Abstract:
The present invention generally relates to systems and methods for generating controllable beam of electrons using a hollow-cathode triode electron gun that substantially mitigate impact of back-streaming electrons.

Description:
BACKGROUND 
       [0001]    The present invention relates to systems and methods for generating controllable beam of electrons using a hollow cathode triode electron gun that substantially mitigates the impact of back-streaming of the electrons. 
         [0002]    A vacuum electron device (VED), such as a linear particle accelerator or a Klystron, uses a source of an electron beam which is typically known as an electron gun. 
         [0003]    Conventional electron guns are of two types. The first type of electron guns is the diode electron gun which has two electrodes; namely a cathode and an anode. The second type of electron guns is the triode electron gun which has three electrodes; namely a cathode, an anode, and a grid. 
         [0004]    The triode electron gun has operational advantages over the diode electron gun. One advantage is allowing for fast changes in the electron beam current produced by the electron gun. In the case of the diode electron gun, changing the electron beam current is done by changing a high-voltage difference between the cathode and the anode which is normally thousands of volts. In the case of the triode electron gun, changing the electron beam current is done by changing a voltage difference between the cathode and the grid which is normally a few or less than 100 volts. Thus, changing the electron beam current can be done faster and in a more controlled way. 
         [0005]    A major use of a triode electron gun is to supply electron beam current to a linear particle accelerator (Linac). A common problem associated with Linacs is that some electrons entering the Linac&#39;s RF Structure are out of synchronism with the RF (electromagnetic energy) and are reflected back towards the electron gun at accelerated velocities and this is commonly called back-streaming electrons. These back-streaming electrons impact its cathode and raise its temperature. The cathode is normally impregnated with a material, such as Barium, that enhances electron emission by lowering the cathode&#39;s work function. The rise of the cathode temperature increases the evaporation rate of the impregnating material. Over time this same impregnate material adheres to all surfaces that are line-of-sight, mainly the gun&#39;s grid which is directly in front of the cathode&#39;s emitting surface. The grid is kept at a voltage very near the same potential voltage as the cathode and thus sees a voltage gradient between it and the anode which is at ground potential. The back-streaming electrons impact the grid, raising its temperature. With the deposit of the impregnating material on the grid and the rise of its temperature due back streaming of electrons, the grid can emit unwanted electrons and in an uncontrolled way. 
         [0006]    The back-streaming electrons also impact the center portion of the cathode&#39;s emitting surface, raising its temperature and consequently increasing the evaporation rate of the impregnating material. This excess impregnating material will adhere to the grid and can lead to unwanted emission due to high DC field gradients and will also adhere to other line-of-sight surfaces, including the Linac&#39;s RF structure that is down-stream from the cathode. The Linac structure also has high RF field gradients and when its surfaces become coated with the impregnating material it would experience field emission of unwanted and uncontrolled electrons which form what is commonly known as “dark current.” 
         [0007]    It is therefore clear that an urgent need exists for an improved electron gun that is a triode and can substantially mitigate impact of back-streaming of the electrons and addresses the above described problem of the emission of unwanted and uncontrolled electrons. The present invention is concerned with a triode electron gun. Particularly, relates to a triode electron gun with hollow cathode used with vacuum electron devices (VED&#39;s). 
       SUMMARY 
       [0008]    A vacuum electron device (VED), such as a linear particle accelerator (Linac) or a Klystron, uses a source of an electron beam which is typically known as an electron gun. A typical triode electron gun is comprised of a cathode to emit electrons, an anode to attract and focus these electrons and a grid to control and/or modulate the flow of the electrons. 
         [0009]    When the electron gun is used with a VED such as a Linac, some electrons emitted from the cathode of the electron gun, that enter the RF structure, can stream back towards the electron gun impacting the grid and cathode, causing the grid and cathode temperature to rise above their normal operating temperatures. This results in a shorter life for the electron gun, by increasing the evaporation rate of the cathode&#39;s impregnating material and it causes the grid to also emit unwanted electrons that will be detected as high-voltage DC leakage current and unwanted and uncontrolled electrons commonly known as “dark current” producing unwanted radiation exiting the Linac. 
         [0010]    The present invention mitigates the adverse effect of the back-streaming electrons by using a hollow cathode and a hollow grid in the triode electron gun and including a post as an integral part of the hollow cathode electron gun. Inclusion of the post is an essential feature of this present invention that helps eliminate the emission of unwanted and uncontrolled electrons and in the same time providing for a well behaved converging electron beam. 
         [0011]    Note that the various features of the present invention described above may be practiced alone or in combination. These and other features of the present invention will be described in more detail below in the detailed description of the invention and in conjunction with the following figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    In order that the present invention may be more clearly ascertained, some embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0013]      FIG. 1  is a basic schematic of an linear particle accelerator with an electron gun; 
           [0014]      FIG. 2  depicts a cross-sectional view of a hollow cathode electron gun with a post and a few cavities of the linear particle accelerator; 
           [0015]      FIG. 3  is a detailed cross-sectional view of the hollow cathode electron gun with the post; and 
           [0016]      FIG. 4  is a simplified graphical illustration of the role of the post in preventing the collapse of an emitted electron beam in the hollow cathode electron gun. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The present invention will now be described in detail with reference to several embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. It will be apparent, however, to one skilled in the art, that embodiments may be practiced without some or all of these specific details. In other instances, well known process steps and/or structures have not been described in detail in order to not unnecessarily obscure the present invention. The features and advantages of embodiments may be better understood with reference to the drawings and discussions that follow. 
         [0018]    Aspects, features and advantages of exemplary embodiments of the present invention will become better understood with regard to the following description in connection with the accompanying drawing(s). It should be apparent to those skilled in the art that the described embodiments of the present invention provided herein are illustrative only and not limiting, having been presented by way of example only. All features disclosed in this description may be replaced by alternative features serving the same or similar purpose, unless expressly stated otherwise. Therefore, numerous other embodiments of the modifications thereof are contemplated as falling within the scope of the present invention as defined herein and equivalents thereto. Hence, use of absolute and/or sequential terms, such as, for example, “will,” “will not,” “shall,” “shall not,” “must,” “must not,” “first,” “initially,” “next,” “subsequently,” “before,” “after,” “lastly,” and “finally,” are not meant to limit the scope of the present invention as the embodiments disclosed herein are merely exemplary. 
         [0019]    In addition, as used in this specification and the appended claims, the singular article forms “a,” “an,” and “the” include both singular and plural referents unless the context of their usage clearly dictates otherwise. Thus, for example, reference to “a piston” includes a plurality of springs as well as a single piston, reference to “an outlet” includes a single outlet as well as a collection of outlets, and the like. 
         [0020]    A common problem associated with the use of electron guns with linear particle accelerator is that some electrons are injected into the accelerator out of phase with the RF and are accelerated backwards towards the electron gun&#39;s grid and cathode. These back-streaming electrons can have significant energy and impact the grid and cathode causing the grid and cathode temperature to rise above their normal operating temperatures. The area of impact is usually spread over the centermost region of the grid and cathode&#39;s emitting surface resulting in a predominantly higher temperature in those regions, but also causing the entire surfaces to increase in temperature as well. The cathode is normally impregnated with a material that includes Barium, which enhances electron emission by lowering the cathode material&#39;s work function. The evaporation rate of the Barium is strongly dependent on the cathode temperature and the rise of the cathode temperature due to back-streaming electrons quickly increases the evaporation rate of the impregnating material. Over time, this same evaporated impregnate material adheres and builds-up to all surfaces that are line-of-sight, which include but are not limited to the electron gun&#39;s grid which is normally positioned directly in front of the cathode&#39;s emitting surface, the electron gun&#39;s anode and the accelerating structure of the Linac. The grid also sees a voltage gradient between it and the anode which is normally at ground potential. The grid&#39;s potential is close to the potential voltage of the cathode. The back-streaming electrons impact the grid and cause its temperature to rise. With the deposit of the impregnating material on the grid and the rise of its temperature due to back streaming of electrons, the grid will begin emitting unwanted electrons and in uncontrolled way. 
         [0021]    The back-streaming electrons also impact the center portion of the cathode&#39;s emitting surface, raising its temperature and consequently increasing the evaporation rate of the impregnating material. This excess impregnating material will adhere to the grid and other surfaces, including the Linac structure that is down-stream from the cathode. The Linac structure also has high field gradients and when its surfaces become coated with the impregnating material, it would experience high-field emission of unwanted and uncontrolled electrons which form what is commonly known as “dark current” in the Linac. 
         [0022]    Dark current is particularly problematic for Linac&#39;s electron radiation applications, where small amounts of current (of the order of hundreds of micro-amps) are used and therefore small amounts of unwanted and uncontrolled emission of electrons can significantly change the planned-for electron radiation. 
         [0023]    One solution that can be used on triode electron guns is the coating (for example, by sputtering) the electron gun&#39;s grid (which is made of Molybdenum (Mo), as an example) with a material such as Zirconium (Zr) whereby the Zr reacts chemically with a impregnating material, such as Barium, deposited on the grid to inhibit the unwanted and uncontrolled emission of electrons from the grid. However, in this approach the center regions of the grid and the cathode still get very hot due to the impact of back-streaming electrons and the presence of excessive impregnating material from the cathode to the RF Structure will lead to dark current. Also, as the back-streaming electrons impact the center portion of the cathode&#39;s emitting surface and thus raising its temperature, there will be increase in the evaporation rate of the impregnating material and consequently, the useful life of the cathode becomes shorter. 
         [0024]    An alternative approach to address the issue of back-streaming electrons and the associated problem of dark current is used with diode electron guns (which have only two electrodes, a cathode and an anode and no grid). In this approach, a hollow-cathode is employed together with a center post that is thermally isolated from the cathode. In this configuration, the back-streaming electrons would miss the cathode and instead impact the post. In a diode electron gun the cathode is pulsed from zero (ground potential) to full cathode potential (normally kilo volts) when electron flow is wanted. Although the post will get coated with impregnating material, such as Barium, and experience increased heat from the back-streaming electrons, when the cathode and post are pulsed off at zero volts, there is no DC field gradient and no unwanted electron flow between pulses. The post is not impregnated, but a very small amount of cathode&#39;s impregnating material, such as Barium does adhere to it and can be liberated, but at such a small amount that no meaningful amount of dark current is created. However, this approach is limited to diode electron guns. 
         [0025]    On a triode electron gun, the cathode remains at full potential voltage and the grid voltage is pulsed positively, with respect to the cathode, to allow and/or enhance electron flow from the cathode and pulsed negatively with respect to the cathode to inhibit electron flow from the cathode. The use of triode electron guns has important advantages over diode electron guns. One example is when a triode electron gun is used to provide an electron beam to a Linac. The use of a triode gun allows for ultra-fast current pulsing, much faster than that of a diode electron gun, and the faster pulse repetition rate facilitates faster inspections in industrial screening applications. The use of a triode electron gun also allows for ultra-fast changes in beam current in the Linac which lends itself to multi-energy Linac operation, which is highly advantageous in industrial screening applications when different energies are needed to discriminate home-made-explosives (HME&#39;s) and other forms of contraband. For medical applications, the use of a triode electron gun to provide an electron beam to a Linac would allow the accelerator to operate at multiple energies very similar to industrial Linacs described above. Thus, one accelerator-based system would be able to handle both imaging and a multitude of treatments covering a broad spectrum of patients and types of cancer. 
         [0026]    The present invention addresses the above-described problem of the emission of unwanted and uncontrolled electrons. This invention is concerned with a triode electron gun. Particularly, relates to a triode electron gun with hollow cathode used with a vacuum electron device (VED), such as a linear particle accelerator or a Klystron, wherein the Klystron can be a single-beam klystron or a multi-beam klystron. 
         [0027]    The hollow cathode triode electron gun of this invention can also have advantageous use as a source of electrons for a multiple of devices that requires an electron beam. 
         [0028]    The hollow cathode triode electron gun according to one embodiment of the present invention can be used with many types of Linacs for medical, industrial, and security applications. This includes: standing wave Linacs and traveling wave Linacs. The standing wave Linacs can be of the bi-periodic axially coupled type or the magnetically side-coupled type or the bi-periodic magnetically coupled type. 
         [0029]    Also the hollow cathode triode electron gun according to one embodiment of the present invention can be used with deferent Linac designs such as Linacs designed based on the constant impedance approach or Linacs designed based the constant gradient approach. 
         [0030]    The present invention represents a practical solution to the above-described problem based on a triode electron gun employing a hollow cathode, a post and a grid with a center hole to receive the post. Incorporating a grid with a hollow cathode provides the benefits of using a triode electron gun without the disadvantages that a grid or cathode suffers due to heating caused by the impact of back-streaming electrons. 
         [0031]    One embodiment of this invention is also concerned with a shadow gridded electron gun which is basically, a triode electron gun having a shadow grid connected directly to the cathode in addition to the control grid. 
         [0032]    Using incorporated figures, the present invention of the hollow cathode triode electron gun is described hereafter in more detail. 
         [0033]      FIG. 1  shows a basic schematic  100  of an exemplary linear particle accelerator (Linac)  110  with an electron gun  120  emitting an electron beam  130  along an axis  105  which is the common axis for both the electron linear accelerator  110  as well as the electron gun  120 . The electron beam  130  is being accelerated through cavities  140   a,    140   b,    140   c,  . . . ,  140   n  which are powered by microwave power  150 , also known as RF power or electromagnetic power. The exemplary electron linear accelerator  110  thus produces a high-energy electron beam  160  as its output. It is to be noted that some of the electrons emitted from the electron gun  120  can arrive in the cavities of the electron linear accelerator at a wrong phase and thus they form a back-streaming electron beam  170 . 
         [0034]      FIG. 2  depicts a cross-sectional view  200  of a hollow cathode electron gun  300  according to the present invention which is emitting the electron beam  130  along the axis  105  towards an anode  210  which is connected mechanically and electrically to the exemplary Linac  110 . The electron beam  130  passes through a center aperture  215  in the anode  210  onto the Linac  110 . Only the first three cavities  140   a,    140   b  and  140   c  of the electron linear accelerator are shown. The center of anode aperture  215  is aligned with the axis  105  which is the common axis for both the hollow cathode electron gun  300  and the Linac  110 . The hollow cathode electron gun  300  is affixed to the Linac  110  by mating a weld flange  223  of the hollow-cathode electron gun  300  to a weld flange  113  of the Linac  110 . 
         [0035]      FIG. 3  depicts details of the hollow cathode electron gun  300  according to the present invention. The hollow cathode electron gun  300  is comprised of a hollow cathode  310 , a grid  320 , a heating filament  330 , a post  340 , a focusing electrode  350 , and a high-voltage insulator  360  enclosing all the hollow-cathode electron gun&#39;s constituent components and all are centered on the axis  105  which is the common axis for both the hollow cathode electron gun  300  and the Linac  110  (only the edge of the accelerator is shown). Each of the hollow cathode electron gun  300  constituent components is described hereafter in more detail. 
         [0036]    The hollow cathode  310  is of concave shape and has a center hole  311  which is centered on the axis  105 . The hollow cathode  310  is made of a material, such as impregnated porous Tungsten, that can emit electrons easily when heated to elevated temperatures (thermionic emission). The hollow cathode is normally impregnated with a material, such as Barium, that enhances electron emission by lowering the cathode material&#39;s work function. The hollow cathode  310  is affixed in place by a cathode support  312  or series of support structures. The cathode support  312  is typically a metal tube, cylinder and/or conical cylinder made of Molybdenum, Molybdenum-Rhenium, Tantalum or similar low vapor pressure material also centered on the emission axis  105 . The cathode support  312  is connected to a focus electrode  350  and also a cathode support sleeve  313  which is typically made of Molybdenum or Molybdenum-Rhenium or other suitable low vapor pressure material, which acts to as a thermal choke, keeping the heat generated by the heating filament  330  from being thermally conducted away from the hollow cathode  310  allowing the hollow cathode to achieve and maintain high temperature operation that can be greater than 1000 C for an impregnated dispenser cathode. Similar structures are used to maintain high temperatures in coated cathodes, oxide cathodes, reservoir cathodes and other types of cathodes used in electron guns. The cathode support  312  is attached to a cathode connector  314 , which is brazed between the cathode-to-grid insulator  324  and the filament insulator  334 . The cathode support  312  is also welded to a post support  341  and the post support is welded to the post  340  keeping it centered on axis  105  and held in this centered position relative to the hollow cathode  310 , the grid  320  and the anode  210 . The hollow cathode  310  is connected to a power supply (not shown) through the cathode connector  314 . The power supply provides the cathode with a biasing negative voltage which is normally of tens of kilo volts. 
         [0037]    It is to be noted that according to one embodiment of the present invention, one type of the hollow cathode is a “dispenser B cathode” which is a metal matrix of porous Tungsten impregnated with a mixture of Barium Oxide (BaO), Calcium Oxide CaO, and Aluminum Oxide (2Al2O3) having, for example, the mole-ratio of 5 BaO:3 CaO:2Al2O3, also known as “5-3-2 impregnation”. Other common mole-ratios include 3:1:1, 4:1:1, and 6:1:2. Other impregnation rations can also be used. Another type of dispenser cathode is the “dispenser scandate cathode” which is impregnated with Scandium Oxide (Sc2O). A yet another cathode type according to one embodiment of this invention is a dispenser B cathode with a thin layer of Os—Ru (Osmium-Rhenium), which is known as an “M-coated cathode”. A fourth cathode type which can be used according to one embodiment of the present invention is an “oxide cathode”. 
         [0038]    The grid  320  is of a concave shape as the hollow cathode  310  and is placed in a close proximity, typically as close as a few mils to tens of mils, to the emitting surface of the hollow cathode  310  and having approximately the same curvature of the cathode as needed to achieve the proper emission and beam trajectories  130 . The position and shape of the grid  320  as well as its openings are chosen to optimally control the passage of the electrons emitted from the cathode. Grid  320  is secured by a metal supporting tube or cone called a grid support  322 , which can be made up of multiple components and is typically Molybdenum and/or the same material as the grid and is centered on the common axis  105 . The grid support  322  constitutes an extension of a coaxial cavity, which is centered on the common axis  105 . The grid support  322  is fixed in position by welding or brazing to the high voltage insulator  360  typically made from alumina (94%-99.8% pure) and a cathode-to-grid insulator  324  which is also made from alumina and exits the vacuum wall to provide a means of connecting a grid power supply (not shown) to the electron gun  300  at a grid connector  323 . 
         [0039]    The heating filament  330  is connected to a filament leg  331  which extends from the back of the hollow cathode  310  and is connected to a filament rod  332 , typically made from Kovar or Nickel, by a metal conductor ribbon  333  made of Platinum or other suitable metal. The filament rod  332  is welded to a filament cap  335  such that the weld creates a hermetic seal and proper electrical contact with a filament connector  336  that is connected to a filament power supply (not shown). The cathode connector  314  is electrically isolated from the filament connector  336  by an alumina filament-heater isolator  334 . 
         [0040]    When a current is supplied to the heating filament  330 , the filament wire increases in temperature due to resistive heating and the heat from this wire is conducted to the cathode, raising the temperature of the hollow cathode  310  and thus allowing it to emit electrons from its impregnated concave surface. The presence of the focusing electrode  350  keeps unwanted electrons from emitting out the sides of the cathode and also helps focus the emitted electrons, from the face of the cathode, into a properly shaped electron beam having proper electron trajectories  130  along the axis  105 . 
         [0041]    An essential feature of this invention is the inclusion of the post  340  as an integral part of the hollow cathode electron gun  300 . The post  340  is placed at the center of the hollow cathode  310  and is affixed in place by the post support  341  typically made from Kovar or Nickel 
         [0042]    A hollow cathode without a post such as the post  340  in the center of the hollow cathode through its hole will emit less desirable electrons with poor trajectories from its inside diameter. One embodiment of the present invention prevents this effect by adding a solid post such as the post  340  positioned in the center of the hollow cathode  310 . The said post can be of cylindrical or conical shape. It is thermally isolated, but electrically connected to the hollow cathode and is therefore at the same potential as the cathode and will therefore inhibit any emission from the cathode&#39;s inside diameter. Without such post, the electrons coming off the cathode will have collapsing trajectories under the absence of any space charge in the center of the emitted beam. A post whose potential voltage is the same as the cathode will effectively repel electrons with the same potential voltage and keep the electron beam from collapsing, improving the electron trajectories, providing for a well behaved converging electron beam. 
         [0043]    The configuration  400  in  FIG. 4  illustrates the role of the post in preventing the electrons coming off the cathode from having collapsing trajectories. The electron beam is emitted from a surface  315  of the hollow cathode  310 . The cathode is normally biased at a negative voltage potential of tens of kilo-volts and the grid  320  is pulsed positively to allow electrons flow from the cathode forming the emitted electron beam  130 . The post  340  is positioned in the center of the hollow cathode  310  and according to one embodiment of the invention is electrically connected to the hollow cathode  310 . Thus, both the cathode surface  315  and a post surface  345  will have the same potential and therefore inhibit any undesirable emission, such as electron rays  410 , from the cathode&#39;s inside diameter. A post whose potential voltage is the same as the cathode and that is positioned axially such that the end of the post is in front of the cathode will effectively repel electrons with the same potential voltage and keep the electron beam from collapsing, improving the electron trajectories, providing for a well behaved converging electron beam. The position of the post relative to the grid is also important such that the gap between the two can be full cut-off when the grid is pulsed negatively. Too large a gap will allow the field from the anode to bend inward toward the cathode surface allowing it to bias a small amount of electrons when the beam should be fully turned off. 
         [0044]    It is to be noted that in the presence of the impregnated cathode, the post  340  will eventually get coated with the impregnating material, such as Barium, lowering the post&#39;s material work function. As the back-streaming electrons impact the post, they will result in an increase in temperature of the post and consequently emission of unwanted and uncontrolled electrons from the post. In one embodiment according to the present invention, the post can be made of a material such as Zirconium (Zr) or Hafnium (Hf) or another metal or composite that reacts with the impregnating material, such as Barium, to inhibit or completely stop emission. 
         [0045]    In yet another embodiment of the present invention the post can be made of a material such as Molybdenum, Tungsten or another low vapor pressure material and then coated (for example by sputtering, chemical vapor deposition, or other means of coating) with Zirconium (Zr) or another element that reacts chemically with the impregnating material, such as Barium, to inhibit electron emission. 
         [0046]    According to one embodiment of the present invention, the post is thermally isolated from the cathode and has a heat-sink path to keep the post material from melting. 
         [0047]    According to one embodiment of the present invention, the post can be shaped as a hollow cylinder or a hollow cone such that the back-streaming electrons will impact the inside of the post over a larger surface area, providing for a lower power density and less heat created by the back-streaming electrons. 
         [0048]    According to yet another embodiment of the present invention, the post can be positioned in a preferred position such as to help focus the electrons emitted from the hollow cathode  310  into a properly shaped electron beam. 
         [0049]    In still another aspect of the invention, the post can be positioned in a preferred position such as to allow the electron beam  130  to be cut-off when the grid voltage is lowered or run at a slight negative voltage with respect to the cathode&#39;s voltage. 
         [0050]    While this invention has been described in terms of several embodiments, there are alterations, modifications, permutations, and substitute equivalents, which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, modifications, permutations, and substitute equivalents as fall within the true spirit and scope of the present invention.