US2994800A

US2994800A - High-power, high-frequency amplifier klystron tube

Info

Publication number: US2994800A
Application number: US11535A
Authority: US
Inventors: Richard F Lazzarini
Original assignee: Eitel Mccullough Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1960-02-29
Filing date: 1960-02-29
Publication date: 1961-08-01
Anticipated expiration: 1978-08-01

Description

1961 R. F. LAZZARINI 2,994,800

HIGH-POWER, HIGH-FREQUENCY AMPLIFIER KLYSTRON TUBE Filed. Feb. 29. 1960 2 Sheets-Sheet 1 I? INVENTOR. RICHARD I.- LAZZAR/N/ BY Mk 8% {U r W ATTORNEYS 1961 R. F. LAZZARINI 2,994,800

HIGH-POWER, HIGH-FREQUENCY AMPLIFIER KLYSTRON TUBE Filed Feb. 29, 1960 2 Sheets-Sheet 2 42 t 44 lull 3 l CBS:

420 3 EH29 i 24 H11 F M 1 EL g. Z

INVENTOR. RICHARD F LAZZAR/N/ A 7' TORNEYS United States Patent HIGH-POWER, HIGH-FREQUENCY ANIPLIFIER KLYSTRON TUBE Richard F. Lazzarini, San Jose, Calif., assignor to Eitel- McCullough, Inc., San Bruno, Calif, a corporation of California Filed Feb. 29, 1960, Ser. No. 11,535 4 Claims. (Cl. '31'55.53)

This invention relates to a high-power high-frequency (H.F.) amplifier klystron tube and, more particularly, to a means for coupling high-frequency electromagnetic energy from the output cavity of the klystron.

A klystron tube comprises an electron gun for generating an electron beam which travels through a drift tube to a collector. The drift tube is divided into spaced sections to form interaction gaps therebetween and each gap is surrounded by a resonant cavity coaxial with the drift tube. An amplifier klystron has two or more interaction gaps and therefore two or more resonant cavities. The cavity nearest the electron gun is the control, or input, cavity and velocity modulates the electrons in the beam in relation with the input signal to form electron bunches. The cavity nearest the collector is the output cavity and is excited to an amplified power level of electromagnetic energy by the electron bunches passing therethrough. Output energy levels above one megawatt at frequencies above 400 megacycles per second have been obtained using amplifier klystrons. The amplified electromagnetic energy is coupled from the output cavity to a load.

Klystrons that have their resonant cavities and tuning mechanism outside the vacuum envelope have an advantage over klystrons that have their resonant cavities and tuning mechanism within the vacuum envelope in that, if the tube fails, only the vacuum portion is required to be replaced while the cavities and tuning mechanism may be reused. A klystron having external cavities thus requires that tubular dielectric vacuum walls be provided surrounding the interaction gap between adjacent drift tube sections. In high-power klystrons the vacuum walls usually comprise tubular alumina sections. Electromagnetic energy may be coupled from the output cavity through a coupling loop protruding through the wall of the exterior portion of the cavity or through an iris or window in the cavity wall.

When no power is extracted from the output cavity of a klystron, the electromagnetic (magnetic and electrostatic) fields in the cavity are symmetrical about the tube and cavity axis. The tubular dielectric section, being coaxial with the tube axis, is located in a uniform field region. When power is extracted from the cavity by the coupling loop which is located in the exterior portion of the cavity or by a window which is located in the exterior portion of thecavity, the symmetry of the electromagnetic fields. in the cavity is distorted. The amount of field distortion isproportional to the amount of power extracted. When a high-power tube is heavily loaded, the electromagnetic fields in the cavity become highly distorted and the distorted electrostatic fields across the dielectric become large in some areas and low in other areas, producing large thermal stresses in the dielectric which, in some cases, may cause the dielectric to crack. Then, in other cases, when a heavily loaded high-power klystron is suddenly mismatched to its load, whereby the conductance through the coaxial line or waveguide from the coupler approaches infinity, the relatively high electromagnetic fields that are formed in the cavity also causes the ceramic to crack. The ceramic cracks in these cases because the large electrostatic fields increase the losses therein and overheat the ceramic. The mismatched load effect, or the latter effect, is much larger than the disice torted electromagnetic fields efiect. One method of preventing ceramic failure in the latter cases is to have a cavity with a relatively high internal resistance so thatv a klystron which does not distort the fields within theresonant cavity.

It is another object of the present invention to prevent electrostatic fields of large values in a cavity which happens to be instantaneously mismatched from its load.

It is still another object of the present invention to provide a means for coupling energy from an output cavity of a klystron which means is at the region close to maximum electrostatic field value.

It is yet another object of the present invention to provide a coupling window for an output cavity of a klystron which Window has a fixed opening and a broad frequency response band.

It is a further object of the present invention to provide an improved klystron tube.

In terms of broad inclusion, the improved klystron tube comprises coupling means which is provided by an aperture disposed in the output cavity wall as close to the drift tube as practical so that the aperture is located in a region of relatively high electrostatic fields. The aperture opens into a waveguide, which is disposed trans: verse to and enclosing the drift tube with one end of the waveguide conducting the power to a load and the other. end of the waveguide is shorted. A vacuum wall of dielectric material is disposed within the waveguide to provide a vacuum wall between the vacuum chamber of the tube and the waveguide.

The invention possesses other objects and features of advantage, some of which with the foregoing, will be set forth in the following description of the invention. It is to be understood, of course, that the invention is not limited to the disclosed embodiment but includes other variant embodiments thereof within the scope of the claims.

Referring to the drawings:

FIGURE 1 is a view of a multi-cavity klystron having demountable exterior cavities.

FIGURE 2 is an enlarged sectional view of the output cavity of the klystron in FIGURE 1.

Referring to the drawings in greater detail, andFIG- URE l in particular, there is shown a multi-cavity klystron with an electron gun 10 at one end and a collector 12 surrounded by a water jacket at the other end. Th electron gun 10 directs a beam of electrons along the axis of a drift tube 13 defined by a plurality of aligned drift tube sections which are spaced from each other to form interaction gaps. Around each of the drift tube sections are disposed water cooling jackets 14 and around each of the interaction gaps are disposed tubular ceramic insulators 16 which form a portion of the vacuum wall. To complete the klys-tron, resonant cavities would be mounted around each of the ceramic insulators 16. These cavities are similar to the output cavity 18-which is shown mounted on the klystron. Cavity 18- maybe a cylindrical tube with its axis at right angles to the drift tube 13 axis, or preferably, may be a rectangular tube also oriented with its axis at right angles to the drift tube axis.

Referring to FIGURE 2 the region of the klystron around the output cavity is shown enlarged and in crosssection. The ceramic insulator 16 encloses a working gap 20 formed by drift tube sections 22 and 24. Suitable ceramic-to-metal seals 25 bond the ceramic insulator 16; to two

transverse metal plates

26 and 28 which are part of the electrical circuit of the output cavity 118,.

The cavity 18 is tuned by conventional tuning doors 29. Electrical contact between the doors 29 and the cavity walls is made by contact fingers 30. The doors are actuated by an adjustment nut 31 threaded on a post 32.

The electron gun (FIGURE 1) projects a beam of electrons along the axis to the collector 12. According to well known klystron principles, the electron beam is velocity modulated at the first interaction gap. The modulated electron beam will form bunches and the bunches passing through the working gap 20 form amplified'I-LF. electromagnetic oscillations in the cavity 18. The electromagnetic energy is coupled from the cavity 18 by a novel coupling means which reduces distortion of eitherthe electrostatic fields or the magnetic fields and excessive values of electrostatic fields in the cavity 18, and the novel coupling means provides broadband coupling. 7 V

The novel coupling means is formed by an aperture 33 in plate 26. Aperture 33 is shown with the drift tube section 22 protruding through it and the periphery of the aperture is spaced from the drift tube section to form an annular opening which is continuous 360 around the drift tube section 22 although a non-continuous opening or series of openings less than 360 around the drift tube section may be used without departing from the invention. If theaperture 33 is a single opening less than 360 around the drift tube, the fields in the cavity may not be as symmetrical as when the aperture 33 is an opening continuous 360 around the drift tube. The aperture 33 in the form of a series of openings may or may not give perfectly symmetrical fields because this depends on the location of the openings. But an aperture 33 of any embodiment provides built in resistance for the cavity without any increase in losses (1 R).

The aperture 33 communicates with a waveguide 34 which is disposed at right angles to and crosses the tube axis. The waveguide 34 is preferably of rectangular tubular cross-section, but it may be round tubular crosssection or other convenient shape. Since aperture 33 is located within the vacuum envelope, a suitable vacuum wall is required within the waveguide 34 around the aperture. The vacuum wall is formed by a short tubular ceramic window 36 placed within the waveguide and coaxial with the tube axis. The ceramic window 36 is sealed to plate 26 and to another plate 38 by suitable ceramic-to-metal seals 40 similar to seals 25.

Plates

26 and 38 are part of the electrical circuit of the waveguide. The drift tube 22 is electrically connected to plate 38, and therefore, to one wall of the waveguide, and protrudes into the interior of the output cavity 18 where it is influenced by the H.F. energy. A door 42 with contact fingers 44 is used for shortening one end of the waveguide 34 and for matching the circuit for the different frequencies and thereby providing for a coupling means with a broad frequency band. An adjustment nut 46 that is threaded on post 48 controls the position of door 42.

Energy is believed to be coupled from the cavity 18 to the waveguide 34 by the drift tube section 22, since H.F. electrostatic fields are maximum across the interaction gap 20 and the H.F. magnetic fields are maximum near and'are parallel to the cavity Walls. The tip of drift tube section 22 at the gap 20 is subjected to the H.F. electrostatic fields. The plate 26 is a relatively thin plate in comparison to the length of one-quarter of a wave of the H.F. electromagnetic energy. Therefore the H.F. electromagnetic energy passes through the aperture 33 and is not choked off by plate 26. The drift tube, being at right angles to the waveguide, is believed to act as an antenna and to radiate H.F. energy into the waveguide. Since the drift tube section 22 is located at the center of the symmetrical electromagnetic fields, the novel coupling means according to this invention does not appreciably distort the symmetry of the fields. Also, since the aperture is located in the region of high elec- I 4 V trostatic energy, a broad frequency band coupling is produced by simply moving door 42 toward or away from the drift tube section 22. Therefore, the teachings of this invention are useful in an integral-cavity (a cavity which is evacuated) klystron to obtain the broad frequency band coupling without adjusting the opening of the iris; for example, an integral-cavity can be made by removing the ceramic insulator 16 and applying a suitable vacuum seal such as metal bellows around the post 32 between the door 29 and the walls of the cavity as is common in the art.

When the invention is used with an external-cavity tube, as shown, the insulator 16 does not crack when there is a mismatch in the load. This phenomenon may be explained mathematically. The conductance of the waveguide approaches infinity during mismatch or, since conductance is the reciprocal of resistance, the resistance approaches zero. The reason why the insulator 16 in this invention does not crack is believed to be that when the mismatching occurs, the current (I) in the cavity is large, and since the resistance (R) of the cavity is constant, the voltage (V) across the insulator must increase to a value which may crack the insulator. Therefore, this invention prevents excessive voltage (V) across the insulator 16 because some of the resistance in the waveguide acts as part of (built-in) the cavity resistance and therefore the total effective R of the cavity is much higher.

I claim:

1. A klystron comprising a vacuum-tight envelope, an electron gun for projecting an electron beam through a drift tube, and a collector for collecting said beam, said drift tube comprising at least two sections forming an interaction gap between said sections, a resonant cavity including evacuated and non-evacuated portions enclosing said gap, one of said sections of said drift tube protruding through one wall of said cavity and forming electrical contact between said one section and said one wall whereby no energy is radiated between the two, another of said sections of said drift tube protruding through a second wall of said cavity forming an aperture between said other section and said second wall whereby energy can radiate between the two, and a waveguide section disposed adjacent said second wall with said aperture communicating the interior of said waveguide with said cavity, said other section disposed protruding through said waveguide and forming electrical contact between said other section and the wall of said waveguide opposite said aperture whereby no energy is radiated between the two.

2. A klystron comprising a vacuum-tight envelope, an electron gun for projecting an electron beam through a drift tube, and a collector for collecting said beam, said drift tube comprising at least two sections forming an interaction gap between said sections, a resonant cavity enclosing said gap, one of said sections of said drift tube protruding through one wall of said cavity and forming electrical contact between said one section and said one wall whereby no energy is radiated between the two, another of said sections of said drift tube protruding through a second wall of said cavity forming an aperture between said other section and said second wall whereby energy can radiate between the two, and a waveguide section disposed adjacent said second wall with said aperture communicating the interior of said waveguide with said cavity, said other section disposed protruding through said waveguide and forming electrical contact between said other section and the wall of said waveguide opposite said aperture whereby no energy is radiated between the two, and a tubular ceramic Window hermetically sealed to opposing walls of said waveguide and disposed around said other section with said aperture disposed within said tubular window.

3. A klystron comprising a vacuum-tight envelope, an

electrcngun for projecting an electron beam through a drift tube, and a collector for collecting said beam, said drift tube comprising at least two sections forming an interaction gap between said sections, a resonant cavity enclosing said gap, one of said sections of said drift tube protruding through one wall of said cavity and forming electrical contact between said one section and said one wall whereby no energy is radiated between the two, another of said sections of said drift tube protruding through a second wall of said cavity forming an aperture between said other section and said second wall whereby energy can radiate between the two, a tubular ceramic window connected between said first and second cavity walls coaxial with said drift tube sections, said tubular window constituting part of said envelope and dividing said resonant cavity into a portion internal of the envelope and a portion external of the envelope, tuning means in said external portion, a waveguide section disposed adjacent said second wall with said aperture communicating the interior of said waveguide with said cavity, said other section disposed protruding through said waveguide and forming electrical contact between said other section and the wall of said waveguide opposite said aperture whereby no energy is radiated between the two, and a tubular ceramic window hermetically sealed to opposing walls of said waveguide and disposed around said other section.

4. A klystron comprising a vacuum-tight envelope, an electron gun for projecting an electron beam through a drift tube, and a collector for collecting said beam, said drift tube comprising at least two sections forming an interaction gap between said sections, a resonant cavity enclosing said gap, one of said sections of said drift tube protruding through one wall of said cavity and forming electrical contact between said one section and said one wall whereby no energy is radiated between the two, another of said sections of said drift tube protruding through a second wall of said cavity forming an aperture between said other section and said second wall whereby energy can radiate between the two, a tubular dielectric window connected between said first and second cavity walls coaxial with said drift tube sections, said tubular window constituting part of said envelope and dividing said resonant cavity into a portion internal of the envelope and a portion external of the envelope, tuning means in said external portion, a waveguide section disposed adjacent said second wall with said aperture communicating the interior of said waveguide with said cavity, said other section disposed protruding through said waveguide and forming electrical contact between said other section and the wall of said waveguide opposite said aperture whereby no energy is radiated between the two, and a dielectric window hermetically sealed to the wall of said waveguide to form a portion of said vacuum-tight envelope.

References Cited in the file of this patent UNITED STATES PATENTS 2,372,193 Fisk Mar. 27, 1945 2,667,598 binder Jan. 26, 1954 2,910,614 Bondley Oct. 27, 1959 2,947,908 Orapuchettes Aug. 2, 1960