LONDON, Simon, Y. (2016 Dundee Road, Rockville, MD, 20850, US)
| WHAT IS CLAIMED IS: 1. A bipolar puise generator comprising: a first transmission line structure including a switch; a second transmission tine structure; a load coupled between the first transmission Sine structure and the second transmission line structure; a voltage source that charges the first transmission line structure when the switch is in the open state; and wherein the bipolar pulse generator generates a puise inducting a gap between a negative sub-puise and a positive sub-pu!se when the switch is in the closed state. 2, A bipolar pulse generator as claimed in claim i , wherein the first transmission Sine structure includes a first transmission line, the second transmission line structure, includes a second transmission line connected to a third transmission line and the load is positioned between the second transmission fine and the third transmission line 3. A bipolar puise generator as claimed in claim 2, wherein the first, second and third transmission lines are each two conductor transmission lines, 4, A bipolar pulse generator as claimed in claim 2, wherein the distal end of the first transmission line is coupled to the near end of the second transmission ϊine through the switch, the distal end of the second transmission line is coupled to the ioad, the near end of the third transmission line is coupled to the ioad and the distal end of the third transmission line is short-circuited. 5. The bipolar pulse generator according to claim 1, wherein the first transmission line structure comprises a three conductor transmission line structure and the second transmission line structure comprises a two conductor transmission line structure. 6. The bipolar pulse generator according to claim 5, wherein the three conductor transmission line structure includes a first conductor, a second conductor and a central conductor, wherein the first conductor and the central conductor comprises a first transmission fine and the second conductor and the centra! conductor comprises a second transmission line, and wherein the switch is provided between first conductor and the central conductor of the first transmission line at a near end thereof. 7. The bipolar puise generator according to ciaim 6. wherein a distal end of the three conductor first transmission line structure is coupled to the load, wherein the load is connected to the near end of the second transmission line structure, which is a two-conductor transmission line short-circuited at its distal end. 8. The bipolar puise generator according to ciaim 6, wherein the voltage source charges the first transmission line and the second transmission line to opposite polarities. 9. The bipolar pulse generator according to claim 5 wherein the positive sub- pulse and the negative sub-pulse are of equal length, and the gap between the positive sub-pulse and the negative sub-pulse is the same length as the length of positive sub-pulse and the negative sub-pulse. 10. A bipolar putse generator as claimed in claim 6, wherein the second transmission structure includes a third transmission Sine cascade connected to a fourth transmission line and the ioad is positioned between the third transmission line and the fourth transmission line. 11. The bipolar pulse generator according to claim 10, wherein a distai end of the fourth transmission line is short-circuited. 12. The bipolar puise generator according to claim 6, wherein the first transmission line is a multi-step transmission line including n steps, 13. The bipolar pulse generator according to claim 12 wherein the positive sub-pulse and the negative sub-pulse are of equal length, and the gap between the positive sub-pulse and negative sub-pulse is the equal to n times the length of the positive sub-pulse or the negative sub-pulse. 14. The bipolar pulse generator as claimed in claim 12, wherein the second transmission structure includes a third transmission line cascade connected to a fourth transmission line and the load is positioned between the third transmission line and the fourth transmission line. 15. The bipolar puise generator according to claim 14: wherein a distai end of the fourth transmission line is short-circuited. 16. The bipolar pulse generator according to claim 12, wherein the stepped transmission line includes two steps 17. The bipolar pulse generator according to claim 12, wherein the stepped transmission line includes three steps . 18. The bipolar pulse generator according to claim 12, wherein the impedance of each successive step increases. 19. The bipolar pulse generator according to claim 12, wherein the electrical length of the fourth transmission line, is π+1 times the electrical iength of each step of the first transmission line. |
BACKGROUND
[0001] The invention relates generally to bipolar pulse generators. More specifically, the invention relates to bipolar pulse generators that incorporate voltage multiplication (transformation) circuitry and time separation between positive and negative sub-pulses.
[0002] Recent development trends in pu!se power microwave sources for a variety of applications have been directed to increasing power and efficiency as well as energy density (energy per volume). Transmission line type pufse generators with different kinds of fast switches, including tight activated photoconductive switches, can achieve some of the best results. In particular, such transmission line type pulse generators are compact and provide a very fast puise rise time and a very high power.
[0003] 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. The low range of characteristic impedances frequently causes problems for coupling with typically load impedances, for example 50 Ohm or higher, or radiating impedances, which introduces a problem with high ratio impedance transformation.
[0004] There are many different applications of bipolar puise generators, for example, in industry, physics and medicine. Very often bipolar pulse generators with time separation between positive and negative sub-pulses are preferable or required. Bipolar pulse generators capable of separation between positive and negative sub- pulses are certainly known. Examples of such pulse generators are disclosed in "Design of Bipolar Pulse Generator for Ferroelectric Electron Emission Extraction", Feng Chen et a!.. Pulsed Power IEE Symposium, 2000, US Patent 6,214,297 issued to Zhang et a!, entitled High Voltage Pulse Generator 11 , and SU Patent 1 254 994 A1 issued to Remnev G. E. et al in 1994 entitled "Powerful Generator of Twin Pulses", the content of each of which is incorporated herein by reference,
[0005] There are, however, disadvantages associated with the above- referenced bipolar pulse generators. For example, a!! of the above-referenced generators do not provide voltage (impedance) transformation without an additional pulse transformer, in addition, the conventional pulse generators are genera) complex in nature, require more than one switch, and can be difficult to implement in real world applications, especially for high power applications. Still further, the switching elements required in US Patent 6,214,297 and SU Patent 1 254 994 A1 require very short (sub-nanosecond range) rise limes, which are almost impossible to realize.
[0006] The present applicant has previously developed an efficient transmission line based pulse generator which is described in US Patent Application 2007/165,839 entitled "Bipolar Pulse Generators with Voltage Multiplication", the content of which is incorporated herein by reference, which provides ail required voltage/impedance transformation and high power pulses with a single switch. Any type of switch can be used in described puise generator, including those in which it is necessary to generate nanosecond range pufses. Further, because only a single switch is utilized, there are no problems associated with switching time synchronization. The bipolar puise generators in the above-referenced patent application, however, do not have any gap between positive and negative sub- pulses.
[0007] In aii cases, energy stored in a voltage charged transmission line ss proportional to the reverse value of line's characteristic impedance, In order to make a comparison of different generator's circuits, the total energy stored in all equally voltage charged transmission Sines could be related to the energy stored in a transmission line with critical (minimum) characteristic impedance as a reference. The iower characteristic impedance implies the lower space between line ' s conductors and the higher electric field, which is a limitation for selected voltage defined by switch.
[00083 Accordingly, there remains a need for a bipolar pulse generator solution based on voltage charged transmission lines which provides separation between positive and negative sub-pulses, as well as a need for a bipolar pulse generator with pulse separation that provides high pulse power, and that also provides high voltage/impedance transformation.
[0009] Sn view of the above, it would be desirable to provide a bipolar pulse generator that can address the needs set forth above, that can be implemented in a simple transmission Sine structure with a single switch, which has a relatively smal! total size, and that allows simple access by fibers to a dosing photoconductive switch(s) that actuates the bipolar pulse generator.
SUMMARY OF THE INVENTION
[0010] The invention provides a bipolar pulse generator based on voltage charged transmission lines, which provides a separation between positive and negative sub-pulses. The bipolar pulse generator also produces high puise power while providing high voltage/impedance transformation. In addition, the bipolar pulse generator of the invention can be implemented in a simple structure with a single switch, has a relatively small total size, and allows simple access by fibers to a photoconductive switch(s) that can be used to actuate the bipolar pulse generator.
[0011] The bipolar pulse generator of the invention preferably includes a first voltage charged transmission line structure including a switch, a second non-charged transmission line structure, a load positioned in the second transmission line structure, and a voltage source that charges the first transmission line structure when the switch is in the open state. The bipolar pulse generator is activated and generates a bipolar pulse, including a separation or gap between a negative sub- pulse and a positive sub-puise thereof, when the switch is dosed.
[0012] In one preferred embodiment, the first transmission line structure includes a first transmission line, the second transmission structure includes a second transmission line connected to a third transmission line and the load is positioned between the second transmission line and the third transmission line of the second transmission ϋne structure,
|G013] In a further embodiment, the first, second and third transmission lines are preferably composed of two conductor transmission lines, wherein a distal end of the first transmission line is coupled to a near end of the second transmission line, a dista! end of the second transmission line is coupled to the load, a near end of the third transmission line is coupled to the load and a distal end of the third transmission line is short-circuited. fOO14] In another embodiment, the first transmission line structure includes a three conductor transmission Sine structure and the second transmission ϋne structure includes a two conductor transmission line structure. The three conductor transmission fine structure preferably includes a first conductor, a second conductor and a central conductor, wherein the first conductor and the central conductor comprises a first transmission line and the second conductor and the central conductor comprises a second transmission line, and wherein the switch is provided between first conductor and the centra! conductor of the first transmission line at a near end thereof.
[0015] Accordingly, in one embodiment, the invention provides a simple bipolar pulse generator that includes three two-conductor transmission lines coupled together with a load positioned between the second and the third non-charged transmission lines. Each conductor of a transmission line can be defined as a segment. The two-segment first transmission line is charged and switchably coupled to the two-segment second transmission Sine to produce a bipolar puise on the matched load. The distant end of the third transmission line is short-circuited.
[0016] Accordingly, in another embodiment of the present invention, a bipolar pulse generator may include two transmission line structures coupled together with a load positioned between the two transmission fine structures. The first charged transmission line structure may include an embedded (third) charged transmission line segment. A switch is coupled between a first (grounded) transmission line segment and the second transmission line segment of the transmission line structure at their near end. During operation, the second transmission line segment is charged equally with respect to the first and to the third segments of the first transmission line structure and the charging voltage exists on the open position switch, When the switch is closed the first transmission line structure starts to discharge and, with the second non-charged transmission line structure, generates a bipolar pulse on matched load with specified separation between positive and negative sub-pulses.
[0017] in another embodiment of the present invention, the bipolar pulse generator may further include a charged stepped transmission Sine between the switch at its near end and the embedded transmission Sine segment at its distant end that facilitates voltage /impedance transformation. The impedance of the stepped transmission fine may increase for each successive step. Moreover, the load and the second transmission line structure may have impedances that are higher than the maximum characteristic impedance of the stepped transmission line.
[0018] According to still another embodiment of the present invention, the bipolar puise generator may further include additional non-charged transmission Sine interconnected between a first (charged) transmission line structure and the load to provide specified separation (gap) between positive and negative sub-pulses.
[0019] The bipolar pulse generator according to the invention is useful in HPM generation, in particle acceierators and in other high voltage physical instruments and test equipments. These and other advantages and features 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 QF THE DRAWiNGS
[0020] The invention will be described with reference to certain preferred embodiments thereof and the accompanying figures, wherein;
Fig. 1a depicts a schematic of a bipolar pulse generator according to the prior art with a switch positioned inside structure;
Fig. 1b depicts an ideal pulse form provided by the generator illustrated in Fig. 1a;
Fig, 2a depicts a simple schematic of a bipolar pulse generator according to the prior art with a switch that can be positioned outside structure;
Fig. 2b depicts an ideal pu!se form on the load provided by the generator illustrated in Fig 2a;
Fig. 3a depicts a schematic of a single-stage bipolar pulse generator according to the prior art with increased impedance transformation;
Fig. 3b depicts an idea! puise form on the load provided by the generator iilustrated in Fig. 3a;
Fig. 4 depicts a schematic of an N-stage bipolar pulse generator according to the prior art with a charged stepped transmission line that provides high impedance and voltage transformation;
Fig. 5 depicts a schematic of an N-stage bipolar puise generator according to the prior art with a charged stepped transmission line in which first stage consists of n identical switched stacked transmission lines in a first stage that provides increased power/energy and impedance transformation by a factor n;
Fig. 6a depicts a simple structure of a bipolar pulse generator with any specified gap between sub-pulses according to an embodiment of the present invention; Fig, 6b depicts an idea! pulse form provided by the generator illustrated in Fig. 6a;
Fig. 7a depicts a schematic of a single-stage, single-step bipolar pulse generator with a gap between sub-pulses according to an embodiment of the present Invention:
Fig. 7b depicts an ideal pulse form provided by the generator illustrated in Fig. 7a;
Fig, 8a depicts a schematic of a single-stage, singie-step bipolar pulse generator with limited by 2t specified gap between sub-pu!ses according to an embodiment of the present invention;
Fig. 8b depicts an ideal pulse form provided by the generator illustrated in Fig, 8a.
Fig, 9a depicts a schematic of a single-stage, two-step bipolar pulse generator with the gap between sub-pulses equal to the double length of sub-pufse according to an embodiment of the present invention;
Fig. 9b depicts an idea! pulse form provided by the generator illustrated in Fig. 9a.
Fig. 10a depicts a schematic of a single-stage, two-step bipolar pulse generator with limited by 4t specified gap between sub-puises according to an embodiment of the present invention;
Fig. 10b depicts an ideal pulse form provided by the generator iliustrated in Fig. 10a;
Fig. H a depicts a schematic of a single-stage, three-step bipolar pulse generator with a gap between sub-puSses equal to the length of three sub-puises according to an embodiment of the present invention;
Fig. 11 b depicts an idea! pulse form provided by the generator illustrated in Fig. 11a, Fig. 12a depicts a schematic of a single-stage, n-step bipolar pulse generator with a gap between sub-pulses equal to the length of n sub-pulses (2nt) according to an embodiment of the present invention;
Fig. 12b depicts an idea! pulse form provided by the generator illustrated in Fig. 12a;
Fig. 13a depicts a schematic of a single-stage, n-step bipolar pulse generator with limited by 2nt specified gap between sub-pu!ses according to an embodiment of the present invention;
Fig. 13b depicts an ideal pulse form provided by the generator illustrated in Fig. 13a;
Fig. 14a depicts a schematic of two-stage bipolar pufse generator with a gap equal to the length of sub-pulse according to an embodiment of the present invention;
Fig. 14b depicts an ideai pulse form provided by the generator illustrated in Fig. 14a;
Fig. 15 depicts a schematic of two-stage bipolar pulse generator with a gap equal to the length of two sub-pulses according to an embodiment of the present invention;
Fig. 16 depicts a schematic of two-stage bipolar pulse generator with a gap equal to the length of three sub-pulses according to an embodiment of the present invention;
Fig. 17 depicts a schematic of two-stage bipolar putse generator with a gap equal to the length of four sub-pulses according to an embodiment of the present invention;
Figs. 18a and 18b respectively depict a schematic of six-stage bipolar pulse generator with a gap equal to the length of sub-pulse according to an embodiment of the present invention and resulting waveforms; Fig. 19 depicts a schematic of two-stage bipolar pulse generator with a gap equal to the length of two sub-pulses according to an embodiment of the present invention;
Figs. 20a and 20b respectively depict a schematic of four-stage bipolar pulse generator with a gap equal to the length of two sub-pu!ses according to an embodiment of the present invention and resuiting waveforms;
Fig. 21 depicts a schematic of two-stage bipolar pulse generator with a gap equal to the length of three sub-pulses according to an embodiment of the present invention;
Fig. 22 depicts a schematic of three-stage bipolar pulse generator with a gap eqυaf to the length of three sub-pulses according to an embodiment of the present invention;
Fig. 23 depicts a schematic of two-stage bipolar pulse generator with a gap equal to the length of four sub-pulses according to an embodiment of the present invention;
Fig. 24 depicts a schematic of three-stage bipolar pulse generator with a gap equal to the length of four sub-pulses according to an embodiment of the present invention;
Fig, 25 presents the table of normalized characteristic impedances of transmission lines and load impedances for all combinations of values of gaps between sub-pulses (1 , 2, 3, 4} and number of stages (1 , 2, 3, 4, 5, 6) for bipolar pulse generators according to an embodiment of the present invention;
Fig. 26 presents the table of normalized characteristic impedances of transmission lines and load impedances for all combinations of values of gaps between sub-pulses (1 , 2, 3 : 4) and number of stages (7, 8, 9, 10, 11} for bipolar pulse generators according to an embodiment of the present invention; and
Fig. 27 presents the table of normalized characteristic impedances of transmission lines and load impedances for all combinations of values of gaps between sub-pulses (5, 6) and number of stages (1 , 2, 3, 4, 5. 6) for bipolar puise generators according to an embodiment of the present invention.
DETAiLED DESCRIPTION OF THE PREFERRED EMBODiMENTS [0021] Figs. 1-5 illustrated various-ratio impedance transformed bipoiar pulse generators according to the prior art. Fig. 1a, for example, illustrates a bipolar pulse generator with a switch positioned inside a structure. Fig 2a depicts a schematic diagram of a simple bipolar puise generator with a switch that can be positioned outside a structure. Fig. 3a depicts a schematic of a single-stage bipolar pulse generator with increased impedance transformation. Fig. 4 depicts a schematic of an N-stage bipolar pulse generator according to the prior art with a charged stepped transmission line that provides high impedance and voltage transformation. Fig. 5 depicts a schematic of an N-stage bipolar pulse generator with a charged stepped transmission line in which first stage consists of n identical switched stacked transmission lines in a first stage that provides increased power/energy and impedance transformation by a factor n. The illustrated generators generate bipolar pulses without gaps between the positive and negative sub-pulses with different amplitudes depending on number of charged transmission line steps.
[0022] in contrast to the pulse generators of the prior art, the present invention provides a pulse generator that generates bipolar pulses with separation or gaps in time between positive and negative sub-pulses. As shown in Fig. 6a, for example, a bipolar puise generator is provided that includes three two-conductor transmission lines 12, 14 and 16 coupled together with a load 15 positioned between the second transmission line 14 and the third transmission lines 16. For the purposes of this discussion, the first transmission line 12, which is voltage charged, corresponds to a first transmission structure, and the second transmission line 14 and the third transmission line 16 both correspond to a second transmission line structure and are not charged. Further, each conductor of a transmission Sine will be termed a segment, with each transmission iine including at least two conductors, i.e. two segments, in the illustrated embodiment.
[0023] As shown in Fig. 6a, a switch 10 couples a segment 11 of the first transmission line 12 to a segment 13 of the second transmission iine 14. The second transmission iine 14 is connected to the load 15 at an end opposite to the switch 10 (distal end) and to an end (near end) of the third transmission line 16. The transmission line 18 is short-circuited at a second end (distal end) opposite the end connected with the second transmission line 14, The illustrated bipolar pulse generator may be implemented in a flat or a folded design.
[0024] During operation, the first transmission line 12 is charged and is switchabiy coυpied to the second transmission line 14 via the switch 10 to produce a bipolar pufsε on the load 15, Initially , the switch 10 is open, which allows the two segments of first transmission line 12 to be charged by a voltage supply VO as indicated in Fig 6a. The switch 10 is subsequently closed, for example at time tO = 0, which causes the discharge of transmission iine 12 into the second (currently non- charged) transmission line structure, which includes the cascade-connected second transmission iine 14 and third transmission line 16, with the load 15 positioned between second transmission iine 14 and third transmission line 16. At the moment transmission line 12 with the characteristic impedance Z starts to discharge to the non-charged transmission line 14 with the same characteristic impedance Z, a pulse with magnitude VO/2 starts to propagate on transmission line 14 toward the load 15. At the same moment of time when the switch 10 is closed, the voltage at the end of transmission line 12 drops from VO to VO/2. Accordingly, a reflected negative polarity pulse with magnitude -VO/2 starts to propagate on transmission line 12, with its time delay t toward its open end.
[0025] At time delay t1, after switching the positive pulse with magnitude V0/2 reaches the load 15 and sees the resulting load impedance, which is equal to parallel connection of load resistance Z/2 and characteristic impedance of the third transmission line 16 which is equal Z. The resulting bad impedance is equal to Z/3 and, consequently, the reflection coefficient is equal to (Z ~ ZJd)I(Z + 2/3} = 1/2, which means that the voltage on the bad 15 and magnitude of forward wave that continues to propagate on the third transmission line 16 toward the short-circuited end is equal VO/4, The reflected wave, which starts to propagate on second transmission line 14 towards the dosing switch 10 and to the open end of transmission tine 12, is equal - VO/4. The voltage on the load 15, which is equal VO/4 starts at time deiay t1 after switching and will be continua! during the period of time 2t as illustrated on Fig. 8b. Therefore, at the moment of time !1 +2t after switching, the voltage on the load 15 drops to zero. During the period 2t, halve of energy stored in transmission line 12 is dissipated on the load 15.
[0026] The voltage on the load 15 wilt be still equal zero during the time period 2t1 (double transient time of transmission line 14) before the two waves with equal negative magnitudes (-V0/4) arrives at the load 15. One wave with duration 2t is reflected from the short-circuited end of transmission line 16 with reversed polarity (from positive to negative). The second identical wave (also with negative polarity) is reflected at the same moment of time (f + 2t1) after switching at the open end of transmission Sine 12 without changing polarity. Each wave transfers 1/4 of the energy initially stored in transmission line 12. The load 15 is matched (non-reffected) load for each of those equat magnitude and negative pofarity waves. Therefore, during additional time interval 2t, the negative polarity pulse {-VO/4) will be dissipated on the load 15 and no waves will be traveling on transmission lines after this time The full energy initially stored In the first transmission line 12 is now dissipated on the load 15. Fig. 6b depicts resulting idea! pulse on the load.
[00271 The simple circuit illustrated in Fig. 6a is universal and valid for any values of t > 0 and t1 ≥ 0. However, the generator according to Fig.6a does not provide any impedance transformation and power/energy on the load is less than compared to what is possibly achievable. [0028] Fig. 7a depicts a single-stage bipolar pulse generator according to another embodiment of the present invention, wherein the gap between the positive and negative sub-pulses is equal to the length of sub-pulse. Referring to Fig. 7a, the first transmission line structure is a three conductor transmission line structure including a first conductor, a second conductor and a central conductor. The first conductor and the centra! conductor correspond to a first transmission Sine 20 and the second conductor and the centra! conductor correspond to a second transmission line 21, with a switch 22 connected to the first transmission Sine 20 a first end (near end) of the first transmission iine structure. The first transmission iine 20 and the second transmission Sine 21 have equal characteristic impedances and are charged to opposite polarities,
[0029] The output voltage of the first transmission iine structure is connected to a second (non-charged) transmission line structure, which includes a non-charged third transmission line 24 short-circuited at an end (distal end) opposite to the end connected to the first transmission iine structure. A load 23 is positioned between interconnected nodes of first transmission line structure and the second transmission line structure. The first transmission iine 20 and the second transmission iine 21 , each with a normaiized characteristic impedances equal to one (1), have the same time delay t. The third transmission line 24 has a normalized characteristic impedance equal to two (2) and a time delay equa! 2t. The normalized resistive impedance of the load 23 is equal to one (1 ), and defined as a parallel connection of equal characteristic impedances ' , first transmission line structure (two series connected impedances of the first transmission line 20 and the second transmission line 21) and impedance of the third transmission line 24.
[0030] After closing switch 22, for example at time to = 0, a negative pulse with voltage (-V0) starts to propagate on the first transmission line 20 from the near end, where the voltage drops to zero toward its distal end. After a time deJay t, the pυise reaches the distal end of transmission line 20 and summarized in-phase with charged voltage (-VO) on the transmission iine 21. The resulting voltage (-2V0) as the output voltage of the first transmission line structure is applied to the parailβ! connected ioacf 23 and the near end of third transmission line 24 which has its distal end short-circuited. The resulting normalized load impedance for the source, i.e. for the first transmission line structure (two series connected transmission fines 21 and 22), is equal to 2/3. With source voitage - 2VO, normalized source impedance equal 2 and load impedance equal 2/3, the voltage on the ioad 23 is equal to -V0/2, The same magnitude -V0/2 voitage pulse also starts to propagate on the third transmission ϋne 24 toward its short-circuited end. At the same time, the refiected positive voltage pulse 2VO -V072 = 3V0/2 divides equally between equal characteristic impedance's transmission lines 20 and 21, and starts to propagate as two 3V0/4 pulses toward the open end of the second transmission line 21 and toward the short-circuited (by switch 22} end of first transmissfon line 20. After time delay f, these pulses are reflected back. However, the puise on transmission ϋne 20 is reflected at the closing switch 22 with an opposite polarity. After additional time delay t, these two pulses reach the load 23 and now they are in-phase as one pulse with magnitude VQ/2. At the same moment of time, another puise traveling initiaiiy on the third transmission line 24 with magnitude (-V0/2) returns back to the load 23 as positive pulse V0/2 after reflection at short-circuited end of transmission line 24. For each these equal puises with magnitude VO/2, the load 23 is a matched load (twice the load Impedance) and in result a positive puSse with magnitude V0/2 will be on the load 23 during period of time 2t. Fig. 7b depicts resulting ideal puise on the load. [0031] The generator according to Fig.7a, and all the following generators presented as embodiments of the present invention, generates pulse power on the load that exceeds by a factor of 1.5 the pulse power generated by the singie-switch generator presented in mentioned above US Patent Application US2007/165839. The circuit shown on Fig.7a can be modified to achieve any specified gap between sub-pulses which is longer than 2t. JO032J Fig. 8a illustrates a bipolar puise generator in accordance with a further embodiment of the present invention. With respect to Fig.7a, the embodiment of Fig. 8a includes an additional intermediate non-charged transmission line 25 in the second transmission structure with a specified time deiay t1 and normalized characteristic impedance equal 2, which is the sum of normalized characteristic impedances of first and second transmission lines 20 and 21. In addition, the iength of the third transmission line 26 is increased by t1 relative to third transmission line 24 illustrated in Fig.7a. The wave's propagation process in the bipolar pυ!se generator illustrated in Fig.8a in principle is the same as in the generator shown on Fig.7a including only the additional effect of the time delay t1 .
£0033] it is noted that the structure of Fig.7a is a Biumiein pulse generator with an additional double iength transmission line 24 connected to the load at its near end and short-circuited at its distant end. The impedance of load 23 is twice less than in a Biumiein pulse generator, and is equal to the impedance of parallel connected transmission line 24 with impedance of series connected transmission lines 20 and 21. in the case of these impedances and lengths, half of the energy stored in transmission lines 20 and 21 is delivered by the first sub-pulse, and the second half of energy is delivered by the second sub-pulse.
[0034] It is well-known that a Biumiein generator is a single-step (particular case) of stepped-line Darlington unipolar puise generator. Fig. 9a illustrates a bipolar pulse generator according to an embodiment of the present invention, which is a modified two-step Darlington generator with an additional transmission fine 33 short- circuited at its distant end. By analogy with Fig. 7a, a characteristic impedance of transmission line 33 is equai to the sum of characteristic impedances of the transmission line 31 and a second-step line in a two-step transmission line 30. These lines are connected in series with respect to a load 32 and their resulting normalized impedance is equal to 4.5, which is equal Io the impedance of transmission line 33 and is twice the impedance of the load 32. The electrical length of the transmission line 33, also by analogy with circuit Fig, 7a, is longer by time delay t than the electrical length (2t) of two-step Darlington structure (from switch 34 to the end of second step with normalized characteristic impedance equal 3).
[0035] Analysis of the wave propagations on the two-step transmission iine 30 and the transmission lines 31, 33 gives the resulting pulse on the ioad, which is illustrated on Fig. 9b, In this case also, half of the energy stored in the two-step transmission line 30 and the transmission line 31 is delivered to the load 32 by the first sub-pu!se, and the second half of the energy is delivered by two identical waves with one quarter of the total energy in each coming on transmission line 33 after reflection at its short-circuited distant end and on the two-step structure 30 with the transmission line 31.
[0036] By analogy with the circuit of Fig.7b, to increase the separation between the sub-pulses above 3t, an additional matched transmission iine 35 is inserted between the output of the two-step Darlington generator structure and the load 32, As illustrated in a further embodiment of the invention illustrated on Fig, 10a, an electrical length of the transmission line 36 is short-circuited at its distant end is increased by a time delay t1 relative to the transmission line 33 as illustrated in Fig.9a, The resulting pulse on the load 32 initiated by closing switch 34 is illustrated on Fig, 10b.
[0037J Fig.11a depicts a single-stage (Darlington-type) three-step bipolar pulse generator in accordance with another embodiment of the present invention. It consists of three-step transmission line 40 with a switch 44 at its near end and a transmission iine 41 that forms a transmission-Sine part of a Darlington generator, and an additional transmission line 42 connected to the ioad at its near end and short- circuited at its distal end. All characteristic impedances in three-step transmission line 40 and characteristic impedance of the transmission line 41 are the same as for a Darlington generator; however the impedance of load 43 is one half of that for a Darlington generator to achieve matching in full bipolar pulse generator including added the transmission line 42. The electrical length of the transmission line 42, by analogy with circuits Fig.7a-Fig10a : is longer by a time delay t than the total electrical length (3t) of a three-step line Darlington's generator. The resulting pulse on the load 43 is illustrated on Fig.11b.
[0038] The illustrated principle of creating bipolar pulse generators as shown in Figs. 7a-11a. namely adding a transmission line (with specific characteristic impedance and time delay) connected to the load at its near end and short-circuited at its cfistai end, can be extended for any number of steps n and termed as a single stage, n-step bipolar pulse generator. For example, Fig. 12a that depicts a bipolar pulse generator according to a further embodiment of the present invention, in which an n-step transmission line 50 with transmission line 51 and switch 54 forms a charging structure of a Darlington pulse generator. An additional transmission line 52 is connected to the load 53 and short-circuited at its distant end, and provides a bipolar pulse with 2nt separation time between sub-pulses as illustrated on Fig. 12b. The electrical length of the transmission line 52 is equal to {n+1)t where t is the electrical length of each step. The normalized values of step's characteristic impedances and load impedance related to the impedance of the first step are equal: Zi = i(i+1)/2, where i =1 , 2, 3...., n number of steps (1)
Zc = (n+1)/2 (2)
ZL = (n+1) 2 /2 (3)
ZR = [(n+1)/2] 2 =Zc 2 (4)
It should be noted that equations (1) and (2) define a Darlington transmission line structure, while equations (3) and (4) give bipolar pulse with the gap between sub-pulses equal 2nt.
[0039] Referring to Fig.13a, an n-step bipolar pulse generator according to another embodiment of the present invention, is shown in which an additional non- charged matched transmission line 55 with electrical length 11 is inserted between an output of charged structure (stepped line 50 with line 51) and the load 52 in a similar way as illustrated on Fig. 8a and Fig. 10a. The electrical length of transmission line 56 which is short-circuited at its distal end also should be increased by time t1 and equal to (n+1)t + 11. The resulting pulse on the load 52 activated by switch 54 is illustrated on Fig.13b,
Single-stage (Darlington-type), π-step bipolar pulse generators Fig.12a provides increasing voltage/impedance transformation only by increasing the time delay between positive and negative sub-puises.
[0040] Fig. 14 depicts a two-stage (s = 2) bipolar pulse generator according to another embodiment of the present invention, in which a charged transmission line section 81 is a first stage and two foliowed charged steps 82 form a second stage, These two steps 62 with charged transmission lines 63, non-charged transmission Sine 66 and load 65 are defined by transformation of transmission lines 21 , 24 and load 23 in the circuit of Fig, 7a, In this case, transformation in voltage/impedance is increased without changing the gap between sub-puises, which is equal to the length of sub-pulse. The resulting puise on the load 85 initiated by closing switch 84 is illustrated on Fig 14b.
[0041] In ali bipolar pulse generators according to embodiments of the present invention, the first stage is a Darlington n-stepped line (n > 1} that determines the gap between sub-pulses. This gap is equal to or exceeds (in the case of additional non-charged Sine between charged Darlington's structure and the load) the double transit time of Darlington's stepped line. AH followed stages provide only voltage/impedance transformation.
[0042] Fig, 15 depicts a two-stage {s - 2) bipolar pulse generator according to another embodiment of the present invention, in which the gap between sub-pulses equal to the iength of two sub-puises {g = 2), which is simitar to a single-stage generator according to Fig, 9a. In the referenced generator of Fig, 15, the first two- step stage 70. which determines the gap between sub-pulses (4t), is the same as the two-step line 30 in Fig.9a. The values of elements of the second three-step stage 71 , as we!! as elements 72, 73 and 74, are obtained by applying similar transformation (as in the circuit of Fig.14) to elements 31 , 32 and 33 of the circuit shown on Fig.9a. As a result, a generator with the same gap as for initial circuit Fig.9a provides voltage/impedance transformation, which is higher as compared to the ones for the generator illustrated in Fig.9a.
[0043] Fig.16 depicts a two-stage (s = 2} bipolar pulse generator according to another embodiment of the present invention, In which the first three-step stage 80 that determines the gap between sub-pulses (6t) is the same as for the generator illustrated in Fig, 11a. Element's values of the second four-step stage 81 and the elements 82, 83 and 84 are obtained by applying (the same transformation as for circuit Fig.14) to elements 41 , 42 and 43 of circuit in Fig. 1 1a, i.e. to elements 51, 52 and 53 of circuit Fig.12a, assuming n = 3. In the resulting circuit shown in Fig.16, the voltage/impedance transformation is increased as compared with the circuit in Fig.11a.
£0044] Fig.17 depicts another two-stage (s ~ 2) bipolar pulse generator according to another embodiment of the present invention, in which a first four-step stage 90 is a Darlington-type four-step transmission line that determines the gap between sub-pulses, which is equal to the length of four sub-pulses, or to the double transit time of the first stage 90. Normalized characteristic impedances of steps in this stage are defined by equation (1). The impedances of five-step second stage 91 and impedances of transmission iines 92, 93 and load 94 are defined by applying as before circuit transformation to elements 51 , 52 and 53 of circuit Fig, 12a for n = 4. These three impedances are defined by equations (2) t (3) and (4), assuming n = 4 and the electrical length of line short-circuited at its distant end is equal to 5t.
[0045] Fig, 18a depicts a six-stage (s - 6} bipolar pulse generator according to another embodiment of the present invention, in which a single-step first stage 101 determines the gap between sub-pulses equal to the length of sub-pulse (double transit time of first stage 101 ), Ail followed five two-step stages 102, 103, 104, 105 and 106 provide impedance/voltage transformation. Characteristic impedances of all lines and impedance of a load 109 are presented in analytical form,
|0048] Fig.19 depicts a two-stage (s = 2} bipolar puise generator according to another embodiment of the present invention, in which a first two-step stage 111 thai determinates the gap between sub-pulses (4t) is a Darlington stepped line for n = 2. Step impedances of a second three-step stage 112, the impedances of transmission lines 113, 114 and the impedance of load 1 15 are presented on Fig, 19 in analytical form,
[0047] Fig. 20a depicts a four-stage 120 (s - 4) bipolar pulse generator according to another embodiment of the present invention, in which a first two-step stage is the same as in Fig.19 that determinates the same gap between sub-pulses (4t). Step impedances of alt the following three stages and the impedances of transmission lines 121, 122 and the impedance of ioad 123 are presented in Fig.20a in analytical form.
[0048] Fig, 21 depicts a two-stage 130 (s = 2) bipolar pulse generator according to another embodiment of the present invention, in which a first three-step stage determines the gap between sub-pulses equal 6t. Four step impedances of the second stage and the impedances of transmission lines 131 , 132 and the impedance of load 133 are presented in anaiytica! form.
[0049] Fig, 22 depicts a three-stage 140 (s - 3) bipolar pulse generator according to another embodiment of the present invention, in which a first three-step stage determines the same gap between sub-puises (6t) as for the generator according to that shown in Fig.21. All step impedances of the next two stages and the impedances of transmission iines 141, 142 and the impedance of load 143 are presented in analytical form.
[0050] Fig.23 depicts a two-stage 150 (s = 2) bipolar pulse generator according to another embodiment of the present invention, in which a first four-step stage determines the gap between sub-pulses (8t) as a double transit time of this stage. A!i step impedances of the next stage and the impedances of transmission iines 151 , 152 and the impedance of load 153 are presented in analytical form.
[0051] Fig. 24 depicts a three-stage 180 (s = 3) bipolar pulse generator according to another embodiment of the present invention, in which a first four-step stage is the same as for the generator shown in Fig.23 and determines the same gap (8t) between sub-puises. Ail step impedances of the next two stages and the impedances of transmission iines 161 , 162 and the impedance of !oad 163 are presented In analytical form
[0052] Fig.25 is a table of normalized characteristic impedances of transmission lines and load impedances in analytical form for all combinations of gaps between sub-pulses (g = 1 , 2. 3, 4) relative to the length of sub-pulse and number of stages s= 1 , 2, 3, 4, 5, 6.
[0053] Fig. 26 is a table of normalized characteristic impedances of transmission lines and load impedances in analytical form for ail combinations of gaps between sub-pulses (g = 1 , 2, 3, 4) relative to the length of sub-pulse and number of stages s= 7, 8, 9, 10, 11.
[0054] Fig.27 is a table of normalized characteristic impedances of transmission lines and load impedances in analytical form for all combinations of gaps between sub-pulses (g = 5, 6} relative to the length of sub-pulse and number of stages s= 1 , 2, 3, 4, 5, 8.
[005S] The tables illustrate the principle of determination of ali impedances for any given numbers g and s for a bipolar pulse generators according to an embodiment of the present invention.
[0056] The invention has been described with reference to certain preferred embodiments thereof. It will be understood, however, that modifications and variations are possible within the scope of the appended claims.
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