Abstract:
A compact multiple generator system offering high voltage, high repetition rate customizable output waveforms, including rectangular waveforms and variable pulse spacing.

Description:
[0001]    This invention was made with Government support under FA9451-07-C-006 awarded by the United States Air Force. The Government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention pertains to the field of electronic pulse generation, namely pulsed power sources, and is an improvement over existing Marx generator-type circuits that produce high voltage pulses. 
       BACKGROUND OF THE INVENTION 
       [0003]    The several variations of a Marx-type generator, commonly known in the electronics industry and herein simply defined and referred to as Marx generator, is a voltage multiplying circuit in which N capacitors are charged, with a power source, in parallel, to an input voltage V ch , after which the charged capacitors are switched into a series configuration so that the output voltage, in a temporary short burst, equals the sum of the voltages across each of the capacitors, or N·V ch . This voltage multiplication enables the designer to achieve extremely high output voltages with a relatively low input voltage power supply. 
         [0004]    Each Marx generator stage typically incorporates a switch designed to close at a predetermined voltage. At closure, the capacitor stages add, or, in the commonly understood industry terminology, “erect,” to form an overall capacitance that is equal to the individual stage capacitance divided by the number of stages, and the resultant output voltage is the individual stage voltage multiplied by the number of stages. 
         [0005]    The simple Marx generator circuit, schematically depicted in  FIG. 1 , illustrates a resistively charged circuit, or one in which the stage capacitors, C s =C stage  ( 1 ), are charged via resistive elements, R ch  ( 3 ). The stage capacitors  1  are additionally connected via switches S ( 2 ), so that with nearly simultaneous closure, the stage capacitors  1  are connected in a series configuration. The circuit is charged by input HV, and the resistive load is denoted by R Load . Thus a single stage may be defined by the stage capacitor  1 , two charging resistors  3 , and a switch  2 . For charge voltages from tens of kilovolts (kV), spark gap switches are employed. 
         [0006]    Once erected, the Marx generator dumps its energy into the load, which is resistive, capacitive, inductive, or some combination of the three, such as a lossy transmission line. Assuming a resistive load for simplicity, the voltage pulse delivered by the Marx generator, illustrated in  FIG. 2 , is characterized by the voltage risetime  4 , and a fall (or decay) time  5 , referred to as a double exponential. For many applications, this waveform is acceptable. However, for some load applications such as High Power Microwaves, or HPMs, a longer duration peak voltage, as depicted in  FIG. 3 , is desired. Typical Marx generators provide relatively short duration voltage peaks with undesirably long decay times, whereas the present invention offers customizable output waveforms. The system was first presented by the inventor at the 2009 IEEE Pulsed Power Conference, in Washington, D.C. on Jul. 2, 2009. See Mayes and Hatfield,  Development of a Sequentially Switched Marx Generator for HAM Loads , Conference Proceedings of the 2009 IEEE Pulsed Power Conference. 
         [0007]    Several geometries employ Marx generators as base devices for Pulse Forming Networks (PFNs). In a published patent application (US 2008/0036301 A1), McDonald offers a good summary of common Marx generator-based PFN geometries, but merely describes and claims switching with photon-initiated semiconductors instead of spark gap switches. 
         [0008]    Illustrated in  FIG. 4 , a Marx generator  6  is loaded by series LC tank circuits  7 , which are included to shape the double exponential waveform of  FIG. 2  into the rectangular shape of  FIG. 3 . This technique is described by McDonald in his 2008 publication, and reported by Mayes in a report to the Ballistic Missile Defense Organization, under U.S. Army contract DASG60-00-M-0082. This geometry is commonly referred to as a Type A PFN, utilizing a Marx generator with a single capacitor  8  and a single inductor  9 . Several, similar geometries employ Marx generators as base devices for Pulse Forming Networks (PFNs). 
         [0009]    Another technique replaces the simple capacitors of the Marx generator of  FIG. 1  with transmission lines  10 , shown in  FIG. 5 . This technique was first used at Sandia National Laboratory, and revisited by McDonald supra. In such geometry the transmission lines  10  are momentarily added in a manner identical to the manner in which Marx generator stages are added. However, instead of the capacitive discharge, the stacked transmission lines simultaneously release their energy, and the result is a rectangular shape having an amplitude similar to the added voltages of the transmission lines. This technique was reported by Mayes to the Defense Advanced Research Projects Agency (DARPA), in April 2002, in a final report titled “A Compact Quantum Pulse Power Module”, under DARPA/CMO contract #MDA972-01-C-0014. 
         [0010]    Another geometry uses multiple Marx generators within a PFN. As shown in  FIG. 6 , several parallel Marx generators  11  are connected via series inductors  12  in a geometry commonly referred to as a Type E PFN network. 
       SUMMARY OF THE INVENTION 
       [0011]    One objective of this present invention is the provision of a Marx-type high voltage generator that delivers a rectangular-shaped voltage pulse. 
         [0012]    A further objective of the present invention is the provision of a very compact generator. 
         [0013]    A further objective of the present invention is the provision of a Marx-type generator capable of highly flexible delivery of unique pulse shapes and load interactions. 
         [0014]    A further objective is a system in which the failure of an individual generator does not cause overall system failure. 
         [0015]    In the preferred embodiment, multiple commonly-housed Marx generators share a common output connection and are sequentially switched so that energy from each generator is uniquely or individually delivered to the common output. In the fundamental process, the generators sequentially deliver their respective energy pulses with short time delays between pulses. However, the geometry naturally lends itself to custom temporal spacing, since each generator is individually triggered by any number of various triggering, devices commonly known in the industry. See, for example, Mayes et al. (U.S. Pat. No. 7,741,735 B2). 
         [0016]    One advantage of the present invention is the use of multiple Marx generators sequentially delivering energy to a common load so that a rectangular voltage pulse is realized. The geometry of the present invention leads to a very compact configuration. 
         [0017]    An additional advantage of the present invention is the graceful failure of the device. Each Marx generator can be individually charged and controlled. If an individual Marx generator fails, the remaining generators may continue to function with a somewhat reduced width in the delivered rectangular voltage pulse. 
         [0018]    An additional advantage of the present invention is the ability to generate alternate waveforms. Since each Marx generator can be individually and uniquely charged and controlled, each generator can deliver variable amplitudes. Furthermore, each generator can be controlled to deliver its energy at any unique, selectable time. 
         [0019]    The impedance of each Marx generator is matched to the load impedance. Each Marx generator is inductively isolated from the load, either with an inductor or through geometric inductance such that no generator is affected by operation of any neighboring generator. The Marx generators are housed in a common metal vessel. 
         [0020]    The Marx generators can either share a common power supply, or each can be uniquely charged with an independent power supply. The Marx generators can be sequentially triggered from a common trigger circuit and unique trigger delay lines between each generator and the trigger circuit. Alternatively, the Marx generators can be triggered by independent trigger circuits. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic of the simple Marx generator circuit. 
           [0022]      FIG. 2  depicts a Gaussian-like, or double exponential pulse shape. 
           [0023]      FIG. 3  depicts a rectangular-shaped pulse. 
           [0024]      FIG. 4  is a schematic of a Type A pulse forming network-based Marx generator circuit. 
           [0025]      FIG. 5  is a schematic of a transmission line-based Marx generator. 
           [0026]      FIG. 6  is a schematic of a Type E pulse forming network-based Marx generator circuit. 
           [0027]      FIG. 7  depicts the formation of a rectangular-shaped pulse using a closely-spaced sequence of Gaussian pulses. 
           [0028]      FIG. 8  depicts a distorted waveform in which the Gaussian-like pulses are too closely spaced. 
           [0029]      FIG. 9  depicts a rectangular waveform with substantial ripple due to the Gaussian-like pulses being delivered too far apart. 
           [0030]      FIG. 10  is a schematic describing the present invention, in which multiple Marx generator-like circuits are individually charged and triggered to deliver unique waveforms and pulse delivery times to a common load. 
           [0031]      FIG. 11  depicts a synthesized sine wave from the present invention using dual polarity Marx generator-like circuits. 
           [0032]      FIG. 12  depicts a synthesized sine wave from the present invention using the ability to charge the individual sub-Marx generators to different voltage levels. 
           [0033]      FIG. 13  depicts a pulse-coded waveform in which a burst of eight pulses is delivered. However, several pulses are selected to not be delivered so as to form a binary code. 
           [0034]      FIG. 14  depicts the present invention configured to deliver closely spaced pulses from the sub-Marx generators to form bursts of pulses at high repetition rates. 
           [0035]      FIG. 15  depicts the present invention configured to deliver equi-spaced pulses from the sub-Marx generators. 
           [0036]      FIG. 16  depicts the present invention operating with variable temporal spacing between the pulses from the sub-Marx generators. 
           [0037]      FIG. 17  is a schematic for the single-point triggering method, utilizing a single trigger switch connected to the sub-Marx generators with unique, various-length connecting cables. 
           [0038]      FIG. 18  depicts the housing structure for the present invention. 
           [0039]      FIG. 19  depicts a cross sectional view of the internal structure, illustrating the plastic insulator and the radial placed sub-Marx generators. 
           [0040]      FIG. 20  depicts the present invention built with individually-packaged sub-Marx generators that may individually be removed from the housing. 
           [0041]      FIG. 21  depicts a Marx generator-circuit stage platter containing a capacitor, a spark gap, and the charge elements for one sub-Marx generator. 
           [0042]      FIG. 22  depicts the air handling for each platter. 
           [0043]      FIG. 23  depicts the construction of a platter capturing the key Marx generator circuit components for a single Marx generator stage. 
           [0044]      FIG. 24  depicts an assembled platter, or module, and illustrating the electrical connections between neighboring modules. 
           [0045]      FIG. 25  depicts the stacking of modules. 
           [0046]      FIG. 26  depicts the output of the preferred invention, including the tailbiter circuit and saturable inductors which provide sub-Marx generators with isolation. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0047]    A rectangular voltage pulse as in  FIG. 3  can be constructed from the sequential delivery of short duration Gaussian-like pulses like the ones depicted in  FIG. 2 . As shown in  FIG. 7 , closely spaced pulses  13  can produce a substantially rectangular waveform  14 . With careful design, the capacitance of the load will integrate, or smooth the waveform to more closely approximate the rectangular waveform. To achieve high voltage levels, Marx generators are used to generate the short duration pulses, and multiple Marx generators can be sequentially triggered to deliver the closely spaced Gaussian-like pulses  13 . 
         [0048]    The timing of the pulse arrival at the load is not necessarily critical; however, the timing does affect the amount of ripple and distortion that will be seen on the flattop portion of the waveform. Gaussian-like pulses  15  delivered too closely will result in more dramatic peaks in the pulse  16  delivered to the load, as illustrated in  FIG. 8 ; and Gaussian-like pulses  17  delivered with too much separation will result in more dramatic valleys in the pulse  18  delivered to the load, as illustrated in  FIG. 9 . By carefully tuning the delivery time of each pulse, the ripple of the rectangular pulse can be minimized. 
         [0049]    The schematic of  FIG. 10  provides a simple circuit description of the present invention. In general, multiple Marx generators  19 , each now referred to now as “sub-Marx” generators, are placed in a parallel configuration and connected to a common output load  20 . Between each sub-Marx generator  19  and the common load connection  20  should be an inductive isolation element  21  that protects each sub-Marx generator  19  from neighboring sub-Marx generator effects such as pre-triggering. 
         [0050]    The preferred embodiment of this invention powers each sub-Marx generator  19  with an individual power supply  22  and triggers each sub-Marx generator with an individual trigger unit  23 . There are several advantages of providing each sub-Marx generator with its own power supply and trigger source—namely, graceful failure of the system, unique waveform generation, and source impedance flexibility. 
         [0051]    Graceful failure is a unique concept to pulse power systems, since typical pulse power systems cease to function with the failure of any single component. In the present invention the pulse power system is comprised of multiple sub-Marx generators, each operating autonomously, and thus, operating with redundancy. Thus, if one sub-Marx generator fails, it does not bring the whole system down. Instead, the system continues operating with one less sub-Marx generator. 
         [0052]    Since each sub-Marx generator is charged and triggered independently of neighboring sub-Marx generators, output waveform, spacing, and timing flexibility are inherent. In general, each sub-Marx generator can be charged to deliver a wide range of voltages of positive or negative polarity. Each sub-Marx generator can be triggered to deliver energy at any point in time, or it can be selectively silenced. Non-exclusive system variability can include, but is not limited to the example waveforms depicted in  FIGS. 11-16 . 
         [0053]      FIG. 1I  depicts closely-space bipolar pulses, or a positive polarity Gaussian-like pulse  24  followed by a negative polarity Gaussian-like pulse  25  that together simulate a sine wave  26 . The bipolar pulses are achieved using dual polarity charging power supplies.  FIG. 12  demonstrates the invention&#39;s capability to vary the magnitude of the charge voltage on each sub 
         [0054]    Another advantage provided by the individual triggering feature of this invention is impedance matching. A system designed for use with a certain impedance load has the flexibility to be used with loads of various other impedances. The individual sub-Marx generators can all be constructed with identical or different impedances, and those various impedances can be selectively combined for a desired output impedance through the selective triggering capability of this invention. 
         [0055]    The pulse power system of this invention may also rely on a single power supply and a single triggering unit. A single power supply is simply connected to the parallel sub-Marx generators. However, such an embodiment lacks the capability to charge the sub-Marx generators with different voltage levels. Similarly, a single trigger unit may be used to trigger the multiple sub-Marx generators. However, as depicted in  FIG. 17 , sequential generator triggering requires that the trigger connections for the individual sub-Marx generators  28  (Marx  1 ,  2 ,  3 , and  4 ) have unique predetermined electrical transmission properties. For example, the lengths of the trigger connection cables that connect each sub-Marx generator to the main trigger switch  32  can be chosen for provision of a desired trigger delay time for each sub-Marx generator. Marx  1  generator might be triggered at 10 ns, with trigger cable  27  having an approximate length of 2.5 m. Marx  3  might have a trigger cable  31  approximately 11.7 in long 
         [0056]    The preferred embodiment of this invention localizes the sub-Marx generators into a common conductive housing structure, as shown in  FIG. 18 . Ancillary components such as a power supply, or power supplies, and the triggering unit, or triggering units, are located in a separate but connected conductive housing. This configuration minimizes the volume required for the system. 
         [0057]    The sub-Marx generators  33  housed in a common containment structure are radially located inside the cylindrical housing  34 , shown in  FIG. 19 . The preferred embodiment lines the inside of the cylinder with a plastic material  35  to prevent the sub-Marx generators  33  from arcing to the cylinder  34 , thus short circuiting the Marx generator circuit. The plastic material  35  is preferred over air insulation, so that the sub-Marx generator  33  can be located very close to the ground potential provided by the electrically conductive cylinder  34 . Such grounding is referred to as capacitive coupling to the ground potential. 
         [0058]    Capacitive coupling the sub-Marx generators to the ground potential is an important feature of the present invention system. Without a strong reference to the ground potential, triggering any sub-Marx generator can cause all of the other sub-Marx generators to self-trigger. However, with a good reference to the ground potential, self-triggering of sub-Marx generators can be avoided. 
         [0059]    The sub-Marx generators  33  can be individually packaged, so that each sub-Marx generator  33  can be individually removed from the central housing  34 , as depicted in  FIG. 20 . The geometry of this alternate embodiment provides for easy construction and maintenance. However, the preferred embodiment of this invention integrates like stages of each sub-Marx generator into a single disc-like structure, or platter. This embodiment provides for a geometry much more compact than that of the  FIG. 20  embodiment. For example, a system of 8 sub-Marx generators, each comprised of 20 Marx generator stages, would consist of 20 platters, with each platter holding one stage for each of the 8 sub-Marx generators, including the spark gap  38 , the stage capacitor  39 , and the charging elements  40 , as depicted in  FIG. 21 . The stage platters stack vertically to complete the cylindrical system package. 
         [0060]    Since the sub-Marx generators are located radially near the cylindrical housing structure, the central area of each platter  41  is available and used as a central air duct  42 . As depicted in  FIG. 22 , material is removed from this region and o-ring seals  43  are located so that air does not escape from between the stage platters  41 . For each stage platter  41 , small holes  44  are drilled from the central duct  42  to each spark gap switch region  45 , so that during the operation of the system, fresh air flows into the spark gap region  45 . 
         [0061]    The side view of the pre-assembled stage insulator is shown in  FIG. 23 . Two machined ABS discs, a top plate  46  and a bottom plate  47 , encompass the parallel sub-Marx generator stage capacitors  48 . “Tongue and groove” slots  49  are designed to ensure electrical isolation between neighboring sub-Marx generators.  FIG. 24  is a side view of the stage insulator assembly  50  showing insulated stage charge interconnections. Male charge connections  51  connect to the female charge connections  52  of the adjacent (next-in-line) Marx generator stage.  FIG. 25  depicts several platter assemblies, or modules  53 , stacked together, with o-rings  54  between each platter for sealing of the central air duct. 
         [0062]    The output section is defined by two key components—the isolation platter and the tailbiter, or crowbar switch. Shown in  FIG. 26 , the isolation platter encases the isolation inductors in a manner similar to that in which the generators are encased. The isolation platter makes the common electrical connection between the sub-Marx generators, before making contact with the output-feed through. 
         [0063]    The output feed-through is designed with a tailbiter circuit including an integrated crowbar switch, which is included to produce a more dramatic fall time on the output voltage pulse. The crowbar switch should have extremely low inductance. The preferred embodiment, shown in  FIG. 26 , uses a spark gap switch  55 , aided with a saturable inductor  56 . In this configuration most of the voltage drop will be realized across the inductor  56 ; however, once the inductor  56  saturates, the spark gap  55  will be over-voltaged and will close, thus short circuiting the system and extinguishing the voltage on the load. Alternatively, a single magnetic saturable switch can be designed to shunt the voltage at the appropriate time. Either method will quench the trailing voltage tail of a rectangular pulse. 
         [0064]    Each sub-Marx generator connects to the final platter  57  via a spring interconnection  58 . A small saturable ring  59 , such as a ferrite torroid, is placed around the electrical feed  60  to provide some isolation from neighboring sub-Marx generators. On the output side of each saturable element  59 , a common plate  61  connects all sub-Marx generators to the common output feed  62 .