US 3781702 A
A traveling wave amplifier having a slow-wave circuit in the form of a stack of thin, flat elements with identical spiral pairs radiating from a common axis, the inner ends of the spirals of each pair being 180 DEG apart and equidistant from the axis, and each element having a perforation for an electron beam between the inner end of its two spirals. RF that is coupled into the spirals of the element at one end of the stack is propagated by the spirals and radiated element-to-element and extracts energy from the electron beam. The RF, amplified, is taken from the spirals of the element at the other end of the stack.
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Description (OCR text may contain errors)
United States Patent [191 Jasper, Jr. Dec. 25, 1973  TWT WITH STACKED SPIRAL-PAIRS 3,181,024 4/1965 Sensiper 330/43 X SLOW WAVE CIRCUIT 2,951,174 8/1960 Mourier et al 3l5/3.5 2,952,795 9/1960 Craig et al. 315/35 Louis J. Jasper, Jr., Neptune City, NJ.
Filed: Sept. 19, 1972 Appl. No.: 290,287
US. Cl 330/43, 315/395, 330/44, 315/35 Int. Cl. H03F 3/58 Field of Search 315/395, 3.5; 330/43 References Cited UNITED STATES PATENTS Bates 315/35 Primary ExaminerNathan Kaufman Attorney-l-1arry M. Saragovitz  ABSTRACT A traveling wave amplifier having a slow-wave circuit in the form of a stack of thin, flat elements with identical spiral pairs radiating from a common axis, the inner ends of the spirals of each pair being 180 apart and equidistant from the axis, and each element having a perforation for an electron beam between the inner end of its two spirals. RF that is coupled into the spirals of the element at one end of the stack is propagated by the spirals and radiated element-to-element and extracts energy from the electron beam. The RF, amplified, is taken from the spirals of the element at the other end of the stack.
3 Claims, 2 Drawing Figures RF SOURCE &2
RF LOAD TWT WITH STACKED SPIRAL-PAIRS SLOW-WAVE CIRCUIT BACKGROUND OF THE INVENTION If the slow-wave structure of a traveling wave tube were made in such manner that it could be specified entirely by angles, then the performance of the traveling wave tube would be essentially independent of wavelength because the dispersion characteristics of the slow-wave structure would exhibit ultra-broadband properties.- Since dimensions of prior art slow-wave structures for use at microwave frequencies are finite, the structures could not be specified entirely by angles; at least the length of the structure required for interaction needed to be specified. Actually, at least two linear dimensions had to be specified. For example, in a helical slow-wave structure, radius and length dimensions had to be specified; if the radius of the helix was A, and an electron beam directed through the helix had radius R, the maximum bandwidth attainable was related to R/A. The maximum possible numerical value of R/A was less than unity since the inside diameter of the helix was less than the helix diameter by an amount equal to the diameter of the wire. R/A was further restricted by focusing difficulties that precluded a beam diameter equal to the inside diameter of the helix.
SUMMARY OF THE INVENTION This invention concerns a slow-wave structure which can be specified by angles and which is not subject to the R/A ratio restriction of the helix so that its performance is essentially independent of frequency. The preferred emobdiment of this invention includes a stack of thin, flat dielectric laminae having the form of disks. The disks have center perforations for a longitudinal electron beam. Each disk has a pair of conductive spirals. There is no RF conduction between spirals of successive disks. RF is coupled into the spirals on the disk at the beam entering end of the stack and amplified RF is coupled out of the spirals on the disk at the beam exiting end of the stack. Desired overall gain determines the total number of disks in the stack. A stack may have fifty disks, the number being an order of magnitude and not a limitation. Effectiveness of the amplifier is related to the efficiency of coupling RF energy from the conductive spirals on the first disk through spiral structures on the successive disks to the spirals on the last disk and to the generation of proper longitudinal fields for space charge modulation. Spiral-arm length is greater than one wavelength at the lowest frequency of operation to a sufficient extent for the near field on the axis to approach circular polarization and for essentially frequency independent operation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a diagrammatic layout of a preferred embodiment of a traveling wave RF amplifier according to this invention that includes a stack of laminae; and
FIG. 2 is a perspective view, partly broken away of the embodiment of FIG. 1 showing a sampling of the laminae in the stack.
The amplifier embodiment shown in FIGS. 1 and 2 includes an evacuated elongated metallic envelope 12 supporting an electron beam source 14 at one end for projecting a focused electron beam 16 to a collector anode 18 supported at the other end. The beam source 14'and collector anode l8, represented symbolically by flat-headed studs, are conventional prior art structuresand are conductively joined to connecting terminals 20 and 22. A uniform longitudinal magnetic field indicated by legend H is provided in any suitable manner to resist spreading of the beam. A stack 24 of thin, flat, identical dielectric laminae 26 is fixedly supported centrally between the electron beam source and the collector anode. Though shown as disks the laminae 26 may be rectangular, hexagonal, or other configuration. Each disk 26 has a central circular perforation 28 for electron beam 16 and several perforations 30 near the periphery, only two of which are shown in FIG. 2, for
registration with dielectric support rods 32 that extend the length of housing 12. Alternatively, rods 32 may register with notches, not shown, formed in the edges of disks 26. Each disk has identical two-arm multi-turn planar equiangular spirals 34 of conductive material. The loci of the spiral arms are the axes of the perforations 28; the inner ends of each pair of spiral anns are equidistant from the axis and degrees apart. Spiral arms 34 are deposited by sputtering, vacuum deposition, masking and spraying, or other known techniques. Rods 32 threaded through perforations 30 orient the disks so that the spiral arms are in line.
Conductors 36 and 38 extend across that end face of the stack 24 which is closest to the beam source 14, and extend through the end disk to electrically connect to the inner ends of the pair of spirals carried by the end disk. The other ends of conductors 36 and 38 are provided with RF connectors 40, 42 respectively. An output terminal of an RF source 44 is coupled to both terminals 40 and 42.
Conductors 46 and 48 extend across the end face of the stack 24 which is closest to the collector anode 18 to electrically connect to the inner ends of the spirals on that end disk. The other ends of conductors 46 and 48 are provided with RFconnectors 50 and 52, respectively. An RF load 54 is coupled to the RF connectors 50, 52.
Though the .input connections are made to the inner ends of the first pair of spirals and the output connections are made to the inner ends of the last pair of spi-' rals, these relationships are not intended as limitations; the input connections may be made either to the inner or to the outer ends of the first pair of spirals and the output connections may be made either to the inner or to the outer ends of the last pair of spirals.
Though not shown, it is to be understood that all the spiral conductors are connected to a common source of DC potential through RF blocking means, as is conventional in traveling wave tubes.
An alternative structural arrangement for the stack 24 is to provide a pair of spirals on both faces of each disk 26 and to provide a spacer disk of the same material and thickness between each pair of disks 26.
The two spiral arms 34 on the disk nearest the beam source are driven in phase at their inner ends by RF source 44 and radiate the RF between the inner and outer ends of the two spirals; RF radiated by the first pair of spiral arms is intercepted by the second pair of spiral arms and is reradiated by the second pair of spiral arms and is intercepted by the'third pair of spiral arms. The action repeats through every pair of spiral arms. For radiation tooccur disk-.to-disk, where the spiral arms are fed in phase, the outer circumference of the spiral structure should-be larger than 2A,, at the lowest frequency of operation; A, is free space wavelength. For radiation to occur disk-to-disk, where the spiral arms are fed 180 out of phase, A, instead of 2A,, is the criterion.
As input frequency is increased, radiation disk-todisk occurs closer to the axis of the device. Consequently, an RF wave propagates faster through the stack as the frequency is increased. Radiation of the spiral circuit determines the frequency bandwidth and thus the center frequency of operation.
Amplified RF is delivered by the spiral arms of the last disk to an RF load 54. If the coupling between conductor-arm spiral structures of adjacent disks is sufficient, and it should be as high as possible, the circularly polarized field generated by the energized spiral structure of the first disk incident on the spiral structure of the next disk results in an induced field that is also circularly polarized. The sense of the circularly polarized field of the spiral structures of consecutive disks is in opposite directions; a circularly polarized field incident on a spiral structure induces currents to flow either inward toward the center or outward toward the outer ends of the spiral arms. This is true since the direction of current flow depends on the sense of the incident field and the sense of the spiral structure. Current reaching either the inner or outer unterminated ends of a spiral structure reflects and travels back out to be reradiated and this gives rise to a field having polarization of opposite to that of the current inducing field. The induced field has the same type polarization as the polarization of the field of the first spiral conductor structure so the effect is the same as obtains if the second spiral structure is directly excited from the RF source and of opposite sense to the field of the first spiral conductor structure. The net effect is that the electron beam encounters an electromagnetic field with polarizations of opposite senses upon entering and leaving, respectively, the plane of a planar spiral conductor structure. This phenomenon sets up a slow-wave that travels inward toward one side and outward from the other side of the spiral structure. The cumulative effect is a slow wave propagating in the longitudinal direction.
The dielectric disks shown in FIG. 1 are stacked against each other. Whether the disks are stacked against each other or are stacked slightly spaced apart by use of spacers on the dielectric rods, or equivalent techniques, only the RF field distribution close to the planar spiral conductor arms is significant. If the length L of the spiral conductor arms is greater than one wavelength at the lowest frequency of operation, the near field along the axis is elliptically or circularly polarized. This field is more nearly circularly polarized if frequency is increased or if length L of the spiral conductor amrs is increased. Because of this phenomenon and because of the different senses of polarization, within a short distance from the spiral conductor arm structure, longitudinal electric fields are present.
In the described and illustrated embodiment, the spirals of metal conductors are on a dielectric substrate. A variation within the scope of this invention is a stack of metal laminae e.g. 1/16 inch thick with perforations corresponding to those through the dielectric laminae 26 and having two spiral cutouts of the same geometry as the spiral arms on the dielectric laminae 26. RF is coupled into the spiral cutouts at one end of the stack and out of the spiral cutouts at the other end of the stack by conventional well known impedance methods.
In all embodiments, bandwidth may be broadened by having successive pairs of spirals rotated or indexed a predetermined angular amount in the same direction relative to the axis. The sum of the angular rotations of the successive pairs of spirals in a stack of elements may be several times 360 and the multiple is not necessarily a whole number.
The multi-arm equiangular planar spiral operates as the basic propagating structure for the electromagnetic energy. The equiangular spiral is described by the general equation P no p 4 an] where p and 45 are polar coordinates. p and b are the initial radial and phase coordinates respectively, and a is a constant that determines the tightness of the spiral. The outer edge of one spiral conductor arm is defined by the equation p1 P0 p and the inner edge of the same spiral conductor arm is defined by the equation p2 p0 pl 1 o)l= P] where K is a constant and is a measure of the angular width of the arms. Likewise, the second arm which originates at an angle that is displaced by 1r radians from the one arm is defined by corresponding equations 4 p0 p pa for its outer and inner edges respectively. The two edges of each spiral conductor arm are identical curves and give the arm finite width. The length L of the spiral, defined as the distance from the inner end to the outer end measured along the center of the spiral arm is approximated as follows An equiangular flat spiral is specified by three variables, arm length L and the constants p and K. ln order that there be spacing between the arms, there must be a lower limit on K. Therefore K is ordered as follows exp (-onr) K l If a given diameter structure is to have as wide bandwidth as possible, the spiral should be as tight as possible, i.e., the smallest practical value.
' For small values of a, the phase fronts are approximately circular and the center is located on the-axis nearly in the plane of the spiral arms. in the immediate vicinity of the plane of the spiral arms, there is generated a slow wave moving radially inward or outward depending on the mode excited. It is this slow wave that interacts with the electron beam projected axially through the centers of the dielectric disks. Lower cutoff frequency is related to the magnitude L and to the magnitude of K; therefore, for a slow-wave structure of planar equiangular spiral arms of a given magnitude of diameter to have the lowest cutoff frequency, the combination of values of L and K is optimized. The upper cutoff frequency is a function of fineness of construction of the conductor arms at the feedpoints. In genera], the one length needed to specify the spiral structure, the arm length L, needs to be at least one wavelength at the lowest frequency of operation, in order that performance be essentially independent of frequency. With the restrictions specified, the planar equiangular spiral structure possesses inherently ultra-wide band characteristics.
Characteristic impedance of the planar equiangular spiral is approximately 50 ohms. As frequency increases, the input impedance converges rapidly toward 50 ohms and remains relatively constant at frequencies such that arm length is greater than one wavelength. Since the spiral structures are of finite thickness, the characteristic impedance is not entirely constant but for very thin structures, as in this invention, it is reasonably constant.
The device described is an amplifier. With modifications well known in the art, it can be made to operate as a backward-wave oscillator, or it can be made for modulating an electron beam.
While the spiral structures described employs the two-arm planar equiangular spiral, a multi-armed spiral can be used and also the planar archimedian spiral as well as other planar spiral configurations can be used as the electromagnetic energy propagation structure, though the others have the disadvantage of being more frequency dependent. Also, while the description specified conductive spiral structures deposited on thin dielectric disks, analogous structures for serving the same function can be made by etching or punching spirals out of thin disks of selected conductor material. The latter disks may be coated with insulating material and stacked against each other or may be supported slightly spaced apart in the same manner as the dielectric disks. RF is coupled to the first conductor disk with a coupling loop just as in conductor-to-waveguide couplings and RF is coupled out from the last disks in similar manner. All of the metal disks are conductively connected in common to a source of DC potential.
What is claimed is:
1. In a traveling wave tube having an electron beam source and a collector anode spaced apart for flowing an axial electron beam therebetween, and a' combined slow-wave propagation and beam-wave interaction means between the electron beam source and the collector anode, said means comprising:
a stack of disks between the electron beam source and the collector anode, each disk having a plurality of flat RF propagating spirals and each disk having a central circular perforation, the perforations of all the. disks being in line with a common axis that extends between the electron beam source and the collector anode to provide an electron beam path, the inner ends of the spirals on all the disks being equidistant from the axis and being equiangularly spaced apart relative to the axis,
means for coupling RF into the spirals closest to the electron beam source, and
means for coupling the RF, amplified, out of the spirals closest the collector anode.
2. In an RF amplifier having an electron beam source and collector anode spaced apart, and a combined slow wave propagation and beam-wave interaction means between the electron beam source and the collector anode and comprising:
a stack of essentially identical dielectric disks having an axial perforation for the electron beam from said source, supported coaxially adjacent one another, each of said disks having deposited thereon a plurality of conductor spirals wherein the two edges of each spiral are described by the equation P A. pl 4 0)] where p and 4) are polar coordinates, p, and 4),, are the initial radial and phase coordinates respectively, and a is a constant selected according to the desired tightness of the spiral, the outer edge of a conductor spiral being defined by the equation P1 p0 P( and the inner edge of the same conductor am being defined by the equation and corresponding edges of the plurality of spiral conductor arms being apart at their inner ends,
conductor means for coupling RF from an input source to the conductor spirals on the disk closest to the electron beam source, and conductor means connected to the conductor spirals on the disk closest to the collector anode for coupling the RF, amplified, to an RF load,
said stack being supported in line with and between said electron beam source and said collector.
3. In the RF amplifier defined in claim 2 wherein said conductor spirals on each disk have several turns, and the conductor spirals on the successive disks end-toend of the stack are indexed around the axis in one direction a predetennined angular amount.