Abstract:
A linear beam device having a multi-stage depressed collector is provided with a single crowbar circuit that quickly removes all voltages from the collector electrodes upon detection of an arc between the collector electrodes. The linear beam device comprises a cathode and an anode spaced therefrom that are operable to form and accelerate an electron beam. A collector having a plurality of successive collector electrodes is arranged downstream from the anode to collect electrons of the electron beam. At least one power supply is coupled to the collector and provides a plurality of distinct voltage levels to respective ones of the collector electrodes. The power supply comprises a plurality of serially coupled filter capacitors, with each one of the filter capacitors being charged to a respective difference between adjacent ones of the distinct voltage levels. A crowbar circuit is coupled across the plurality of filter capacitors. The crowbar circuit reduces the total voltage across the plurality of filter capacitors to zero upon detection of an arc between any two of the collector electrodes. According to an embodiment of the invention, the power supply further comprises a plurality of diodes respectively coupled across the plurality of serially coupled filter capacitors. The diodes prevent reversal of voltage of a corresponding one of the filter capacitors upon operation of the crowbar circuit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to linear beam devices having multi-stage depressed collectors. More particularly, the invention relates to a crowbar circuit for use in protecting a linear beam device from damage due to arcing between the internal electrodes by quickly removing voltages from each of the electrodes upon detection of an arcing condition. 
     2. Description of Related Art 
     Linear beam electron devices are used in sophisticated communication and radar systems that require amplification of a radio frequency (RF) or microwave electromagnetic signal. Conventional klystrons, traveling wave tubes and inductive output tubes are examples of linear beam electron devices. In a linear beam device, an electron beam originating from an electron gun having a cathode is accelerated by a voltage differential with an anode spaced from the cathode. The electron beam passes through a drift tube containing an RF interaction structure. The electron beam may be amplitude modulated by applying an RF input signal between a grid and the cathode. Alternatively, the RF interaction structure of the drift tube may further include an RF circuit used to induce a modulation on the electron beam. Either way, the modulation results in electron bunching due to electrons that have had their velocity increased gradually overtaking those that have been slowed. The accelerated electrons of the electron beam give up varying amounts of their energy to the RF electric fields of traveling or standing wave circuits of the RF interaction structure. The energy removed from the electron beam in this manner may be subsequently removed from the device in the form of an amplified RF signal. 
     A known technique for increasing the efficiency of a linear beam device is to collect the electrons of the electron beam after they pass through the RF interaction structure using a multi-stage depressed collector. A multi-stage depressed collector includes plural collector electrodes that are each connected to power supplies that provide DC voltages ranging between the cathode potential and the maximum potential to which the electrons of the beam were accelerated. The spent electrons of the beam have various energy levels remaining as they exit the RF interaction structure, and the electrons are collected on individual ones of the collector electrodes having DC voltage levels corresponding to their remaining energy level. This way, it is possible to recover some of the remaining energy of the spent electrons that was not given up to the RF electric fields without losing this energy from conversion into heat. 
     A drawback with high power linear beam devices is that they sometimes arc internally, and it then becomes necessary to quickly remove all voltages from the device in order to prevent damage to the internal electrodes. By way of example, an electric arc that delivers one Joule of energy to a copper electrode can leave a hemispherical pit slightly over one millimeter in diameter. The filter capacitors of the power supplies -connected to the electrodes of a high power linear beam device typically store energy on the order of thousands of Joules. Thus, an arc discharge from one of the filter capacitors could cause substantial damage to the internal electrodes. To avoid such catastrophic events from occurring, it is known to connect each filter capacitor to a spark-gap, or “crowbar,” circuit to discharge the capacitor and thereby remove the stored energy very quickly (i.e., within a few microseconds) after an arc is sensed. A single crowbar circuit has been impractical when multi-stage depressed collectors are used with a linear beam device, since each collector electrode generally utilizes a separate power supply and an associated filter capacitor to provide the corresponding voltage, and thus a separate crowbar circuit would generally be included with each filter capacitor. This duplication of circuitry tends to substantially increase the complexity and hence the cost of equipment using such a high power linear beam device. 
     Accordingly, it would be very desirable to provide a linear beam device with a multi-stage depressed collector in which only a single crowbar circuit is needed to remove voltages from all of the internal electrodes of the device upon the detection of an arc. 
     SUMMARY OF THE INVENTION 
     The present invention provides a linear beam device having a multi-stage depressed collector in which a single crowbar circuit can quickly remove all voltages from the collector electrodes upon detection of an arc. 
     More particularly, the linear beam device comprises a cathode and an anode spaced therefrom. The anode and cathode are operable to form and accelerate an electron beam. A collector having a plurality of successive collector electrodes is arranged downstream from the anode to collect electrons of the electron beam. A power supply conditions electrical power from an input AC line and provides a plurality of distinct voltage levels to respective ones of the collector electrodes relative to the cathode. The power supply comprises a plurality of serially coupled filter capacitors, with each one of the filter capacitors being charged to the difference between adjacent ones of the distinct voltage levels. A single crowbar circuit is coupled across the plurality of filter capacitors. The crowbar circuit reduces the total voltage across the plurality of filter capacitors to zero upon detection of an arc between any two of the collector electrodes. 
     According to an embodiment of the invention, the power supply further comprises a plurality of diodes respectively coupled across the plurality of serially coupled filter capacitors. The diodes prevent reversal of the voltage of a corresponding one of the filter capacitors upon operation of the crowbar circuit. In another embodiment of the invention, a current sensing device is associated with plural ones of the collector electrodes in order to detect arc current between any two of the collector electrodes. The current sensing device can detect arc current flowing either to or from at least one of the collector electrodes. The current sensing device may further comprise a transformer coupled to at least one of said collector electrodes and a bridge rectifier coupled to the transformer. 
     A more complete understanding of the crowbar circuit for a linear beam device having a multi-stage depressed collector will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings that will first be described briefly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a first embodiment of a linear beam device having a multi-stage depressed collector with a single crowbar circuit coupling all electrode stages; 
     FIG. 2 is a block diagram of a second embodiment of a linear beam device as in FIG. 1, with diodes used to block voltage reversal of filter capacitors during operation of the crowbar circuit; and 
     FIG. 3 is a block diagram of a third embodiment of a linear beam device as in FIGS. 1 and 2, with current sensing devices used for triggering the crowbar circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention satisfies the need for a crowbar circuit for a linear beam device having a multi-stage depressed collector to remove voltages from each of the internal electrodes upon the detection of an arc. In the detailed description that follows, like element numerals are used to describe like elements illustrated in one or more of the figures. 
     Referring first to FIG. 1, an exemplary linear beam device  10  comprises a generally cylindrical-shaped tube having a thermionic cathode  12  disposed at a first end thereof. A control grid  14  is positioned closely adjacent to the surface of the cathode  12 , and an anode  16  is disposed downstream from the cathode and control grid. The anode  16  defines an opening into a drift tube through which a beam of electrons emitted by the cathode  12  are drawn. The drift tube includes a gap that is coupled to a cavity  18 . The linear beam device  10  further includes a multi-stage depressed collector that includes a plurality of collector electrode stages  22 ,  24 ,  26 ,  28  and  32 . As shown in FIG. 1, the anode  16  is coupled to ground, and a highly negative voltage (V K ) is applied to the cathode  12 , which draws a beam of electrons therefrom. The electron beam may be density modulated by a control signal applied to the grid  14 , thereby inducing an output voltage in the cavity  18 . After passing the cavity  18 , the electrons of the electron beam are collected on the various electrode stages  22 ,  24 ,  26 ,  28  and  32 . 
     The electrode stages each have different respective voltages applied thereto ranging between the anode voltage (e.g., ground) and the cathode voltage (V K ). More particularly, the first electrode stage  22  is electrically connected to the anode  16  and is at ground potential. V 1  is the potential between the first and the second electrode stages  22 ,  24 . V 2  is the potential between the second and third electrode stages  24 ,  26 . V 3  is the potential between the third and fourth electrode stages  24 ,  26 . V 4  is the potential between the fourth and fifth electrode stages  26 ,  28 . V 5  is the potential between the fifth electrode stage  32  and the cathode  12 . The cathode voltage V K  is equal to the sum of the foregoing voltage potentials V 1  through V 5 . As known in the art, the voltage applied to each successive collector electrode is generally an increasing percentage of the cathode voltage, with the fifth electrode stage  32  being at or near the cathode voltage (V K ). The current flowing to/from the electrodes stages  24 ,  26 ,  28 , and  32  is identified in FIG. 1 as I 2 , I 3 , I 4 , and I 5 , respectively. The current flowing to the cathode  12  is identified as I K , and the current flowing to ground is identified as I 1 . 
     Persons having ordinary skill in the art will recognize the exemplary linear beam device  10  illustrated in FIG. 1 as comprising an inductive output tube. An example of an inductive output tube having a multi-stage depressed collector is provided by U.S. Pat. No. 5,650,751, for INDUCTIVE OUTPUT TUBE WITH MULTISTAGE DEPRESSED COLLECTOR PROVIDING NEAR CONSTANT EFFICIENCY, which is incorporated by reference herein. Nevertheless, it should be appreciated that the present invention is not limited to use with inductive output tubes. Particularly, it is anticipated that the present invention be utilized with any type of linear beam device utilizing a multi-stage depressed collector, such as a klystron or traveling wave tube. 
     Further, the number of collector electrodes shown in FIG. 1 are entirely exemplary, and the present invention would be applicable to a linear beam device having any number of electrodes. 
     As shown in FIG. 1, the voltages V 1  through V 5  are provided by separate power supplies  42 ,  52 ,  62 ,  72  and  82  coupled together in series. The power supplies  42 ,  52 ,  62 ,  72  and  82  receive an alternating current (AC) input that may be single-phase, bi-phase or poly-phase. The power supplies may further comprise half-wave or full-wave rectification as generally known in the art, depending upon the particular power requirements of the linear beam device  10 . Each power supply provides a direct current (DC) output between positive (+) and negative (−) output terminals thereof, with a filter circuit comprising a capacitor and an inductor coupled to the output terminals. Particularly, power supply  42  has a filter circuit comprising inductor  44  and capacitor  46 ; power supply  52  has a filter circuit comprising inductor  54  and capacitor  56 ; power supply  62  has a filter circuit comprising inductor  64  and capacitor  66 ; power supply  72  has a filter circuit comprising inductor  74  and capacitor  76 ; and power supply  82  has a filter circuit comprising inductor  84  and capacitor  86 . The filter capacitors  46  through  86  are coupled together in series. The aforementioned voltages V 1  through V 5  are thereby defined across the filter capacitors  46  through  86 , respectively. The power supplies  42  through  82  may be provided by variable voltage transformers, power conditioners or phase control circuits using solid state or gas tube components, enabling the DC output voltages to be varied as desired by a particular application. Alternatively, if variable voltage control is not desired, the power supplies could be provided by fixed transformers, or a single transformer having plural secondary windings with one or more associated rectifier circuits. Such power supplies are generally well known in the art, and further description herein is therefore deemed unnecessary. 
     For the reasons described above, it is desirable to reduce all of the voltages V 1  through V 5  to zero as quickly as possible upon detection of an arc between any two of the aforementioned collector electrodes  22 ,  24 ,  26 ,  28 , and  32 , or between the cathode  12  and the anode  16  connected to the first collector electrode  22 . To accomplish this, in a first embodiment of the invention, a single crowbar circuit  40  is coupled across the serially coupled capacitors  46  though  86 . More particularly, the crowbar circuit  40  has a first lead connected to the positive terminal of the first power supply  42  and a second lead connected to the negative terminal of the fifth power supply  82  such that the entire voltage across the capacitors  46  through  86  may be shorted to zero. The crowbar circuit  40  comprises a high speed switch, and may be provided by an ionized plasma switch, such as a tetrode, pentode or triode thyratron, or ignition or triggered spark gap device, or a solid state switch, such as a field effect transistor (FET). 
     A drawback of this first embodiment of the invention is that the crowbar circuit  40  does not necessarily shunt all of the stored energy of the capacitors to ground, even though the total voltage across the serially coupled capacitors equals zero. When the crowbar circuit  40  closes, the same current will flow through all of the capacitors  46  through  86 . If the capacitors are of unequal capacitance and/or are charged to unequal voltages, the discharge will end when there is no voltage across the crowbar circuit  40 , i.e., the sum of the voltages V 1  through V 5  will be equal to zero. The final voltages (V f ) across the capacitors nevertheless will be positive and negative as determined in accordance with the following equation:          V   f     =       V   0     +       1   C            ∫   0   ∞          i           t                                    
     in which i is the current, C is the capacitance of the individual capacitor, V 0  is the initial voltage across the capacitor, and wherein the current i falls to zero at time t equal to infinity. For a string of n capacitors of differing capacitance (C i ), charged to differing initial voltages (V 0i ), connected in series so that a total voltage (V t ) and a capacitance (C t ) exist across the string, the final voltage (V fi ) across each capacitor following shorting by the crowbar is determined in accordance with the following equation:          V   fi     =       V     0      i       -       V   t            C   t       C   i                                  
     Notably, the final voltage (V fi ) across each capacitor is independent of any resistance in the circuit. This will not provide the desired situation after a crowbar operation. For example, if an arc should occur between two of the collector electrodes of FIG. 1 (e.g., electrodes  24 ,  26 ), the capacitor  56  between these two electrodes would be partially discharged by the arc before the crowbar circuit  40  is triggered. Then, when the crowbar circuit  40  triggers, charge present in the other capacitors will recharge the capacitor  56  between the arcing electrodes with the opposite polarity. As a result, each of the capacitors will be charged to a final voltage (V f ), even though the total voltage will be zero. 
     FIG. 2 illustrates a second embodiment of the invention that is substantially identical to the first embodiment described above, except that a high-voltage, high-current diode is coupled across each respective capacitor. Particularly, diode  48  is coupled across capacitor  46 , diode  58  is coupled across capacitor  56 , diode  68  is coupled across capacitor  66 , diode  78  is coupled across capacitor  76 , and diode  88  is coupled across capacitor  86 , with the anode of each respective diode coupled to the positive (+) terminal of the corresponding power supply. All other elements of FIG. 2 are otherwise identical to FIG. 1 described above, and description of these elements is therefore not repeated herein. When the crowbar circuit  40  is triggered, current will flow through a corresponding diode after the voltage across the associated capacitor falls to zero, thereby preventing the capacitor from being recharged to a negative voltage as occurred in the embodiment of FIG.  1 . Accordingly, when the crowbar circuit  40  is triggered, the voltage across each capacitor will fall to zero in a succession determined by the amount of charge in each capacitor and its corresponding capacitance. As a result, all arcs occurring in the linear beam device  10  will be extinguished and the final voltage (V f ) across each capacitor will be zero. 
     FIG. 3 illustrates a third embodiment of the invention that is substantially identical to the second embodiment described above, further including plural current sensing devices to detect arcing conditions on the various electrodes of the linear beam device  10  and a trigger generator  50 . The crowbar circuit  40  is triggered by arc current flowing in any two or more of the leads from the power supplies to the corresponding collector electrodes. Such arc currents may be sensed with a current transformer on each lead, but the direction of the current flow cannot be known in advance except in the most positive and most negative leads. Thus, in an embodiment of the invention, current sensing devices are provided on certain leads in order to detect arc current flowing either to or from a corresponding electrode. The trigger generator  50  receives signals from plural current sensing devices and provides a trigger signal to the crowbar circuit  40  if the detected current exceeds a predetermined level. 
     Particularly, the linear beam device  10  is provided with current sensing devices  150 ,  160 ,  170 ,  180  and  190 . Current sensing device  150  detects the current  12  flowing to/from the second collector electrode  24 ; current sensing device  160  detects the current  13  flowing to/from the third collector electrode  26 ; current sensing device  170  detects the current  14  flowing to/from the fourth collector electrode  28 ; current sensing device  180  detects the current  15  flowing to the fifth collector electrode  24 ; and current sensing device  190  detects the current I K  flowing to the cathode  12 . The current sensing device  150  comprises a transformer  152  and a bridge rectifier comprising diodes  154 ,  155 , 156  and  157 . The bridge rectifier provides a DC current signal to the trigger generator  50 . The current sensing devices  160  and  170  have identical construction as the current sensing device  150 , and are adapted to detect current flowing either to or from the corresponding collector electrode. For example, the current sensing device  150  is adapted to detect current flowing either to or from the second collector electrode  24 , since an arc may occur either between the first and second collector electrodes  22 ,  24  or between the second and third collector electrodes  24 ,  26 . The current sensing devices  180  and  190  are adapted to detect current flowing in only a single direction, as they are coupled to the most positive and negative electrodes, respectively. Current sensing device  190  includes a transformer  192  and a diode  194 , and current sensing device  180  has substantially identical construction. It should be appreciated that other known types of current sensing devices, such as utilizing series resistors rather than transformers, could also be advantageously utilized. 
     Arc current detected by any one of the current sensing devices  150 ,  160 ,  170 ,  180  and  190  that exceeds a predetermined threshold will be sufficient to cause the trigger generator  50  to generate the trigger signal. The diodes of the current sensing devices  150  through  190  (see, e.g., diodes  156 ,  157  of current sensing device  150 , and diode  194  of current sensing device  190 ) preclude current from flowing back into the current sensing devices from any other one of the current sensing devices. As a result, the current sensing devices provide a logical OR function, whereby detected arc current from current sensing device  150  or current sensing device  160  or current sensing device  170  or current sensing device  180  or current sensing device  190  will cause the trigger generator  50  to generate the trigger signal. 
     Having thus described a preferred embodiment of a crowbar circuit for a linear beam device having a multi-stage depressed collector, it should be apparent to those skilled in the art that certain advantages of the within system have been achieved. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. The invention is defined by the following claims.