Abstract:
A bipolar pulse generator is implemented in a simple structure while providing a high efficiency design having a relatively low total size, while still allowing access by fibers used to control a photoconductive switch that activates the generator. The bipolar pulse generator includes a stacked Blumlein generator structure with an additional transmission line connected to a load at its near end and short-circuited at its distal end. An extra transmission line is positioned between the Blumlein generator&#39;s structure and the load provides specified limited gap between positive and negative sub-pulses. The bipolar pulse generator further includes a bended Blumlein generator structure, in which an existing intrinsic “stray” transmission line is used to provide the bipolar pulse. Still further, bipolar pulse generator includes stepped transmission lines, with additional switches positioned between steps, which are charged by different voltages.

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a divisional of and claims the benefit of priority to prior U.S. patent application Ser. No. 12/404,061 by inventor Simon Y. London, entitled “HIGH POWER BIPOLAR PULSE GENERATORS” filed on Mar. 13, 2009. 
    
    
     BACKGROUND 
     The present invention relates in general to pulse generators. More specifically, the present invention relates to bipolar pulse generators that provide a high power/energy pulse on a load. 
     Recent trends in the development of pulse power microwave sources for a variety of applications have been directed to increasing power, efficiency and energy on the load. Transmission line pulse generators with different kinds of fast switches, including light activated photoconductors, can achieve some of the best results in generating high power short duration pulses. For a given limited charging voltage of transmission lines defined by high-current switches, high powered and high energy density transmission lines imply low characteristic impedances. This low range of characteristic impedances, however, frequently causes problems for coupling with typically used load impedances, 50 ohm or higher, for example, radiating impedances, which introduces a problem with efficient high ratio impedance transformation. 
     Bipolar pulse generators very often have significant advantages compared to unipolar pulse generators, with just one example being UWB radiation. Further, there are many potential applications of bipolar pulse generators, for example in industry, physics and medicine, where very often bipolar pulse generators with time separation between positive and negative sub-pulses are preferable or required. Today, however, there are only various types of high power and high energy unipolar pulse generators (Marx generator and stacked Blumlein generator in various modifications). 
     For example, a high energy Marx generator with coaxial cable to provide rectangular unipolar pulse is known and described in “A PFN Marx Generator Based on High-Voltage Transmission Lines”, by S. M. Turnbull et al., presented in Meas. Sci. Technol.11 (2000) N51-N55. Further, a stacked Blumlein generator with a single switch has been proposed in U.S. Pat. No. 2,769,101 issued to R. D. Drosd. This type of generator has been designed and presented in various publications including, for example, “Modeling of Wound Coaxial Blumlein Pulsers”, by Jose O. Rossi et al:, published in IEEE Transactions on Plasma Science”, Vol. 34, No. 5, October 2006, “Design of a 150 kV, 300 A, 100 Hz Blumlein Coaxial Pulser for Long Pulse Operation”, presented in IEEE Transactions on Plasma Science”, Vol. 30, No. 5, October 2002. Still further, some modifications of stacked (cascade) Blumlein generators are presented in “A Combined High-Voltage, High-Energy Pulse Generator”, by S. J. MacGregor et al., published in “Meas. Sci. Technol” 5 (1994), pp. 1580-1582, and “A Novel HV Double Pulse Modulator”, published in “Meas. Sci. Technol” 5 (1994), pp. 1407-1408. Finally, another type of high-power generator, namely, a “Multi-Stage Blumlein” is proposed by J. Yampolsky in US Patent Application 2005/0174715 A1, 2005. The content of each of the above-reference documents is incorporated herein by reference. All of the above-referenced generators produce only a unipolar pulse and do not provide voltage (impedance) transformation, with the exception of the proposed multi-stage Blumlein disclosed in US Patent Application 2005/0174715A1, which provides moderate transformation but requires a substantial number of switching devices. 
     A transmission line “High-Voltage Pulses Generator” has also been described in SU Patent 1098502 A1 issued to Bosamykin V. S. et al, 1996, which provides bipolar pulse by a single switch. However, the power/energy on load is much less compared to that provided by the above-mentioned unipolar generators. In addition, impedance transformation in the device is low. 
     The applicant has also previously described a transmission line in U.S. Patent Application 2007/0165839 A1 entitled “Bipolar Pulse Generators With Voltage Multiplication”, which provides a device with a single switch with all of the required voltage/impedance transformation. However, in a stacked configuration with several switches, the energy provided by this type of generator is less compared to the above mentioned Blumlein-based stacked unipolar generators with less number of switches. 
     Accordingly, there remains a need for a bipolar pulse generator solution based on voltage charged transmission lines, which provides high power and high energy. Further, there remains a need for high power/energy bipolar pulse generator, which can provide voltage/impedance transformation. Still further, there remains a need for a high power/energy bipolar pulse generator with pulse separation between positive and negative sub-pulses. 
     It would be desirable to provide a bipolar pulse generator that could meet all of the above needs while being implemented in a simple structure, preferably with a single switch, and preferably in a high efficiency design that has a relatively low total size, while still allowing simple access by fibers to a closing photoconductive switch that actuates the bipolar pulse generator. 
     SUMMARY OF THE INVENTION 
     The present invention provides a bipolar pulse generator that can be implemented in a simple structure while providing a high efficiency design having a relatively low total size and still allowing access by fibers used to control a photoconductive switch that activates the generator. 
     In a preferred embodiment of the invention, a bipolar pulse generator includes a stacked Blumlein generator structure with an additional transmission line connected to a load at its near end and short-circuited at its distal end. An extra transmission line is positioned between the Blumlein generator&#39;s structure and the load provides specified limited gap between positive and negative sub-pulses. 
     According to a further preferred embodiment of the present invention, the bipolar pulse generator further includes a bended Blumlein generator structure, in which an existing intrinsic “stray” transmission line is used to provide the bipolar pulse. 
     According to a still another embodiment of the present invention, the bipolar pulse generator consists of stepped transmission line with additional switches positioned between steps, which are charged by different voltages. 
     The bipolar pulse generator according to the invention generates high power/energy pulses in a compact design with access to fibers for activating photoconductor switches. Bipolar pulse generators according to the invention are useful for HPM generation, in particle accelerators and in other high voltage physical, industrial, medical and test instruments. 
     Other features, uses, advantages, embodiments, etc. of the invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to certain preferred embodiments thereof along with the accompanying figures, wherein: 
         FIG. 1  depicts a schematic of classic n-stacked Blumlein pulse generator according to the prior art; 
         FIG. 2  depicts a schematic of double stacked bipolar pulse generator according to the prior art; 
         FIG. 3  depicts a schematic of two-step bipolar pulse generator as an expansion of the generator in  FIG. 2  for impedance transformation according to the prior art; 
         FIG. 4  depicts a schematic of double Blumlein pulse generator according to the prior art; 
         FIG. 5  depicts a schematic of two series connected double Blumlein pulse generators according to the prior art; 
         FIG. 6  depicts a schematic of bipolar pulse generator with two switches positioned in first two successive steps according to the prior art; 
         FIG. 7   a  depicts a schematic of transmission line Marx-based bipolar pulse generator according to an embodiment of the present invention; 
         FIG. 7   b  depicts a pulse shape on the load of generator shown on  FIG. 7   a;    
         FIG. 8   a  depicts a schematic of an n-stacked Blumlein based bipolar pulse generator according to an embodiment of the present invention; 
         FIG. 8   b  depicts a pulse shape on the load of generator shown on  FIG. 8   a;    
         FIG. 9  depicts a schematic of a three-step, two-stacked Blumlein based bipolar pulse generator according to an embodiment of the present invention; 
         FIG. 10  depicts a table of normalized element&#39;s values of an N-step, two-stacked Blumlein-based bipolar pulse generators according to an embodiment of the present invention; 
         FIG. 11   a  depicts a schematic of a double Blumlein-based bipolar pulse generator according to an embodiment of the present invention; 
         FIG. 11   b  depicts a pulse form on the load for generator according to  FIG. 11   a;    
         FIG. 12   a  depicts a schematic of a double Blumlein-based bipolar pulse generator with their intrinsic transmission lines according to an embodiment of the present invention; 
         FIG. 12   b  depicts a pulse form on the load for generator according to  FIG. 12   a;    
         FIG. 13   a  depicts a schematic of two series connected double Blumlein based bipolar pulse generators with their intrinsic transmission lines according to an embodiment of the present invention; 
         FIG. 13   b  depicts a schematic of two series connected double Blumlein based bipolar pulse generators in case of neglecting of intrinsic transmission lines according to an embodiment of the present invention; 
         FIG. 14  depicts a schematic of a series connected N double Blumlein based bipolar pulse generators with their intrinsic transmission lines according to an embodiment of the present invention; 
         FIG. 15   a  depicts a schematic of double single-stage bipolar pulse generator with their intrinsic transmission lines that provides a bipolar pulse without a gap between sub-pulses according to an embodiment of the present invention; 
         FIG. 15   b  illustrates the pulse form on load for generator according to  FIG. 15   a;    
         FIG. 16  depicts a schematic of series connected two single-stage double bipolar pulse generators with their intrinsic transmission lines according to an embodiment of the present invention; 
         FIG. 17  depicts a schematic of series connected two single-stage, double bipolar pulse generators without (neglecting) their intrinsic transmission lines according to an embodiment of the present invention; 
         FIG. 18   a  depicts a schematic of two-step, double bipolar pulse generators with their intrinsic transmission lines according to an embodiment of the present invention; 
         FIG. 18   b  illustrates the pulse form on the load provided by the generator according to  FIG. 18   a;    
         FIG. 19  depicts a schematic of three-step bipolar pulse generator with two switches in first two successive steps and with the gap between sub-pulses equal to the length of sub-pulse as an embodiment of the present invention; 
         FIG. 20  illustrate the table of multi-steps bipolar pulse generators with two switches in first two successive steps and with the gap between sub-pulses equal to the length of sub-pulse according to an embodiment of the present invention; 
         FIG. 21   a  depicts a totally folded design of N-step bipolar pulse generator with two switches in first two successive steps and with the gap between sub-pulses equal to the length of sub-pulse according to an embodiment of the present invention; and 
         FIG. 21   b  depicts a partial-folded design of N-step bipolar pulse generator with two switches in first two successive steps and with the gap between sub-pulses equal to the length of sub-pulse according to an embodiment of the present invention 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  depicts a well-known stacked Blumlein pulse Generator that provides a high-energy unipolar pulse on a matched load.  FIG. 2  depicts a schematic of a prior art double stacked bipolar pulse generator of the type described in US Patent Application 2007/0165839 A1. The stored energy and the energy on the load, however, is 75% of the energy provided by double stacked (n=2) unipolar generator presented on  FIG. 1 .  FIG. 3  depicts a schematic of a prior art bipolar pulse generator, which is an extended type of generator shown in  FIG. 2  with and additional impedance transformation step.  FIG. 4  depicts a schematic of a prior art unipolar pulse generator, which is a double Blumlein pulse generator (with interconnected open-circuited charged transmission lines) presented by S. J. MacGregor et al. discussed above.  FIG. 5  depicts a schematic of a prior art unipolar pulse generator, which is two series connected double Blumlein pulse generators (with interconnected open-circuited charged transmission lines) presented by S. J. MacGregor et al. discussed above.  FIG. 6  depicts a schematic of a prior art bipolar pulse generator with two switches in first two successive steps presented in US 2007/0165839 A1. The power/energy of the generating pulse is not a maximum that could be achieved in similar structure with two switches positioned in first two successive steps. The invention will be described in part with reference to prior art structures such as those discussed above. 
       FIG. 7   a  is a schematic of Marx-based transmission line bipolar pulse generator according to an embodiment of the present invention. The generator may consist of any number (n) of identically charged transmission lines  10 . Each transmission line  10  is connected to a corresponding individual switch  11  and to a corresponding individual charging element  12  (resistor R or inductance L). Instead of typical direct connection to the load  15 , the load  15  is connected through an additional transmission line  13  of a specified length. In addition, a transmission line  14  is connected to the load  15  at its near end and is short-circuited at its distal end. The electrical length of the transmission line  14  is equal to the sum of length of each charged line  10  and the length of line  13 . The described arrangement provides a specified gap between positive and negative sub-pulses that is equal double the transit time of transmission line  13 . 
     In operation, all of the charged transmission lines  10  are charged by their individual charging element  12 . Once all the charged transmission lines  10  are fully charged, all of the switches  11  are closed at the same moment of time, thereby causing the charged transmission lines  10  to operate as n series connected generators. As a result, a bipolar pulse with a predicted time space or gap between positive and negative sub-pulses is realized on the load  15  as is illustrated in  FIG. 7   b.    
       FIG. 8   a  is a schematic of a stacked Blumlein-based bipolar pulse generator according to an embodiment of the present invention. The generator consists of a charging structure  30  with any number n of identically first charged transmission lines  31  with switches at their near ends, and n oppositely charged second transmission lines  32  with the same length and characteristic impedances as for the first transmission lines  31 . The output of this stacked Blumlein structure  30  is connected to the near end of an additional non-charged transmission line  33  with a specified electrical length t 1  and characteristic impedance equal to 2nZ 0 , where Z 0  is a characteristic impedance of each charged first transmission lines  31  and the second transmission lines  32 . The load  34  is connected to the distal end of the transmission line  33 . In addition, another transmission line  35  is provided, which is connected to the load  34  at its near end and is short-circuited at its distal end. The load impedance is equal to nZ 0 , while the characteristic impedance of transmission line  35  is the same as for transmission line  33 . The electrical length of the transmission line  35  is equal 2t+t 1 , where t is electrical length of each of the first transmission lines  31  and the second transmission lines  32 . 
     During operation, all the transmission lines  31  and the transmission lines  32  are charged by a voltage supply V 0 . All of the n switches are then closed simultaneously and a wave propagation process occurs. Identical waves propagate on all of the charged transmission lines  31  and the same is true for all of the charged transmission lines  32 . The resulting pulse on the load is illustrated on  FIG. 8   b  and minimum separation between sub-pulses is equal 2t. 
     Referring to  FIG. 9 , a schematic of a three-step two stacked Blumlein based bipolar pulse generator according to an embodiment of the invention is illustrated. The generator starts from a generator according to  FIG. 8   a  for particular case n=2 (transmission lines  41  and transmission lines  42 ) and t 1 =0. Extra step transmission lines  43  and  44  are provided as well as a transmission line  45  with specific characteristic impedances, obtained by impedance transformation procedure applied to initial circuit ( FIG. 8   a  for n=2 and t 1 =0). This provides additional impedance/voltage transformation. The charged structure  40  of this generator consists of transmission lines  41 ,  42 ,  43 ,  44  and  45 . Load  46  is positioned between charged transmission line structure  40  and a transmission line  47  that is connected to the load  46  and short-circuited at its distal end. The bipolar pulse is initiated by simultaneously closing two switches  48 . 
       FIG. 10  is a table of normalized characteristic impedances of transmission lines as well as load impedances for odd numbers of steps 1, 3, 5 . . . 19. The table illustrates the rate of increasing impedance transformation by increasing the number of steps. The pulse form is independent on the number of steps. Only the magnitude of pulse is increased from step to step. 
       FIG. 11   a  is a schematic of a double Blumlein based bipolar pulse generator according to an embodiment of the present invention. This generator consists of a known double Blumlein unipolar pulse generator structure (transmission lines  60 ,  61 ,  62  and switch  63 ) with additional transmission lines  64  and  65 . A transmission line  64  with time delay t 1  is connected between the output of the double Blumlein unipolar pulse generator structure and a load  66 . Transmission line  65  is connected to the load  66  at its near end and is short-circuited at its distal end. A characteristic impedance of the transmission lines  64 ,  65  is twice the impedance of the load  66  and four times more then the impedance of each of the transmission lines  60 ,  61  or  62 . The electrical length of the transmission line  65  is twice the length of transmission lines  60  or  61  and is equal to the length of transmission line  62 . Line  62  could also be separated in the middle by two identical length transmission lines without any change in operation and in pulse form on the load  66 . 
     Ideal operation of this generator is similar to that for generator according to  FIG. 8   a  when the number of switches equal two (n=2). The resulting pulse form on the load  66  is illustrated on  FIG. 11   b . The ideal operation of the generator according to  FIG. 11   a  assumes that there are no inductances by outer conductors, or more correctly, no transmission lines associated by outer conductors of transmission lines  60  and  61 , i.e. between nodes a 1  and a 2 , as well as between nodes a 1  and b 1  by outer conductors of transmission line  62 . However, between these nodes, there always exists intrinsic (stray) transmission lines short-circuited at their distant ends in practice. These transmission lines with specific characteristic impedances and electrical length could be used instead of transmission line  65  (or in addition to transmission line  65  with increased characteristic impedance) to provide a bipolar pulse. However, it is valid only for the case t 1 =0 and illustrated on  FIG. 12   a.    
     Referring to  FIG. 12   a , which is a schematic of double Blumlein-based Bipolar Pulse generator as an embodiment of the present invention. This generator consists of known double Blumlein unipolar pulse generator structure with transmission lines  70 ,  71 ,  72  and switch  73 . This structure is similar to the structure with transmission lines  60 ,  61 ,  62  and switch  63  of  FIG. 11   a . However, instead of using transmission line  65  with time delay 2t (t 1 =0), which is short-circuited at its distal end, there are two intrinsic transmission lines  74  and  75  formed by outer conductors of transmission lines  70 ,  71  and by a folded outer conductor of transmission line  72 . Transmission lines  74  and  75  are connected in series relative to load  76  with resulting characteristic impedance equal to 4Z and operate in the same manner as the transmission line  64  of  FIG. 11   a . The electrical length of each of these lines should also be equal to 2t. 
     It should be noted that combined design of  FIG. 11   a  for t 1 =0 and  FIG. 12   a  is also possible. Accordingly, in addition to the two intrinsic transmission lines  74  and  75  of  FIG. 12   a  (with impedances more then 2Z each), the transmission line  65  of  FIG. 11   a  with characteristic impedance more then 4Z could be used. 
       FIG. 13   a  is a schematic of a series connected two double Blumlein based bipolar pulse generators according to an embodiment of the present, invention. This generator consists of a double Blumlein-based bipolar pulse generator&#39;s structure (transmission lines  80 ,  81 ,  84 ,  86  and  88 ), which is the same as the generator on  FIG. 12   a  and it is connected in series with exactly the same generator&#39;s structure (transmission lines  82 ,  83 ,  85 ,  87  and  89 ). Both switches  91  and  92  should be closed simultaneously. These two switches could be replaced by a single switch or by any number of simultaneously closed switches. It should be noted that intrinsic transmission lines  86 ,  87 ,  88  and  89  (if they are neglected) could be replaced by a single (or two) transmission line(s) as shown in  FIG. 13   b  (transmission line  95  or transmission lines  94  and  95  if t 1 &gt;0). In the case of two lines, transmission line  94  with time delay t 1  provides an additional 2t 1  separation between sub-pulses, i.e. total time separation 2(t+t 1 ) could be implemented. Any negative effect by the intrinsic transmission lines can be minimized by proper design. 
       FIG. 14  is a schematic of series connected N double Blumlein based bipolar pulse generator according to an embodiment of the present invention. This generator consists of double Blumlein-based bipolar pulse generator structure (transmission lines  100 ,  101 ,  106 ,  109  and  112 ), which is the same as generator in  FIG. 12   a  and is connected in series with exactly the same generator structure (transmission lines  102 ,  103 ,  107 ,  110  and  113 ). This second double Blumlein-based bipolar pulse generator structure is also connected in series with the next the same generator structure and finally with the last N-th generator structure (lines  104 ,  105 ,  108 ,  111  and  114 ). All simultaneously closed N switches  116 ,  117  . . .  118  could be replaced by a single switch or by any number of switches. The load  115  is a result of series connection matched loads of individual double Blumlein-based Bipolar Pulse Generators with their summarized impedance 2NZ. 
     By analogy with the generators shown in  FIG. 13   a  and  FIG. 13   b , all of the intrinsic transmission lines  109 ,  110 , . . .  111  and  112 ,  113 , . . .  114  could be replaced by a single line connected to the load  115  at its near end and short-circuited at its distal end, which is similar to transmission line  95  ( FIG. 13   b ) when t 1 =0. For an extended gap between sub-pulses (t 1 &gt;0), an additional transmission line as transmission line  94  in  FIG. 13   b  should be used. If various combined solutions for short-circuited at their distal end transmission line and intrinsic lines as discussed above with respect to  FIGS. 11   a ,  12   a  and  FIG. 13   a , could be used depending on specific designs issues. It should be noted for all generators shown in  FIGS. 11   a ,  12   a ,  13   a  and  14 , various positions of ground connections can be used including single ground or no connections to ground. 
       FIG. 15   a  is a schematic of double single-stage bipolar pulse generator according to an embodiment of the present invention. By analogy with the double Blumlein-based bipolar pulse generators according to  FIG. 12   a , this generator is obtained by interconnection of two bipolar pulse generators. However, in this case there is no gap between positive and negative sub-pulses. Two switched transmission lines  120  and  121  are combined with a single switch  126  and a single non-switched transmission line  122  is provided with double length  2   t . Two intrinsic equal-length transmission lines  123  and  124  with impedances Z 2  and Z 1 , respectively, are provided. For this generator, no connections to ground or different ground connections, including shown on  FIG. 15   a , could be used. Independent on connections to ground, different combinations of characteristic impedances Z 1  and Z 2  without deterioration of the generating pulse are acceptable. Assuming Z=1 (normalization) some of these combinations are presented in Table 1 below. 
     
       
         
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Z1 (Z2) 
                 8 
                 7 
                 6 
                 5 
                 4 
                 3 
                 2.5 
                 2 
               
               
                   
                 Z2 (Z1) 
                 8 
                 9.2 
                 11 
                 14 
                 20 
                 38 
                 74 
                 ∞ 
               
               
                   
                   
               
             
          
         
       
     
       FIG. 15   b  illustrates pulse shape on the load  125  independent on values Z 1  and Z 2 . In the case when Z 1  and Z 2  are very high (Z 1 , Z 2 &gt;&gt;Z, i.e. intrinsic lines  123  and  124  are neglected), the bipolar pulse according to  FIG. 15   b  could be achieved by using a transmission line connected to the load  125  at its near end and short-circuited at its distal end, as was shown on  FIG. 11   a  and  FIG. 13   b . To provide separation between sub-pulses an extra transmission line like transmission line  64  in  FIG. 11   a  or transmission line  94  in  FIG. 13   b  should be used. 
     Referring to  FIG. 16 , which is a schematic of generator, which consists of two series connected double single-stage bipolar pulse generators shown on  FIG. 15   a  as an embodiment of the present invention. This generator consists of double single-stage bipolar pulse generator structure (lines  130 ,  131 ,  134 ,  136  and  138 ), which is the same as generator structure on  FIG. 15   a  and it is connected in series with exactly the same generator structure (transmission lines  132 ,  133 ,  135 ,  137  and  139 ). Both switches  141  and  142  should be closed simultaneously. These two switches could be replaced by a single switch or by any number of simultaneously closed switches. 
     In the case when impedances Z 1  and Z 2  of intrinsic transmission lines  136 ,  137 ,  138  and  139  are much more compared to Z (and 3Z) the structure  FIG. 13   b  is valid with specific values of load impedance  140 , which should be equal to 32Z/3 as shown on  FIG. 17 . Characteristic impedance of transmission line  143  should be equal to 32Z and their electrical length is equal t. Separation between sub-pulses will be also equal zero. 
       FIG. 18   a  is a schematic of bipolar pulse generator as an embodiment of the present invention. This generator consists of two-step, double single-stage bipolar pulse generator structure with intrinsic transmission lines that provides bipolar pulse. This generator operates, in principle, as generator according to  FIG. 7   a  in mentioned above US Patent Application 2007/0165839 A1. First step with transmission lines  150  and  152  is connected to the second step with transmission lines  151  and  153 , while transmission line  154  with double electrical length  2   t  play the same role as two transmission lines  340  in mentioned above generator according to  FIG. 7   a . Intrinsic lines  155  and  156  connected in series play the same role as transmission line  385  in mentioned above generator according to  FIG. 7   a .  FIG. 18   b  illustrates the pulse form on the load provided by generator according to  FIG. 18   a.    
       FIG. 19  is a schematic of bipolar pulse generator with two switches in first two successive steps as an embodiment of the present invention. This generator provides bipolar pulse with gap between sub-pulses equal to the length of sub-pulse. The charging voltage 2V of second and third steps is twice the charging voltage V of the first step. In this case voltages on both switches  164  and  165  are identical and equal V. Mostly because of characteristic impedance of the second step line  161  is about twice the characteristic impedance of the first step line  160  the total energy stored in this generator, i.e. in lines  160 ,  161 ,  162  and  163  is much higher compared to single-switch stepped-line generator, as well as two-switches generator according to FIG. 20 in US Patent Application 2007/0165839 A1. 
     During operation if switch  164  is turned ON (closed) at time t 0 , the second switch  165  should be turned ON (closed) at time t 0 +t, i.e., at time slightly less than t after t 0  to prevent overvoltage on switch  165 . However, in the case of switch  165  is a spark-gap it will be turned ON automatically due to overvoltage. The impedance transformation (ZL/Z) as a ratio of load impedance  167  to the lowest impedance Z of the first step  160  and ratio of inductive stub  166  to the load impedance will be increased by increasing the number of steps without deterioration the pulse shape.  FIG. 20  shows a table of normalized element&#39;s values for N-step (N=3, 5, 7 . . . 23) bipolar pulse generator according to  FIG. 19  and illustrate the increasing transformation ratios. 
       FIGS. 21   a  and  21   b  illustrates folded and partly folded designs of bipolar pulse generator according to  FIG. 19 , as an embodiment of the present invention. Both switches  164  and  165  are positioned outside structure that is preferable for practical implementation. 
     The invention has been described with reference to certain preferred embodiments thereof. It will be understood by those skilled in the art that modifications and variations are possible with the scope of the appended claims.