Abstract:
The invention provides an apparatus and method of switching more than one bias voltage within an electron beam tube in order to achieve electron beam cutoff. The invention is particularly useful for high-perveance electron tubes in which a large change in focus-electrode-to-cathode or anode-cathode voltage might otherwise be needed to achieve cutoff. In one embodiment of the invention, the cathode and anode bias voltages are both switched by magnitudes well within the capabilities of standard high-voltage switches to achieve beam cutoff.

Description:
RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/242,308, filed Sep. 14, 2009. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to the field of electron beam tubes and more particularly to an ungridded electron gun having an electrically isolated focusing electrode and a modulating anode. 
     2. Description of Related Art 
     In travelling wave tubes (TWTs) used in microwave power modules, it is desirable to be able to selectively shut off the electron beam current or at least reduce its magnitude to a tolerably low level. This can be accomplished by switching the focusing electrode of the electron gun to a voltage potential that is negative with respect to the cathode. Alternatively, the cathode voltage potential can be switched towards ground in order to establish a negative bias on the cathode with respect to the focusing electrode. In a typical modulating anode electron gun, the anode is generally switched from essentially ground potential to a potential approximately equal to the cathode potential, thereby reducing the electron beam current effectively to zero. 
     However, many electron guns are designed to exhibit a high perveance, which is defined as the ratio of the space-charge-limited beam current to the gun cathode-to-anode voltage raised to the three halves power. A higher perveance thus indicates that the emitted electron beam is more heavily influenced by space-charge effects. In such a system, the voltage that must be applied to the focusing electrode in order to completely cut off the beam current becomes unacceptably large. For example,  FIG. 1  is a plot of a normalized focusing electrode cutoff voltage as a function of anode microperveance. The open circles, e.g.,  110 , are measured cutoff voltage ratios of various electron guns having different microperveance values. The microperveance of a given electron gun is a function of its geometry. It can be observed from  FIG. 1  that the relationship of the cutoff voltage ratio to the microperveance is quite linear. This linear relationship is illustrated by curve  102 , which is an empirical fit to the measured cutoff ratios of several different electron gun designs. A linear fit  106  to the data, shows that the normalized cutoff voltage is related to microperveance by a ratio of approximately 0.44. Thus, as the anode microperveance approaches a value of 2.1, the voltage that must be applied to the focusing electrode approaches the level of the cathode-to-anode voltage itself. Switching a voltage of this magnitude, which may be several thousand volts, poses a difficult challenge because modern solid-state voltage switches cannot easily handle voltage magnitudes greater than approximately 2000 volts. Thus, it would be desirable to provide a system for switching voltages within a high-perveance electron gun to achieve full beam cutoff while overcoming the difficulties of switching high-magnitude voltages described above. 
     SUMMARY OF THE INVENTION 
     The invention provides an apparatus and method of switching one or more bias voltage within an electron beam tube in order to achieve electron beam cutoff. In a first embodiment of an electron beam tube in accordance with the present invention, an electron beam tube comprises a tube body and a cathode mechanically affixed to the tube body through an insulating element, wherein the cathode is adapted to emit an electron beam at an operating electron beam current. The cathode is further connected to a cathode bias circuit adapted to apply a cathode bias voltage to the cathode. The electron beam tube further includes an anode mechanically affixed to the tube body through an insulating element wherein the anode is connected to an anode bias circuit adapted to apply an anode bias voltage to the anode. The electron beam tube further includes a focusing electrode mechanically affixed to the tube body through an insulating element wherein the focusing electrode is connected to a focusing electrode bias circuit adapted to apply a focusing electrode bias voltage to the focusing electrode. The electron tube includes a switching mechanism that may comprise a plurality of individual switches in some embodiments or a single switch in other embodiments. The switching mechanism is adapted to switch the bias voltage applied to at least one of the cathode, the anode, or the focusing electrode. As a result of actuating the switching mechanism and changing the selected bias voltage or voltages, the electron beam current drops from its operating current to a current that is substantially equal to zero. 
     In one particular embodiment of an electron tube in accordance with the present invention, the switching mechanism is implemented as a single switch configured to switch a floating power supply that may be connected to multiple ones of the cathode bias circuit, the anode bias circuit, and the focusing electrode bias circuit. 
     In another embodiment of an electron tube in accordance with the present invention, the electron tube is configured to operate at a microperveance that is greater than approximately 0.8. Two exemplary embodiments described in detail below discuss an electron gun with a microperveance of 0.87 and an electron gun with a microperveance of 2.0. While the invention is particularly useful for electron guns operating at high perveance, it applies equally to a gun of any perveance value. 
     In an embodiment of an electron gun in accordance with the present invention, the switching mechanism is configured to switch one or more voltages by a magnitude of less than approximately 2000 volts while still achieving cutoff of the electron beam. A typical system may have a maximum voltage magnitude of approximately 7500 volts applied to any of the cathode, anode, or focusing electrode during normal operation. Thus, this means that the switching mechanism is configured to switch voltages of a magnitude less than roughly one third of the maximum bias voltage employed during normal operation. The invention would similarly be applicable to electron guns having a maximum operating voltage other than 7500 volts, and the switching mechanism would accordingly operate to provide beam cutoff by switching voltages smaller than roughly one third of the maximum operating voltage. 
     In one embodiment of an electron gun in accordance with the present invention, the switching mechanism is configured to switch the cathode bias voltage and the anode bias voltage. Typically, upon actuation of the switching mechanism, the cathode will be switched to a voltage that is less negative and the anode will be switched to a voltage that is more negative than in normal operating mode. Thus, the voltage difference between the cathode and anode will decrease to a sufficient extent, and electron beam cutoff will be achieved. In some embodiments, the magnitude by which the cathode and the anode are switched will be equal. In other embodiments, the cathode and anode may be switched by different magnitudes. 
     In another embodiment of an electron gun in accordance with the present invention, the switching mechanism is configured to switch the anode bias voltage and the focusing electrode bias voltage. The magnitudes of the voltages by which the anode and focusing electrode are switched may be equal to one another or may differ. Typically, the focusing electrode and the anode will both be switched such that their biases become more negative in order to achieve electron beam cutoff. 
     In another embodiment of an electron gun in accordance with the present invention, the switching mechanism is configured to switch the cathode bias voltage and the focusing electrode bias voltage. Again, the magnitudes of the voltages by which the cathode and focusing electrode are switched may be equal to one another or may differ. 
     In still another embodiment of an electron gun in accordance with the present invention, the switching mechanism may be configured to switch the voltage biases applied to all three of the cathode, the anode, and the focusing electrode. The magnitudes by which each bias is switched may or may not be equal to each other. 
     In some embodiments of an electron gun in accordance with the present invention, the electron gun may further include a collector unit configured to collect the spent electron beam. The collector may comprise a multistage depressed collector having one or more stages that are depressed in voltage potential with respect to the tube body. 
     Those skilled in the art will recognize other benefits and applications of the disclosed invention, and such would also fall within the scope and spirit of the present invention. A detailed description of the preferred embodiments follows with reference to the attached sheets of drawings, which will first be described briefly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is plot of normalized electron beam cutoff voltage as a function of microperveance; 
         FIGS. 2   a - 2   c  depict a cross section of a 0.87 microperveance electron gun in accordance with an embodiment of the present invention; 
         FIGS. 3   a - 3   b  depict a cross section of a 2.0 microperveance electron gun in accordance with an embodiment of the present invention; 
         FIG. 4  is a curve summarizing beam cutoff versus microperveance for various switching schemes; 
         FIG. 5  illustrates the results of an electromagnetic simulation of a 0.87 microperveance electron gun in normal operation; 
         FIG. 6  illustrates a simulated attempt to cut off the electron beam current by biasing the focusing electrode more negative than the cathode; 
         FIG. 7  illustrates a simulated attempt to cut off the electron beam current by biasing the focusing electrode negative with respect to the cathode and the anode negative with respect to ground in accordance with an embodiment of the present invention; 
         FIG. 8  illustrates the simulated effect of biasing the anode 1700 volts below ground; 
         FIG. 9  illustrates the simulated effect of biasing the cathode positive by 1700 volts toward ground; 
         FIG. 10  illustrates that complete beam cutoff is achieved by biasing the cathode positive by 1700 volts, and the modulating anode negative by 1700 volts, in accordance with an embodiment of the present invention; 
         FIG. 11  depicts a simplified circuit diagram of a power supply switching circuit capable of producing cathode and modulating anode switching in accordance with an embodiment of the present invention; 
         FIG. 12  depicts a simplified circuit diagram of a power supply switching circuit according to another embodiment of an electron beam tube switching a floating power supply in accordance with the present invention; 
         FIG. 13  depicts a simplified circuit diagram of a power supply switching circuit in accordance with another embodiment of an electron beam tube in accordance with the present invention; and 
         FIG. 14  depicts a simplified circuit diagram of a power supply switching circuit in accordance with yet another embodiment of an electron beam tube in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention provides an apparatus and method for switching voltages within a high-perveance electron gun to achieve full beam cutoff. In a preferred embodiment of an electron gun in accordance with the present invention, selective full-beam cutoff is achieved by switching both the cathode and the modulating anode voltages using two moderate-voltage switches. This switching scheme is illustrated both for a gun with a microperveance of 0.87, shown parametrically in  FIG. 1  at marker  104 , and for a gun with a high microperveance of 2.0, as illustrated at marker  108 .  FIGS. 2   a - 2   c  depict a cross section of a preferred embodiment of an electron gun in accordance with the present invention that operates with a microperveance near 0.87. Distance (Z) along the beam path is shown along the horizontal axis, and the radial distance (R) from the center of the electron beam is depicted along the vertical axis. Referring to  FIG. 2   a , the electron gun includes a cathode  208  that produces electrons to form an electron beam  204  that propagates within the electron gun. The electron gun also includes a focusing electrode  206 , and a modulating anode  210 . The cathode  208 , focusing electrode  206 , and modulating anode  210  are typically mounted on insulating elements (not shown) and thereby affixed to a tube body  202 . In normal operation, the cathode  208  is held at a potential of −7500 volts with respect to the tube body  202 , which is held at ground potential, or 0 volts. The focusing electrode  206  is held at −7500 volts, and the modulating anode  210  is held at body potential or ground. In this configuration, a large electron beam current  204  is achieved. Further, a magnetic field is typically applied within the body of the device to counteract space charge effects that can disperse the beam. Within the body of the device and under these conditions, the focused beam is generally cylindrical in shape. 
     It should be noted that while the focusing electrode  206  is depicted as being at the same potential as the cathode  208 , it may be desirable to bias the focusing electrode  206  slightly negative with respect the cathode, for example by −10 volts or so, in order to reduce electron emission from the side of the cathode and to improve uniformity of the current density near the edge of the cathode. In addition, it may be desirable to bias the modulating anode  210  about −100 volts or so below (or +100 volts or so above) ground potential in order to adjust the current emitted from the electron gun in its normal beam-on operational mode. The basic switching principles presented herein do no preclude the application of such bias voltages; in fact, they may serve to enhance the switching methods described below. 
     In  FIG. 2   b , the effect of switching the cathode  208  toward ground by 1700 volts is shown. In this configuration, the cathode potential has been shifted to −5800 volts, meaning that there is now a potential difference of 5800 volts from the modulating anode  210  to the cathode  208 . This results in a reduction in the beam current, shown schematically at  220 . However, a sizeable beam current remains. 
     In  FIG. 2   c , in accordance with an embodiment of the present invention, both the cathode and the modulating anode voltages are switched by 1700 volts. The voltage of the cathode  208  is switched from −7500 volts to −5800 volts, and the voltage of the modulating anode is switched from ground to −1700 volts. This double switching operation reduces the beam current to zero or near zero, as indicated at  224 . 
     While the cathode and the modulating anode were both described as being switched by 1700 volts in the embodiment depicted in  FIG. 2   c , it is not necessary to switch them symmetrically. For example, the cathode could be switched by 1900 volts and the modulating anode could be switched by −1500 volts, and a similar effect on the beam current would be produced. Systems that are switched asymmetrically, as described above, would also fall within the scope and spirit of the present invention. 
     Mathematically, for the system illustrated in  FIGS. 2   a - 2   c , switching just the focusing electrode by 1.7 kV creates an anode potential difference of −5.8 kV. From  FIG. 1 , it can be seen that at a microperveance of 0.87, cutoff voltage is approximately 0.4 times the anode voltage. Thus, multiplying by 0.4, a voltage of −2.32 kV is required for full cutoff. Since −2.32 kV has a larger magnitude than −1.7 kV, the electron beam current is not completely cut off. On the other hand, when both the cathode and anode are switched by 1.7 kV in accordance with the present invention, as illustrated in  FIG. 2   c , the potential difference becomes −4.1 kV. When multiplied by 0.4, this results in −1.64 kV. Because −1.64 kV is smaller in magnitude than −1.7 kV, the electron beam current is completely cut off. 
       FIGS. 3   a  and  3   b  illustrate an alternative embodiment of an electron gun in accordance with the present invention for a gun with a microperveance of 2.0. In  FIG. 3   a , the focusing electrode  306  is at a potential of −5.35 kV, and the modulating anode  310  and body  302  are set at a potential of 0 kV. The cathode  308  is switched from −5.35 kV toward ground by 1.7 kV, as shown at  308 , to create a potential difference of −3.65 kV between the cathode  308  and modulating anode  310 . From  FIG. 1 , element  108 , a gun operating at a microperveance of 2.0 requires a cutoff voltage of 91% of the cathode-to-anode voltage. Because 0.91 multiplied by −3.65 kV is −3.33 kV, which is larger in magnitude than −1.7 kV, the electron beam is not fully cut off, as illustrated at  304 . However, in  FIG. 3   b , the modulating anode  310  is also switched by −1.7 kV, in accordance with the present invention. This creates a potential between the cathode and modulating anode of 1.95 kV. Multiplied by 0.91, this gives −1.78 kV as the cutoff voltage, which is very close to −1.7 kV, resulting in the electron beam&#39;s being nearly completely cut off, as indicated at  324 . 
     While the embodiments depicted in  FIGS. 2   a - 2   c  and  3   a - 3   b  illustrate switching of the cathode and the modulating anode, it is also possible to produce similar beam cutoff effects by switching the voltage of the focusing electrode, i.e., element  306  in  FIGS. 3   a  and  3   b . Thus, control of the electron beam current can be achieved by switching a single element, such as the cathode; by switching two elements, such as the cathode and modulating anode, or the focusing electrode and modulating anode; or by switching all three elements. 
     The effects of these different switching schemes can be described mathematically as follows, with reference to the elements depicted in  FIG. 3   a . First, consider case A, a standard electron gun control scheme whereby the cathode  308  and anode  310  are held constant, and the focusing electrode  306  is switched. As shown in  FIG. 1 , for a given microperveance, the ratio of the cutoff voltage, V co , to the anode voltage, V a , is constant, or V co /V a =K. From  FIG. 1 , a gun with a microperveance of 0.87 has K=0.408. In such a gun operating with the focusing electrode  306  and the cathode  308  at −7500 volts with respect to the modulating anode  310 , the cutoff voltage is thus V co =K*V a =0.408*−7500=−3060 volts. In other words, the focusing electrode would have to be switched negative by −3060 volts with respect to cathode, or to −10,560 volts with respect to ground, in order to achieve full beam cutoff. 
     Next, consider case B, a single switch scheme whereby the focusing electrode  306  and the modulating anode  310  are held constant and the cathode  308  is switched. Again, we start with V co /V a =K. Switching the cathode voltage produces a new effective anode voltage of V a −V co ′, where V co ′ is a new effective cutoff voltage. So K=V co ′/(V a −V co ′), which can be rewritten as K=(V co ′/V a )/(1−V co ′/V a ). Then, defining K′ as the new constant, such that K′=V co ′/V a , we see that K=K′/(1″K′). Manipulating this expression to solve for K′, we can see that K′=K/(1+K). From the curve of  FIG. 1 , we can see that a gun with a microperveance of 0.87 has K=0.408. Thus, K′=0.408/(1+0.408)=0.28977 for a cathode-switched gun. Thus, for a gun operating at a 7500 volt cathode-to-anode voltage and using cathode switching, cutoff can be achieved by switching the cathode by V co ′=0.28977*7500=2173.295 volts. 
     Next, consider case C, a dual-switched scheme in accordance with the present invention whereby the focusing electrode  306  is held fixed and the cathode  308  and anode  310  are both switched. In this case, the effective cathode-anode difference becomes V a −2V co ′ because the anode and cathode are switched toward one another. Thus, K=V co ′/(V a −2V co ′)=(V co ′/V a )/(1−2V co ′/V a ). Or in other words, K=K′/(1−2K′), and solving for K′ gives K′=K/(1+2K). So in this case, for a gun of microperveance 0.87 and K=0.408, K′=0.224670. For a cathode-anode difference of 7500 volts, cutoff can be achieved by switching the cathode and anode by V co ′=0.224670*7500=1685.02 volts. 
     Finally, consider case D, a second dual-switched scheme in accordance with the present invention whereby the cathode  308  is held constant and the focusing electrode  306  and the anode  310  are both switched. In this case, the effective cathode-anode difference is V a −V co ′, so K=V co ′/(V a −V co ′). The analysis is thus the same as for the cathode-switched case, and K′=0.408/(1+0.408)=0.28977, producing a cut-off switching voltage V co ′=0.28977*7500=2173.295 volts. While this switching voltage magnitude is higher than for case C, described above, this embodiment has the advantage of leaving the cathode voltage constant. Because the cathode draws significant current, it is much simpler to switch the focusing electrode or the modulating anode than it is to switch the cathode. 
       FIG. 4  is a graphical depiction of the cutoff voltage to anode voltage ratio versus the anode microperveance for cases A, B, C, and D, described above. The standard cutoff case, case A, is depicted by curve  350 , corresponding to curve fit A, y=0.43865x+0.024432. Case B, the cathode-switched case, and case D, the second dual-switched case, both correspond to curve  360 , which is fit to a fourth-order polynomial, y=−0.016660x 4 +0.096922x 3 −0.23827x 2 +0.45003x+0.022180. Finally, case C, the first dual-switched case, corresponds to curve  370 , which is also fit to a fourth-order polynomial, y=−0.026379x 4 +0.14477x 3 −0.31458x 2 +0.41225x+0.022430. Using the curves in  FIG. 4 , the cutoff voltage ratio for an electron gun using any of the cutoff switching schemes A, B, C, or D, can be predicted for a gun of any microperveance. As can be seen from the figure, the dual switching schemes depicted at  360  and  370  become particularly advantageous for high microperveance guns, reducing the switching voltage required for cutoff by a factor of two or more over the standard switching mode  350 . 
       FIGS. 5-10  depict the results of several electromagnetic simulations using an electron transport code known as DEMEOS of an electron gun in accordance with an embodiment of the present invention. A 7500-volt electron gun is illustrated, showing that full beam cutoff can be achieved using two 1700-volt switches.  FIG. 5  shows the gun during normal operation with 100% beam transmission. At a microperveance of 0.87, the cathode  408  and focusing electrode  406  are held at a potential of −7.5 kV. The modulating anode  402  and body  404  are at ground potential. The periodic permanent magnet or PPM field on axis used to focus the beam in this case is shown in the figure at  412 . In actuality, the average sinusoidal field level is displaced but a small amount from zero Gauss. In this configuration, electron beam  410  is fully transmitted at a power level of 4244.3 watts and a current of 565.9 mA. 
     In  FIG. 6 , the focusing electrode  406  is switched 1700 volts negative with respect to the cathode  408 . This results in a reduction of the electron beam flux  510 . The emitted beam current is approximately 39 mA, resulting in a net power in the beam of 290 watts. Since approximately 60.7% of the beam current is transmitted, net power on the body beam shaver is 114.2 watts. This may result in a heat load high enough to cause melting. The remainder of the beam current that makes its way down the travelling wave tube (TWT) or other linear beam microwave tube can provide amplification of noise, which may be unacceptable. 
     In  FIG. 7 , the focusing electrode  406  is switched 1700 volts negative with respect to the cathode  408 , and the modulating anode  402  is also switched 1700 volts negative with respect to ground. Beam current and body power are both significantly lower in comparison to the case shown in  FIG. 6 . 
     It should be noted that the voltages achieved in the system depicted in  FIG. 7  can also be achieved using a single switching element. The anode  402  may be biased to +7500 volts with respect to the focusing electrode  406  using a fixed-voltage floating power supply or similar device connected between the anode  402  and the focusing electrode  406 . When the focusing electrode  406  is switched to −1700 volts below the potential of the cathode  408 , the voltage of the anode  402  will follow, bringing it to −1700 volts below ground. A simplified configuration implementing this scheme according to an embodiment of the present invention is depicted in  FIG. 12 , described further below. 
     In  FIG. 8 , only the modulating anode  402  is switched negative by 1700 volts. Beam current  710  is reduced with respect to the configuration depicted in  FIG. 5  but still propagates in a stable, focused manner out of the electron gun. 
     In  FIG. 9 , the cathode  408  is instead switched 1700 volts positive with respect to the focusing electrode  406 . In this case, the emitted beam current is 8.3 mA and the net beam power is 48.2 watts. This level of current is similar to the case shown in  FIG. 7 . In the scheme depicted in  FIG. 9 , the beam transmission is 64.6%, so the power load on the power shaver is 17.1 watts. While this level of body power is not unduly high and will not cause damage to the tube, the beam current progressing through the circuit can lead to increased amplified noise that may be undesirable in certain systems. 
     In  FIG. 10 , full beam cutoff is achieved by switching both the cathode and the anode by 1700 volts. Here, the cathode  408  is switched 1700 volts positive with respect to the focusing electrode  406 . The anode  402  is switched to −1700 volts below ground, thus reducing the potential difference between the cathode and the anode to 4100 volts. This configuration achieves hard cutoff, as can be seen from the simulation depicted in  FIG. 10 . Since no beam current is emitted, there is no extra heat load on the input of the tube, and there is no beam current flowing through the circuit that can produce unwanted amplified noise. 
       FIG. 11  depicts an embodiment of an electron beam tube power supply switching circuit in accordance with the present invention that can be used to provide the dual switching function described above. The electron tube comprises a cathode  802 , a focusing electrode  804 , a modulating anode  806 , a main body  808 , and a multi-stage depressed collector unit comprising a first collector  810  and a second collector  812 . Of course, collectors having a single stage or three or more individual stages may also be used. A power supply  814  is used to supply current to a cathode heater  820 . Further voltage sources  822 ,  824 ,  826 ,  828 , and  830  are used to bias various components of the electron tube. Cathode bias circuit  840  is used to supply a bias voltage to the cathode  802 . Focusing electrode bias circuit  842  is used to apply a voltage bias to the focusing electrode  804 , and anode bias circuit  844  is used to supply a voltage bias to anode  806 . In this example, focusing electrode bias circuit  842  is fixed at −7.5 kV by series power supplies  822 ,  824 , and  830 . Also in this example, the cathode is normally at −7.5 kV, when switch  816 , connected to cathode bias circuit  840 , is in the position indicated by the solid arrow. By throwing the first 1700-volt switch  816  to the position indicated by the dashed arrow, the cathode voltage can be reduced in magnitude to −5.8 kV. A second 1700-volt switch  818  is connected to anode bias circuit  844  and can be used to switch a bias applied to the modulating anode from ground (solid arrow position) to −1.7 kV (dashed arrow position). The combined effect of the switching operation comprising changing the position of the two switches  816  and  818  results in a full cutoff of beam current, as described previously. 
       FIG. 12  is a simplified schematic drawing of another embodiment of an electron beam tube power supply switching circuit in accordance with the present invention. The electron tube includes a cathode  902 , a focusing electrode  904 , an anode  906 , and a tube body  908 . A power supply  916  supplies current to a cathode heater  918 . In this embodiment, a fixed floating power supply  914  is situated between the focusing electrode  904  and the anode  906  to maintain a constant potential difference between these two components. Focusing electrode bias circuit  932  and anode bias circuit  934  are connected to the two terminals of floating power supply  914 . The switching mechanism comprises a single switch  920  operatively coupled to both the focusing electrode bias circuit  932  and the anode bias circuit  934 . Actuating switch  920  alternatively ties the negative terminal of the floating supply  914  to the negative terminal of the cathode power supply  910  or to the negative terminal of the 1.7 kV floating power supply  912 . Thus, in normal operation, with switch  920  in the position shown by the solid arrow, the anode  906  and tube body  908  are both held at ground potential, the focusing electrode  904  is held at −7.5 kV by the cathode power supply  910 , and the cathode  902  is held at −7.5 kV by the cathode power supply  910 . Switch  920  can then be thrown to the position indicated by the dashed arrow, and both the anode bias circuit  934  and the focusing electrode bias circuit  932  are pulled negative by 1.7 kV by connection to the negative terminal of the 1.7 kV floating power supply  912 . Thus, the anode  906  will end up at −1.7 kV and the focusing electrode  904  will move to −9.2 kV, cutting off the electron beam current, as shown previously in the simulation depicted in  FIG. 7 . Thus, the dual switching of the focusing electrode  904  and the anode  906  is achieved with a single switch  920 . Of course, the floating power supply could also be connected between elements other than the focusing electrode and the anode depending on the desired switching configuration. 
       FIG. 13  depicts an alternative embodiment of a power supply switching circuit for an electron beam tube in accordance with an aspect of the present invention. The figure depicts a cathode  1002  coupled to a cathode heater  1018  and cathode heater power supply  1016 . The cathode  1002  is further connected to a cathode bias circuit  1030  that applies a voltage bias of −7.5 kV to the cathode via power supplies  1010  and  1014 . Focusing electrode  1004  is connected to focusing electrode bias circuit  1032 , which is in turn coupled to switch  1020 . In its normal operating position (solid arrow), switch  1020  bypasses power supply  1012  such that the focusing electrode remains at a potential of −7.5 kV. When switch  1020  is actuated to the position shown by the dashed arrow, the bias voltage of the focusing electrode is switched to −92 kV. At the same time, the anode, which is normally held at ground potential via anode bias circuit  1034  and switch  1022  (solid arrow position), is switched to −1.7 kV by actuating switch  1022  to the position shown by the dashed arrow. This results in electron beam cutoff according to the teachings of the present invention.  FIGS. 5 and 7 , described previously, illustrate a simulation of the switching between these two states. 
       FIG. 14  depicts an alternative embodiment of a power supply switching circuit for an electron beam tube in accordance with an aspect of the present invention. Cathode  1102  is coupled to a cathode heater  1118  and cathode heater power supply  1116 . The cathode  1102  is further connected to a cathode bias circuit  1130  that applies a voltage bias of −7.5 kV to the cathode via power supplies  1110  and  1114 . Focusing electrode  1104  is connected to focusing electrode bias circuit  1132 , which is in turn coupled to switch  1120 . In its normal operating position (solid arrow), switch  1120  bypasses power supply  1112  such that the focusing electrode remains at a potential of −7.5 kV. When switch  1120  is actuated to the position shown by the dashed arrow, the bias voltage of the focusing electrode is switched to −9.2 kV. In this embodiment, anode  1106  is connected to anode bias circuit  1134 , which includes a voltage divider formed by resistors R 1  ( 1140 ) and R 2  ( 1142 ) connected between −1.7 kV and ground. Because the anode does not draw significant current, R 1  can be selected to be a large value, such as 10 MΩ. If R 2  is then selected to have a value of 0Ω, the leakage current when switch  1122  is in the normal operating position (solid arrow) is only 0.17 mA. In such a configuration, the anode  1106  is held at ground potential in normal operating mode, and when switch  1122  is opened (dashed position), the anode drops to −1.7 kV, and the leakage current drops to zero. 
     In more general terms, when switch  1122  is closed (solid position), the anode is set to a bias voltage given by the voltage of power supply  1114  multiplied by R 2  and divided by R 1 +R 2 . For example, if R 2  is selected to be 1.11 MΩ, and R 1  is set at 10 MΩ, the anode bias voltage when switch  1122  is closed will be −170 V. As discussed previously, it can be desirable to bias the anode  1106  to such a voltage below ground potential in order to adjust the current emitted from the electron gun in its normal beam-on operational mode. The embodiment shown in  FIG. 14  thus provides one method of achieving this goal. In normal operating mode with switch  1122  closed, the leakage current in this example would be only 0.153 mA. When a switching operation is performed that moves switches  1120  and  1122  to the position shown by the dashed arrows, the focusing electrode potential drops to −92 kV, and the anode potential drops to −1.7 kV. This cuts off the electron beam as illustrated in the simulations discussed previously with respect to  FIGS. 5 and 7 . Of course, the resistive divider network illustrated in  FIG. 14  is merely exemplary of a design for configuring power supply switches to perform dual switching in accordance with the principles of the present invention. Other configurations would similarly fall within the scope and spirit of the present invention. 
     While the invention has been described in terms of various specific embodiments, these are simply meant to be illustrative of certain aspects of the invention. Other biasing and switching configurations are possible and would similarly fall within the scope of the present invention. It may also be desirable to switch all three of the cathode, anode, and focusing electrode in order to achieve electron beam cutoff. Similarly, electron guns operating at voltages other than 7.5 kV and switches that switch more or less than 1.7 kV would also fall within the scope of the present invention, as will be appreciated by those skilled in the art. The invention thus provides a novel apparatus and method for fully cutting off beam current within a high perveance electron beam tube while overcoming the difficulties of switching large-magnitude voltages. It should be clear to those skilled in the art that certain advantages of the invention have been achieved. Other advantages, applications, and modifications of the invention may also be evident to those skilled in the art and would also fall within the scope and spirit of the present invention. The invention is further defined by the following claims.