WO1997038436A1

WO1997038436A1 - Single-beam and multiple-beam klystrons using periodic permanent magnets for electron beam focusing

Info

Publication number: WO1997038436A1
Application number: PCT/US1997/005795
Authority: WO
Inventors: George Caryotakis; Robert M. Phillips; Daryl W. Sprehn; Neville C. LUHMANN, Jr.
Original assignee: The Board Of Trustees Of The Leland Stanford Junior University
Priority date: 1996-04-08
Filing date: 1997-04-08
Publication date: 1997-10-16

Abstract

A klystron using periodic permanent magnet arrays (PPMs) (2a, 2b) for focusing the electron beam. The PPMs (2a, 2b) are selected to have magnetic periods that are less than 1/4 of the plasma wavelength of the electron beam for low beam interception, enabling use with high power, pulsed klystrons. The use of permanent magnets reduces power consumption and cost, and provides adjustable field amplitudes along the length of the drift tube (12). In multiple beam klystrons, each electron beam can be individually focused, and a coaxial transmission line (42a, 42b) can be used to provide feedback for self-oscillation.

Description

Single-Beam and Multiple-Beam Klystrons Using Periodic Permanent Magnets For Electron Beam Focusing

RELATED APPLICATIONS

Thi s appl icat ion cl aims priori ty from copending U . S . provisional patent applications 60 / 015 , 204 and 60 / 017 , 167 f i led 04 / 08 / 96 and 5 / 16 / 96 , respectively, which are hereby incorporated by reference .

FIELD OF THE INVENTION

This invention relates to high power, single-beam and multiple-beam klystrons, and, more specifically, to focusing the electron beams in such klystrons with periodic permanent magnet arrays .

BACKGROUND OF THE INVENTION

Klystrons are devices used for the amplification or generation of high frequency electromagnetic energy. They can be made to operate at up to tens of gigahertz and are capable of very high power and high gain (30-60 dB) . Klystrons can operate in cw or pulsed mode. Klystrons have applications in television transmission, radar, and scientific instruments such as particle accelerators.

A klystron comprises an electron gun, an electron beam drift tube, a collector, resonant cavities, and a magnetic field for focusing the electron beam. Electrons accelerated by the electron gun travel down the drift tube where they pass through a first resonant cavity. The cavity resonates at the klystron operation frequency and is externally pumped with input energy. In other words, the energy to be amplified is input to the first cavity. The cavity is aligned with the drift tube such that the resonant electric field in the cavity is alternately parallel and antiparallel with the electron beam. The electrons in the beam are thus alternately slowed and accelerated. This velocity modulation of the beam causes the formerly uniform electron beam to become bunched. The bunched electron beam continues down the drift tube where it similarly interacts with successive resonant cavities that are disposed along the drift tube in predetermined locations. A typical klystron may have 4-6 resonant cavities. These cavities further velocity modulate the beam and are designed to produce an optimally bunched beam. The tighter the bunching, the higher the efficiency of the klystron, but also the greater the radial spreading of the bunches due to the higher charge density of the bunches. At the end of the drift tube is an output circuit which converts the energy of the electron bunches into an electromagnetic wave which exits the klystron through an output waveguide. Sapped of energy, the electron beam is absorbed by the collector which is located behind the output circuit.

As the electron bunches travel down the drift tube, electrostatic repulsion causes them to spread radially towards the drift tube walls. Opposing this tendency to spread is the focusing magnetic field. Without such a field the beam will collide with the walls of the drift tube. Typically, focusing is provided by a solenoid electromagnet wound around the klystron to provide an axial magnetic field (either parallel or antiparallel with the electron beam) . The resistance of the electron bunches to spreading is described as beam stiffness and is a function of the magnetic field of the drift tube. High beam stiffness (greater resistance to spreading) is achieved with high fields and field profiles with higher magnitudes near the walls of the drift tube than at the center. An important consideration in klystron design is preventing the electron beam from colliding with the walls of the drift tube. The percentage of electrons that collide with the drift tube walls is called the beam interception. High beam interception is caused by radial beam spreading which can be a result of low beam stiffness. High beam interception can be very destructive to high power klystrons .

A severe problem with solenoidal magnetic focusing in some applications is that it consumes large amounts of electric power. High power klystrons, such as those used in particle accelerators, may require tens of kilowatts of electricity to provide the necessary focusing magnetic fields. Superconducting magnets, which require very little power, may be used, but they have well known disadvantages of complexity and expense. Cylindrical permanent magnets may be used, but they suffer from high flux leakage and thus have difficulty providing the fields necessary for proper focusing.

Solenoidal focusing has the disadvantage of forcing a tradeoff between low beam interception and low power consumption. The problem is worsened in high power klystrons that require high fields for focusing.

Beam interception is a problem with solenoidal focusing because the magnetic field strength does not increase rapidly with distance from the center of the drift tube. Thus, a high average field is necessary for focusing.

High beam interception problems become worse when solenoidal focusing is used with multibeam klystrons. This is because the drift tubes in a multibeam klystron cannot all be axially symmetric (axisymmetric ) with respect to the focusing solenoidal field. Solenoid magnets are too large to put around each drift tube. When the field and drift tube are not axisymmetric, elaborate measures must be taken to straighten the field to become parallel with the drift tubes. Thus, solenoidal focusing of a multibeam, high power klystron is complex, expensive, and often results in high beam interception and high power costs.

Since the electron bunches become tighter as they travel down the drift tube, the downstream end of the drift tube needs a higher focusing field than the upstream end of the drift tube. This adjustment is commonly performed in single-beam, solenoid-focused klystrons by appropriately shaping the solenoid magnet. This technique cannot be adapted to multiple-beam klystrons, however. Shaping the solenoid necessarily distorts the magnetic field such that the solenoid can then only focus a single axisymmetric electron beam. Individual solenoids cannot be provided for each beam in a multiple-beam klystron because there is not enough space between the drift tubes. What is needed is a focusing technique that is adjustable along the length of the drift tube and does not require much space.

The relatively large weight of solenoid magnets used for high power klystrons is another disadvantage, for obvious reasons.

Another option for electron beam .focusing is periodic permanent magnet (PPM) focusing. PPM focusing uses an array of permanent magnets with alternating magnetic orientations to produce a focusing magnetic field. The focusing field produced by PPM focusing is axial, as in solenoidal focusing, but alternates direction, unlike solenoidal focusing. PPM focusing has been used for years for beam focusing in traveling wave tubes (TWTs) . The problem with PPM focusing is that, in the form used in TWTs, it does not provide focusing adequate : r the beam in a high power klystron.

Long period PPM focusing such as used in TWTs applied to high power klystrons can result in high beam interception because the focusing field is periodic rather than continuous.

Electrons with less energy than a termed stop-band voltage will not be focused by the field and will instead collide with the drift tube walls. The stop band voltage is a measure of the minimum energy an electron must have in order to be focused by the periodic magnetic field. Electrons slowed by velocity modulation, for example, may have less energy than the stop band voltage. Using PPM focusing as applied in TWTs in a klystron will result in a high stop band voltage and therefore a high beam interception. Further, TWT-type PPM focusing will produce a field with only a small field increase from drift tube center to wall, limiting beam stiffness.

Therefore, there exists a need for a beam focusing technique in high power klystrons that does not require large amounts of power, does not require large amounts of space, is lightweight, provides low beam interception, is able to provide adjustable focusing along the length of the drift tube, is relatively inexpensive, and is able to individually focus beams in a multiple-beam klystron.

OBJECTS AND ADVANTAGES OF THE INVENTION

Accordingly, it is an object of the present invention to provide a magnetic focusing technique for single beam and multiple beam klystrons that:

1. does not require a large amount of power,

2. does not require a large amount of space,

3. is lightweight,

4. provides low beam interception,

5. is able to provide adjustable focusing along the length of the drift tube,

6. is inexpensive, and 7. is able to focus the electron beams of a multiple beam klystron individually.

SUMMARY OF THE INVENTION

These objects and advantages are attained by using a PPM array with a short magnetic period to provide a periodic axial magnetic field for focusing the electron beam. The magnetic field period is less than 1/4 of the plasma wavelength of the electron beam. A short magnet period compared to the plasma wavelength minimizes the perturbations caused by nonideal magnets .

Short period magnetic focusing fields also produce low stop- band voltages, thereby reducing beam interception. Also, short periods produce magnetic field profiles that are much higher at the drift tube walls than at the center of the drift tube. This feature further reduces beam interception.

The PPM array may use axially magnetized or radially magnetized permanent magnets to provide the periodic magnetic field. The magnets have polarities with alternating orientations .

The PPM array uses only permanent magnets and thereby requires no power.

The permanent magnets of a PPM array are small and commercially available, and are thereby lightweight and inexpensive. Preferably, the magnets are toroidally shaped. Also preferably, the permanent magnets are made of a high energy product magnetic material such as samarium-cobalt or neodymiu -iron-boron.

The permanent magnets of a PPM array are individually adjustable in size and so can be adjusted to provide desired magnetic field amplitude distributions. For example, larger magnets can be used at the downstream end of the drift tube where larger magnetic fields are needed for focusing.

Since PPM arrays are small and lightweight, an array can be provided for each electron beam in a multiple beam klystron. Thus, each beam in a multiple beam klystron can be individually focused with its own dedicated PPM array. Each focusing magnetic field is then axisymmetric with respect to each electron beam.

In a multiple beam klystron where the drift tubes are closely spaced, neighboring PPM arrays can distort each other's focusing fields. Therefore, it may be necessary to asymmetrically shape the permanent magnets to produce a focusing field that is axisymmetric with each drift tube. Alternatively, a ferromagnetic pole piece and symmetrical magnets can be combined to achieve the same result.

DESCRIPTION OF THE FIGURES

Fig. 1 is a cross sectional side view of a preferred embodiment of a PPM focused klystron drift tube using axially magnetized magnets. Fig. 2 is a cross sectional side view of an alternative embodiment of a PPM focused klystron drift tube using axially magnetized magnets. Fig. 3 is a cross sectional side view of a PPM focused klystron drift tube using radially magnetized magnets. Fig. 4 is a graph of an electrical pulse as applied to a pulsed klystron illustrating the effect of the stop band voltage on beam interception. Fig. 5 is a graph of the RMS axial field amplitude in a specific embodiment of the present invention. Fig. 6 is a graph of the axial magnetic field along the length of the drift tube illustrating the key features of the field. Fig. 7A is a cross sectional view of a multiple beam klystron wherein the magnets are circular and symmetrically positioned around the drift tubes.

Fig. 7B is a cross sectional view of a multiple beam klystron wherein the magnets are shaped to provide a magnetic field axisymmetric with the electron beams.

Fig. IC is a cross sectional view of a multiple beam klystron illustrating an alternative means to provide a focusing field that is axisymmetric with the electron beams. Fig. 8 is a cross sectional side view of a multiple beam klystron illustrating a novel feedback circuit that can be used with PPM focused klystrons. The feedback circuit makes the klystron self-oscillating.

Fig. 9A is a cross sectional view illustrating a technique for proper coupling to a coaxial transmission line used for feedback in a multiple beam klystron.

Fig. 9B is a cross sectional view illustrating an alternate technique for proper coupling to a coaxial transmission line used for feedback in a multiple beam klystron.

DETAILED DESCRIPTION

In all the embodiments of the present invention, the focusing magnetic field along the beam axis is axial, periodic, and varies approximately sinusoidally.

The amplitude (RMS value) of the axial field can vary over the length of the drift tube. In fact, it is beneficial for a klystron to have a larger magnetic field amplitude near the downstream end of the drift tube. The magnetic field period can also change over the length of the drift tube to compensate for the larger amplitude, i.e. , to maintain a substantially constant stop band voltage. These field distributions can be produced with either axially magnetized or radially magnetized periodic permanent magnet (PPM) arrays. The axial magnetic field period is the same as the period of the magnets in the PPM array. The RMS value of the field amplitude should be approximately equal to the solenoidal field required to focus the same beam. It is well known in the art how to focus a given electron beam with a solenoidal field.

In both the axial magnet and radial magnet cases, the magnets are oriented with alternating, opposite polarities, as in all PPM arrays. Axial magnets are magnetized parallel with the electron beam and radial magnets are magnetized perpendicular to the electron beam.

The magnets used are made of high energy product magnetic material, preferably with an energy product of at least IO⁷ Gauss-Oersteds. Samarium-cobalt and neodymiu -iron-boron magnets are examples of magnets that can be used. Design and construction of magnetic circuits based on these materials is well known in the art .

A preferred embodiment that uses axially magnetized toroidal permanent magnets 2a, 2b is shown in the cross-sectional side view of Fig. 1. A drift tube axis 4 along which the electron beam travels is shown. Resonant cavities 8 are also shown. Lines 13, represent the magnetic field. Ferromagnetic pole pieces 10 are located between the magnets 2. In this drawing, the pole pieces 10 and magnets 2a,2b comprise the drift tube 12. Arrows within the magnets 2 indicate their polarity. The pole pieces 10 serve to increase the magnetic flux in the drift tube 12 and provide an axial magnetic field that varies sinusoidally along the length of the drift tube axis 4. The magnetic field will have a radial component in the regions off-axis. Arrows 14 along the axis 4 indicate the direction of the axial magnetic field. The magnets 2b on top of the cavities 8 are made larger or more powerful than the magnets between the cavities 2a. This is necessary to maintain a consistent sinusoidal axial field in the vicinity of the cavity 8 because the cavity 8 necessitates the use of a larger diameter magnet 2b. The diameter difference would otherwise introduce unacceptably large deviations from the preferred sinusoidal axial field. The size difference of the cavity magnets can be completely compensated for. Inconsistent (nonperiodic) variations in the axial field are undesirable because they cause beam interception. It is well known in the art how to adjust the size of the magnets 2b to achieve the desired, consistent sinusoidal axial field.

Fig. 2 shows an alternate embodiment that also uses axially magnetized toroidal permanent magnets 3a, 3b. In this embodiment, the magnets 3a, 3b are arranged such that they are interleaved with the cavities 8. The magnets 3b adjacent to the cavities 8 are made larger or more powerful than the magnets 3a between the cavities 8. This is necessary to maintain a relatively consistent sinusoidal axial field in the vicinity of the cavity 8 because the cavity 8 precludes the placement of a pole piece 10. The absence of a pole piece 10 would otherwise introduce unacceptably large deviations from the preferred sinusoidal axial field. The absence of a pole piece 10 cannot be completely compensated for, but the field distortion can be reduced to insignificant levels. Inconsistent (nonperiodic) variations in the axial field are undesirable because they cause beam interception. It is well known in the art how to adjust the size of the magnets 3b to achieve the desired, consistent sinusoidal axial field.

An embodiment of the invention that uses radially magnetized toroidal magnets 16 is shown in the cross sectional side view of Fig. 3. This figure shows a drift tube axis 4, resonant cavities 8, and arrows 14 that indicate the direction of the axial field. Here, pole pieces 18 are placed under the magnets 16 (between the drift tube 12 and magnet 16) . Because of this geometry, the cavities 8 do not prevent the placement of a pole piece 18 or require a larger diameter magnet. A magnetic return pole piece 20 is placed on the outside of the magnets 16 to complete the magnetic circuit. Radially magnetized magnets can provide the same sinusoidal axial fields as axially magnetized magnets. In some embodiments the return pole piece 20 may not fit over the cavity 8, necessitating a hole in the return pole piece 20. This is roughly equivalent to an absent pole piece in the embodiment of Fig 2. In this case, the magnets adjacent to the cavities 8 must be adjusted in size or strength to produce the required magnetic field

There is a subtle difference between the three embodiments that bears mentioning, although it is inconsequential to electron beam focusing. In the axial magnet case of Fig. 2, the axial magnetic field amplitude is zero in the plane midway between neighboring magnets and a maximum in the plane of each magnet. In the plane 24 bisecting a cavity, for example, there is no magnetic field; this is where the magnetic field reverses . The situation is reversed in the radial magnet case of Fig. 3 and the axial magnet case of Fig. 1. The axial magnetic field is a maximum in the plane midway between magnets and zero in the plane of each magnet. In a plane 26 of Fig. 4 and a plane 27 of Fig. 1 bisecting a cavity, for example, there is a magnetic field maximum. The magnetic field reverses in the plane of the magnets. This difference is simply a result of the different magnet geometries for the three cases.

It is noted that the field ceases to be periodic in the vicinity of the output circuit. A substantially uniform field is created by using a long axially magnetized toroidal magnet or using a combination of axial and radial magnets. Nonperiodic magnetic fields in the output circuit region are commonly used in the art.

An important parameter used in the design of the present invention is the plasma wavelength of the electron beam. The plasma wavelength, λ, is given for a nonrelativistic electron beam by the equation:

ll λ_=V ^{3 / 4} /J¹ ''² ,

where V is the accelerating voltage of the electron beam (operating voltage of the klystron) , and J is the current density of the electron beam. The plasma wavelength should be at least 4 times as long as the magnetic period of the axial magnetic field. The preferred embodiment of the present invention has a plasma wavelength of 10 centimeters or 5 times the magnet period (P), which is 2 centimeters, i.e., λ/P=5.

Conventional traveling wave tubes, by comparison, typically have λ/P=2. To facilitate this requirement, it may be necessary to produce a beam with a long plasma wavelength. This can be done by using a combination of low perveance and high accelerating voltage. The preferred embodiment of the present invention operates at approximately 400-500 kilovolts and has a perveance of .6xl0^~6 for example. Perveance is a commonly used parameter in klystron design and is equal to 1/V³^², where I is the beam current and V is the accelerating voltage.

One benefit of a high λ/P ratio is that it smoothes out the effect of random distortions in the focusing field. Since the electron beam passes through several (approximately 10 in the preferred embodiment) magnetic field reversals per plasma wavelength, the perturbing effect of any single nonideal magnet is greatly reduced. This effect reduces the beam interception and makes the PPM arrays much easier to manufacture.

Another important consideration in klystron design is the stop band voltage. The stop band voltage in a PPM focused electron beam is proportional to both the square of the RMS field amplitude and the square of the magnet period. The stop band voltage is expressed as a percentage of the operating voltage of the device. In TWTs, the stop band voltage is typically 30% of the operating voltage. In the klystrons of the present invention, the stop band voltage is approximately 6% of the operating voltage.

The stop band voltage is an especially important consideration in high power, pulsed klystrons. In a pulsed klystron, the electron beam accelerating voltage is pulsed. Therefore, for a period of time at the beginning and end of each pulse, the accelerating voltage will be less than the stop band voltage. None of the electrons emitted while the accelerating voltage is less than the stop band voltage will be focused. They will all collide with the drift tube walls and contribute to beam interception. This effect is illustrated in Fig. 4. The shaded regions 28 represent the portion of the electron pulse that will not be focused.

A high stop band voltage can also cause beam interception while the klystron is operating at its normal operating voltage. Electrons with an energy greater than the stop band voltage may be slowed below the stop band voltage by velocity modulation and then contribute to beam interception. This mechanism affects both pulsed and cw operation. It can be seen that the higher the stop band voltage, the higher the energy that is absorbed by the drift tube walls.

The stop band voltage, and therefore beam interception, in a PPM focused klystron, would be unacceptably high if λ/P=2, as in traveling wave tubes. By making P as short as possible, the stop band voltage can be reduced to acceptable levels .

In the preferred embodiment of the invention, the RMS axial magnetic field amplitude increases towards the downstream end of the drift tube, i.e., toward the collector. Fig. 5 shows the variation of the RMS axial field value as a function of axial position in the drift tube for a specific embodiment of the present invention. As the electron beam travels down the drift tube (left to right) , the axial magnetic field required for optimum focusing increases. This is because the electron bunches spread radially as they travel. The required increase in the magnetic field amplitude is easily achieved by using either larger size or higher energy product magnets in the downstream region where a higher RMS field is desired. The use of focusing fields that increase towards the downstream end of the drift tube is well known in the art of klystron design.

Since the magnetic field amplitude increases toward the downstream end of the drift tube, the stop band voltage will also increase at the downstream end if all other parameters are equal. To produce a relatively constant stop band voltage along the entire length of the drift tube (which is desired) , the magnetic period is shortened toward the downstream end. The amount of period shortening is commensurate with the amount of magnetic field increase (the amount of magnetic field increase is commensurate with the focusing requirements of the electron beam) . These changes are only introduced at the downstream end because shorter period, higher amplitude fields require larger size, higher energy product (more expensive) magnets.

Fig 6 shows a plot of the axial magnetic field amplitude as a function of position for a PPM focused klystron according to the present invention. The electron beam travels from left to right. This Figure is merely illustrative of the features described above and is not an accurate representation of a working klystron.

With the short period PPM arrays of the present invention, the field amplitude at the drift tube walls can be twice that at the center of the drift tube. The higher field at the walls is able to focus a beam having a current density almost four times that which can be focused on the drift tube axis. In other words, an electron traveling near the drift tube walls experiences a much stronger focusing force than it would at the center of the drift tube. The result is reduced beam interception. The magnetic field profile in the drift tube depends upon the inner diameter of the PPM array compared to the magnetic period. Short magnet periods result in steeper field profiles from drift tube center to drift tube wall, but also reduced on-axis field amplitudes. It will be obvious to one skilled in the art how to adjust the geometry of a PPM array to achieve a desired magnetic field profile.

The diameter of the drift tube is also an important parameter in the design of the present invention because it establishes the inner diameter of the PPM array. A smaller drift tube diameter allows for a smaller P in the PPM arrays for a given desired axial magnetic field amplitude (a purely geometrical magnetic effect) . Excessively small drift tubes can result in higher current densities, which decreases λ and therefore decreases the λ/P ratio.

Another important parameter in PPM klystron design is the cathode immersion. The cathode immersion is the percentage of the magnetic flux that threads the electron beam that also threads the cathode. High cathode immersion techniques as used in solenoid focused klystrons cannot be used in the PPM focused klystrons of the present invention. The electron beam will reflect from the magnetic reversals, resulting in high beam interception. Partial (reduced) cathode immersion solves this problem. In the preferred embodiment of the invention, the klystron is constructed with a cathode immersion of 50% or less. Partial cathode immersion techniques are well known in the art. A preferred technique uses an electromagnet disposed near the cathode for electrically controlling the percentage cathode immersion.

The PPM focusing technique of the present invention is particularly well suited for multiple beam klystrons. This is because a PPM array is small enough to be provided for each electron beam. Thus, each electron beam can be axisymmetric with its focusing field. Further, PPM arrays are lightweight, so a multiple beam klystron can be built at lower cost.

Fig. 7A shows a cross sectional view (parallel with the electron beams) of a multiple beam (six beam) klystron. The electron beams 30 are represented as X's. The magnets are toroidal and symmetrically located around each drift tube 34, as in the single beam case. In most multiple beam klystrons this arrangement will provide adequate focusing. However, some klystrons, if neighboring drift tubes 34 are close together, will experience focusing problems due to the magnetic interactions between neighboring PPM arrays . The focusing field in each drift tube 34 will not be axisymmetric with each beam 30. The magnetic flux will be concentrated on the side of each electron beam 30 facing the middle of the drift tube ring. For proper focusing, the fields must be axisymmetric with each electron beam. A solution is shown in Fig. 7B. Here, each magnet 35 is displaced away from the center of the drift tube ring. This redistributes the magnetic flux, and, for a certain degree of asymmetry, results in an axisymmetric field distribution for each electron beam 30. Generally, there needs to be more permanent magnet material outside the circle of drift tubes 34 than on the inside. The magnets 35 can be elliptical, round or other shapes. It is well known in the art how to shape and position the magnets to produce the desired axisymmetric fields.

An alternative solution to this problem is shown in Fig. 7C. Here, symmetrical magnets are used with a ferromagnetic field shaping piece 36. The field shaping piece 36 is shaped to perturb the magnetic field such that axisymmetric magnetic fields are provided for each electron beam 30.

For some klystron applications, such as high power, it is desirable to make the klystron self oscillating. This is typically done by coupling a small portion of the output energy into the first resonant cavity. Alternatively, the third, fourth, or other downstream cavities can be used as a source of feedback energy. Provisions to control the amplitude and phase of the feedback energy can be included.

Fig. 8 is a cross sectional side view of a novel approach for energy feedback that can be used in PPM focused multiple beam klystrons . This particular embodiment sources feedback energy from the third cavity 38, although in principle any cavity (except the first) or the output can be used. Because PPM arrays do not require much space, there is space between the drift tubes 40 when they are arranged in a circle. The feedback mechanism of the present invention uses a coaxial transmission line 42a, 42b disposed in this space between the drift tubes 40. The inner conductor 42a and outer conductor 42b are shown. Energy from the third cavity 38 enters the coaxial line 42a, 42b through openings 43 and propagates to the first cavity 46, maintaining the self-oscillation. Not shown in Fig. 8 is the collector and the output circuit.

An important consideration in the construction of the transmission line 42a 42b is the coupling of the line 42a 42b to the third cavity 38 and the first cavity 46. The electromagnetic energy must be both sourced and distributed equally among the electron beams 30. This requires that the energy must oscillate in a mode inside the transmission line 42a 42b that results in a uniform energy distribution. The amount of coupling and the mode are influenced by the placement of the openings 43.

Fig. 9A is a cross sectional view in plane 39 (in Fig. 8) illustrating a suitable placement of the openings 43 for proper coupling. Visible are the electron beams 30, the inner conductor 42a, the outer conductor 42b, the drift tubes 40, and the cavity 38, which is shared among the six beams 30. Six identical openings 43 (one for each beam 30) are symmetrically disposed around the outer conductor 42b of the transmission line. Preferably, each opening 43 is facing an electron beam 30, as in Fig. 9A. This alignment of the openings 43 provides for an equal contribution of energy by the beams 30. Alternatively, the openings can be located midway between the beams 30 as shown in Fig. 9B. It will be obvious to one skilled in the art how to adjust the size of the openings 43 to acheive a desired amount of coupling and a uniform energy distribution inside the transmission line 42.

A specific embodiment of the present invention is a 50 megawatt peak power, pulsed klystron operating at 11.424 gigahertz. The magnet period is 2.5 centimeters or less and the drift tube has a diameter of 1 centimeter. The beam voltage is 465Kv and the beam current is 200 amps. The beam diameter is .5 centimeter and is solid. The focusing magnetic field is sinusoidal and has a peak value of 2000 gauss on axis. The peak value of the field increases monotonically with distance down the drift tube to a maximum value of 3000 gauss near the output circuit where it becomes nonperiodic and is maintained at 3000 gauss. The field amplitude is made to increase in amplitude at a rate which prevents ream interception and the period is reduced at a rate which maintains a substantially constant stop band voltage along the length of the drift tube

It will be clear to one skilled in the art that the above embodiments may be altered in many ways without departing from the scope of the invention. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.

Claims

What is claimed is: 1. A klystron comprising an electron beam and an array of periodic permanent magnets surrounding said electron beam, wherein said array produces a periodic magnetic field axial with said electron beam and having a λ/P ratio of at least 4; wherein said electron beam is focused by said periodic axial magnetic field produced by said periodic permanent magnet array.

2. The klystron of claim 1 wherein said electron beam has a plasma wavelength of at least 10 centimeters.

3. The klystron of claim 1 wherein said PPM array has a period of less than 2.5 centimeters .

4. The klystron of claim 1 wherein said klystron is a multimegawatt, pulsed klystron.

5. The klystron of claim 1 wherein said klystron has an operating voltage of approximately 400-600 kilovolts .

6. The klystron of claim 1 wherein said periodic axial magnetic field has a maximum value of approximately 2000 gauss in the upstream end of said klystron and approximately 3000 gauss in the downstream end of said klystron.

7. The klystron of claim 1 wherein said periodic permanent magnets are axially magnetized.

8. The klystron of claim 7 wherein said permanent magnets are positioned such that said axial magnetic field has maximum amplitude at the axial location of the midpoint of said resonant cavities .

9. The klystron of claim 8 wherein said permanent magnets are interleaved with ferromagnetic pole pieces .

10. The klystron of claim 7 wherein said permanent magnets are positioned such that said axial magnetic field has minimum amplitude at the axial location of the midpoint of said resonant cavities.

11. The klystron of claim 10 wherein a subset of said permanent magnets are not in contact with said cavities, and wherein the magnets in the subset are interleaved with ferromagnetic pole pieces .

12. The klystron of claim 1 wherein said permanent magnets closest to said resonant cavities are selected to maintain a substantially consistent RMS axial magnetic field amplitude in vicinity of said resonant cavity.

13. The klystron of claim 1 wherein said periodic permanent magnets are radially magnetized.

14. The klystron of claim 13 wherein said periodic axial magnetic field has maximum amplitude at the axial location of the midpoint of said resonant cavities .

15. The klystron of claim 13 wherein ferromagnetic pole pieces are disposed between said permanent magnets and said drift tube.

16. The klystron of claim 1 wherein said permanent magnets are toroidally shaped and disposed such that said electron beam travels through opening of said toroidally shaped permanent magnets .

17. The klystron of claim 1 wherein said permanent magnets have an energy product of at least IO⁷ Gauss-Oersteds.

18. The klystron of claim 17 wherein said permanent magnets are selected from the group consisting of samarium cobalt magnets and neodymium iron boron magnets.

19. The klystron of claim 1 wherein said klystron operates at a single predetermined frequency in the range of 1-12 Gigahertz .

20. The klystron of claim 1 wherein amplitude of said periodic axial magnetic field increases in the downstream direction to compensate for electron beam spreading.

21. The klystron of claim 20 wherein said periodic axial magnetic field has a shorter period toward the downstream end of said drift tube to maintain a relatively constant stop band voltage.

22. The klystron of claim 1 wherein said periodic axial magnetic field is substantially sinusoidal.

23.A klystron comprising:

A) a plurality of electron beams,

B) an array of periodic permanent magnets surrounding each said electron beam, wherein said array produces a periodic magnetic field axial with each said electron beam and having a λ/P ratio of at least 4;

wherein said electron beams are individually focused by means of said periodic axial magnetic field produced by said periodic permanent magnet arrays.

24. The klystron of claim 23 wherein said electron beams are parallel and arranged in a circle.

25. The klystron of claim 24 further comprising a coaxial transmission line disposed inside said circle of electron beams and parallel with said electron beams, said coaxial transmission line providing feedback to facilitate self oscillation of said klystron.

26. The klystron of claim 25 wherein said coaxial transmission line provides a first resonant cavity with resonant electromagnetic energy.

27. The klystron of claim 23 wherein said periodic permanent magnet arrays are asymmetrically shaped such that they each produce a focusing magnetic field that is substantially axisymmetric with respect to each said electron beam.

28. The klystron of claim 23 further comprising an article of ferromagnetic material of predetermined shape, shaped and disposed in said klystron such that the focusing magnetic field for each said electron beam is substantially axisymmetric with each said electron beam.

29. The klystron of claim 23 wherein said periodic axial magnetic field has a maximum value of approximately 1500 gauss in the upstream end of said klystron and approximately 2500 gauss in the downstream end of said klystron.

30. The klystron of claim 23 wherein said permanent magnets are toroidally shaped and disposed such that each said electron beam travels through the opening of said toroidally shaped magnets.

31. The klystron of claim 23 wherein said klystron is a multi egawatt, pulsed klystron.

32. The klystron of claim 23 wherein said klystron has an operating voltage of approximately 400-600 kilovolts.

33. The klystron of claim 23 wherein said periodic permanent magnets are axially magnetized.

34. The klystron of claim 33 wherein said permanent magnets are positioned such that said axial magnetic field has maximum amplitude at the axial location of the midpoint of said resonant cavities.

35. The klystron of claim 34 wherein said permanent magnets are interleaved with ferromagnetic pole pieces.

36. The klystron of claim 33 wherein said permanent magnets are positioned such that said axial magnetic field has minimum amplitude at the axial location of the midpoint of said resonant cavities.

37. The klystron of claim 36 wherein said permanent magnets not in contact with said cavities are interleaved with ferromagnetic pole pieces .

38. The klystron of claim 23 wherein said permanent magnets closest to said resonant cavities are selected to maintain a substantially consistent RMS axial magnetic field amplitude in vicinity of said resonant cavity.

39. The klystron of claim 23 wherein said periodic permanent magnets are radially magnetized.

40. The klystron of claim 39 wherein said periodic axial magnetic field has maximum amplitude at the axial location of the midpoint of said resonant cavities.

41. The klystron of claim 39 wherein ferromagnetic pole pieces are disposed between said permanent magnets and said drift tube.

42. The klystron of claim 23 wherein said permanent magnets have an energy product of at least IO⁷ Gauss- Oersteds.

43. The klystron of claim 42 wherein said permanent magnets are selected from the group consisting of samarium cobalt magnets and neodymium iron boron magnets .

44. The klystron of claim 23 wherein said klystron operates at a single predetermined frequency in the range of 1-12 Gigahertz.

45. The klystron of claim 23 wherein amplitude of said periodic axial magnetic field increases in the downstream direction to compensate for electron beam spreading.

46. The klystron of claim 45 wherein said periodic axial magnetic field has a shorter period toward the downstream end of said drift tube to maintain a relatively constant stop band voltage.

47. The klystron of claim 23 wherein said periodic axial magnetic field is substantially sinusoidal.