Patent Publication Number: US-8975816-B2

Title: Multiple output cavities in sheet beam klystron

Description:
FIELD 
     This application relates generally to radio-frequency (RF) source, such as a klystron, and more specifically, to klystron that generates a sheet beam. 
     BACKGROUND 
     A klystron is a device that converts the kinetic energy of a direct current (DC) electron beam into radiofrequency (RF) energy. Klystrons have been used in a variety of applications. For example, klystron may be used to provide RF energy to a particle accelerator, such as an electron accelerator, to cause the accelerator to generate a particle beam with a certain desired characteristic. In some cases, the particle beam may be used to produce a radiation beam for treatment or diagnostic purpose. Klystrons may also be used to produce reference signals for superheterodyne radar receivers, and high-power carrier waves for communications. 
     A klystron may include an electron gun, two or more resonant cavities through which the electron beam propagates, and a collector which captures the spent electron beam and dissipates the resultant heat. The simplest klystron has two cavities—an input cavity and an output cavity. In the input cavity, microwave energy excites the cavity resonance. The resultant electric field that is produced in the beam tunnel modulates the DC electron beam. In one half period of the RF wave, the electrons lose energy from the electric field in the resonator and decrease velocity. In the next half period, the electrons gain energy and increase in velocity. The change in velocity is small but the sinusoidal variation in beam velocity causes the electrons to bunch together and produce a sinusoidally varying RF beam current. 
     The output cavity of the klystron may be situated at the position along the beam path where the RF current has reached a desired value, e.g., a maximum value. As the electron bunches pass through the output cavity, they induce currents on the surface of the cavity walls, which in turn produce a resonant mode in the output cavity. The resonant mode produces an electric field that decelerates the electron bunches and converts the electron beam kinetic energy into RF energy. The RF energy is then coupled out from an output cavity at the resonator. In some cases, additional resonant cavities may be placed between the input and output cavity to increase the gain of the klystron, or to modify the frequency response and bandwidth of the device. 
     Existing klystrons produce a cylindrical electron beam with a circular cross section that propagates down a cylindrical beam tunnel and interacts with resonant cavities that are figures of revolution. However, Applicants determine that it may be desirable to have klystrons that produce an electron beam with an elongate cross section. In addition, Applicants determine that it may be desirable to provide more than one output cavities at the klystron that are uncoupled from each other. 
     SUMMARY 
     In accordance with some embodiments, a RF generator includes a structure having an input section, an output section, and an opening extending between the input section and the output section, wherein the output section has a first cavity and a second cavity, and wherein the first and second cavities are spaced apart from each other so that they are electromagnetically uncoupled from each other. 
     In accordance with other embodiments, a method of providing RF energy, includes receiving an electron beam, providing a first RF energy through a first cavity, wherein the first RF energy is generated using the electron beam, and providing a second RF energy through a second cavity, wherein the second RF energy is generated using the electron beam, wherein the first cavity and the second cavity are spaced apart from each other so that they are electromagnetically uncoupled from each other. 
     Other and further aspects and features will be evident from reading the following detailed description of the embodiments, which are intended to illustrate, not limit, the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DAWINGS 
       The drawings illustrate the design and utility of embodiments, in which similar elements are referred to by common reference numerals. These drawings are not necessarily drawn to scale. In order to better appreciate how the above-recited and other advantages and objects are obtained, a more particular description of the embodiments will be rendered, which are illustrated in the accompanying drawings. These drawings depict only typical embodiments and are not therefore to be considered limiting of its scope. 
         FIG. 1  illustrates a klystron in accordance with some embodiments; 
         FIG. 2  illustrates an electron source in accordance with some embodiments; 
         FIG. 3  is a diagram comparing a sheet beam with a circular beam; 
         FIG. 4A  illustrates a distal end of a klystron in accordance with some embodiments; 
         FIG. 4B  illustrates a perspective view of the distal end of the klystron of  FIG. 4A  in accordance with some embodiments; 
         FIG. 5A  illustrates a distal end of a klystron in accordance with other embodiments; 
         FIG. 5B  illustrates a perspective view of the distal end of the klystron of  FIG. 5A  in accordance with some embodiments; 
         FIG. 6A  illustrates four modes that are associated with four output cavities; 
         FIG. 6B  illustrates separation of modes in accordance with some embodiments; 
         FIG. 7  illustrates a system for delivering radiation that includes a klystron in accordance with some embodiments; and 
         FIG. 8  illustrates the radiation source of  FIG. 7  in accordance with some embodiments. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Various embodiments are described hereinafter with reference to the figures. It should be noted that the figures are not drawn to scale and that elements of similar structures or functions are represented by like reference numerals throughout the figures. It should also be noted that the figures are only intended to facilitate the description of the embodiments. They are not intended as an exhaustive description of the invention or as a limitation on the scope of the invention. In addition, an illustrated embodiment needs not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments even if not so illustrated. 
       FIG. 1  illustrates a klystron  10  in accordance with some embodiments. As used in this specification, the term “klystron” may refer to any device that is capable of converting a kinetic energy of a DC electron beam into energy, such as RF energy. The klystron  10  may be a linear-beam vacuum tube for use as an amplifier at microwave or radio frequencies to produce a driving force for a device, such as a particle accelerator. However, in other embodiments, the klystron  10  may be configured to operate in any frequency, such as W-band, X-band, S-band, L-band, etc. Thus, the term “klystron” should not be limited to any particular operating frequency or range of operating frequencies. In other embodiments, the klystron  10  may be used to produce low-power reference signals for superheterodyne radar receivers. In further embodiments, the klystron  10  may be used to produce high-power carrier waves for communications. In further embodiments the klystron may be used as a power source to provide energy for material processing, curing of materials, or cooking. 
     As shown in the figure, the klystron  10  includes a structure  12  having a first end  14  with an input section  15 , a second end  16  with an output section  17 , and a body  18  extending between the ends  14 ,  16 . As used in this specification, the term “input section” may refer to any part of the klystron  10  that includes a component for receiving energy. Similarly, as used in this specification, the term “output section” may refer to any part of the klystron  10  that includes a component for outputting energy. The structure  12  also includes an opening  20  extending between the ends  14 ,  16 , and a plurality of intermediate cavities  22   a - 22   d  arranged in series. Although only four intermediate cavities  22  are shown in the illustrated embodiments, in other embodiments, the klystron  10  may include less than four intermediate cavities  22  or more than four intermediate cavities  22 . In the illustrated embodiments, the resonant cavities  22  are waveguide sections operating at cutoff frequency. Each of the cavities  22  is separated from an adjacent cavity  22  by a drift space area. In the illustrated embodiments, each cavity  22  has an elongate (e.g., rectangular) cross section. The vertical extent  50  of the cavity&#39;s cross section is larger than the horizontal extent  52 , and therefore the resonant frequency of the cavity  22  is determined by the vertical extent  50 . In other embodiments, each cavity  22  may have other cross sectional shapes. 
     An input cavity  24  is provided at the input section  15 , which includes an input  25  (e.g., in a form of a passage way) formed by part of the structure  12  for directing/coupling energy into the input cavity  24  through an opening  28 . Also, a plurality of output cavities  26   a - 26   d  are provided at the output section  17 , which includes a plurality of outputs  27   a - 27   d  (each in a form of a passage way) formed by part of the structure  12  for directing/coupling energy out of the output cavities  26   a - 26   d , respectively, through openings  29   a - 29   d . In some cases, the lumen in each output  27  may be considered to be a part of the corresponding output cavity  26 , in which case, the output cavity  26  would include the space of the output  27 . In the illustrated embodiments, the input section  15  is at the first end  14 , and the output section  17  is at the second end  16 . In other embodiments, the input section  15  and the output section  17  may be located at other positions. 
     The klystron  10  also includes a first magnetic structure  30  and a second magnetic structure  32  located above and below, respectively, the structure  12 . Each of the magnetic structures  30 ,  32  includes a plurality of magnets and polepieces (e.g., iron bars) arranged in a series along the length of the structure  12  in an alternating manner. The magnetic structures  30 ,  32  are configured to provide magnetic field along the length of the structure  12  to thereby confine an electron beam inside the structure  12 . 
     The klystron  10  also includes an electron source  40  (e.g., an electron gun) coupled to the first end  14  of the structure  12 , and a collector  42  coupled to the second end  16  of the structure  12 . The electron source  40  is configured to provide an electron beam  44 , which enters into the opening  20  of the structure  12 . The electron beam  44  is used to produce DC energy, which is converted to RF energy and coupled out from the output cavities  26   a - 26   d . The collector  42  is configured to collect spent electron beam, with reduced energy. In some embodiments, the collector  42  may be a depressed collector, which recovers energy from the beam before collecting the electrons. 
       FIG. 2  illustrates the electron source  40  in accordance with some embodiments. The electron source  40  includes a cathode  60 , an anode  62 , and a voltage generator  64  coupled to the cathode  60  and the anode  62 . During use, the voltage generator  64  provides a differential voltage between the cathode  60  and the anode  62 , thereby generating the electron beam  44 . As shown in the figure, the anode  62  has an elongate opening  68 , and the cathode  60  has a track  66  that is longer in one direction than in an orthogonal direction. Such configuration allows a beam with an elongate cross section to be produced. As used in this specification, the term “elongate” refers to any shape, such as a rectangle, an ellipse, etc., in which one dimension of the shape measured in one direction is longer than another dimension of the shape measured in another direction that is orthogonal to the first direction. Similarly, the term “sheet beam” should not be limited to a beam having a sheet-like configuration, and may refer to any beam with an elongate cross section in which one dimension measured in one direction is longer than another dimension measured in another direction that is orthogonal to the first dimension. 
       FIG. 3  is a diagram comparing a sheet beam with a circular beam that has a same thickness. In the figure, J is the beam current density and B is the magnetic flux density. It should be noted that the illustration is made with reference to some examples of operating parameters for the beam  44 , and that embodiments described herein should not be limited to the examples shown. Providing the beam  44  with an elongate cross section has several advantageous. First, the beam  44  with an elongate cross section provides an increased surface area that supports higher peak and average power. Given the same beam voltage and current, the surface area of the beam tunnel and resonant cavities are larger in a sheet beam klystron than in a round beam klystron that produces a circular beam with a same thickness. Assuming the same beam thickness (height) for both round and elongate beams, then the aspect ratio of the elongate beam is also the ratio of beam areas. Also, for the same beam current, the elongate beam will have space charge defocusing forces (i.e., forces resulted from electrons that are bunched together) that are much less than that of the round beam. This will help reduce the magnetic field required to focus the beam. Further, the beam  44  with the elongate cross section may result in a lower impedance, but may still provide a same power compared to a circular beam with a same thickness. 
       FIG. 4A  illustrates the output section  17  of the structure  12  in accordance with some embodiments. As shown in the figure, the output section  17  includes the four output cavities  26   a - 26   d  with respective outputs  27   a - 27   d . Each of the four cavities  26   a - 26   d  may have a unique resonant frequency. In the illustrated embodiments, each of the cavities  26  has an elongate shape (e.g., a rectangular shape). However, in other embodiments, each of the cavities  26  may have other shapes. In the illustrated embodiments, each output cavity  26  has a rectangular cross section (when viewing the output cavity  26  from a side), with a vertical extent  80  that is longer than a horizontal extent  82 . In other embodiments, each cavity  26  may have other cross sectional shapes, such as a square, a circular, an elliptical, or other customized shapes. Each output  27  has a thickness that is less than the horizontal extent  82  of the cavity  26 . In other embodiments, each output  27  may have a thickness that is the same as the horizontal extent  82  of the cavity  26 . 
     The output cavities  26   a - 26   d  are coupled, via outputs  27   a - 27   d , respectively, to a waveguide  100 , which is configured to transmit RF power from the output cavities  26   a - 26   b  to another device  150 , such as an accelerator. In the illustrated embodiments, the waveguide  100  has a tree configuration. In particular, the waveguide  100  has a plurality of tubes  120   a - 120   d  coupled to respective output cavities  26   a - 26   b . The tubes  120   a  and  120   b  are coupled to tube  130   a , and the tubes  120   c  and  120   d  are coupled to tube  130   b . The tubes  130   a ,  130   b  are, in turn, coupled to tube  140 , which is configured to deliver RF energy to the device  150 . Although only four output cavities  26   a - 26   d  are shown in the illustrated embodiments, in other embodiments, the klystron  10  may have less than four output cavities  26  (e.g., two output cavities  26 ) or more than four output cavities  26 . Accordingly, in other embodiments, the klystron  10  may have less than four outputs  27  or more than four outputs  27 , with the number of outputs  27  corresponding to the number of output cavities  26 . 
     A perspective view of the device of  FIG. 4A  is shown in  FIG. 4B . The component  42  and the waveguide  100  are omitted for clarity purpose. As shown in the figures, the outputs  27  of the output cavities  26  extend towards a top side of the structure  12 . Such configuration may require the magnetic structure  30  to have one or more openings for accommodating the tubes  120   a - 120   d  that connect to the outputs  27   a - 27   d , respectively. 
     In other embodiments, the outputs  27   a - 27   d  of the output cavities  26  may extend towards a side of the structure  12 , such as that shown in  FIG. 5A  (wherein the component  42  and the waveguide  100  are omitted for clarity).  FIG. 5B  illustrates a perspective view of the device of  FIG. 5A . In the illustrated embodiments, because the outputs  27   a - 27   d  extend from the respective lateral sides of the respective output cavities  26 , the waveguide  100  is coupled to the outputs  27  at a lateral side of the structure  12 . Such configuration is advantageous in that it obviates the need to provide opening(s) at the magnetic structure  30  for accommodating the tubes  120  of the waveguide  100 . 
     The klystron  10  is configured to amplify RF signals by converting the kinetic energy in the electron beam  44  into radio frequency power. During use of the klystron  10 , the electron source  40  produces the electron beam  44  with an elongate cross section to form a sheet beam. The electron beam  44  is injected into the opening  20  of the structure  12 , and is transmitted downstream along the length of the structure  12 . A RF signal is fed into the input cavity  24  at or near its natural frequency to produce a voltage which acts on the electron beam  44 , and the structure  12  functions as a high frequency circuit which interacts with the beam  44  of electrons to thereby velocity modulate the electron beam  44 . As a result, electrons that pass through during an opposing electric field are accelerated and later electrons are slowed, thereby causing the electron beam  44  to form bunches at the input frequency, and resulting in current modulation. The resonant cavities  22   a - 22   d  are used to increase the current bunching to a desired level, e.g., a maximum value. The current bunches induce RF currents in the gap of each of the output cavities  26   a - 26   d . The impedance of each of the output cavities  26   a - 26   d  produces a gap voltage, which decelerates the bunched electron beam  44  and converts the beam&#39;s kinetic energy into RF output power. 
     The developed RF energy is then coupled out from the output cavities  26   a - 26   d  via outputs  27   a - 27   d  at the output section  17  of the structure  12 . In particular, the RF output power from the cavities  26   a ,  26   b  are delivered via outputs  27   a ,  27   b  to the tubes  120   a ,  120   b , respectively, which transmit the power to the tube  130   a  to combine the RF power from the cavities  26   a ,  26   b . Similarly, the RF output power from the cavities  26   c ,  26   d  are delivered via outputs  27   c ,  27   d  to the tubes  120   c ,  120   d , respectively, which transmit the power to the tube  130   b  to combine the RF power from the cavities  26   c ,  26   d . The tubes  130   a ,  130   b  in turn deliver the power to the tube  140  to thereby combine the power from the cavities  26   a - 26   d . The combined RF power is then output to the device  150 . The electron beam  44  downstream from the cavities  26   a - 26   d , with reduced energy, is captured by the collector  42  distal to the output cavities  22   a - 22   d.    
     In the illustrated embodiments, the outputs  27   a - 27   d  allow RF power to be separately extracted from each of the output cavities  26 . Thus, instead of developing a single gap voltage that is equal to or greater than the DC beam voltage, the klystron  10  distributes the voltage used to decelerate the beam  44  over several output cavities  26   a - 26   d . Since the ohmic loss in each output cavity  26  is proportional to V 2 /R (wherein V is voltage and R is resistance), splitting the total voltage V t  into multiple cavities (V 1 +V 2 + . . . +V n =V t ) reduces the total ohmic loss (V t   2 &gt;&gt;(V 1   2 +V 2   2 + . . . +V n   2 ). Also, use of multiple output cavities  26  to extract RF power is beneficial in that the individual cavity impedances sum to give a higher total impedance (compared to that of single output cavity) and hence provides a better circuit efficiency. Therefore, the embodiments of the klystron  10  provide a significant advantage in performance over RF source with a single output cavity. When a single output cavity is used to output the RF energy, a large fraction of the output power is consumed in ohmic losses in the output cavity, resulting in a low circuit efficiency for the RF source. This is because the single resonant output cavity results in a high capacitance that reduces the cavity&#39;s impedance, and makes it difficult to develop adequate gap voltage in the gap of the single output cavity without compromising the circuit efficiency. 
     In the illustrated embodiments, the output cavities  26  (and their corresponding outputs  27 ) are spaced apart from each other such that they are electromagnetically uncoupled from each other. Electromagnetically uncoupling the output cavities  26  from each other allows the resonant frequencies of the output cavities  26  to be independent of one another, thereby preventing, or at least reducing, mode competition compared to output cavities that interact with each other. Competing modes are not desirable for the operation of the device  10  because energy generated in the second mode (and higher mode) may be lost and not captured by the device  10 , thereby making the device  10  less efficient.  FIG. 6A  illustrates four modes that are present when the individual cavities are closely spaced (electromagnetically coupled) and the field from an individual gap couples strongly with neighboring gaps. In this case, the cavities  26   a - 26   d  are actually one extended interaction cavity with four possible mode patterns shown graphically in  FIG. 6A .  FIG. 6B  illustrates separation of modes that is resulted from spacing apart the output cavities  26   a - 26   d  so that the field produced in one cavity does not couple to an adjacent cavity. As shown in  FIG. 6B , two output cavities  26  are considered to be electromagnetically uncoupled from each other if they are spaced apart such that at least one of the respective curves  600   a ,  600   b , representing the electric field vector in the direction of electron beam propagation, has a value of 1% or less, or more preferably 0.5% or less (e.g., 0.2%), of the maximum level at the midpoint between two output cavities. In the example, the maximum level is normalized to be 1, and the curve  600   a  has a value of 0.002 at the midpoint between the two output cavities  26   a ,  26   b . In some embodiments, the output cavities  26  are separated from each other by an electron bunch phase value of at least 2π, and more preferably 4π Embodiments of the klystron  10  eliminate the risks (e.g., mode competition, oscillation, reduced efficiency) associated with undesired modes from extended-interaction output circuits. In some cases, providing uncoupled output cavities  26  allows many complicated factors associated with the design of the klystron  10  to be eliminated. 
     It should be noted that the uncoupling of output cavities  26  are suitable for beam with any cross sectional shape, but are especially beneficial for sheet beam. This is because in sheet beam, the impedance (i.e., that is associated with the response of the cavity to the bunches) may be significantly less than that for the circular beam. So providing a plurality of output cavities  26  would allow the device  10  to produce the required impedance to stop the beam. Thus, for the embodiments in which the klystron  10  is configured to generate a sheet beam, the reduced shunt impedance R/Q may make it desirable to use multiple output cavities to achieve sufficient voltage for slowing the beam. On the other hand, the electromagnetically uncoupled cavities may not be necessary in circular beam tubes because their interaction impedance may be high enough that only one cavity is required to decelerate the beam. 
     In some embodiments, the klystron  10  is configured to provide RF energy to an accelerator, in which case the device  150  is an accelerator, or a part of an accelerator. The accelerator may be a component of a medical device. For example, in some embodiments, the accelerator may be a part of a treatment device for delivering a treatment beam, such as x-ray, a proton beam, etc., for treating a patient. In other embodiments, the accelerator may be a part of a diagnostic device for delivering an imaging beam for imaging a portion of a patient. In still other embodiments, the accelerator may be a part of an object inspection device, such as a security system, for scanning object. In further embodiments, the klystron  10  may be used to produce low-power reference signals for superheterodyne radar receivers. In further embodiments, the klystron  10  may be used to produce high-power carrier waves for communications, in which case, the klystron  10  is a part of a communication system. In other embodiments, the klystron  10  may be a part of a radar system. In still further embodiments, the klystron  10  may be a part of a material processing system, e.g., for drying wood, curing ceramics, drying adhesives, cooking food or other industrial heating processes. 
       FIG. 7  illustrates a radiation system  700  that utilizes the klystron  10  in accordance with some embodiments. The system  700  includes a gantry  712  (in the form of an arm), a patient support  714  for supporting a patient, and a control system  718  for controlling an operation of the gantry  712 . The system  700  also includes a radiation source  720  that projects a beam  726  of radiation towards a patient  728  while the patient  728  is supported on support  714 , and a collimator system  722  for controlling a delivery of the radiation beam  726 . The radiation source  720  can be configured to generate a cone beam, a fan beam, or other types of radiation beams in different embodiments. In the illustrated embodiments, the radiation source  720  is coupled to the arm gantry  712 . Alternatively, the radiation source  720  may be located within a bore. 
     In the illustrated embodiments, the control system  718  includes a processor  754 , such as a computer processor, coupled to a control  740 . The control system  718  may also include a monitor  756  for displaying data and an input device  758 , such as a keyboard or a mouse, for inputting data. In the illustrated embodiments, the gantry  712  is rotatable about the patient  728 , and during a treatment procedure, the gantry  712  rotates about the patient  728  (as in an arch-therapy). In other embodiments, the gantry  712  does not rotate about the patient  728  during a treatment procedure. In such case, the gantry  712  may be fixed, and the patient support  714  is rotatable. The operation of the radiation source  720 , the collimator system  722 , and the gantry  712  (if the gantry  12  is rotatable), are controlled by the control  740 , which provides power and timing signals to the radiation source  720  and the collimator system  722 , and controls a rotational speed and position of the gantry  712 , based on signals received from the processor  754 . Although the control  740  is shown as a separate component from the gantry  712  and the processor  754 , in alternative embodiments, the control  740  can be a part of the gantry  712  or the processor  754 . 
     As shown in  FIG. 8 , the radiation source  720  includes an electron beam standing wave accelerator  730 . The accelerator  730  includes an electron source  734  for generating electrons, and a main body  736  coupled to the electron source  734  for bunching and accelerating the electrons. The main body  736  includes a plurality of cavities  738  (electromagnetically coupled resonant cavities) that are coupled in series. The accelerator  730  also includes a plurality of coupling bodies  739 , each of which having a coupling cavity (not shown) that electromagnetically couples to two adjacent resonant cavities via irises or openings. Although the coupling bodies  739  are illustrated as side coupling bodies that are coupled to sides of the main body  736 , in other embodiments, the coupling bodies  739  can be implemented as on-axis coupling cells to reduce the overall profile of the accelerator  730 . During use, the electron source  734  generates electrons  735 , and injects them into the accelerator  730 . The standing wave accelerator  730  is excited by microwave power delivered by the klystron  10  at a frequency near its resonant frequency, for example, between 1000 MHz and 20 GHz, and more preferably, between 2800 and 3000 MHz. The klystron  10  may be any of the embodiments of the klystron  10  described herein. The RF power from the klystron  10  enters one of the resonant cavities  738  along the chain, through an opening (not shown). As a result, standing waves are induced in the resonant cavities  738  by the applied RF energy. The excited accelerator  730  accelerates the electrons  735 , which interact with a target material (not shown) to generate the radiation beam  726 . As shown in the figure, the electron beam  735  is deflected using magnets (not shown) so that it is transmitted towards a desired direction. 
     In the illustrated embodiments, the radiation source  720  is a treatment radiation source for providing treatment energy. In other embodiments, in addition to being a treatment radiation source, the radiation source  720  can also be a diagnostic radiation source for providing diagnostic energy. In such cases, the system  700  will include an imager, such as the imager  800 , located at an operative position relative to the source  720  (e.g., under the support  714 ). In some embodiments, the treatment energy is generally those energies of 160 kilo-electron-volts (keV) or greater, and more typically 1 mega-electron-volts (MeV) or greater, and diagnostic energy is generally those energies below the high energy range, and more typically below 160 keV. In other embodiments, the treatment energy and the diagnostic energy can have other energy levels, and refer to energies that are used for treatment and diagnostic purposes, respectively. In some embodiments, the radiation source  720  is able to generate X-ray radiation at a plurality of photon energy levels within a range anywhere between approximately 10 keV and approximately 20 MeV. In further embodiments, the radiation source  720  can be a diagnostic radiation source. 
     It should be noted that the radiation system  700  may have different configurations in different embodiments, and that embodiments of the klystron  10  may be used with radiation systems that are different from the example shown. 
     Although the electromagnetically uncoupled output cavities  26  have been described with reference to the klystron  10  (which may be considered a type of RF source), in other embodiments, the electromagnetically uncoupled output cavities  26  may be provided for other devices. For example, in other embodiments, the electromagnetically uncoupled output cavities  26  may be parts of a RF source, such as an inductive output tube (IOT), which may or may not be considered a klystron. 
     In further embodiments, the electromagnetically uncoupled output cavities  26 , and/or the sheet beam feature, may be part of an active denial system (ADS), which is a non-lethal weapon that may be used for crowd control. The ADS is configured to direct electromagnetic radiation, such as high-frequency microwave radiation at a certain frequency (e.g., 95 GHz at wavelength of 3.2 mm) toward a person, or persons. The waves excite water molecules in the epidermis to a high temperature (e.g., 55° C.) to thereby cause the person(s) to feel intense pain without injuring the person(s). In some cases, the focused beam can be directed at the person(s) from a distance that is anywhere from 1 yard to 500 yards away. In other embodiments, the focused beam may be directed at the person(s) from a distance that is more than 500 yards away. In some embodiments, the uncoupled output cavities  26 , and/or the sheet beam feature, may be part of a microwave generator for generating high-frequency microwave radiation, wherein the microwave generator is a component of the ADS. The output radiation from the klystron is fed to a high gain antenna such as a parabolic antenna. The antenna focuses the radiation into a narrow beam that can be precisely positioned on target. The advantage of using a sheet beam klystron over the current RF source for ADS is that the startup time for the klystron is related to the time to heat the cathode in the electron gun. This is advantageous over existing ADS sources that require long cool down times for cryogenic beam focusing magnets, in excess of 12 hours before the device is ready to operate. 
     Although particular embodiments have been shown and described, it will be understood that they are not intended to limit the present inventions, and it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present inventions. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense. The present inventions are intended to cover alternatives, modifications, and equivalents, which may be included within the spirit and scope of the present inventions as defined by the claims.