A cascaded magnetron device has an elongate cathode shank extending along its axis and a series of tubular anode elements placed end to end in a linear cascade surrounding the cathode shank along at least part of its length. Each adjacent pair of anode elements is separated by a conductive, annular pin down disc, and the cathode shank has a series of spaced bands of field emitting material separated by non-emitting regions, each band being located within a respective one of the anode elements and spaced inwardly from the ends of that element. Suitable power inputs and magnetic field generators are provided for generating electron emission and oscillation in the interaction zone between each emitting band and the anode element surrounding that band, and suitable extraction devices are provided for extracting power from each of the interaction zones, the arrangement producing phase-locking of the cascaded magnetron bodies.

CROSS-REFERENCES TO RELATED APPLICATION 
This application is related to a co-pending application Ser. No. 07/602,549 
for "Single Body Relativistic Magnetron" by the same applicants, filed 
Dec. 21, 1990. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to cold or field emission cathode 
relativistic magnetron devices. 
2. Description of Related Art 
The conventional magnetron is a well-known and very efficient source of low 
frequency microwaves. Its operating principles have been known since at 
least 1921, and the first pulsed resonant cavity magnetron (3 GHz), built 
by the British in 1940, can be considered the germinal point of modern 
microwave radar. Today, magnetrons can be found in every home possessing a 
microwave oven. 
A typical single body magnetron is a coaxial vacuum device consisting of an 
external cylindrical anode (the positive electrode, which attracts 
electrons) and an internal, coaxial cylindrical cathode (the negative 
electrode, which emits electrons). In many designs, rectangular resonator 
cavities are cut into the anode block in a gear tooth pattern. During 
operation, a constant axial (perpendicular to the plane of the page) 
magnetic field fills the vacuum annulus, and an electric potential is 
placed between the anode and cathode. The number and shape of the 
resonator cavities, and the dimensions of the anode and cathode are 
arbitrary design features which determine the magnetron's frequency and 
operating characteristics. 
Deficiencies in the present high power microwave magnetron technology are 
evident, with the most serious being the inability to generate pulse 
lengths of a microsecond or greater. This is particularly critical for 
increasing the energy per pulse being produced. A magnetron producing 500 
MW for 3 .mu.s would represent an order-of-magnitude increase in energy 
per pulse over the present experimental devices, and is greatly desired 
for practical applications. 
Attempts have been made in the past to achieve higher output power by phase 
locking separate magnetrons. However, magnetrons are historically 
notorious for their inability to be phase locked. Efforts are being made 
to achieve injection phase locking of several distinct or separate 
magnetron bodies having a common master input signal. Efforts have also 
been made to achieve bootstrap phase locking of several distinct magnetron 
bodies arranged side by side in a hexagonal array by energizing them 
simultaneously without a common master input signal, but with pair-wise 
waveguide connections between the magnetrons. The communication between 
the magnetron bodies via the waveguides is tenuous at best in this 
arrangement, and neither approach can be considered a significant solution 
to the phase-locking problem. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide an improved magnetron design 
in which several relativistic magnetrons can be bootstrap phase locked to 
achieve higher output power. 
According to the present invention, a relativistic magnetron device is 
provided which comprises an elongate, cathode shank extending along the 
axis of the device and a plurality of anodes placed end to end in a 
cascade with an annular pin down disc separating each adjacent pair of 
anodes, the anodes surrounding the cathode shank along at least part of 
its length, and the cathode shank having a series of spaced electron 
emitting bands of field emitting material separated by non-emitting 
regions, each band of emitting material being located within a respective 
one of the anodes. A suitable power input or driver for applying an 
electric field between each anode and the cathode is provided, the driver 
impedance being matched to the total impedance of the cascaded magnetron 
units, and a suitable magnetic field generator is provided for creating an 
axial magnetic field of predetermined strength in the annular cavity 
between each anode and the enclosed emitting band of the cathode. 
This arrangement produces a multi-body cascade of magnetrons which are 
phase-locked and in which the output powers can be coherently added. 
The pin-down discs are used to pin down the nodes of any axial mode 
generated by a magnetron unit. In this way, if the magnetrons are properly 
started up in the .pi.-mode, they would behave as autonomous single body 
magnetrons in their lowest axial mode. However, because they share a 
common annular volume, communication exists and the magnetrons should 
phase lock together. 
Preferably, a symmetrical current feed is provided to both ends of the 
cathode shank, using dual, synchronized pulsed power units or dual 
transmission lines from a single power unit. In either case, the total 
impedance of the cascaded magnetron and the drivers or drive units must be 
matched. For example, if each magnetron has an impedance of Z ohms, then N 
bodies in cascade would exhibit an impedance of Z/N ohms. The two pulse 
forming units driving each end of the cascade should each have an 
impedance of 2Z/N ohms. 
In a preferred embodiment of the invention, each anode has an even number 
of resonator cavities facing the cathode, and alternate cavities are 
coupled to suitable microwave extraction devices such as waveguides or the 
like to extract energy from the cavities. The magnetic field generator 
preferably comprises a series of electromagnets or permanent magnets 
placed in the gaps between the output waveguides of adjacent anodes and 
outside the outermost anodes at each end of the cascade.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The drawings illustrate a cascaded relativistic magnetron device 10 
according to a preferred embodiment of the present invention. The device 
basically comprises a plurality of separate tubular anode elements 12 
arranged end to end in a linear cascade with an annular, conductive pin 
down disc 14 separating each adjacent pair of anode elements 12, as 
illustrated in FIG. 1. A cathode shank 16 extends co-axially within the 
anode elements along the length of the device, with opposite ends of the 
cathode shank projecting outwardly from the outermost anode elements and 
mounted in a suitable end supporting structure, for example as illustrated 
in FIG. 4. 
Each anode element of the cascade has a series of identical resonator 
cavities 22 cut into its inner surface (see FIG. 2). Preferably, each 
anode element has the same number of resonator cavities. In the embodiment 
illustrated, the anode elements each have 8 cavities, but a greater or 
lesser number may be used in alternative embodiments. If the number of 
resonator vanes is an integer power of two, an HPM (high power microwave) 
phased array antenna may be conveniently driven by the cascade with a 
minimum of splitters and collateral waveguide plumbing. Alternative 
cavities of each anode element are each cut through to the outside of the 
anode and coupled to an external load via separate extraction devices, 
which in the preferred embodiment illustrated comprise output waveguides 
24 (see FIG. 2). A suitable quarter wave transformer coupling iris 26 
connects the respective alternate cavity to its respective waveguide 24, 
and the waveguide is sealed off to maintain vacuum integrity by means of a 
suitable vacuum tight dielectric window 28. The magnetron is preferably 
designed to operate in the S- band (2.60 to 3.95 GHz), in which case the 
waveguides may comprise standard WR-284 or S-band waveguides, and each 
transformer iris is used to match the impedance between the magnetron 
output vanes or cavities and the waveguide. However, the magnetron may 
alternatively be designed for operation at other frequencies. 
Annular cavities 30 are defined between each anode element and the area of 
the cathode shank within that cavity. The cathode is of an anodized, 
non-emitting material and has spaced bands or layers 32 of emitting 
material applied to predetermined regions of its surface, one of the bands 
being located within each of the interaction cavities 30 and spaced 
inwardly from the outermost ends of the cavity. Thus, each emitting band 
32 is separated from the emitting band in the next adjacent cavity by a 
non-emitting gap 34. This, in addition to the separation discs, physically 
separates the interaction area or chamber in each magnetron cavity from 
the next adjacent magnetron cavity in the cascade. In the preferred 
embodiment of the invention, the emitting band is of a cathode material 
having a non-smooth, fibrous or fuzzy surface texture which is bonded to 
the cathode shank in the desired areas. One suitable material is a 
graphite felt cathode material as produced by Quantum Diagnostics, Ltd. of 
Hauppage, New York. This material will ignite at relatively low electric 
field stresses and has a relatively high current density, improving 
operation efficiency of the magnetron, and also has a relatively long shot 
lifetime of several hundreds of shots or ignitions before it must be 
replaced. By spacing the emitting inwardly from the ends of the adjacent 
magnetron cavities, the cavities can be linearly assembled and undesirable 
end effects can be reduced or avoided. 
The projecting end portions of the cathode shank are surrounded by co-axial 
waveguide members 35 of conductive material which project outwardly from 
the outer end of each of the outermost anode member. The members 35 have 
annular projecting rings or flanges 36 at their innermost ends which form 
end caps for the outermost ends of the resonator cavities of the outermost 
two anode elements, as best seen in FIG. 1. The waveguide members 35 each 
have a flared outer end portion 37 which is secured in the end supporting 
structure, as described below in connection with FIG. 4. The annular 
waveguide endspaces 38 defined between the projecting ends of the cathode 
shank and the surrounding waveguide members are designed to be in cut-off 
at the operating mode of the magnetron. In a preferred embodiment of the 
invention, the waveguide members are designed to be in cut-off at the 
desirable .pi.-mode. In a magnetron, various resonant modes occur, and the 
magnetron should be designed such that only one mode of oscillation, or 
resonant frequency, is dominant. Preferably, this should be the .pi.-mode 
since this provides the most stable operation. Thus, in the illustrated 
embodiment, other modes having frequencies greater than cut-off will 
escape the magnetron cavity by propagating into the end spaces. Any modes 
which cannot propagate into the end spaces will be trapped in the 
magnetron cavity and the magnetron resonates at this frequency. The end 
spaces are sealed off to maintain vacuum integrity within the magnetron, 
by means of suitable end structures, for example as illustrated in FIG. 4. 
The desired axial magnetic field is provided in the magnetron cavities via 
permanent magnets or electromagnets 40 which are located in the spaces 
between the output waveguides of adjacent magnetron elements and larger 
magnets 42 located outside the waveguides at the outermost ends of the 
magnetron cascade. The magnetic field is designed to be constant in the 
annular interaction spaces 30 between each anide element and the 
respective cathode emitting band within that anode element. 
A suitable electric field is applied between the anode elements and cathode 
in order to induce electron emission. The magnitude of the electric and 
magnetic fields required for magnetron operation can be estimated 
theoretically in a similar manner to the single body magnetron as 
explained in our co-pending application Ser. No. 07/602,459 entitled 
"Single Body Relativistic Magnetron", filed Dec. 24, 1990. In the 
preferred embodiment of the invention illustrated, the anode elements are 
connected to ground while a symmetrical current input is applied at 
opposite ends of the cathode shank via input leads connected to respective 
synchronized pulsed power units or drivers 44, 46, as illustrated 
schematically in FIG. 1. The impedance of units 44, 46 is matched to that 
of the cascaded magnetron. Thus, where each element of the cascade has an 
impedance of Z ohms, and there are N magnetron elements in the cascade, 
the total impedance will be Z/N, and the two pulse forming elements 
driving each end of the cascade should each have an impedance of 2Z/N. 
Preferably, the mounting structure at opposite ends of the magnetron 
includes a radial voltage grading structure for reducing the risk of 
arcing or voltage breakdown, as best illustrated in FIG. 4. This structure 
is the same as that described in our co-pending application Ser. No. 
07/602,549 referred to above entitled "Single Body Relativistic 
Magnetron", and basically comprises an outer wall or sleeve 50 of 
metallic, conductive material, secured to the outer end of each of the 
endspace waveguide members and surrounding the cathode shank, and annular 
ring 52 of plastic material mounted on a flared portion 53 at the end of 
the cathode shank and extending between the cathode shank and the outer 
wall. The ring 52 has a saw tooth pattern on its inner face, and separates 
the magnetron vacuum chamber from an oil chamber 54 located within the 
outer wall outside ring 52, and sealed by a suitable end cap (not 
illustrated). 
The dimensions of each of the magnetron elements in the cascade will be 
selected according to the same criteria as for the single body magnetron 
described in our co-pending application Ser. No. 07/602,549 referred to 
above. The following description represents the design considerations for 
such a single body relativistic magnetron which would be suitable as one 
element of the cascade structure described above. Magnetron operation 
begins when an electric potential is applied between the electrodes. The 
magnetic field acts to insulate the electrodes by confining the electrons 
to the annular region inside the magnetron. The circular motion of 
electrons in the crossed electric and magnetic fields stimulates 
electromagnetic oscillations in the cavity, particularly when the velocity 
of the electrons matches the phase velocity of one of the normal mode 
components. The radiation thus formed is coupled via the waveguides from 
the magnetron cavity. The resonant frequencies of a magnetron can be 
calculated by the standard admittance matching technique, in which the RF 
admittance of the interaction space between the anode and cathode is set 
equal to the RF admittance of the resonator vanes at their common 
interface (see, e.g., Microwave Magnetrons edited by G. B. Collins, MIT 
Radiation Laboratory Series Vol. 6 (McGraw Hill, New York, 1948), for 
standard magnetron design theory. 
The cavity fields can be derived by ignoring the presence of electron space 
charge. Assuming a standard (r,.phi.,z) cylindrical coordinate geometry of 
infinite length, there r is the radial coordinate in a standard 
cylindrical coordinate system .phi. is the azimuthal coordinate, and z is 
the axial coordinate, the fields will have nonzero components E.sub.r, 
E.sub..phi., and B.sub.z, where E.sub.r is the radial electric field, 
E.sub..phi. is the azimuthal electric field, and B.sub.Z is the axial 
magnetic field, as illustrated in FIG. 3. Boundary conditions require 
E.sub.100 =0 on the cathode, and zero everywhere on the anode block except 
where there are gaps, when the field is allowed to have a uniform 
amplitude E. The field varies in phase from gap space to gap space, with a 
phase difference between adjacent gaps of 2.pi.n/N radians, n and N being 
integers. N is the total number of vane gaps, and in the nomenclature of 
magnetron mode identification, n is the mode number. As shown in FIG. 3, d 
is the depth of a cavity while w is the width. In addition r.sub.a and 
r.sub.c are the radii of the anode and cathode respectively. 2.theta. is 
the angle subtended by the gap space between adjacent anode segments, and 
h is the magnetron height. 
A standard technique in boundary value problems is to use a basis set of 
orthogonal functions which satisfy the wave equation. In this case, a 
combination of Bessel and Neumann functions forms a useful basis set 
Z.sub..gamma., defined as: 
##EQU1## 
The wavenumber k=.omega./c where .omega.is the electromagnetic mode 
frequency and c is the speed of light. J.sub..gamma. is a Bessel function 
of the first kind, order .gamma., and Y.sub..gamma. is a Bessel function 
of the second kind (Neumann function), order .gamma., J'.sub..gamma. is 
the derivative of J.sub..gamma. with respect to its argument, and 
Y'.sub..gamma. is the derivative of Y.sub..gamma. with respect to 
argument. For each mode number n, the angular harmonics can be combined to 
satisfy the imposed boundary conditions (see Collins, supra, p. 65): 
##EQU2## 
In the summation, the index .gamma.=n+mN. .THETA. is the half angle 
subtended by the gap space between segments of the anode block. 
.epsilon.is the permitivity of free and .mu. is the magnetic permeability 
of free space. t is the time coordinate, and i is the square root of -1. E 
is the electric field in the anode gap. 
The solution is not complete because the fields in the side cavities must 
be matched to the interaction region fields at the gap space. The fields 
in the vanes are: 
##EQU3## 
The vane coordinates are such that z is along the magnetron axis, and x' 
measures depth into the vane. The orthogonal axis y' is aligned with the 
direction of .phi.. H.sub.z is the axial magnetic intensity. 
As one might expect, the fields will match only at particular frequencies 
which are resonances of the system. The frequencies are found by setting 
the RF admittance of the interaction space equal to the RF admittance of 
the vanes at their common interface. The RF admittance is expressed as a 
spatial average of the Poynting flux, giving the following dispersion 
relation: 
##EQU4## 
The height of an individual magnetron in the cascade is denoted by h. This 
transcendental equation for the frequency is usually solved graphically by 
plotting both the admittances of the interaction space and the vanes (the 
left and right hands sides of the dispersion equation) as a function of 
frequency; points where the lines intersect give .omega.. There are an 
infinite number of resonances for each mode number n, but only the lowest 
ones will be important. 
In one particular example, each anode element had a length of 10 cm while 
the length of the emitting band within the anode element was 8 cm, so that 
it was spaced inwardly 1 cm from each end of the anode element. The length 
of each anode element must be greater than the width of the waveguide 
used, so that the space is sufficient to allow waveguide extraction from 
each of the magnetron bodies in the cascade. However, the anode elements 
are still short enough to avoid higher order axial mode competition in the 
.pi.-mode. The cathode shank had a radius of 1.75 cm while the anode inner 
surface had a radius of 4.61 cm, resulting in an anode to cathode 
separation of 2.86 cm. The anode had 8 vanes or resonator cavities, and 
each cavity had a depth of 1.75 cm and a width of 1.34 cm. With this 
arrangement, a relatively large anode to cathode gap is provided, to 
increase the output pulse length, and at the same time the cathode radius 
is relatively large to provide a large emitting surface area. At the same 
time, the coaxial endspace waveguides, which are of the same radius as the 
anode inner surface, will be in cut-off at the .pi.-mode frequency. Thus, 
frequencies higher than the .pi.-mode are allowed to leak out of the end 
spaces, further establishing the dominance of the .pi.-mode. 
The angles subtended by the resonator cavities and the gap between adjacent 
cavities are approximately equal with this design. 
Each magnetron has its own characteristic scaling parameters which are 
functions of the magnetron dimensions and operating frequency (see G. B. 
Collins, ed., Microwave Magnetrons, MIT Radiation Laboratory Series Vol. 
6, McGraw-hill, New York, 1948, page 416). A single chamber of the 
cascaded magnetron illustrated in the drawings conforms to a 
well-established magnetron known as the 2J32 magnetron, which has the same 
number of vanes and ratio of anode to cathode size, and vane gap to anode 
block spacing. Thus, this can be used to provide a practical operating 
point for the described magnetron, using FIG. 11, page 420 of Collins, 
supra. 
An example of the single body design described in our copending application 
referred to above was tested and demonstrated very clean and stable 
.pi.-mode operation at 3.148 GHz, with an estimated power conversion 
efficiency of 35%. This magnetron produced a power output of 125 MW at 680 
kV in testing, and had a typical pulse length of 80 ns. In the cascaded 
design, if the magnetrons are properly started up in the .pi.-mode, they 
will behave as autonomous single body magnetrons in their lowest axial 
mode, but because they all share a common annular volume, communication 
exists and they should phase lock together. Thus, their output powers can 
be coherently added. With this arrangement, a four body cascade can 
potentially produce at least 1 GW of RF power for microsecond (1 kJ per 
pulse) into sixteen waveguides, with a power conversion efficiency 
estimated to be at least 35%. This combination of peak power, pulse 
duration, and efficiency has not previously been offered by any high power 
microwave source. 
The cascaded magnetron differs from simply lengthening the axial dimension 
of a single body magnetron, which would result in competition between 
axial modes, with each mode absorbing its portion of the injected 
electrical power, and reduction in efficiency. In the cascaded magnetron, 
the pin down discs between adjacent cavities enforce the operation of the 
cascade at the lowest per-body axial mode, by pinning down the nodes of 
any axial mode. If the magnetrons are started up properly in the 
.pi.-mode, they will behave as autonomous single body magnetrons in their 
lowest axial mode. This forces the axial mode 60 into the lowest operating 
mode of one standing half wave per body of the cascade, as illustrated at 
60 in FIG. 1. However, because communication exists between the annular 
cavities, the magnetron bodies should phase-lock together. The cathode 
emitting areas are spaced inwardly from the ends of the anode and thus 
from the pin-down discs, reducing or eliminating the risk of arcing to the 
pin-down discs. 
This arrangement permits intimate communication between different magnetron 
bodies linked in a cascade, since the cascade is intrinsically one body 
composed of many linked chambers. Thus, phase locking of several bodies 
and corresponding higher total output power for the same input can be 
achieved with this design. 
The relatively low operating voltage (around 600 kV), high impedance, and 
high efficiency of this magnetron design permits the use of militarily 
compact, transportable and rugged modulators as the power input. Other 
high power microwave sources typically operate at voltages of the order of 
1 MV, requiring larger enclosures to handle insulation and breakdown 
problems. They usually have much lower efficiency, of the order of 10% or 
less, with size and weight penalties on the modulator to achieve the same 
output power levels. Also, they typically require powerful magnets to 
operate. The magnetic fields required for their operation are typically 
greater than 1.5 T, so that permanent magnets cannot be used. The use of 
electromagnets to generate such fields requires the investment of 
significant energy, reducing the utility of such high power microwave 
sources. 
In contrast, the cascaded magnetron can operate efficiently with relatively 
low operating voltages and magnetic field strengths, allowing the use of 
permanent magnets or electromagnets and reducing power consumption. 
Although a preferred embodiment of this invention has been described above 
by way of example only, it will be understood by those skilled in the 
field that modifications may be made to the disclosed embodiment without 
departing from the scope of the invention, which is defined by the 
appended claims.