US3447018A - High power multicavity amplifier having enlarged drift tube gap defining portions to inhibit electronic feedback - Google Patents

Description

May 27, 1969 R. C. SCHMIDT HIGH POWER MULTICAVITY AMPLIFIER HAVING ENLARGED DRIFT TUBE GAP DEFINING PORTIONS TO INHIBIT ELECTRONIC FEEDBACK Sheet Filed Sept 16, 1966 INVENTOR. ROBERT SCHMIDT BY M V TTORNEY oo 2 1 f TMTE D M PC mm CHIR SPNT Mm C EL RH H MGB TEN

t 7 W 2 S y d e a l l M F FIG. 5.

lb 210 510 TRANSIT ANGLE (mums) 255655 ZQEDQOE FIG. 4

PRIOR ART FIG. 6

INVENTOR. ROBERT C. SCHMIDT United States Patent U.S. Cl. SIS-5.29 3 Claims ABSTRACT OF THE DISCLOSURE A multicavity klystron amplifier tube is disclosed. The tube includes a plurality of cavity resonators successively arranged along the beam path for electromagnetic interaction with the beam. The cavities are tuned to a fundamental mode of resonance having a frequency below 700 mHz. A plurality of axially aligned drift tubes interconnect successive interaction gaps of the re-entrant cavity resonators. The drift tube tunnels have a characteristic diameter of less than 1.5 radians at the output signal frequency and the gap-defining end portions of the drift tube tunnels are outwardly flared and dimensioned to provide a beam filling factor less than 0.5, whereas the beam filling factor for the preponderance of the remaining portion of the drift tube tunnels is greater than 0.6. In this manner, the beam coupling coeflicient is reduced in the interaction gap to prevent reflection of electrons by the strong R.F. fields in the gap, thus avoiding positive electronic feedback while still maintaining normal gain and efiiciency for the tube.

Heretofore UHF video transmitter tubes, operating in the frequency range of 450 mHz. to 900 mHz. at above 16 kv. beam voltage with power gain in excess of 30 db and 20 kw. output, have been plagued with instability when operating near saturated power levels and/or by an abrupt non-linear increase in gain at intermediate drive power levels. In addition, the swept-carrier pass-band response was found to be abnormally sharp at power output levels from approximately half-power to saturation when operated in a synchronously tuned condition. These non-linearities and instabilities resulted in producing nonlinear contrast and loss of synchronism in the received television picture.

It has been discovered that the non-linear gain and instability was being caused by a reflection of electrons in the beam of the klystron amplifier at the interaction gaps and constituted a positive internal electronic feedback. It has been found that this positive feedback is obtained only in tubes operating at frequencies below 700 mHz. where the cavity resonators are relatively large in diameter as compared to the beam diameter and where the beam-field interaction coupling coefiicient is thus relatively high. It is believed, although the theory is not well understood, that with these relatively high beam coupling coefficients that the slower electrons at the leading edges of the electron bunches of the beam are more strongly acted upon by the electric fields of the interaction gap and are actually turned around by the electric fields in the electronic interaction gap. These reversed electrons are thus bunched in the reverse direction and feed energy back to the input end of the driver section of the klystron amplifier. At certain frequencies this feedback is positive and produces an oscillation in the tube.

In the present invention the drift tube end portions which define the electronic interaction gap are outwardly flared to reduce the gap coupling coeflicient and thus prevent reflection of electrons at the gaps. The drift tube internal diameter is maintained at a value which is less than that of the flared end portions over a preponderance of the length of the drift tube segments to maintain relatively high radio frequency output efficiency and gain. It has been found that in UHF amplifier tubes of the present invention that the instabilities and non-linearities of the output performance have been eliminated while retaining normal efiiciency and gain.

The principal object of the present invention is the provision of an improved UHF multicavity velocity modulation amplifier tube.

Onefeature of the present invention is the provision of a drift tube tunnel having an increased diameter at the interaction gap defining end portions to reduce the beam coupling coefficient, whereby instabilities in the output performance of a high power UHF multicavity amplifier is prevented.

Another feature of the present invention is the same as the preceding feature wherein the axial extent of the increased diameter end portions of the drift tube tunnel is less than one half of the axial extent of the drift tube tunnel between successive interaction gaps, whereby normal amplifier gain and efiiciencyare maintained.

Another feature of the present invention is the same as any one of or more of the preceding features wherein the beam filling factor for the drift tube tunnel is greater than 0.6 over a preponderance of the drift tube tunnel length between successive interaction gaps and is reduced to less than 0.5 at the gap defining end portions of the drift tube tunnel.

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:

FIG. 1 is a schematic line diagram of a prior art UHF klystron power amplifier,

FIG. 2 is a plot of UHF power output versus frequency for the prior art tube and for the tube of the present invention,

FIG. 3 is a plot of UHF power output versus UHF power input for the prior art tube and for the tube of the present invention,

FIG. 4 is an enlarged detail view of a portion of the structure of FIG. 1 delineated by line 44,

FIG. 5 is an enlarged fragmentary detail view of a portion of the tube of the present invention corresponding to that portion of the structure of FIG. 1 delineated by line 5-5, and

FIG. 6 is a plot of modulation coefficient squared versus beam transit angle in radians for the prior art tube.

Referring now to FIG. 1 there is shown the prior art multicavity UHF klystron amplifier 1. The tube 1 includes a conventional electron gun assembly 2 for forming and projecting a beam of electrons 3 over an elongated beam path 4 to a conventional beam collector assembly 5. A plurality of cavity resonators 6 are successively arranged along the beam path 4 for electromagnetic interaction with the beam 3.

7 An input signal to be amplified is fed into the input cavity resonator 6' via input coupling loop assembly 7 and input coaxial line 8. A segmented drift tube tunnel 9, through which the beam 3 passes, communicates between successive cavity resonators 6. The drift tube tunnel segments 9 extend re-entrantly and axially into each of the resonators 6. The mutually opposed ends of the drift tube tunnel segments, within each of the cavity resonators 6, defines the electronic interaction gaps 11 therebetween.

The input signal applied to the input cavity resonator 6' excites resonance ofthe input cavity 6' and develops an alternating electric field across the input gap 11. The electric field of the gap 11 velocity modulates the beam 3. In the succeeding drift tube tunnel segment 9 the imparted velocity modulation is converted in the drift space to current density modulation which excites the next two driver cavities 6". These two succeeding driver cavities further velocity modulate the beam 3 which velocity modulation is converted in the drift tunnel 9 into increased current density modulation of the beam 3 as the electrons move toward the collector 5.

In the output cavity resonator 6", the current density modulation of the beam 3 serves to excite the cavity to produce an amplified output signal which is extracted from the output cavity via output coupling loop 12. The output signal is then fed to a suitable load such as a transmitting antenna, not shown, via output coaxial line 13. A solenoid 14 surrounds the tube 1 for producing an axial magnetic field B which confines the electrons of the beam to the desired beam path 4. Capacitive tuning plates 15 bridge the gaps 11 within the cavities 6 for mechanically tuning the operating frequency of the tube to certain frequencies over a band of frequencies such as, for example, from 470 mHz. to 560 mHz.

In a typical example of a prior art tube of FIG. 1 the electron gun 2 produced a beam 3 having a beam voltage of 18 kv. and 4.8 amperes with a perveance of 2X1O The length of various gaps 11 were dimensioned within the range of 0.8 to 1.5 radians of beam transit angle at the center band operating frequency of 515 mHZ. for the tube 1.

The cathode emitter 17 had an emission density of 0.8 amps/cm. of emitting surface. The emitted electrons were converged by an area convergence of 4 into a drift tube tunnel 9 diameter of 0.875" with a beam filling factor for the drift tunnel 9 of 0.7, Where filling factor is defined as the beam diameter D divided by the drift tube tunnel diameter D,. The characteristic drift tube tunnel diameter of 0.875 was approximately equal to one radian at the center band operating frequency and beam voltage.

Tunnel and gap diameters for most klystron amplifiers operating above 700 mHz. fall in the range of 1.75 to 2.5 radians, but smaller gaps 11 generally yield more effective coupling between the beam 3 and fields of the cavity gap 11 (higher beam modulation coefficient) which leads to high gain and efficiency.

The cavities 6 were cylindrical with an inside diameter of 8.00" and a length of 5.4". The drift tube tunnel segments 9 were of copper and had an outside diameter of 1.475" with a wall thickness of 0.300". The drift tube segments 9 in between adjacent gapsll of the first and second and second and third cavities had lengths of 5.1", whereas the last drift tube segment between the third cavity gap 11 and the output cavity gap had a length of 4.6. The gap defining end portions of the drift tube segments 9 were beveled at 60 on their outside surfaces to form knife-edge end portions as shown in FIG. 4 and, in addition, the ends were serrated as shown in the third and output cavity resonators to inhibit multipactor.

The problem with the typical prior art tube as above described was that it exhibited instabilities and non-linear gain characteristics at output power levels from approximately half-power to saturation. More particularly, power output performance curves are shown in FIGS. 2 and 3. FIG. 2 shows the power output versus frequency for a saturating drive power level and for a lower drive level. The saturated bandwidth between 1 db points down from the peak output was 6 to 9 mHz. for a synchronously tuned tube (all cavity resonators tuned to the same frequency). However, at absolute saturation the power output became unstable and produced large notches or sharp discontinuities in the power output response, as shown.

This instability is particularly troublesome in a final television video amplifier tube because the synchronizing pulses to be amplified have amplitudes which drive the tube to saturated power levels as of 30 kw. The black level of the output video signal is about 17 kw. Thus, as seen from the plot of FIG. 2, the instability of the amplifier at saturation could cause the synchronizing pulses not to rise above the black level, resulting in loss of sync. in the television receivers.

In addition, at intermediate drive power levels as encountered in use for amplification of the picture information the gain characteristic of the tube departed substantially from the desired linear characteristic as shown in FIG. 3. This non-linear gain is troublesome as it accentuates the black level in comparison to the white level and distorts the picture contrast.

It has been discovered that the aforementioned problems were being caused by a positive electronic feedback along the beam 3. It was found that certain bunched primary electrons of the beam were being turned around by the fields in the gap 11 and reflected back along the beam path to provide an input. It is believed that the slow electrons at the leading edges of the electron bunches of the beam 3 were being acted upon more strongly and turned around by the tightly coupled gap field. In this regard see FIG. 6 wherein it is shown that for a beam filling factor of 0.7 and a beam transit angle of 1.0 radian that the modulation coeflicieut squared M is 0.7 which is relatively high.

Referring now to FIG. 5 it has been found that the instabilities of the tube 1 at saturated power levels and the non-linear gain characteristic are corrected by decreasing the beam filling factor at the gap defining ends of the drift tube segments 9. More particularly, it has been found that by reducing the beam filling factor from 0.7 in the drift tube tunnel 9 to 0.45 at the end of the tunnel 9 that positive electronic feedback with its attendant problems is prevented as indicated by the dotted lines of FIGS. 2 and 3.

As seen from the plot of FIG. 6 a decrease in the beam filling factor to 0.5 at the gap produces a reduction in the modulation coefficient squared M from 0.7 to approximately 0.66 for a transit angle of 1 radian. However, by outwardly flaring the end portions of the drift tunnel 9 at the gaps 11 as shown in the structure of FIG. 5 the effective gap transit angle is also increased due to the gap fields extending further axially into the end portions of the tunnel 9. Accordingly the effective beam transit angle has been increased to approximately 1.6 radians which further reduces the beam modulation coefiicient squared M to on the order of 0.45.

The same bevel angle of 60 relative to a normal to the beam axis is maintained except that the bevel is on the inside surface of the drift tube tunnel 9 instead of on the outside, as before. The beveled end of the drift tube tunnel 9 is serrated, as before, in the gaps 11 of the output and next to last cavity resonator to further inhibit multipactor.

The flared ends of the drift tube segments 9 did not result in any appreciable loss of gain or efficiency and the tube produced 40 db gain at 38% efficiency.

Although all the drift tube end portions which define the gaps 11 in all the cavities may, if desired, be increased in diameter it is not necessary that the input cavity be so modified as the fields in the input cavity are not strong enough to reflect electrons of the beam and even if they were the reflected electrons would not produce a positive electronic feedback.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A multicavity amplifier tube including, means for forming and projecting a beam of electrons over an elongated predetermined beam path, means forming a plurality of cavity resonators tuned to frequencies below 700 mHz. and arranged along the beam path for successive electronic interaction with the beam to produce velocity modulation of the beam, means for extracting an amplified output radio frequency signal from the velocity modulated beam, means forming a series of axially aligned drift tube tunnels through which the beam passes along the beam path, said drift tube tunnel means having a characteristic diameter of less than 1.5 radians at the output signal frequency and beam voltage with mutually opposed end portions of said tunnel means re-entrantly extending into said cavity resonators to define electronic interaction gaps therebetween, and the cross-sectional area of at least one of said gap defining end portions of at least one of said drift tube tunnels being substantially greater than the cross-sectional area of a preponderance of the remaining portion of said one drift tube tunnel thereby forming an enlarged end portion, and wherein said enlarged end portion of said one drift tube tunnel is dimensioned to have a beam filling factor less than 0.5 and a preponderance of the remaining portion of said one drift tube tunnel dimensioned to have a beam filling factor greater than 0.6, whereby the gap coupling coefficient of the gap defined by said enlarged end portion of said one drift tube tunnel is reduced to prevent positive electronic feedback caused instability in the output signal of the tube while maintaining normal gain and efiiency for the tube.

2. The apparatus of claim 1 wherein the axial extent of each one of said enlarged end portions of said drift 1 tube tunnel means is less than 20% of the length of each drift tube tunnel segment.

3. The apparatus of claim 2 wherein said enlarged end portions of said drift tube tunnel segments are outwardly flared to produce the enlarged end portions and wherein the wall thickness of said drift tube tunnel segments is decreased in the direction taken toward the end portions of said tunnel segments.

References Cited UNITED STATES PATENTS 2/1953 Eitel et a1. 315-5.46 7/1956 Van Iperen 315-5.39

US. Cl. X.R.

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