US3324342A

US3324342A - Traveling wave tube having maximum gain and power output at the same beam voltage

Info

Publication number: US3324342A
Application number: US294564A
Authority: US
Inventors: Charles M Eallonardo
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1963-07-12
Filing date: 1963-07-12
Publication date: 1967-06-06
Anticipated expiration: 1984-06-06
Also published as: DE1491335B2; DE1491335A1; GB1074833A

Description

J1me 1967 c. M. EALLONARDO 3,

TRAVELING WAVE TUBE HAVING MAXIMUM GAIN AND POWER OUTPUT AT THE SAME BEAM VOLTAGE Filed July 12, 1965 2 Sheets-Sheet 1 .E 57.! $1422 i s v, v BEAM VOLTAGE- MIL INVENTOR. CHARLES M. EALLONARDO ATTORNEY J1me 1967 c. M. EALLONARDO 3,324,342

TRAVELING WAVE TUBE HAVING MAXIMUM GAIN AND POWER OUTPUT AT THE SAME BEAM VOLTAGE Filed July 12, 1963 2 Sheets-Sheet 2 INPUT OUTPUT 7 QOOOHOOHHH V BEAM VOLTAGE F:Z .ZA

I INVENTOR.

CHARLES M. EALLONARDO ATTORNEY United States Patent 3,324,342 TRAVELII G WAVE TUBE HAVING MAXIMUM GAIN AND PUWER GUTPUT AT THE SAME BEAM VOLTAGE Charles M. Eallonardo, Sunnyvale, Calif., assignor, by

mesne assignments, to Varian Associates, a corporation of California Filed July 12, 1963, Ser. No. 294,564 7 Claims. (Cl. 3153.6)

This invention relates to electron discharge devices and more particularly to improvements in electromagnetic wave amplifying devices which utilize interaction between a beam of elections and electromagnetic waves guided adjacent the beam of electrons by a low wave structure.

One of the several types of known devices which utilize interaction between a beam of electrons and electromagnetic energy to amplify the electromagnetic energy is the traveling valve tube. A traveling wave tube generally comprises a source of a beam of electrons, a collector for the electrons, and a slow wave structure operatively interposed between the source and the collector for guiding electromagnetic waves adjacent to, and in interaction relationship with, the beam of electrons. By properly adjusting the velocities of the electron beam and the guided electromagnetic wave, they will be in interaction relationship whereby energy is transferred from the electron beam to the electromagnetic wave thereby amplifying the wave. Generally, the velocity of the electromagnetic wave is controlled by the physical dimensions (pitch, period, etc.) of the slow wave structure and the velocity of the electron beam is controlled by a beam voltage applied to the traveling wave tube.

Although it is desirable to operate a traveling Wave tube at maximum power output and maximum gain, this has not been possible with prior art tubes because maximum power and maximum gain occurred at different values of beam voltage. That is, the gain and power output versus beam voltage characteristic curves of previous traveling wave tubes is such that maximum gain is obtained with a lower value of beam voltage than is required to obtain maximum power output. This necessitates that the user of the tube decide whether he desires maximum gain or maximum power output. Further, the slope of the gain curve at maximum power output operating conditions is such that slight changes in beam voltage cause a relatively large change in gain thereby producing unwanted amplitude modulation of the output signal. In order to reduce this amplitude modulation to a minimum, prior art traveling wave tubes have their beam voltage supplied from a very well regulated voltage source.

Accordingly, an object of this invention is to improve electron discharge devices which utilize an interaction relationship between an electron beam and electromagnetic waves.

Another object of this invention is to provide a traveling wave tube having a predetermined gain versus beam voltage characteristic.

Still another object of this invention is to provide a traveling wave tube the gain of which is less sensitive to changes in beam voltage at maximum power output operating conditions.

A further object of this invention is to provide a traveling wave tube having maximum gain and maximum power output at the same value of beam voltage.

A still further object of this invention is to provide a traveling wave tube having a substantially flat gain versus beam voltage characteristic at maximum power output operating conditions.

These and other objects, features and advantages of 3,324,342 Patented June 6, 1967 the present invention are obtained in a traveling wave tube, which utilizes interaction between an electron beam and electromagnetic waves by operatively disposing a unique and novel continuous slow wave structure between a source of a beam of electrons and a collector for the electrons. The novel slow wave structure has a plurality of slow wave sections or portions. Each of the portions or sections are in energy interaction relationship with the beam of electrons and each have distinct uniformly recurrent structures that define distinct and different values of synchronous voltage whereby the gain versus beam voltage characteristic curve for each section intersects or overlaps the gain versus beam voltage curve of at least one other section to form a predetermined gain versus beam voltage curve for the traveling wave tube. For purposes of describing and claiming this invention, the synchronous voltage for any slow wave section or portion is defined as the value of beam voltage that produces the highest gain in the section. The slow wave structure may be formed from a helical conductor or it may comprise a plurality of resonant cavities of the filter type. Although the gain curve is preferably made flat in the area of maximum power output operation, the present invention teaches means for obtaining various predetermined gain versus beam voltage curves.

This invention, as well as other objects, features and advantages thereof will be readily apparent from consideration of the following detailed description relating to the annexed drawings wherein like reference characters designate like or corresponding parts throughout the several views and in which:

FIGURE 1 is a schematic illustration of a traveling wave tube in accordance with one embodiment of the present invention;

FIGURE l-A is a graphic illustration of the gain and power output characteristics of the traveling Wave tube of FIGURE 1 as a function of beam voltage;

FIGURE l-B is a schematic illustration of a traveling wave tube which is a modification of the device in FIG- URE 1;

FIGURE 2 is a schematic illustration of a traveling wave tube in accordance with another embodiment of the present invention; and

FIGURE 2A is a graphic illustration of the gain and output power of the traveling wave tube of FIGURE 2 as a function of beam voltage.

Referring now to FIGURE 1, there is illustrated a traveling wave tube according to one embodiment of the present invention. The traveling wave tube comprises an elongated evacuated envelope portion 11 with a source of a beam of electrons, such as a cathode 12, located at one end of the envelope 11 and a collector 13 for the electron beam located at the opposite end of the envelope. The beam of electrons is indicated by the dashed line 21. Operatively disposed between the cathode 12 and the collector 13 is a helical slow wave structure 14 which lies adjacent the electron beam 21 such that the electron beam 21 passes through the helix. Input and

output terminals

22 and 23, respectively, are coupled to opposite ends of the helical slow wave structure 14 whereby an electromagnetic wave to be amplified is applied to the input terminal 22 and guided adjacent and in interaction relationship with the electron beam 21 by the helical slow Wave structure 14.

The continuous helical slow wave structure 14 contains two equal sections or

portions

15 and 16 each of which have a uniformly recurrent structure, such as a uniform pitch. However, even though each

portion

15 and 16 contains a uniformly recurrent structure, they each have a different value of pitch. For example, the slow wave section 15 adjacent the cathode 12 hasa uniformly recurrent pitch helical slow wave structure 14, where the pitch of the slow wave helical section 16 adjacent the collector 13 is caused to be less than the pitch of the slow wave section 15. The function of the two helical slow wave sections and 16 having ditferent pitch values will 'be described hereinbelow in detail.

A single beam voltage or potential is supplied by the voltage source 18 which has one of its terminals connected to the collector 13 which in turn is electrically connected to the helical slow wave structure 14 by way of the lead or connection 19. The other terminal of the voltage source 18 is coupled to the cathode 12 by way of a lead 20. This beam voltage functions to direct the electron beam 21, which is emitted by the cathode 12, to the collector 13. FIGURE 1 illustrates the beam voltage as being the voltage which exists between the cathode 12 and the helical slow wave structure 14. However, as is well known to those skilled in the art, the beam voltage may be applied in a variety of ways. For example, the helical slow wave structure 14 and/or the collector 13 may be at ground potential and the cathode 12 may be at a negative potential. Also, an accelerating anode (not shown) may be positioned between the helical slow wave structure 14 and the cathode 12 for attracting the electrons from the cathode.

The operation of the device illustrated in FIGURE 1 is such that the beam of electrons 21 produced by the cathode 12 pass through the helical slow wave structure 14 and are collected by the collector 13. An electromagnetic wave to be amplified is applied to the input terminal 22 and is guided adjacent and in interaction relationship with the electron beam 21 by way of the helical slow wave portion 15 and the helical slow wave portion 16 which comprise the helical slow wave structure 14. The amplification of the electromagnetic wave occurs in a well known manner and is coupled out of the traveling wave tube by way of the output terminal 23. The beam of electrons 21 is confined to a narrow area by means of a magnetic field (not shown). Also, conventional attenuation means (not shown) are associated with the helical slow wave structure 14 to suppress backward wave oscillations due to reflected waves. In a traveling wave tube employing the present invention, two

equal length sections

15 and 16 were severed at the point 17 and the two adjacent severed ends were coupled to attenuation material located adjacent point 17. This is possible because the amplitude of the electromagnetic waves being amplified is substantially zero at point 17 due to reflected waves and because the beam voltage 18 was applied to an accelcration anode (not shown) which was located between the slow wave section 14 and the cathode 12.

In accordance with the present invention, the electron beam 21 has associated therewith a slow space charge wave. This space charge wave travels at a rate which is slower but substantially the same as the rate of travel of the electron beam 21. The helical

slow wave portions

15 and 16 are designed to interact with this slow space charge wave to amplify electromagnetic waves applied to the input terminal 22. The Pierces b parameter of each

slow wave section

15 and 16 is such that the phase velocity of the electromagnetic wave applied to the two slow wave portions by way of the input terminal 22 approaches the speed of the slow space charge wave. This is accomplished by properly adjusting the pitch of the helixes in each

helical portion

15 and 16. However, it is to be understood that an adjustment of this parameter of each helical section may be brought about in any other conventional manner. Since the helical

slow wave portions

15 and 16 must contain unlike values of pitch, the Pierces b parameter of both

sections

15 and 16 are also unlike. The amount they difier depends upon the difference in pitch between the

sections

15 and 16. In one embodiment, the pitch of section 15 was no more than 2.0% greater than the pitch of the other section 16. However, differences in pitch up to 20.0% may be utilized, if desired. Also,

since each

section

15 and 16 has unlike values of pitch, they have distinct and unlike values of synchronous voltage. This construction of the helical slow wave structure permits operating the traveling wave tube device of FIG- URE l with a single valued beam voltage that corresponds to maximum power output and maximum gain as is described hereinbelow in detail.

Referring now to FIGURE 1-A, the curve 42 illustrates the power output versus beam voltage for a traveling wave tube which contains the helical slow wave structure of FIGURE 1, or an equal length of a single uniformly pitched slow wave structure (not shown) as heretofore in the prior art has been well known. The dotted curve 41 illustrates the gain versus beam voltage characteristic of a traveling wave tube having a single pitched slow wave helical structure.

With a single pitched slow wave helical structure, maximum gain is obtained for a beam voltage value V that is smaller in magnitude than the value V of beam voltage that is required to obtain maximum power output indicated at the point 46. Accordingly, heretofore in the prior art, a traveling wave tube would be operated with a value of beam voltage corresponding to V or V,, depending on whether the user desired maximum gain or maximum power output, respectively. Also, the slope of the gain curve 41 at maximum power output operating conditions is so steep that slight changes or variations in beam voltage produce corresponding large changes in gain which amplitude modulates the output signal.

These disadvantages have been overcome in the present invention by the two interconnected distinct slow wave portions or

sections

15 and 16 illustrated in FIGURE 1. Each of the

sections

15 and 16 have unlike Pierces b parameter values and each have different values of pitch. The gain versus beam voltage curve of the helical slow wave section 16 is illustrated by the curve 44 and the corresponding curve for the helical slow wave section 15 is illustrated by the curve 43. As is illustrated in FIGURE 1-A, the maximum gain for the helical slow wave portion 16 occurs for a value of V of beam voltage. As discussed hereinabove, the value of beam voltage corresponding to maximum gain is known as the synchronous voltage of a slow wave section. As is well known to those skilled in the art, the synchronous voltage of a helical slow wave section is determined by its pitch. The synchronous voltage V of the helical slow wave structure 15 is of a larger magnitude than the synchronous voltage V of the helical slow wave section 16 because the pitch of the slow wave section 16 is less than the pitch of the slow wave section 15.

The curves 44 and 43 intersect at a point 47 which corresponds to a value V of beam voltage that produces maximum power'output. The gain versus beam voltage characteristic, 44 and 43, respectively, of each of the

slow wave portions

16 and 15 combine algebraically to form a composite gain versus beam voltage characteristic for the helical slow wave structure 14 which is indicated by the curve 45. Reference to FIGURE 1-A shows that for a beam voltage value of V which corresponds to maximum power output operating conditions, the composite gain curve 45 provides maximum gain and is substantially flat for a wide variation of beam voltage. Also, the gain provided is equal to that provided by a conventional helical slow wave structure at maximum power output operating conditions. In one embodiment of the present invention, the beam voltage varied 12% without producing any appreciable change in gain. Accordingly, since the gain of the device of FIGURE 1 is substantially independent of relatively large changes in beam voltage, the supply of beam voltage 18 need not be well regulated and little or no amplitude modulation of the output signal on the output terminal 23 will be present for relatively large changes of beam voltage.

Applicants improved traveling wave tube is not limited to a helical slowwave structure, for other slow wave structures may also incorporate applicants inveniton. For example, FIGURE 1-B is a schematic illustration of a traveling wave tube substantially similar to that in FIG- URE 1 but which utilizes a loaded filter type slow wave guiding structure in place of the helical slow wave structure of FIGURE 1. The slow wave circuit comprises a first portion 26 adjacent the cathode 12 comprising a linear array of cavity resonators formed by an outer cylinder 30 coaxial with the electron beam 21 and containing regularly spaced transverse partitions 28. The partitions are centrally apertured as at 29 to pass the electron beam and are formed with coaxial reentrant portions 31 rimming their central apertures 29 in a well known manner. A second section or portion 27 adjacent the collector 13 substantially similar to the first portion 26, is also provided but it contains resonant cavities of smaller volume than those present in the first section. Each of the two

sections

26 and 27 are slow wave structures having unlike valued Pierces b parameter values and interact with the slow space charge wave associated with the beam of electrons 21 in a manner as described above in conjunction with FIGURES 1 and 1A. Accordingly, each of the two

sections

26 and 27 have separate and distinct values of synchronous voltage, which is determined by their periodic waveguide structure, and intersecting gain versus beam voltage characteristic curves substantially similar to the curves 43 and 44, respectively, illustrated in FIGURE 1A. The operation of the device illustrated in FIGURE 1-13 is substantially similar to the operation of the device shown in FIGURE 1 and therefore need not be repeated.

For some applications of a traveling Wave tube device, it may be desirable that the tube have ,a gain versus beam voltage characteristic which is even more independent of variations in beam voltage than the device illustrated in FIGURE 1A. This may be accomplished in a traveling wave tube in a manner as is illustrated in FIGURE 2 wherein the conductive helical slow wave structure 32 contains three portions or

sections

33, 34 and 35 of equal length. Each

section

33, 34 and 35 has an unlike pitched uniformly recurrent structure which is obtained in a manner as described hereinabove in detail. Each of the slow wave

helical sections

33, 34 and 35 have individual gain versus beam voltage curves 38, 37 and 36, respectively, as illustrated in FIGURE 2-A. These curves combine algebraically to form a composite gain versus beam voltage curve 39 for the traveling wave tube illustrated in FIGURE 2. Reference to FIGURE 2-A shows that the composite gain curve 39 is very flat and maximum gain occurs at a value V of beam voltage which corresponds to maximum power output indicated by the point 46 on the power output curve 42.

It is to be understood that the present invention is not limited to obtaining a flat gain versus beam voltage characteristic at maximum power output operating conditions, for various predetermined gain versus beam voltage characteristics may be obtained by utilizing the principles of this invention. For example, by utilizing different lengths of unlike pitched uniformly recurrent slow wave structures described hereinabove in detail, various predetermined composite gain versus beam voltage characteristic curves may be obtained. Since the maximum gain of any such slow wave section is directly proportional to its length, individual gain versus beam voltage curves, each having difierent maximum gain values, can be obtained to acquire, when combined, various composite gain versus beam voltage characteristic for a traveling wave tube.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention set forth in the appended claims.

What is claimed is:

1. A traveling wave tube comprising: an electron beam source, said electron beam having a slow space charge wave that has a rate of travel substantially equal to said electron beam, a collector for said electron beam, a slow wave structure operatively interposed between said source and said collector for guiding electromagnetic waves adjacent to and in interaction relationship with said space charge wave, said slow Wave structure having a plurality of distinct slow wave sections, each of said slow wave sections being in interaction relationship with said space charge wave and having a uniformly recurrent structure that determines the synchronous voltage of each section, each of said plurality of sections having difierent synchronous voltage values, the synchronous voltage of the sections more proximate the electron beam source higher than the synchronous voltage of the sections more proximate the collector, and a voltage source coupled to apply a beam voltage between the electron beam source and collector, said voltage source adjusted to provide a beam voltage in the range between the synchronous voltage of the interaction section proximate the electron beam source and the synchronous voltage of the interaction section proximate the collector.

2. A traveling wave tube comprising: an electron beam source, a collector for said electron beam means for applying a beam voltage to said traveling wave tube, a slow wave structure operatively interposed between said source and said collector for guiding electromagnetic waves adjacent to and in interaction relationship with said beam of electrons and to provide a predetermined gain versus beam voltage characteristic for said discharge device, said slow wave structure having a plurality of distinct continuous slow wave sections, each of said section characterizer as being in interaction relationship with said electron beam and having a uniformly recurrent structure that determines the synchronous voltage of each section, said plurality of sections having different synchronous voltage values and overlapping gain versus beam voltage characteristic curves, the synchronous voltage of the sections more proximate the electron beam source higher than the synchronous voltage of the sections more proximate the collector, said voltage source adjusted to provide a beam voltage in the range between the synchronous voltage of the interaction section proximate the electron beam source and the synchronous voltage of the interaction section proximate the collector.

3. A traveling wave tube comprising: a source of a beam of electrons, a collector for said beam of electrons, said traveling wave tube adapted to receive a single value of beam voltage, means for obtaining a predetermined gain versus beam voltage characteristic including a continuous slow wave structure operatively interposed adjacent said beam of electrons and between said source of electrons and said collector, said slow wave structure having a plurality of distinct slow wave interaction sections, each said section interacting with said electron beam and having a distinct synchronous voltage whereby the gain versus beam voltage curve for each section intersects the gain versus interaction voltage curve of at least one other section to form a predetermined composite gain versus beam voltage curve for said electron discharge device, the synchronous voltage of the sections more proximate the electron beam source higher than the synchronous voltage of the sections more proximate the collector, and a voltage source coupled to apply a beam voltage between the electron beam source and collector, said voltage source adjusted to provide a beam voltage in the range between the synchronous voltage of the interaction section proximate the electron beam source and the synchronous voltage of the interaction section proximate the collector.

4. A traveling wave tube comprising: an electron source; a collector for said electrons; means for directing a beam of electrons from said source to said collector; said electron beam having at least a slow space charge wave, said space charge wave traveling at a rate substantially the same as said electrons; helical slow wave means adjacent said electron beam and operatively interposed between said electron source and said collector, said slow wave means including a first slow wave section more proximate said electron source and a second slow wave structure more proximate said collector; means for propagating an electromagnetic wave along said first and second helical slow wave sections; the pitch of said first section being selected to provide an interaction between said electromagnetic wave and said slow space charge wave and the pitch of said second section being selected to be less than the pitch of said first section and to provide interaction between said slow space charge wave and said electromagnetic wave, said first section having a synchronous voltage greater than that of said second section; and a voltage source coupled to apply a beam voltage between said electron source and said collector, said voltage source adjusted to provide a beam voltage in the range between the synchronous voltage of the first helical section and the synchronous voltage of the second helical section.

5. A traveling wave tube comprising: an electron beam source; a collector for said electrons; and a slow wave structure operatively interposed between said source and said collector for guiding electromagnetic waves adjacent to and in interaction relationship with said beam of electrons; said slow wave structure including at least first and second slow wave interaction sections; said first section having one end thereof adjacent said source of electrons, having its length in interaction relationship with said electron beam and having a distinct synchronous voltage; said second section having one end thereof adjacent said collector, having its length in interaction relationship with said electron beam and having a distinct synchronous voltage less than the synchronous voltage of said first section; and a voltage source coupled to apply a beam voltage between said electron beam source and said collector, said voltage source adjusted to provide a beam voltage in the range between the synchronous voltage of the first interaction section and the synchronous voltage of the second interaction section.

6. A traveling wave tube comprising: an electron beam source, a collector for said electrons, an elongated helical conductor slow wave structure operatively interposed between said source of electrons and said collector for guiding electromagnetic waves adjacent said electron beam, at least a portion of said slow wave helical conductor adjacent said collector having a uniform pitch and being in interaction relationship with said electron beam, at least a portion of said slow wave helical conductor adjacent said source of electrons being in interaction relationship with said electron beam and having a uniform pitch greater than the pitch of the helix adjacent said collector, said helical conductor portion of uniform pitch adjacent said collector having a synchronous voltage less than the helical conductor portion of uniform pitch adjacent said electron beam source, and a voltage source coupled to apply a beam voltage between the electron beam source and collector, said voltage source adjusted to provide a beam voltage in the range between the synchronous voltage of the helical conductor portion of uniform pitch adjacent said electron beam source and the synchronous voltage of the helical conductor portion of uniform pitch adjacent said collector.

7. A traveling wave tube comprising: an electron beam source, a collector for said electrons, and a loaded filter type slow wave guiding structure operatively interposed between said source of electrons and said collector for guidmg electromagnetic waves adjacent said electron beam, at least a portion of said slow wave guiding structure adjacent said collector being in interaction relationship with said electron beam and having a uniformly recurring structure characterized as having a first distinct synchronous voltage, at least a portion of said slow wave guiding structure adjacent said source of electrons being in interaction relationship with said electron beam and having a uniformly recurrent structure characterized as having a second distinct synchronous voltage, said synchronous voltage of said slow wave guiding structure adjacent said electron beam source greater than said synchronous voltage of said slow wave guiding structure adjacent said collector, and a voltage source coupled to .apply a beam voltage between the electron beam source and collector, said voltage source adjusted to provide a beam voltage in the range between the synchronous voltage of the slow wave guiding structure adjacent said electron beam source and the synchronou voltage of the slow wave guiding structure adjacent said collector.

References Cited UNITED STATES PATENTS 2,851,630 9/1958 Birdsall 315-3.5 2,882,441 4/ 1959 Coulson 3153.6 2,922,920 1/ 1960 Convert 315-306 2,925,529 2/1960 Cutler 315-293 3,092,750 6/1963 Haus et al 315-3.6

HERMAN KARL SAALBACH, Primary Examiner.

R. COHN, Assistant Examiner.

Claims

1. A TRAVELING WAVE TUBE COMPRISING: A ELECTRON BEAM SOURCE, SAID ELECTRON BEAM HAVING A SLOW SPACE CHARGE WAVE THAT HAS A RATE OF TRAVEL SUBSTANTIALLY EQUAL TO SAID ELECTRON BEAM, A COLLECTOR FOR SAID ELECTRON BEAM, A SLOW WAVE STRUCTURE OPERATIVELY INTERPOSED BETWEEN SAID SOURCE AND SAID COLLECTOR FOR GUIDING ELECTROMAGNETIC WAVES ADJACENT TO AND IN INTERACTION RELATIONSHIP WITH SAID SPACE CHARGE WAVE, SAID SLOW WAVE STRUCTURE HAVING A PLURALITY OF DISTINCT SLOW WAVE SECTIONS, EACH OF SAID SLOW WAVE SECTIONS BEING IN INTERACTION RELATIONSHIP WITH SAID SPACE CHARGE WAVE AND HAVING A UNIFORMLY RECURRENT STRUCTURE THAT DETERMINES THE SYNCHRONOUS VOLTAGE OF EACH SECTION, EACH OF SAID PLURALITY OF SECTIONS HAVING DIFFERENT SYNCHRONOUS VOLTAGE VALUES, THE SYNCHRONOUS VOLTAGE OF THE SECTIONS MORE PROXIMATE THE ELECTRON BEAM SOURCE HIGHER THAN THE SYNCHRONOUS VOLTAGE OF THE SECTIONS MORE PROXIMATE THE COLLECTOR, AND A VOLTAGE SOURCE COUPLED TO APPLY A BEAM VOLTAGE BETWEEN THE ELECTRON BEAM SOURCE AND COLLECTOR, SAID VOLTAGE SOURCE ADJUSTED TO PROVIDE A BEAM VOLTAGE IN THE RANGE BETWEEN THE SYNCHRONOUS VOLTAGE OF THE INTERACTION SECTION PROXIMATE THE ELECTRON BEAM SOURCE AND THE SYNCHRONOUS VOLTAGE OF THE INTERACTION SECTION PROXIMATE THE COLLECTOR.