in forward wave mode by reversing the beam direction such that the electron beam enters at  164  and exits at  162 , and the beam shaper is placed at the same height  166 , but at  164 . In this manner, forward waves co-propagating with the electron beam enter at  164 , travel through gain section  161  and co-propagate to exit aperture  158 , where they combine with reflected counter-propagating waves from reflection region  159 . As described earlier, higher electron beam velocities are used for forward wave devices compared to the backward wave devices description of  FIG. 4 . 
     FIG. 6  shows a multi-wavelength reflection slow wave structure  180 . Electron beam  184  travels down a beam tunnel having in sequence a beam tunnel entrance  182 , a beam shaper  181 , a plurality of elongate pintles  185  having one end attached to a substrate  181  and an opposing end which is in proximity to the electron beam  184 , the plurality of pintles formed into a reflection region comprising a plurality of pintles cut to decreasing first depths  207 ,  206 ,  204 , a half wavelength region having the plurality of pintles cut to a second depth, an output aperture  208 , and a plurality of pintles  185  at a second depth  202 . Each change in pintle depth in the reflection region is spaced a half wavelength from the exit aperture  208  for a given output wavelength. By selecting the particular corresponding wavelengths for these depth changes in the reflection region  196 , it is possible to optimize the operation band of the device over a wide range of wavelengths. The plurality of the opposing ends of pintles  185  may be substantially planar with the beam shaper  181  and substantially co-planar with the electron beam  184  axis. The RF counter-propagating with the electron beam  184  travels past the output aperture  208  for the removal of RF energy, and the plurality of pintles  185  changes to a second depth  204  at a first half wavelength distance  190 . The pintle depth is again changed to a third depth  206  at a second half wavelength distance  192 , and may also continue to subsequent depth  207  at additional half wavelength  194 . Each half wavelength distance  190 ,  192 ,  194  is associated with a particular half wavelength of RF counter-propagating with electron beam  184  which is reflected as a co-propagating RF wave to sum with the counter-propagating RF wave and couple to output aperture  208 . The half wavelength separation distances  190 ,  192 ,  194  may also be any integer multiples of wavelength such as (n+1)λ/2 where n is an integer &gt;0, as was described in  FIG. 4 . As was described in  FIG. 5   a , a row or more of pintles may be removed and the waveguide  208  centered in the resulting gap to enhance coupling of reflected RF energy to the output aperture  208 . 
   The pintles  154  and  160  of  FIG. 4 , and  185  of  FIG. 6  may be made in a variety of shapes, and arranged in a variety of forms. The pintles may be rectangular or circular, and they may be formed by machining substrates  151 ,  181 , or by chemical etching or electro-discharge machining (EDM) of the substrate, as is known in the art of machining metallic substrates  151  and  181 . For any of these machining processes, it is desirable to have the structures formed from a planar surface, as shown in the figures of the present invention. The pintles  154  and  160  of  FIG. 4 , and  185  of  FIG. 6  may comprise corrugations perpendicular to the axis of the electron beam, or they may include slots which are parallel to the axis of the electron beam, and the beam shaper  153  of  FIG. 4 and 181  of  FIG. 6  may or may not be present, depending on the accuracy of alignment of the electron beam  152  of  FIG. 4 and 184  of  FIG. 6 . In general, the structures of the pintles and beam shaper are formed from a planar substrate. 
   The reflector structures shown in  FIGS. 4 and 6  may be combined in a variety of ways to form traveling wave tube oscillators and amplifiers using forward wave region or backward wave region operation, for which two examples are shown in  FIGS. 7 and 8 . 
     FIG. 7  shows the present invention used as a tunable wideband oscillator  220  in backward wave mode. A cathode  222  in proximity with an anode  226  has an applied voltage  224  which causes the cathode  222  to emit a beam of electrons  234  in the backward wave region of  FIG. 2 , which may be focused using an external axial magnetic field (not shown), as known to one skilled in the art. Slow wave structure  221  includes an electron beam entrance  228 , a beam shaper  238  followed by a plurality of pintels  236  forming reflector section  232  comprising a plurality of pintles of decreasing depths each successively positioned one half wavelength from output aperture  242  as was described in  FIG. 6 , an output aperture  242 , and a gain section  240 . The spent electron beam  234  dissipates in collector  230 . RF noise in the gain section  240  is amplified in counter-propagating waves, which are reflected in reflector region  232  to co-propagating waves which combine with the counter-propagating wave and couple into output  242 . The internal coupling of forward and reflected waves causes an oscillation at a particular frequency, which is tunable with cathode voltage  224 , and the reflector  232  provides for gain over a range of frequencies for which the device may operate. 
     FIG. 8  shows the present invention used as an forward wave amplifier  260 .  FIG. 8  shows a pair of RF reflectors of  FIG. 4  arranged in a mirror fashion as an input reflector  268  and an output reflector  276 . Cathode  264  in conjunction with voltage source  262  and anode  266  supplies a beam of electrons  280  in forward wave mode, which is shaped to the height of the pintels by beam shaper  267 , as before. The beam shaper  267  may include slots parallel to the electron beam  280  axis at the same depth as the pintels in the gain section  272 . Input RF energy is coupled into port  270 , which is coupled into the beam tunnel, whereby some RF energy is directly coupled co-propagating towards collector  278  and some RF energy is reflected by input reflector  268 , summing in phase with incoming energy from port  270 . The RF co-propagates through gain section  272 , and is coupled to output  274  with output reflector  276 , as before. The spent beam passes to collector  278 . For the amplifier configuration of  FIG. 8 , the voltage  262  is adjusted to a voltage in the forward wave region of  FIG. 2  about which a range of wideband amplification may take place. 
   While a specific illustration for the backward wave structure has been shown for the purposes of illustration, it is clear that the reflector structure described in  FIGS. 4 and 6  may be scaled to any wavelength, and is suitable for frequencies in the thousands of Ghz (Thz) region. It is clear that the reflector comprising a plurality of pintles attached to a common conductive substrate, the pintles having a common height substantially co-planar to an electron beam, a first section which includes an output port, and a reflection section located a multiple of a half wavelength from the output aperture, the reflection section comprising pintles at the same height as the pintles of the first section, but with greater depth distance to the substrate. The structure may be formed from corrugations without any slots substantially co-planar to the electron beam axis, or the corrugations may include slots parallel to the axis of the electron beam, which may improve the coupling efficiency of co-propagating and counter-propagating RF to the output aperture. The structures may be operated in the forward wave region with the RF co-propagating with the electron beam, or in the backward region with the RF counter-propagating with the electron beam according to  FIG. 2 . Using combinations of the structure described herein, amplifiers and oscillators using forward or backward mode suitable for sub-millimeter RF waves may be formed.