Stand-Alone, FCG-Driven High Power Microwave System
A. Young, A. Neuber, M. Elsayed, J. Walter, J. Dickens, M. Kristiansen, L.L. Altgilbers1) The Center for Pulsed Power and Power Electronics, Texas Tech University, MS 43102, Lubbock,
TX, 79409, USA 1) SMDC, U.S. Army, Huntsville, AL, USA
Abstract
An explosively driven High Power Microwave (HPM) source has been developed that is based on the use of a Flux Compression Generator (FCG) as the primary driver. Four main components comprise the HPM system, and include a capacitor-based seed energy source, a dual-staged FCG, a power conditioning unit and an HPM diode (reflex-triode vircator). Volume constraints dictate that the entire system must fit within a tube having a 15 cm diameter, and a length no longer than 1.5 m. Additional design restrictions call for the entire system to be stand-alone (free from any external power sources). Presented here are the details of HPM system, with a description of each subcomponent and its role in the generation of HPM. Waveforms will be shown which illustrate the development of power as it commutates through each stage of the system, as well as power radiated from the diode. Analysis and comparisons will be offered that will demonstrate the advantages of an explosively driven HPM system over more conventional pulsed power devices.
I. INTRODUCTION The progression of technology has led to the interest of
using High Power Microwaves (HPM) in a broadening range of applications. Examples of more modern HPM-related applications include particle accelerators, plasma heating, space propulsion, power beaming, high power radar and directed energy weapons [1]. The limiting factor for some of these applications is the energy density and transportability of the HPM system. Field-deployed devices, such as active denial, require the system to be mounted on a vehicle and transported to where it is needed. More conventional HPM and pulsed power systems can be somewhat bulky, and are typically not suited for such applications. Although ideas for new, more compact conventional pulsed power systems are constantly being reported, some classes of pulsed power are more inherently suited for these uses.
There have been significant efforts to design HPM systems based on the use of explosively-driven pulsed power. While overall system volume has not necessarily played a significant role in these designs, recent research indicates that there is interest in developing compact HPM systems with the aid of explosive drivers. Xingen
[2] claimed to have achieved 38 MW of radiated power by driving a virtual cathode oscillator (vircator) with an FCG driven system. Polevin [3] reported the design of an HPM system which used two FCGs to drive a resonant relativistic Backwards Wave Oscillator (BWO), where radiated microwave power was claimed to be in the gigawatt range. With these and other similar reported results, it is evident that there is great interest in the development of FCG based systems for use in modern HPM applications.
A research effort is underway at Texas Tech University to develop a compact, explosively-driven HPM device. The main objective of this effort is to design a system which is stand-alone (i.e. no external power source) and compact. Compactness defined here means that the system must be able to fit into a tube having a 15 cm diameter, and have a minimal length (1.5 m as an initial goal). Another requirement of this design is that it needs to be an orderly working totality [1], a term used to describe a system which is complete end-to-end. This means that the system must contain all the electrical components necessary for HPM production. In standard practice, this corresponds to an initial prime power source, a pulsed power device and the microwave generator. The prime power source supplies DC or slow transient energy to the pulsed power device, which transforms this energy into a fast pulse (10’s to 100’s of nanoseconds) used to drive the HPM generator. The microwave source may also include a mode converter and an antenna to direct the radiated power. A first generation prototype such a system was designed, fabricated and tested. This system was composed of a capacitor based seed energy source, Helical Flux Compression Generator (HFCG), Power Conditioning System (PCS) and reflex-triode vircator. Microwave radiation with power levels in the tens of megawatts was reported in 2008 [4]. Since that time, a second generation system has been established (see Fig. 1), where improvements and additions have been made to increase the overall performance of the system as well as completely conform to the specified system constraints. A description of these improvements and additions will be made herein, with data and waveforms provided to show the present performance capability of the HPM system, as well as analysis and conclusions on the possible advantages of explosively driven pulsed power over other systems.
Figure 1. HPM System Diagram: (A) Seed Energy Source (B) Helical Flux Compression Generator (C) Power Conditioning (D) Sealed-Tube Reflex-Triode Vircator
II. EXPERIMENTAL SETUP
A. Seed Energy Since the HPM system is designed to be single-shot (i.e
expendable), minimizing the number of components in the seed energy source was a major design goal. Also, minimizing the number of components allowed the overall volume of the seed energy source to remain constant with the addition of an Exploding Bridge Wire (EBW) fire set for the HFCG (to be described shortly). The circuit diagram of the seed energy source can be seen in Fig. 2.
Figure 2. Seed Energy Source Circuit Schematic [5]
To initiate energy flow into the circuit, a 9 V lead acid battery is discharged into 4 parallel, conventionally available, HV DC-to-DC converters. The combined output of these converters (5 kV @ 8 mA) charges a 50
μF capacitor (with an energy density of 0.673 J/cm3) in approximately 30 seconds. As can be seen in Fig. 2, the HFCG is placed on high potential terminals of the capacitor, which allows for a simpler switching scheme. To initiate current flow through the HFCG-capacitor circuit, a 10 V TTL pulse is transmitted to the lower of two BJT transistors arranged in series (see Fig. 2). Once current begins to flow in the circuit, it quickly exceeds the rated value of the transistors, destroying them in the process. The simplicity and relatively inexpensive cost of this circuit makes it well suited for use in a single-shot application.
With an energy transfer efficiency of 82%, the seed energy source can drive ~360 J into a 5.2 μH inductor from the capacitor with a ~4.25 kV charge [5].
B. Energy Amplification
The energy amplifier used in this system is a dual-staged HFCG with inductively coupled (flux-trapping) stages, and has been detailed in [7] and [8]. For completeness, few of the major HFCG parameters will be given here, but the reader is encouraged to consult the above sources for details on the HFCG design and performance. The total volume of the generator is ~7 L with an active volume of 0.882 L. The stator has an inner diameter of 76 mm and the armature has an outer diameter of 38 mm, and holds a C-4 explosive charge of ~410 g.
Presently, the HFCG is capable of driving ~5 kJ into a given load inductor in the microhenry range, with achieved energy amplification factors of thirty or greater.
(A)
(B)
(C)
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This increase in performance from previous reports is attributed more to the improved design of the energy storage inductor in the power conditioning system (to be discussed shortly) than to any design changes in the HFCG.
1) HFCG Fire Set
The first generation HPM system was dependent on an external fire set to detonate the explosive charge in the HFCG. A new circuit, derived on the same principles of the seed energy source (i.e. simple, low volume), was designed and fabricated to deliver sufficient energy to an RP501 EBW to initiate the explosive detonation.
A 9 V lead acid battery is used to charge a 1 μF capacitor to 1 kV, which in turn is discharged into the EBW through the closure if a BJT transistor. As previously mentioned, the volume savings in the seed energy source enabled the introduction of the EBW fire set with no impact on the overall volume of the system. A more detailed description of the device’s topology and behavior can also be found in [6].
C. Power Conditioning
The power conditioning system is designed to transform the relatively slow-rise current pulse from the HFCG to a fast-rise voltage pulse which is characterized by a decrease in rise-time by three or four orders of magnitude. This is achieved through the use of an energy storage inductor in series with a fuse-opening switch, and a spark gap.
The energy storage inductor in the first generation system consisted of 5-AWG 12 stranded copper wires connected in parallel and wound around an 89 mm diameter PVC mandrel. The resistance of the inductor was ~200 mΩ. This could be attributed to the highly non-uniform current distribution in the conductor cross-section due to the skin and proximity effects which are inherent in these types of circuit elements. To promote a more uniform current through the conductor cross-section, the energy storage inductor was wound with AWG-12 Litz wire, and the conductor cross-section was also increased by a factor of two. With these changes, the resistance of the inductor decreased by about an order of magnitude to ~30 mΩ, while the inductance remained approximately the same (~3 μH), thus reducing the I2R losses as the current in the inductor increases.
The fuse-opening switch in the first generation system was composed of 0.127 mm gold wires wound around brass gears. The design of this fuse was revised, with the aim of fabricating a more expendable, lighter fuse with a more sound electrical connection between the fuse wires and the rest of the power conditioning system. A detailed description of the new fuse design, as well as experiments with the fuse on a non-explosive FCG test-bed can be found in [9]. A new design feature not shown in [9] is that the fuse wire was twisted ninety degrees in the azimuthal direction at set intervals in the fuse winding. Thus, the
overall length of the fuse was decreased while the length of the fuse wire remained constant. Using this method, overall fuse lengths were decreased by approximately 50% from the straight wire value. For the experiments shown here, fourteen 0.127 mm diameter silver wires served as the fuse conductors. The length of the fuse wire was 35.5 cm, while the overall length of the fuse was 19 cm.
A spark gap in the power conditioning serves to decouple the inductor and fuse from the load before the fuse has opened and the voltage pulse has been formed. The gap is a simple self break device with hemispherical brass electrodes, with a gap distance which can be varied to increase or decrease the hold-off voltage.
The entire power conditioning assembly is placed in a 15 cm diameter PVC vessel with an overall volume of ~11 L and pressurized with 6.8 atm of SF6. The high gas pressure serves as the quenching medium for the fuse, and consequently, the spark gap could be designed more compact due to the smaller gap distances required with a higher pressure. Also, since SF6 provided good insulation between components with a high potential difference, the entire power conditioning assembly could be made more compact.
D. Microwave Generation and Radiation
The first generation microwave diode was a vircator configured in the reflex triode geometry, with a cloth velvet cathode and polycarbonate insulators. Since the system is meant to be self-contained, the vircator had to operate without constant evacuation. To facilitate this, the new vircator design included an aluminum cathode and ceramic insulators. The cathode has a 63.5 mm diameter with a radius of 6.35 mm around the edge. The anode is made from 0.76 mm thick aluminum and machined to have a 70% transparency. The AK gap distance was set at 8 mm. The vircator components are assembled in a 4.8 L stainless steel tube, placed into an oven and evacuated at a temperature of 400 oC. Once the desired vacuum pressure is reached, the vircator is sealed and disconnected from the evacuation system. For more information on the sealed tube vircator, as well as experiments conducted with it when driven by a compact Marx generator, see [10].
III. EXPERIMENTAL RESULTS
Figs. 3 – 6 show the results from one of the initial tests with the second generation system. The seed energy source capacitor was charged to 3.5 kV, and was then discharged into the HFCG, whereupon the explosives were detonated and energy amplification began. The HFCG in this experiment was seeded with ~175 J. The HFCG starts producing an output pulse at t = 42 μs (see Fig.4).
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Figure 3. Seed and Detonator Current Waveforms
At approximately t = 49 μs, 7 μs into the generator runtime, the system experiences electrical breakdown either in the HFCG, power conditioning system, or the connection between the two. As a result, the current deviates from its expected quasi-exponential rise, and the fuse is slightly under-driven. The maximum energy stored in the inductor at the time of current interruption was ~2 kJ. The fuse reaches the high resistance state at t = 51 μs, inducing a high voltage across the inductor and causing the spark gap to close. The voltage pulse seen at the AK gap of the vircator can be seen in Fig. 5.
Figure 4. Fuse Current Waveform
Figure 5. AK Gap Voltage Waveform
The voltage pulse rises sharply, and then drops suddenly to below 100 kV before increasing again to a maximum of ~170 kV. The high voltage across the AK gap drives the vircator into operation, as can be seen in the captured microwave signal in Fig. 6. Note that the
microwave signal was captured with the horn antenna 2 m from the vircator output window. The pulse width of the microwave signal was ~100 ns. Peak electric field measured on the horn antenna was ~27 kV/m, with a calculated peak radiated power of ~25 MW. A time resolved FFT of the microwave signal can be seen in Fig. 7. To create this plot, incremental 5 ns samples of data are taken of the microwave data and then an FFT is taken of each sample.
Figure 6. Radiated Electric Field - Frequency Spectrum Grey scale intensity denotes amplitude, so the darker the spot, the higher the amplitude of that frequency component. Fig. 6 shows that at the beginning of the microwave pulse, the main frequency component of the signal is ~3.9 GHz. As time progresses, the frequency chirps up to ~5 GHz, and then reaches a main component of ~6.2 GHz before the impedance of the vircator collapses and microwave radiation ceases. Compared with other experiments performed with this vircator [10], a 100 ns pulse width is relatively short. It is estimated that the sharp decrease in voltage followed by the subsequent increase to the voltage maximum indicates the time at which the vircator “turns on”, or begins emission.
IV. CONCLUSIONS
A compact, self-contained HPM system has been
demonstrated. Dictated by the design constraints, the HPM system fits into a tube having a diameter of 15 cm and has a length of ~1.5 m. The system is capable of achieving radiated power levels of ~25 MW with voltage pulses of ~170 kV and HFCG driven energies in the kJ range. As of today, a maximum HPM output power level in excess of 100 MW has been achieved.
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V. REFERENCES [1] J. Benford, J.A. Swegle, E. Schamiloglu, High Power Microwaves 2nd Ed. Taylor and Francis Group, New York, 2007. [2] G. Xingen, , G. Shunshou, L. Shoofu, Z. Chuanmin, X. Weiping, D. Bonan, “The Experimental Study of Microwave Radiation by Vircator with the Help of Explosive Energy Source,” Proc. of the 7th International Conference on Megagauss Magnetic Field Generation and Related Topics, pp. 911-913, 1996. [3] S.D. Polevin, E.V. Chernykh, V.E. Fortov, K.V. Gorbachev, S.D. Korovin, G.A. Mesyats, E.V. Nesterov, V.A. Stroganov, M. Yu. Sukhov, “HPM Pulses Generated by S-Band Resonant Relativistic BWO with Power Supply Based on MCGs,” 3rd Euro-Asian Pulsed Power Symposium, 2006. [4] A. Young, T.Holt, M. Elsayed, J. Walter, J. Dickens, A. Neuber, M. Kristiansen, L.L. Altgilbers, A.H. Stults, “A Compact, Self-Contained High Power Microwave Source Based on a Reflex-Triode Vircator and Explosively Driven Pulsed Power,” Proc. of the 2008 Power Modulator Conference, pp. 147-150, May 2008. [5] M. Elsayed, M. Kristiansen, A. Neuber, “Fast-charging compact seed source for magnetic flux compression generators”, Review of Scientific Instruments, Vol. 79, 124702, 2008. [6] M. Elsayed, A. Neuber, M. Kristiansen, L.L. Altgilbers, “Integration of a Self-Contained Compact Seed Source and Trigger Set for Flux Compression Generators,” 17th IEEE International Pulsed Power Conference, June 2009, these proceedings. [7] T. Holt, “Design of a Dual-Staged Flux Compression Generator,” Dissertation for the Degree of Doctor of Philosophy, Texas Tech University, 2008. [8] T.Holt, A. Young, A. Neuber, M. Kristiansen, “A Fabrication Method for a Mid-Sized, High Energy Density, Flux Compression Generator”, Proc. of the 2006 International Conference on Megagauss Magnetic Field Generation and Related Topics, pp. 281-286, September 2006. [9] C. Davis, A. Neuber, A. Young, J. Walter, J. Dickens, M. Kristiansen, “Optimizing Power Conditioning Components for a Flux Compression Generator Using a Non-Explosive Testing System,” 17th IEEE International Pulsed Power Conference, June 2009, these proceedings.
[10] J. Walter, J. Dickens, M. Kristiansen, “Performance of a Compact Triode Vircator and Marx Generator System,” 17th IEEE International Pulsed Power Conference, June 2009, these proceedings.
