US2880354A - Rapid frequency shift traveling-wave tube - Google Patents

Description

March 31, 1959 R. D. wx-:GLIN

RAPID FREQUENCY SHIFT TRAVELING-WAVE TUBE Filed Apr-111, 1957 Wi/vrai Rolf D.Weg|ein,

y gli" (96m.

United States Patent .IO

Rolf D. Weglein, Los Angeles, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application April 1,1957, serial No'. 649,835

-s claims. (ci. B15- 3.5)

This invention relates generally to traveling-wave tubes and more particularly to a traveling-wave tube which is capable -of shifting rapidly from one set of operating conditions to another.

It is often desired to shift the operating conditions of a traveling-wave tube as rapidly as possible as, for example, shifting the frequency of a backward-wave oscillator by applying a step of voltage to the electron stream accelerator. Heretofore, there has been found to exist a relatively long period of time before the new condition has been established, i.e., the new frequency. Delay times or lag times of the order of 50 microseconds have previously been presumed to be inherent in conventional backward-wave oscillators. It is considered that this lag time represents the period required for the electron stream to charge the dielectric such as glass in the proximity of the slow-wave structure and which may be thought of as partially filling the inter-face space of the capacitor represented by the slowwave structure as one plate, with other and distant elements comprising the opposite plate.

The problem has been approached in the past by minimizing this capacity as, for example, by spacing other conducting elements further from the slow-wave structure. The results of such an approach have been to achieve lag times only as low as 50 microseconds.

It is an object of kthe Ipresent invention to provide a traveling-wave ytube' which mayshiftoperating conditions with much less lag time than was heretofore possible. ,p l

It is a further object to provide a traveling-Wave tube such as a backward-wave oscillator in which frequencyl of oscillation may be voltage shifted in a period .of the order of 5 microseconds.

vBrielly, these and other objects are here achieved in the following manner:`

A new approach is utilized to substantially eliminate the dielectric charging which gives rise to the lag time. Instead of attempting to minimize the capacitance between the slow-wave structure and the outer elements, a shield is provided which isolates the slow-wave structure and the dielectric to be charged from the inter-capacity space. In one embodiment, a nonmagnetic conductive tube is disposed closely about the slow-Wave structure, for example, just outside the glass envelope. This shield is electrically connected to the slow-wave structure in a manner to insure that both slow-wave 4structure and shield remain at exactly the same direccurrent potential. An alternative arrangement utilizing dielectric supporting rods between the slow-wave structure and the glass envelope comprises a conductive coating on the supporting rods and on the interior of the glass envelope. In both embodiments a shield is provided which does not appreciably affect the propagation characteristics of the slow-wave structure.

Further objects and advantages of the present invention will become apparent to those skilled in the art upon Patented Mar. 31, 1959 2 a reading of the more detailed description of the accompanying drawing in which:

Fig. lis a partially cut-away schematic view of va backward-wave oscillator constructed in accordance with the present invention; v

Fig. 2 is a sectional view of a portion of the midsection of a traveling-wave tube embodying the present invention; and

Fig. 3 is a sectional view of a portion of a third ern bodiment of the invention.

Referring to the drawing, and particularly to Fig. `1 in which there is shown an example of the present invention in the form .of a backward-wave oscillator 10 which includes aglass envelope 12 supporting and enclosing within its enlarged left end an electron gun 14, and Within its elongated righthand portion a slow-wave structure which in the example chosen is'a helix 16. At the right-hand extremity of glass envelope 12 and sealed thereto is a collector electrode 18.

Closely encompassed about the elongated, more slender portion of envelope 12 and extending somewhat beyond the ends of slow-wave structure 16 is shown a conducting shield 20 which is directly connected to slowwave structure 16 by a short lead 22. Shield 20 may be light weight and thin as, for example, a length of nonmagnetic, metallic foil wrapped about envelope 12, and which may be of copper or aluminum. An output lead 24 is connected to the left extremity of slow-wave structure 16 and is adapted to be coupled in a conventional manner to a transmission line or waveguide as illustrated. Electron gun 14 includes a cathode 26 with a heater 23 for emitting a stream of electrons which are focused by a beam-forming electrode 30. An accelerating electrode 32 is maintained at a high positive potential with respect to cathode 26 and serves to accelerate the electron stream to a high velocity. The cathode 26 and electrodes 30, 32 may be shaped to develop a hollow electron beam asillustrated.

Slow-wave structure 16 is ordinarily maintained somewhat higher in potential than accelerating electrode 32 and its potential with respect to cathode 26 may determine the frequency of oscillation of oscillator`10.' An adjustable contact 34 connected to a source of potential 36 schematically indicates that oscillator 10 is thus tunable by varying the voltage of slow-wave structure 16 with respect to elements of electron gun 14. Collector electrode 18 is shown connected also to voutage source 36 at a point a few hundred volts positive with respect to slow-wave structure 16 whereby the effects of secondarily-emitted electrons may be minimized by holding them near collector electrode 18. An outer solenoid 38, concentric with respect to the longitudinal axis of slow-wave structure 16 and energized by a source of potential 40, provides a steady axial magnetic field for constraining and confining within slow-wave structure 16 the electron stream as it traverses from electron gun 14 to collector electrode 18.

Fig. 2 illustrates a second embodiment of the present invention in which a slow-wave structure 42 is supported within a glass envelope 44 by a set of dielectric rods 46. This technique of support is conventional and may apply, as does this example of Fig. 2, generally either to forward-wave or to backward-wave types of traveling- Wave tubes. Rods 46 and the interior surface of glass envelope 44 are coated with a thin resistive coating 48. The criteria for this coating are that an effective capacitive shielding should be provided about slow-wave structure 42 while not appreciably affecting the radio frequency propagation characteristics thereof. The coating should be thin compared to the depth of skin elect at the microwave frequency being propagated and would ordinarily be less than of the order of .0001 of an inch. Also, the resistance between turns of slow-wave structure 42 should be several orders of magnitude greater than the characteristic impedance of the tube such as to present substantially an open circuit to the energy being propagated. An effective coating for such purposes is stannous chloride evaporated onto rods 46 and the interior surface of envelope 44. The coatings on the rod must be in good electrical contact with the coating on the envelope to provide the prescribed shielding effect.

Referring to Fig. 3, Athere is shown a portion of the mid-section of a typical ribbon helix forward-wave travcling-wave tube amplifier in which, as similarly shown in Fig. 1, ribbon helix 50 is supported by the glass envelope 52 while a conductive shield 54 is provided about the exterior surface of envelope 52 which is directly connected to helix 50 by a short lead 56.

In operation, when the velocity of the electron stream or operating parameters are changed for the purpose of changing the frequency of operation, no dielectric charging or recharging is required before the tube attains the new steady state condition. In Fig. 1 the dielectric of envelope 12 is effectively removed from the inter-capacity space between slow-wave structure 16 and outer elements, such as solenoid 38 and waveguides or any cooling means. This is accomplished by shielding the slow-wave structure by shield 20 and maintaining it through lead 20 at the same direct-current potential as that of slow- .wave structure 16.

In the example of Fig. 2, dielectric charging or recharging of envelope 44 and rods 46 is minimized by conductive, resistive coating 48 which shields slow-Wave structure 42 from all the dielectric involved. The shield 54 of the example of Fig. 3 operates in the same manner as does shield 20 in Fig. l. In both cases, as with Fig. 2, dielectric charging which gives rise to the time lag in arriving at a new steady-state condition of operation is minimized or substantially eliminated by a shielding of the dielectric from the inter-capacity space between the slow-wave structure and the outer elements of the traveling-wave tube circuitry.

What is claimed is:

l. A traveling-wave tube providing a minimum lag time in attaining a change in state of operation comprising: a slow-wave structure; an electron gun for projecting a stream of electrons in energy exchange relationship with said slow-wave structure; dielectric support means for maintaining said slow-wave structure in alignment with the electron stream; and shielding means having a finite electrical conductivity disposed about and substantially encompassing said slow-wave structure and electrically directly connected thereto in a manner to shield said dielectric support means from the inter-capacity space between said slow-wave structure and outer capacitive elements, whereby the charging of said dielectric by said electron stream is substantially eliminated.

2. A backward-wave oscillator traveling-wave tube providing a minimum lag time in attaining a change in frequency of oscillation comprising: a slow-wave structure; an electron gun for projecting a hollow stream of electrons in energy exchange Arelationship with said slow-Wave structure; dielectric support means for maintaining said slow-wave structure in alignment with said electron stream; and shielding means having a nite electrical conductivity disposed about and substantially encompassing said slow-wave structure and electrically directly connected thereto in a manner to shield said dielectric support means from the inter-capacity space between said slow-wave structure and outer capacitive elements whereby the dielectric charging of said support means by said electron stream is substantially eliminated.

3. The device as set forth in claim 2 in which said dielectric support means comprises a glass envelope contiguously surrounding said slow-wave structure, and said shielding means comprises a hollow nonmagnetic conductive cylinder disposed contiguously about said glass envelope and which is electrically directly connected to said slow-wave structure.

4. The device as set forth in claim 2 in which said dielectric support means comprises a glass envelope and a plurality of elongated dielectric rods disposed substantially symmetrically on the interior surface of said glass envelope in a manner to support said slow-wave structure within said envelope radially spaced therefrom by a distance equal to the diameter of said elongated rods; and said shielding means comprises a thin coating of resistive material substantially over the interior surface of said envelope and said elongated rod in a manner to provide electrical connection between said slow-wave structure and said shielding means.

5. The device as claimed and set forth in claim 4 in which said coating comprises an evaporated coating of stannous chloride which has a thickness of the order of .0001 of an inch.

References Cited in the tile of this patent Peter Nov. 12, 1957

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