A Compact Modular 5 GW Pulse PFN-Marx Generator for Driving HPM Source

electronics

Article A Compact Modular 5 GW Pulse PFN-Marx Generator for Driving HPM Source

Haoran Zhang 1,2, Ting Shu 1,2, Shifei Liu 1,2, Zicheng Zhang 1,2,*, Lili Song 1,2,* and Heng Zhang 1,2

1 College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China; [email protected] (H.Z.); [email protected] (T.S.); [email protected] (S.L.); [email protected] (H.Z.) 2 State key Laboratory of Pulsed Power Laser Technology, Changsha 410073, China * Correspondence: [email protected] (Z.Z.); [email protected] (L.S.)

Abstract: A compact and modular pulse forming network (PFN)-Marx generator with output parameters of 5 GW, 500 kV, and 30 Hz repetition is designed and constructed to produce intense electron beams for the purpose of high-power microwave (HPM) generation in the paper. The PFN- Marx is composed by 22 stages of PFN modules, and each module is formed by three mica capacitors (6 nF/50 kV) connected in parallel. Beneﬁting from the utilization of mica capacitors with high energy density and a mini-trigger source integrated into the magnetic transformer and the magnetic switch, the compactness of the PFN-Marx system is improved signiﬁcantly. The structure of the PFN module, the gas switch unit, and the connection between PFN modules and switches are well designed for modular realization. Experimental results show that this generator can deliver electrical pulses with the pulse width of 100 ns and amplitude of 500 kV on a 59-ohm water load at a repetition

rate of 30 Hz in burst mode. The PFN-Marx generator is ﬁtted into a cuboid stainless steel case with the length of 80 cm. The ratio of storage energy to volume and the ratio of power to weight

Citation: Zhang, H.; Shu, T.; Liu, S.; of the PFN-Marx generator are calculated to be 6.5 J/L and 90 MW/kg, respectively. Furthermore, Zhang, Z.; Song, L.; Zhang, H. A utilizing the generator to drive the transit time oscillator (TTO) at a voltage level of 450 kV, a 100 MW Compact Modular 5 GW Pulse microwave pulse with the pulse width of 20 ns is generated. PFN-Marx Generator for Driving HPM Source. Electronics 2021, 10, 545. Keywords: PFN-Marx; compact; modular; trigger source; gas switch; mica capacitor https://doi.org/10.3390/ electronics10050545

Academic Editor: Mikhail Glyavin 1. Introduction Intense electron beams have a wide range of applications in science research and Received: 31 January 2021 industry ﬁelds, such as in the high-power microwave (HPM) and ﬂash X-ray [1–3]. Pro- Accepted: 21 February 2021 Published: 26 February 2021 ducing high voltage (with several hundred kilovolts), rectangular pulses are one of the crucial parts. Recently, with the development of the energy storage technology, pulsed

Publisher’s Note: MDPI stays neutral power technology has been developed in the direction of compactness, miniaturization, with regard to jurisdictional claims in and light weight. Because the pulse forming network (PFN)-Marx generator has the natural published maps and institutional afﬁl- advantage of the integration of pulse modulation and pulse voltage accumulation, it has iations. been developed rapidly in recent years and is regarded as one of the most promising types for compactness, modularization, light weight, and miniaturization realization [4–9]. In this paper, two key points, including the compactness and modularization, of the PFN-Marx generator are discussed in detail. Firstly, the literature on the research of the Marx generator over the past few decades Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. has been reviewed. Institutes around the world, including the French-German Research This article is an open access article Institute of Saint-Louis (ISL), Texas Technology University (TTU), Commissariat à l’Energie distributed under the terms and Atomique et aux Energies Alternatives (CEAE), Applied Physical Electronics L. C. (APELC), conditions of the Creative Commons etc., have developed a series of compact Marx generators with different circuit topologies Attribution (CC BY) license (https:// and the typical parameters of the generators are listed in Table1[ 10–15]. Generally, the ratio creativecommons.org/licenses/by/ of storage energy to volume (E/V) and the ratio of power to weight (P/W) are calculated 4.0/). to describe the compactness of the generator. According to the limited parameters of the

Electronics 2021, 10, 545. https://doi.org/10.3390/electronics10050545 https://www.mdpi.com/journal/electronics Electronics 2021, 10, 545 2 of 13

generator in the literature, the E/V and P/W are calculated to several J/L and dozens of MW/kg, respectively. Actually, a variety of methods, including structure optimization, novel topology circuits, large energy density capacitors, etc., have been studied by researchers to realize the compact design. However, the physical limits of insulation are roughly the same in spite of the circuit topologies and structures. The effective way to improve the compactness of the generator is by using the high energy density capacitors. Generally, the ceramic capacitors and the film capacitors are more popular in Marx generator applications. The ceramic capacitors have the advantages of small size and low self-inductance, and the film capacitors have the advantages of large capacitance and high withstand voltage. In terms of compact Marx generators, ceramic capacitors are used more often because of their small size. However, the energy density of ceramic capacitors is low because it is difficult to balance the large capacitance, high withstand voltage, and small volume. Recently, mica capacitors have gradually drawn researchers’ attention because of their good electric characteristics [11]. Mica capacitors have the advantages of large capacitance, high withstand voltage, and small size, which make them a promising alternative to ceramic capacitors in the compact Marx generators.

Table 1. The parameters of the typical compact pulse forming network (PFN)-Marx generators.

Research Output Output Pulse E/V P/W Institute Power Voltage Width ISL 2.3 GW 342 kV 60 ns 2 J/L 70 MW/kg TTU 2.2 GW 210 kV 200 ns 9 J/L - CEAE 1.6 GW 400 kV 85 ns 2 J/L 36 MW/kg APELC 5 GW 350 kV 200 ns 2 J/L -

Secondly, the modular design of the PFN-Marx generator generally contains two meanings. One is the modular implementation of the PFN-Marx body. Obviously, a variety of PFN-Marx generators with different structures have been designed. However, once the generator is assembled, the PFN module is difﬁcult to replace. The modular design requires that the PFN module can be taken out and replaced from the system individually and therefore the PFN-Marx generator can be convenient for maintenance and parameter adjustment. In this case, the optimization of the PFN module design and assembling method becomes necessary. On the other hand, the PFN-Marx generator system includes many subsystem modules by their functions, which are the primary energy module, PFN- Marx body, trigger source, and vacuum diode. In order to realize a compact PFN-Marx generator system, the design of the subsystem modules should be optimized as well. In this paper, mica capacitors with large energy density are assembled in the PFN- Marx generator to improve its compactness. The PFN module and the integrated gas gap switch are well designed for modular realization. In Section2, the design and the assembly of the PFN module are elaborated in detail and the subsystem module of the PFN-Marx system is introduced in Section3, followed by experimental results and a summary in Sections4 and5, respectively.

2. Modular PFN-Marx Design 2.1. Overall Description The PFN-Marx generator is designed for driving a HPM source. In order to make the generator have a wider range of application, the peak power (P) and the load impedance (R) are designed as 5 GW and 50–60 Ω, respectively. Figure1 shows the schematic circuit of the PFN-Marx. The type-E is applied in this generator, because the equal L-C sections are easy for modular realization and fabrication in practice. The established parameters of the PFN-Marx are listed in Table2. The number of the node capacitor n and the number of the PFN module m can be calculated to 3 and 22, respectively, by Equations (1)–(3). √ τ = 2n LC (1) Electronics 2021, 10, x FOR PEER REVIEW 3 of 14

in practice. The established parameters of the PFN-Marx are listed in Table 2. The number of the node capacitor n and the number of the PFN module m can be calculated to 3 and 22, respectively, by Equations (1)–(3).

Table 2. PFN-Marx generator specifications.

Symbol Description Value P Peak power 5 GW Vload Load voltage 500 kV R Load impedance 50–60 Ω Τ Pulse width 90–100 ns C0 Node capacitor 6 nF L0 Node inductor 15 nH Vch Charging voltage 50 kV Rep Repetition rate 30 Hz

Electronics 2021, 10, 545 3 of 13 τ = 2n LC (1)

r L Rm= L (2) R = m (2) CC

mVmVch VoutputV == ch (3)(3) output 22

FigureFigure 1.1. Schematic circuitcircuit ofof thethe PFN-Marx.PFN-Marx.

In order to realize the repetition operation of the generator, the inductive isolation of Table 2. PFN-Marx generator speciﬁcations. the charging is used and the optimized isolation inductor Liso is calculated to 20 μH. The inductanceSymbol of the switch and the switch Description leading wire Ls is set as 45 nH. Value The simulation result is shownP in Figure 2. The load Peak voltage power waveform characteristics 5 GW include the rise-time of aboutVload 20 ns, pulse width ofLoad about voltage 90 ns, flat-top of about 50 500ns, kVand amplitude of about 530 kVR with the charging voltage Load impedance of 50 kV. 50–60 Ω T Pulse width 90–100 ns C0 Node capacitor 6 nF L0 Node inductor 15 nH Vch Charging voltage 50 kV Rep Repetition rate 30 Hz

In order to realize the repetition operation of the generator, the inductive isolation of the charging is used and the optimized isolation inductor Liso is calculated to 20 µH. The inductance of the switch and the switch leading wire Ls is set as 45 nH. The simulation result is shown in Figure2. The load voltage waveform characteristics include the rise-time of about 20 ns, pulse width of about 90 ns, ﬂat-top of about 50 ns, and amplitude of about 530 kV with the charging voltage of 50 kV. A series of methods are used for the realization of the modular PFN-Marx body, including the PFN module design, the connection method between the PFN module and the electrode of the switch, and the common switch housing. The key components of the modular PFN-Marx generator are illustrated as follows. Electronics 2021, 10, 545 4 of 13 Electronics 2021, 10, x FOR PEER REVIEW 4 of 14

FigureFigure 2 2.. TheThe load load voltage voltage waveform waveform in simulation. in simulation.

2.2.A PFN series Module of methods are used for the realization of the modular PFN-Marx body, in- cludingThe the assembly PFN module diagram design, of the the connection PFN module method is shown between in the Figure PFN3 .module Each PFN and module theconsists electrode of three of the 6000 switch pF,(22 and× the40 common× 95 mm switch3) mica housing. capacitors The connectedkey components in parallel of the by means modularof copper PFN strips,-Marx and generator the copper are illustrated strips also as serve follows. as the node inductors. The mica capacitors × × 2.2.are PFN inserted Module into a nylon case with external dimensions of 178 mm 150 mm 30 mm. The energy density of the mica capacitor is nearly 90 J/L, which is over three times that The assembly diagram of the PFN module is shown in Figure 3. Each PFN module of the commercial ceramic capacitor with 2000 pF (ϕ60 mm × 32 mm). The selection consists of three 6000 pF (22 × 40 × 95 mm3) mica capacitors connected in parallel by meansof the of mica copper capacitor strips, and with the large copper energy strips densityalso serve sets as athe good node foundation inductors. The for mica the compact capacitorsdesign. Each are inserted mica capacitor into a nylon assembly case with was external testedunder dimension the DCs of voltage178 mm of× 150 50 kVmm with × a time 30duration mm. The of energy 1 min beforedensity usage. of the Then,mica capacitor the connected is nearly mica 90 J/L, capacitors which is were over insertedthree into a timesnylon that box of to the ensure commercial the insulation ceramic capacitor distance with between 2000 pF the (φ adjacent60 mm × modules32 mm). The and se- to provide lectionmechanical of the support.mica capacitor Finally, with the large key energy point density of the modularsets a good realization foundation is for the the connection compactbetween design. the PFN Each module mica capacitor and the a switch.ssembly Here,was tested the socket under jointthe DC mode voltage is adopted of 50 kV in which withtwo pluga time electrodes duration of are 1 weldedmin before with usage. the PFNThen, via the two connected high-voltage mica capacitors wires. In were this way, the Electronics 2021, 10, x FOR PEER REVIEWinsertedPFN module into a nylon can be box taken to ensure out individually the insulation from distance the PFN-Marxbetween the bodyadjacent and modules the5 of structure 14 of

andthe PFN-Marxto provide mechanical remains unchanged. support. Finally, the key point of the modular realization is the connection between the PFN module and the switch. Here, the socket joint mode is adopted in which two plug electrodes are welded with the PFN via two high-voltage wires. In this way, the PFN module can be taken out individually from the PFN-Marx body and the structure of the PFN-Marx remains unchanged.

FigureFigure 3. TheThe assembly assembly diagram diagram of the of thePFN PFN module. module.

2.3. Gas Gap Switches Switches, as energy-shifting elements in the pulse power system, play an important role in the pulse compression and power enhancement. Generally, the generation of high voltage of the PFN-Marx generator is that the PFN modules are erected via the Marx-type method. Therefore, the number of switches of the PFN-Marx generator is the same as the number of the PFN modules. For a PFN-Marx generator, the operating characteristics of switches are related to the wave erection. So, the switches are designed specifically to ensure the performance and at the same time to realize the modular function of the PFN-Marx generator. In practice, all of the switch electrodes of the generator are assembled in a nylon housing with external dimensions of 695 mm × 85 mm × 60 mm, as shown in Figure 4. Actually, there are two main advantages of the integrated switch design: one is that the gas pressure of the switch can be adjusted independently, so that the operation voltage of the switch can be adjusted in a wide range; the other advantage is that the ultraviolet light, which is generated when the first few stages of the switches close, facilitates the switching of the later ones. In this way, the erected time of the PFN-Marx can be decreased.

Figure 4. The assembly diagram of the gas gap switches.

The electrode of the switch has a spherical geometry with the material of copper. The diameter of the electrode is 15 mm and the gap distance is 10 mm. These spherical electrodes are assembled with the electrode bases, as shown in Figure 4. A rectangular countersink hole is milled at the end of the electrode base to assemble the plug electrode,

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Electronics 2021, 10, 545 Figure 3. The assembly diagram of the PFN module. 5 of 13

2.3. Gas Gap Switches 2.3. GasSwitches, Gap Switches as energy-shifting elements in the pulse power system, play an important role in the pulse compression and power enhancement. Generally, the generation of Switches, as energy-shifting elements in the pulse power system, play an important high voltage of the PFN-Marx generator is that the PFN modules are erected via the role in the pulse compression and power enhancement. Generally, the generation of high Marx-type method. Therefore, the number of switches of the PFN-Marx generator is the voltage of the PFN-Marx generator is that the PFN modules are erected via the Marx-type samemethod. as Therefore,the number the of number the PFN of switchesmodules. of For the a PFN-Marx PFN-Marx generator generator, is the the same operating as the characteristicsnumber of the of PFN switches modules. are related For a PFN-Marx to the wave generator, erection. the So, operating the switches characteristics are designed of specificallyswitches are to related ensure to the the performance wave erection. and So, at the the switches same time are designed to realize speciﬁcally the modular to ensure func- tionthe performanceof the PFN-Marx and atgenerator. the same In time practice, to realize all of the the modular switch electrodes function of of the the PFN-Marx generator aregenerator. assembled In practice, in a nylon all of housing the switch with electrodes external of dimension the generators of are695 assembled mm × 85 inmm a nylon × 60 mhousingm, as shown with external in Figure dimensions 4. Actually, of 695there mm are× two85 mmmain× advantages60 mm, as shown of the inintegratedFigure4 . switchActually, design: there areone two is that main the advantages gas pressure of the of integratedthe switch switchcan be design:adjusted one independently, is that the gas sopressure that the of operation the switch voltage can be adjustedof the switch independently, can be adjusted so that thein a operation wide range; voltage the ofother the advantageswitch can is be that adjusted the ultraviolet in a wide light range;, which the otheris generated advantage when is the that first the few ultraviolet stages of light, the switcheswhich is generatedclose, facilitate whens the ﬁrstswitching few stages of the of later theswitches ones. In close,this way, facilitates the erected the switching time of theof the PFN later-Marx ones. can In be this decreased. way, the erected time of the PFN-Marx can be decreased.

FigureFigure 4. 4. TheThe assembly assembly diagram diagram of of the the gas gas gap gap switches. switches.

TheThe electrode electrode of of the the switch switch ha hass a a spherical spherical geometry geometry with with the the material material of of copper. copper. TheThe diameter of of the electrode is 15 mm and the gap distance is 10 mm.mm. These spherical electrodeselectrodes are are assembled with with the electrode bases, as shown in Figure4 4.. AA rectangularrectangular counterscountersinkink hole hole is is milled milled at at the the end end of of the the electrode electrode base base to to assemble assemble the the plug plug electrode electrode,, which is shown in Figure3. The plug electrode and the electrode base are secured by a metal screw. Therefore, the switch of the generator is an independent assembly and the PFN module can realize the quick dismantling function. In fact, in order to improve the operation stability of the generator, the switch is normally triggered by a high-voltage electric pulse. The switches of the ﬁrst two stages are a three-electrode conﬁguration constructed by a 2 mm stainless steel needle into a 15 mm brass sphere electrode, and Figure5 shows the schematic diagram of the three-electrode switch. The gap length between triggered electrode and main electrode and between two main electrodes is 3 mm and 10 mm, respectively. The dielectric material is assembled in the electrode to insulate the trigger electrode. The lifetime of the single triggered switch was tested in the experiment. The insulation gas is N2 mixed with 15% SF6 and the gas pressure in the switch house is 80 kPa. The switch was tested in 10 Hz repetition mode with an operation voltage of 50 kV over 10,000 shots. Figure6 shows the photograph of the triggered switch after 100,000 shots. This work indicates that the gas gap switch in Figure4 would have a long lifetime when it periodically changes the working gas. Electronics 2021, 10, x FOR PEER REVIEW 6 of 14

Electronics 2021, 10, x FOR PEER REVIEW 6 of 14

which is shown in Figure 3. The plug electrode and the electrode base are secured by a metal screw. Therefore, the switch of the generator is an independent assembly and the which is shown in Figure 3. The plug electrode and the electrode base are secured by a PFN module can realize the quick dismantling function. metal screw. Therefore, the switch of the generator is an independent assembly and the In fact, in order to improve the operation stability of the generator, the switch is PFN module can realize the quick dismantling function. normally triggered by a high-voltage electric pulse. The switches of the first two stages In fact, in order to improve the operation stability of the generator, the switch is are a three-electrode configuration constructed by a 2 mm stainless steel needle into a 15 normally triggered by a high-voltage electric pulse. The switches of the first two stages mm brass sphere electrode, and Figure 5 shows the schematic diagram of the are a three-electrode configuration constructed by a 2 mm stainless steel needle into a 15 three-electrode switch. The gap length between triggered electrode and main electrode mm brass sphere electrode, and Figure 5 shows the schematic diagram of the and between two main electrodes is 3 mm and 10 mm, respectively. The dielectric mate- three-electrode switch. The gap length between triggered electrode and main electrode rial is assembled in the electrode to insulate the trigger electrode. The lifetime of the sin- and between two main electrodes is 3 mm and 10 mm, respectively. The dielectric mate- gle triggered switch was tested in the experiment. The insulation gas is N2 mixed with rial is assembled in the electrode to insulate the trigger electrode. The lifetime of the sin- 15% SF6 and the gas pressure in the switch house is 80 kPa. The switch was tested in 10 gle triggered switch was tested in the experiment. The insulation gas is N2 mixed with Hz repetition mode with an operation voltage of 50 kV over 10,000 shots. Figure 6 shows 15% SF6 and the gas pressure in the switch house is 80 kPa. The switch was tested in 10 the photograph of the triggered switch after 100,000 shots. This work indicates that the Hz repetition mode with an operation voltage of 50 kV over 10,000 shots. Figure 6 shows gas gap switch in Figure 4 would have a long lifetime when it periodically changes the the photograph of the triggered switch after 100,000 shots. This work indicates that the working gas. gas gap switch in Figure 4 would have a long lifetime when it periodically changes the

Electronics 2021, 10, 545 working gas. 6 of 13

FigureFigure 5. Schematic 5. Schematic diagram of diagram the trigger switch. of the trigger switch. Figure 5. Schematic diagram of the trigger switch.

FigureFigure 6. Photograph 6. Photograph of the trigger of switch. the trigger switch. 2.4. The Modular PFN-Marx Generator Assembly Figure 6. Photograph of the trigger switch. 2.4Twenty-two. The Modular PFN modules PFN- areMarx constructed Generator in the form Assembly of a drawer-type geometry, which can minimize circuit inductance as well as realize the modular function, as shown in FigureTwenty7. There are-two four componentsPFN modules of the PFN-Marx are constructed body, including in three the support form of a drawer-type geometry, rods,2.4 PFN. The modules, Modular and aPFN switch-Marx assembly. Generator Firstly, the PFNAssembly modules are assembled on nylonwhich rods can with nylonminimize screws. Secondly, circuit the inductance switch assembly, as the well nylon rods,as realize and the PFN the modular function, as shown Twenty-two PFN modules are constructed in the form of a drawer-type geometry, modulesin Figure are assembled 7. There on the are nylon four plate. components Finally, the plug electrodes of the are PFN inserted-Marx into the body, including three support electrode bases of the switch and assembled with metal screws. In this way, the PFN module Electronics 2021, 10, x FOR PEER REVIEWwhich can minimize circuit inductance as well as realize7 ofthe 14 modular function, as shown canrods, be taken PFN out individuallymodules and, and the PFN-Marxa switch structure assembly. is kept unchanged. Firstly, Therefore, the PFN modules are assembled on in Figure 7. There are four components of the PFN-Marx body, including three support thenylon convenience rods of with the PFN-Marx nylon generator’s screws. maintenance Secondly, can be the improved switch signiﬁcantly. assembly, the nylon rods, and the rods, PFN modules, and a switch assembly. Firstly, the PFN modules are assembled on PFN modules are assembled on the nylon plate. Finally, the plug electrodes are inserted nylon rods with nylon screws. Secondly, the switch assembly, the nylon rods, and the into the electrode bases of the switch and assembled with metal screws. In this way, the PFN modules are assembled on the nylon plate. Finally, the plug electrodes are inserted PFN module can be taken out individually and the PFN-Marx structure is kept un- into the electrode bases of the switch and assembled with metal screws. In this way, the changed. Therefore, the convenience of the PFN-Marx generator’s maintenance can be PFN module can be taken out individually and the PFN-Marx structure is kept un- improved significantly. changed. Therefore, the convenience of the PFN-Marx generator’s maintenance can be improved significantly.

FigureFigure 7. 7.Assembly Assembly diagram diagram of of the the PFN-Marx PFN-Marx (a—support (a—support rod, rod, b—gas b—gas switch, switch, c—PFN c—PFN module). module).

3. Experimental Setup 3.1. Primary Energy Subsystem A primary energy subsystem was designed and established for repetition operation of the PFN-Marx generator, and the equivalent circuit is shown in Figure 8. At the beginning of the generator operation, the intermediate energy storage capacitor Cf and the primary capacitor of the pulse transformer Cp are charged by an AC transformer TF1, and the maximum charging voltage is 2800 V. The mica capacitors in the PFN-Marx generator are charged by an air-core pulse transformer TF and the maximum charging voltage Vch is up to 50 kV. The charging voltage waveform is shown in Figure 9.

Figure 8. Schematic circuit of the primary energy subsystem.

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Electronics 2021, 10, 545 7 of 13 Figure 7. Assembly diagram of the PFN-Marx (a—support rod, b—gas switch, c—PFN module).

3.3. ExperimentalExperimental SetupSetup 3.1.3.1. PrimaryPrimary EnergyEnergy SubsystemSubsystem AA primary primary energy energy subsystem subsystem was was designed designed and and established established for for repetition repetition operation operation of theof the PFN-Marx PFN-Marx generator, generato andr, theand equivalent the equivalent circuit circuit is shown is shown in Figure in 8Figure. At the 8. beginning At the be- ofginning the generator of the generator operation, operation, the intermediate the intermediate energy storage energy capacitor storage capacitor Cf and the C primaryf and the capacitorprimary capacitor of the pulse of the transformer pulse transformer Cp are charged Cp are bycharged an AC by transformer an AC transformer TF1, and theTF1, maximumand the maximum charging voltagecharging is 2800voltage V. The is 2800 mica V. capacitors The mica in thecapacitors PFN-Marx in the generator PFN-Marx are chargedgenerator by are an air-corecharged pulse by an transformer air-core pulse TF and transformer the maximum TF and charging the maximum voltage Vchargingch is up tovoltage 50 kV. V Thech is charging up to 50 voltagekV. The waveform charging voltage is shown waveform in Figure is9. shown in Figure 9.

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FigureFigure 8. 8.Schematic Schematic circuitcircuit of of the the primary primary energy energy subsystem. subsystem.

FigureFigure 9. 9.The The charging charging voltage voltage waveform waveform of of the the PFN-Marx PFN-Marx generator. generator.

TheThe working working process process of of the the system system in in the the case case of of repetition repetition mode mode is is described described as as follows.follows. The The operation operation sequence sequence of of the the thyristors thyristors is shown is shown in Figure in Figure 10. Firstly, 10. Firstly, the C ftheand Cf Candp are C chargedp are charged to Uf and to U Uf 0and, respectively. U0, respectively Next, when. Next, the when thyristor the thyristor S1 is closed S1 is at closed time T 1at, thetime Cp Twill1, the discharge Cp will discharge to the PFN-Marx to the PFN through-Marx the through TF. After the that,TF. After the C pthat,has the the C negativep has the residualnegative voltage residual Urec voltage because Urec of thebecause principle of the of theprinciple unidirectional of the unidirectional current of the current thyristor. of Then,the thyristor. the thyristor Then, S 2theis closedthyristor at S time2 is closed T2, and at the time voltage T2, and polarity the voltage of the polarity Cp is reversed of the Cp viais reversed the loop ofvia C pthe-S2 -Rloop1-L 1of-R C2-Lp-S2-C2-Rp1.-L Finally,1-R2-L2- theCp. thyristorFinally, the S3 is thyristor closed at S time3 is closed T3, the at C ptimeis charged to U again, and the system waits for the next operation command. So, the key T3, the Cp is 0charged to U0 again, and the system waits for the next operation command. pointSo, the of thekey repetitionpoint of the operation repetition is that operation the control is that of the oncontrol time of the thyristorson time of and the thethy- pulse number is related to the capacitance of the C . ristors and the pulse number is related to the capacitancef of the Cf.

Figure 10. The sequence diagram of the thyristors.

3.2. Trigger Source An important way of improving the synchronization performance of the switch is to reduce the rise-time of the trigger pulse and increase the trigger pulse amplitude. Here, a compact Marx generator, which is based on a magnetic switch, is designed for a trigger source [16]. The key point of this trigger source is that the integrated design of the transformer and the switch is realized, so that the volume of it can be reduced significantly. The basic circuit of the trigger source is shown in Figure 11. The magnetic switch is operated as a transformer during the charging process at first. When the magnetic core is saturated, the inductance of the secondary windings is decreased rapidly. At this time, the energy is sharply delivered to the load.

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Figure 9. The charging voltage waveform of the PFN-Marx generator.

The working process of the system in the case of repetition mode is described as follows. The operation sequence of the thyristors is shown in Figure 10. Firstly, the Cf and Cp are charged to Uf and U0, respectively. Next, when the thyristor S1 is closed at time T1, the Cp will discharge to the PFN-Marx through the TF. After that, the Cp has the negative residual voltage Urec because of the principle of the unidirectional current of the thyristor. Then, the thyristor S2 is closed at time T2, and the voltage polarity of the Cp is reversed via the loop of Cp-S2-R1-L1-R2-L2-Cp. Finally, the thyristor S3 is closed at time Electronics 2021, 10, 545 T3, the Cp is charged to U0 again, and the system waits for the next operation command.8 of 13 So, the key point of the repetition operation is that the control of the on time of the thyristors and the pulse number is related to the capacitance of the Cf.

FigureFigure 10. 10.The The sequence sequence diagram diagram of of the the thyristors. thyristors.

3.2.3.2. Trigger Trigger Source Source AnAn important important way way of of improving improving the the synchronization synchronization performance performance of of the the switch switch is is toto reduce reduce the the rise-time rise-time of of the the trigger trigger pulse pulse and and increase increase the the trigger trigger pulse pulse amplitude. amplitude. Here,Here, a a compact compact Marx Marx generator,generator, whichwhich isis basedbased onon aa magneticmagnetic switch, is designed for for a atrigger trigger source source [16]. [16]. The The key key point point of of this this trigger trigger source source is is that that the the integrated integrated design design of ofthe the transformer transformer and and the the switch switch is isrealized, realized, so sothat that the the volume volume of ofit can it can be bereduced reduced sig- signiﬁcantly.nificantly. The The basic basic circuit circuit of of the the trigg triggerer source isis shownshown inin FigureFigure 11 11.. The The magnetic magnetic switchswitch is is operated operated as as a transformera transformer during during the the charging charging process process at ﬁrst. at first. When When the magnetic the mag- Electronics 2021, 10, x FOR PEER REVIEW 9 of 14 corenetic is core saturated, is saturated, the inductance the inductance of the secondary of the secondary windings windings is decreased is decreased rapidly. Atrapidly. this time,At this the time, energy the is energy sharply is deliveredsharply delivered to the load. to the load.

FigureFigure 11. 11.The The circuitcircuit of of the the trigger trigger source. source.

TheThe triggertrigger sourcesource assemblyassembly inin thetheexperiment experiment isis shownshown inin Figure Figure 12 12.. It It can can be be seen seen thatthat the the trigger trigger source source is is very very compact compact and and its its external external dimensions dimensions are are 150 150 mm mm× ×200 200 mm mm ×× 50 mm. The Marx generator is assembled inin aa dielectricdielectric materialmaterial househouse andand pressurizedpressurized SFSF66gas gas is is used used for electricalfor electrical insulation. insulation. The essential The essential characteristic characteristic of the magnetic of the switchmagnetic is thatswitch the timeis that of the core time saturation of core is saturation consistent is if theconsistent charging if voltagethe charging of the primaryvoltage of capacitor the pri- isma consistent.ry capacitor Therefore, is consistent. the consistency Therefore, ofthe the consistency trigger pulses of the is prettytrigger good, pulses as is shown pretty ingood, Figure as 13shown. The in trigger Figure source 13. The is operatedtrigger source in 30 Hzis operated mode and in the30 Hz experimental mode and resultsthe ex- showperimental that the results amplitude show of that the the trigger amplitude pulses isof over the 70trigger kV and pulses the jitteris over is less70 kV than and 10 ns.the Thisjitter trigger is less sourcethan 10 provides ns. This atrigger stable source and reliable provides trigger a stable pulse and for reliable the switches. trigger pulse for the switches.

Figure 12. The photograph of the trigger source.

Figure 13. Screen shot of 20 pulses at 30 Hz operation of the trigger source.

3.3. Experimental Setup In order to evaluate the performance of the compact modular PFN-Marx generator, the system of the generator was arranged as shown in Figure 14. The generator mainly consists of five parts, including a control subsystem, a trigger source, a primary energy subsystem, a PFN-Marx body, and a load. The charging voltage was measured with a

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Figure 11. The circuit of the trigger source.

The trigger source assembly in the experiment is shown in Figure 12. It can be seen

that the trigger source is very compact and its external dimensions are 150 mm × 200 mm Figure× 50 mm. 11. The The circuit Marx of the generator trigger source. is assembled in a dielectric material house and pressurized SF6 gas is used for electrical insulation. The essential characteristic of the magnetic switchThe istrigger that sourcethe time assembly of core in thesaturation experiment is is consistent shown in Figure if the 12. charging It can be seenvoltage of the pri- thatmary the capacitor trigger source is isconsistent. very compact Therefore, and its external the dimensionsconsistency are of 150 the mm trigger × 200 mm pulses is pretty ×good, 50 mm. as The shown Marx generatorin Figure is 13.assembled The trigger in a dielectric source material is operated house and in pressurized30 Hz mode and the ex- SF6 gas is used for electrical insulation. The essential characteristic of the magnetic perimental results show that the amplitude of the trigger pulses is over 70 kV and the Electronics 2021, 10, 545 switch is that the time of core saturation is consistent if the charging voltage of the9 of pri- 13 majitterry capacitoris less than is consistent. 10 ns. This Therefore, trigger the source consistency provides of the a stabletrigger andpulses reliable is pretty trigger pulse for good,the switches. as shown in Figure 13. The trigger source is operated in 30 Hz mode and the experimental results show that the amplitude of the trigger pulses is over 70 kV and the jitter is less than 10 ns. This trigger source provides a stable and reliable trigger pulse for the switches.

FigureFigure 12. 12.The The photograph photograph of the triggerof the source.trigger source. Figure 12. The photograph of the trigger source.

Figure 13. Screen shot of 20 pulses at 30 Hz operation of the trigger source. Figure 13. Screen shot of 20 pulses at 30 Hz operation of the trigger source. 3.3.3.3.Figure ExperimentalExperime 13. Screenntal SetupSetup shot of 20 pulses at 30 Hz operation of the trigger source. InIn orderorder toto evaluateevaluate thethe performanceperformance ofof thethe compactcompact modularmodular PFN-MarxPFN-Marx generator,generator, Electronics 2021, 10, x FOR PEER REVIEWthethe system system ofof thethe generatorgenerator waswas arrangedarranged asas shownshown inin Figure Figure 14 14.. TheThe generator generator10 of 14 mainlymainly 3.3. Experimental Setup consistsconsists ofof ﬁvefive parts,parts, includingincluding aa controlcontrol subsystem,subsystem, aa triggertrigger source,source, aa primaryprimary energyenergy subsystem,subsystem,In order aa PFN-MarxPFN to- Marxevaluate body,body ,the andand performance aa load.load. TheThe chargingcharging of the voltage voltagecompact waswa smodular measuredmeasured PFN withwith- aMarxa generator, commercialcommercialthe system polestar of the probe probe generator with with 2000:1. 2000:1. wa As A waterarranged water resister resister as divider divider shown wa wass ininserted inserted Figure into into 14. the the The diode generator mainly insulatordiodeconsists insulator to of measure five to measure parts, the loadthe including load voltage. voltage. a It cancontrolIt can be be seen seensubsystem, in in Figure Figure 14 14a that triggerthat thethe compactness com-source, a primary energy assembly is accomplished. After the system is assembled in practice, the length of the pactnesssubsystem, assembly a PFNis accomplished.-Marx body After, theand system a load. is assembled The charging in practice, voltagethe length was measured with a PFN-Marxof the PFN-Marx body body is about is about 80 cm 80 cm and and its its weight weight is is about about 56 56 kg.kg.

Figure 14. Schematic view of the PFN-Marx system. Figure 14. Schematic view of the PFN-Marx system. 4. Experimental Results 4.1. Experimental Results on a Water Load Initial testing of the compact PFN-Marx generator involved firing the system at the charging voltage of 50 kV into a shot, and a 59 Ω coaxial CuSO4 water load was connected to the generator to evaluate the operating characteristic of the generator. The water load was assembled in the vacuum diode as shown in Figure 14. The generator was tested in single shot and burst mode, respectively. The pure SF6 of pressure of 180 kPa was injected into the Marx casing for high voltage isolation, and the gas mixture of the N2 and SF6 with the pressure of 90 kPa was sent to the switch case. In single mode, a maximum 540 kV high-voltage pulse was delivered to the water load with the rise-time of 28 ns and the pulse width of 95 ns, as shown in Figure 15. The peak power of the output pulse was calculated to be about 5 GW (P = U2/R) and the E/V and P/W of the PFN-Marx generator body were about 6.5 J/L and 90 MW/kg, respectively. The generator was also tested in burst mode. The load voltage pulse-train of the generator on the water load, which comprises five pulses at a repetition rate of 30 Hz, is shown in Figure 16. The charging voltage of the PFN-Marx is 45 kV and the amplitude of the load voltage is about 500 kV, which was measured via a resistance voltage divider. The dispersion of the load voltage is less than 3%. The load voltage waveform indicates that the operation of the generator in single mode agrees well with that in burst mode.

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4. Experimental Results 4.1. Experimental Results on a Water Load Initial testing of the compact PFN-Marx generator involved ﬁring the system at the charging voltage of 50 kV into a shot, and a 59 Ω coaxial CuSO4 water load was connected to the generator to evaluate the operating characteristic of the generator. The water load was assembled in the vacuum diode as shown in Figure 14. The generator was tested in single shot and burst mode, respectively. The pure SF6 of pressure of 180 kPa was injected into the Marx casing for high voltage isolation, and the gas mixture of the N2 and SF6 with the pressure of 90 kPa was sent to the switch case. In single mode, a maximum 540 kV high-voltage pulse was delivered to the water load with the rise-time of 28 ns and the pulse width of 95 ns, as shown in Figure 15. The peak power of the output pulse was calculated to be about 5 GW (P = U2/R) and the E/V and P/W of the PFN-Marx generator body were about 6.5 J/L and 90 MW/kg, respectively. The generator was also tested in burst mode. The load voltage pulse-train of the generator on the water load, which comprises ﬁve pulses at a repetition rate of 30 Hz, is shown in Figure 16. The charging voltage of the PFN-Marx is 45 kV and the amplitude of the load voltage is about 500 kV, which was measured via a resistance voltage divider. The dispersion of the load voltage is less than 3%. The load voltage waveform indicates that the operation of the Electronics 2021, 10, x FOR PEER REVIEW 11 of 14 generator in single mode agrees well with that in burst mode. Electronics 2021, 10, x FOR PEER REVIEW 11 of 14

FigureFigure 15.15. TheThe load load voltage voltage waveforms waveforms on the water on the load. water load. Figure 15. The load voltage waveforms on the water load.

Figure 16. Screen shot of 5 pulses at 30 Hz operation. Figure 16. Screen shot of 5 pulses at 30 Hz operation. FigureHowever, 16. Screen the shot rectangular of 5 pulses characteristic at 30 Hz operation.of the load voltage waveform in the ex- perimentHowever, was notthe asrectangular good as in characteristic the simulation. of Thethe loadreason voltage for that waveform contains infive the parts. ex- perimentFirstly, the wa generators not as goodis constructed as in the simulation.by multiple ThePFN reason modules for, thatwhich contain are likes five building parts. Firstly,blocks. So,the thegenerator coupling is constructedcapacitor (C cby) between multiple the PFN adjacent modules PFN, which modules are inevitablylike building ex- c blocks.ists between So, the the coupling PFN modules capacitor, and (C thec) between C , together the adjacent with the PFN PFN modules node inductors inevitably, forms ex- ista parasitics between transmission the PFN modules line (PTL), and with the aC cshort, together characteristic with the time.PFN nodeThe final inductors performance, forms aof parasitic the effect transmission of the PTL is line that (PTL) the high with-frequency a short characteristic oscillation istime. superimposed The final performance on the load ofvoltage the effect waveform. of the PTL This is isthat the the main high reason-frequency why theoscillation load voltage is superimposed waveform’s on quality the load in voltagethe experiment waveform. wa sThis lower is the than main that reason in the why simulation. the load Secondly,voltage waveform the capacitor’s quality in the in thesimulation experiment is anwa s ideallower modelthan that and in the simulation.mica in theSecondly, practical the experimentcapacitor in hasthe simulationself-inductance. is anIn idealfurther model work, and the the measure mica inmethod the practicalof the micaexperiment capacitor has’s self--inductance.inductance willIn befurther studied. work, Thirdly, the themeasure inductance method of the ofwater the resistormica alsocapacitor affects’s selfthe -qualityinductance of the will load be waveform.studied. Thirdly, Fourth the, the inductance load voltage of thewa swater measured resistor by alsoa water affects re- the quality of the load waveform. Fourth, the load voltage was measured by a water re-

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However, the rectangular characteristic of the load voltage waveform in the experiment was not as good as in the simulation. The reason for that contains five parts. Firstly, the generator is constructed by multiple PFN modules, which are like building blocks. So, the coupling capacitor (Cc) between the adjacent PFN modules inevitably exists between the PFN modules, and the Cc, together with the PFN node inductors, forms a parasitic transmission line (PTL) with a short characteristic time. The final performance of the effect of the PTL is that the high-frequency oscillation is superimposed on the load voltage waveform. This is the main reason why the load voltage waveform’s quality in the experiment was lower than that in the simulation. Secondly, the capacitor in the simulation is an ideal model and the mica in the practical experiment has self-inductance. In further work, the measure method of the mica capacitor’s self-inductance will be studied. Thirdly, the inductance of the water resistor also affects the quality of the load waveform. Fourth, the load voltage was measured by a water resister divider with a low impedance type. The sample resistance of the resistor divider is 1 Ω. The deterioration of the rise-edge of the load voltage waveform is significantly affected by the length of the connecting line of the resistance divider, which is the inductance of the connecting line (Lm). The dying oscillation of the load voltage waveform is decreased as the Lm decreases. Finally, in the simulation analysis, the capacitance of the switch and the ground capacitance are ignored. These parasitic parameters also affect the load pulse. Actually, an oscillation damping circuit, which is to add an R-C unit between the last two PFN modules, has been preliminarily investigated in an experiment in another PFN-Marx system [17]. So, future work will also focus on the improvement of the load waveform’s quality.

4.2. Experimental Results on a TTO A miniaturized transit time oscillator (TTO) driven by this generator was initially investigated in an experiment. The setup for TTO operations is similar to the water load testing, with the dummy load removed. The Marx casing was ﬁlled with 160 kPa of pure SF6, and, depending on the charging voltage, 80 kPa gas of N2 mixed with 15% SF6 was sent to the enclosed switch column. The generator was operated in triggering mode, and the output voltage and output current of the generator were measured by a resistance divider and a hand-made Rogovski coil, respectively. The typical waveforms of the generator and the microwave are shown in Figure 17. The pulse characteristics of the voltage waveform, including the voltage of 450 kV, the rise-time of 25 ns, and the pulse width of 90 ns, were measured in the oscilloscope. Obviously, the pulse width of the current waveform is shorter than that of the voltage waveform. The reason could be that the Rogovski coil has the defect during hand making. The sampling loop was not controlled to a minimum and the sampling resistor is not an inductive resistor. Besides, because the Rogovski coil is compact, there may be breakdown inside it during the measuring process. Therefore, distortion of the current signal exists. During the experiment, the Rogovski coil was used only for monitoring the electron beam production. A microwave pulse with power of about 100 MW and a pulse duration of 25 ns is generated when the diode voltage is 450 kV. The HPM signal was estimated to have a peak of 100 MW. The initial experimental results show the ability of the generator to drive the HPM source. The TTO driven by the PFN-Marx generator in burst mode was also investigated initially in an experiment. The experimental results show that the TTO at 30 Hz with ﬁve pulses works well. The microwave power is about 85 MW, with the diode voltage about 440 kV, as shown in Figure 18. Obviously, there are some differences in the consistency of the microwave pulses. The experimental test with the PFN-Marx generator driving the TTO is still under study and detailed results will be published in the next work. Electronics 2021, 10, x FOR PEER REVIEW 12 of 14

sister divider with a low impedance type. The sample resistance of the resistor divider is 1 Ω. The deterioration of the rise-edge of the load voltage waveform is significantly affected by the length of the connecting line of the resistance divider, which is the inductance of the connecting line (Lm). The dying oscillation of the load voltage waveform is decreased as the Lm decreases. Finally, in the simulation analysis, the capacitance of the switch and the ground capacitance are ignored. These parasitic parameters also affect the load pulse. Actually, an oscillation damping circuit, which is to add an R-C unit between the last two PFN modules, has been preliminarily investigated in an experiment in another PFN-Marx system [17]. So, future work will also focus on the improvement of the load waveform’s quality.

4.2. Experimental Results on a TTO A miniaturized transit time oscillator (TTO) driven by this generator was initially investigated in an experiment. The setup for TTO operations is similar to the water load testing, with the dummy load removed. The Marx casing was filled with 160 kPa of pure SF6, and, depending on the charging voltage, 80 kPa gas of N2 mixed with 15% SF6 was sent to the enclosed switch column. The generator was operated in triggering mode, and the output voltage and output current of the generator were measured by a resistance divider and a hand-made Rogovski coil, respectively. The typical waveforms of the generator and the microwave are shown in Figure 17. The pulse characteristics of the voltage waveform, including the voltage of 450 kV, the rise-time of 25 ns, and the pulse width of 90 ns, were measured in the oscilloscope. Obviously, the pulse width of the current waveform is shorter than that of the voltage waveform. The reason could be that the Rogovski coil has the defect during hand making. The sampling loop was not controlled to a minimum and the sampling resistor is not an inductive resistor. Besides, because the Rogovski coil is compact, there may be breakdown inside it during the measuring process. Therefore, distortion of the current signal exists. During the experiment, the Rogovski coil was used only for monitoring the electron beam production. A microwave pulse with power of about 100 MW and a pulse duration of 25 ns is generated Electronics 2021, 10, 545 when the diode voltage is 450 kV. The HPM signal was estimated to have a peak of 10012 of 13 MW. The initial experimental results show the ability of the generator to drive the HPM source.

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Figure 17. The typical waveforms on the transit time oscillator (TTO). Figureof the 17. microwaveThe typical pulses. waveforms The on experimental the transit time test oscillator with (TTO).the PFN-Marx generator driving the TTO is still under study and detailed results will be published in the next work. The TTO driven by the PFN-Marx generator in burst mode was also investigated initially in an experiment. The experimental results show that the TTO at 30 Hz with five pulses works well. The microwave power is about 85 MW, with the diode voltage about 440 kV, as shown in Figure 18. Obviously, there are some differences in the consistency

Figure 18. Screen shot of 5 microwave pulses at 30 Hz operation. Figure 18. Screen shot of 5 microwave pulses at 30 Hz operation. 5.5. Summary Summary TheThe 22-stage 22-stage compact compact PFN-Marx PFN-Marx generator generator based based on on the the mica mica capacitors capacitors has has shown shown successfulsuccessful 5 5 GW GW output output on on a a water water load. load. The The performance performance of of this this PFN-Marx PFN-Marx generator generator in in burstburst mode mode is is also also described described and and the the load load voltage voltage waveform waveform indicates indicate thats that the the operation operation of theof generatorthe generator in single in single mode mode agrees agrees well well with with that inthat burst in burst mode. mode. Typical Typical pulse pulse character- char- isticsacteristics include include a pulse a widthpulse width of 95 ns of and 95 ns a rise-time and a rise of-time 28 ns. of A 28 TTO ns. HPMA TTO source HPM driven source bydriven this generator by this generator was also wa testeds also and tested a 100 and MW, a 100 25 nsMW, microwave 25 ns microwave pulse was pulse obtained was inob- antained experiment. in an experiment. AA series series of of methods methods for for compact compact and and modular modular design design of of the the PFN-Marx PFN-Marx generator generator werewere used, used, including including the the large large energy energy density density mica capacitors,mica capacitors, a mini-Marx a mini trigger-Marx sourcetrigger withsource the with integration the integration of themagnetic of the magnetic transformer transform ander switch, and switch, the PFN the modulePFN modul design,e de- asign, common a common switch switch case, and case the, and connection the connection between between the PFN the module PFN module and the switch.and the Underswitch. theUnder condition the condition of these designs,of these adesigns, single PFN a single module PFN can module be taken can out be from taken the out PFN-Marx from the individually.PFN-Marx individually Finally, the length. Finally, of thethe PFN-Marxlength of the generator PFN-Marx body generator is limited body to 80 is cm limit anded its to weight80 cm isand about its 56weight kg. The is about ratio of56 the kg. energy The ratio storage of the to energy volume storage and the to ratio volume of power and tothe weightratio of of power the 22-stage to weight PFN-Marx of the generator22-stage PFN are up-Marx to 6.5 generator J/L and 90are MW/kg, up to 6.5 respectively. J/L and 90 MW/kg,Actually, respectively. a lot of work could be done to improve the performance of the generator; for example,Actually, increasing a lot theof work pulse could number be anddone improving to improve the the quality performance of the output of the waveform, generator; for example, increasing the pulse number and improving the quality of the output waveform, as well as optimizing the operation of the TTO source driven by the generator. Furthermore, based on our generator, other types of HPM sources, including a relativistic magnetron and a relativistic backward wave oscillator, are being studied.

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as well as optimizing the operation of the TTO source driven by the generator. Furthermore, based on our generator, other types of HPM sources, including a relativistic magnetron and a relativistic backward wave oscillator, are being studied.

Author Contributions: PFN-Marx design and structure design, H.Z. (Haoran Zhang); validation, T.S.; primary energy subsystem, S.L.; methodology, Z.Z.; TTO design, L.S.; control subsystem, H.Z. (Heng Zhang); writing—original draft preparation, H.Z. (Haoran Zhang); writing—review and editing, T.S. and Z.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data that support the findings of this study are available from the corresponding author upon reasonable request. Acknowledgments: The authors wish to thank Bo Liang and Jia Li for their support during the assembly and the experiment. Conflicts of Interest: The authors declare no conflict of interest.

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