AAWSAP DIRD, Pulsed High-Power Microwave Source Technology, January 2010

UNCLASSIFIED//FOR OFFICIAL USE ONLf Defense Intelligence Reference Document Acquisition Threat Support 28 January 2010 !COD: 1 December 2009 DIA-08-0912-005 Pulsed High-Power Microwave Source Technology UNCLASSIFIED//f8rt 8fr1e1At tJ!E eHt I UNCLASSIFIED//F8R 8FFIOIAL l?ISE 8HLY Pulsed High-Power Microwave Source Technology Prepared by: Acquisition Support Division (DW0-3) Defense Warning Office Directorate for Analysis Defense Intelligence Agency Author: AAP Person 64 Administrative Note COPYRIGHT WARNING: Further dissemination of the photographs in this publication is not authorized. This product is one in a series of advanced technology reports produced in FY 2009 under the Defense Intelligence Agency, Defense Warning Office's Advanced Aerospace Weapon System Applications (AAWSA) Program. Comments or questions pertaining to this document should be addressed to !AAP Person 1 IAAWSA Program Manager, Defense Intelligence Agency, ATTN: CLAR/DW0-3, Bldg 6000, Washington, DC 20340-5100. UNCLASSIFIED/~liOA: OFFIOIAL l!f91!! ertti ii UNCLASSIFIED//FOR OFFICIO I. Ulilii 0PtLY Contents Summary................................................................................................................vi Critical Technologies .............................................................................................. 1 Insulation .......................................................................................................... 1 Uniform Homogeneous ....................................................................................... 2 Solid ................................................................................................................... 2 Plastics............................................................................................................ 2 Epoxies ................ ,..............................., .......................................................... 3 Urethanes and Silicones ................................................................................. 4 Liquids ............................................................................................................... 4 Gaseous .............................................................................................................. 4 Laminated .......................................................................................................... 6 Plastic- Paper-Oil ................................................................................................ 6 Plastic-Paper-Epoxy ........................................................................................... 6 Dielectric Tapering ............................................................................................. 7 Cathode Materials .............................................................................................. 7 Velvet................................................................................................................. 9 Carbon ............................................................................................................... 9 Ceramics .......................................................................................................... 10 Cesium Iodide Coated ...................................................................................... 10 High-Voltage Switching .................................................................................... 11 Gaseous Switching ........................................................................................... 12 High-Speed Liquid Switching............................................................................ 14 Solid-State Switching ....................................................................................... 14 High-Voltage Pulse Sources ................................................................................. 15 Marx Generators .............................................................................................. 15 Transformer Based Generators ........................................................................ 16 UNCLASSIFIED//FOR OFFICIO! 1!SF QN1Y iii UNCLASSIFIED//rtiUl 9FFl&IAl W&li 8,.lV Explosively Driven Generators ......................................................................... 16 Pulsed High-Power Microwave Sources................................................................ 17 Pulsed Electron Beam Sources ......................................................................... 17 BWOs, TWTs, and RKAs ..................................................................................... 17 Split-Cavity Oscillators ..................................................................................... 18 Virtual Cathode Oscillators ............................................................................... 18 Magnetrons ...................................................................................................... 18 Gyrotrons ......................................................................................................... 19 Impulse HPM Sources ...................................................................................... 20 SNIPER............................................................................................................. 20 EMBL ................................................................................................................ 20 H-Series HPM Sources ...................................................................................... 20 The Phoenix HPM Source.................................................................................. 22 The GEM II HPM Source ................................................................................... 24 The Jolt HPM source ......................................................................................... 24 Mesoband Sources ........................................................................................... 24 HPM Antennas ...................................................................................................... 25 Narrowband Antennas...................................................................................... 25 Wideband and Ultrawideband Antennas ........................................................... 27 Conclusion............................................................................................................ 29 Figures Figure 1. Paschen Curve for Air ............................................................................ 13 Figure 6. Cross-Section Drawing of HS With Point Geometry Converter, Brewster Figure 7. HS Output Section With the Point Geometry Converter Feeding an Figure 2. Example of Marx Generator Circuit ........................................................ 16 Figure 3. Orion HPM Testing Facility....................................................................... 19 Figure 4. Active Denial System With FLAPS Antenna ............................................ 20 Figure 5. H2 With Large TEM Horn and PGC Output .............................................. 21 Angle Window, and Extended-Ground-Plane Antenna ........................... 21 Extended-Ground-Plane Antenna Through a Brewster Angle Window ... 22 Figure 8. Phoenix Radiated Pulse at 8.5 Meters ................................................... 23 Figure 9. Phoenix Radiated Spectral Content ....................................................... 23 UNCLASSIFIED//fiAR AEEJCJOP 1155 ON! X iv UNCLASSIFIED//FOil 8PPIEIAL U.!E 014Li Figure 10. Jolt Hyperband HPM Source................................................................. 24 Figure 11. Jolt Radiated Electric Field Waveform at 85 Meters ............................. 24 Figure 12. FLAPS Antenna With a Cross-Shorted Dipole Array ............................. 26 Figure 13. Mode Converter Vlasov Antenna and Vlasov Antenna Attached to a Coaxial MILO ....................................................................................... 27 Tables Table 1. Dielectric Properties of Some HPM Plastics ............................................... 3 Table 2. Relative Spark Breakdown Strength of Gases ........................................... S Table 3. Cathode Study Findings .......................................................................... 11 UNCLASSIFIED/ /liiOA OFFICiIAk Wliliii 0,.11¥ V UNCLASSIFIED//FOR OFFI@IAL YSE OHLY Pulsed High-Power Microwave Source Technology Summary This paper provides an overview of the major types of high-power microwave (HPM) sources and the critical technologies required to build them. Pulsed HPM technology has been of scientific and military interest for several decades. Originally, the interest focused on the area of high-altitude electromagnetic pulse (HEMP) concerns. HEMP is produced when a nuclear weapon is detonated high above the earth's surface, creating gamma radiation that interacts with the atmosphere to create an intense electromagnetic (EM) energy field that is harmless to people as it radiates outward but can overload computer circuitry with effects similar to, but causing damage more swiftly than, a lightning strike. HEMP effects became fully known in 1962, when a high-altitude nuclear test (codenamed "Starfish Prime") over the Pacific Ocean disrupted radio stations and electronic equipment 800 miles away in Hawaii. The HEMP effect can span thousands of miles, depending on the altitude, design, and power of the nuclear burst. It is speculated that a single device detonated at high altitude over the central United States could affect the entire country, since the HEMP would be picked up by conductors, such as wires and power cables, acting as antennas to conduct the electrical energy into various electronic systems. This EM radiation was found to have field levels on the order of several hundreds of kilovolts per meter with onset or rise times of a few nanoseconds and a duration of nearly a microsecond. Fields of these magnitudes were found to have severe detrimental effects on numerous electrical systems and items, such as the power grid, automobiles, communications equipment, and aircraft. Much testing was done in the 1960s and 1970s to determine vulnerabilities and to find mitigation solutions. The HEMP is essentially a wideband microwave pulse, and several EMP simulators were devised and built for use in testing its effect on various electronic systems. In addition, several programs were established to investigate the possibility of generating and radiating other spectrums of EM radiation that may have some of these same effects without requiring a nuclear detonation. This was the impetus for the advancement of pulsed HPM technologies for weapons. HPM radiation is composed of shorter waveforms at higher frequencies than is HEMP, which makes it highly effective against electronic equipment and more difficult to harden against. Whereas HEMP weapons are large in scale and require a nuclear capability along with technology to launch high-altitude missiles, HPM weapons are smaller in scale, involve a much lower level of technology, and are within the capability of almost any state. HPMs can damage computers and electronics similar to the way HEMP can, although the effects are limited to a much shorter range. Technical accessibility, lower cost, and the vulnerability of U.S. electronic equipment could make small-scale HPM weapons attractive to terrorist groups. HPM devices are now categorized as directed-energy weapons. UNCLASSIFIED//POlt 8FFI@IAL Y&liii 0PIL¥ vi UNCLASSIFIED//POI\ OPPIClltt tJ.!I! er~LY One major use of HPM by the military is for electronic attack, or what is referred to by the media as an "ebomb." HPM sources developed for this purpose provide peak powers in excess of 10 gigawatts. The goal of such a weapon is to disable communications and computer systems prior to any troop movements and render the enemy unable to stage a response. The technology used to drive such sources has its roots in pulsed power, and for narrowband sources requires the use of tools that have been developed in the plasma physics community. Reliance on microprocessors that have an increasing density of circuits packaged onto each chip makes such systems ever-more vulnerable to HPM effects. As an example of the possible effects of HPM weapons, on 28 May 2001, a U.S. Commanche helicopter, flying in New York state while performing tests involving HPM weapons, was reported to have generated a low-level energy pulse that disrupted the Global Positioning System devices used to land commercial aircraft in Albany. The use of this type of weapon can be based on several scenarios, depending on the asset it will be used against. Sometimes, it may only be necessary to upset a data bus transfer to produce a success; other times, success may not be so simple. Some of the kill mechanisms obtained from microwave weapons include semiconductor overheating or burnout, arc generation, computer upsets, voltage induction into sensitive circuits, display upset, and overvoltage in discrete components. The asset to be neutralized will often determine the specific type of microwave source to be used; for example, assets with slots designed for communications purposes may be most vulnerable to narrowband HPM of a specific frequency, while assets with several computers linked by a communications bus may be more vulnerable to ultrawideband (UWB) pulses of a specific pulse repetition rate (PRR). Often, the variety of EM radiation that would be most effective is not obvious, and therefore several must be evaluated. If EM radiation is able to penetrate a target, the issue then becomes the susceptibility of the many semiconductor devices, which make up the various circuits of the target. Failures in semiconductors owing to thermal effects occur when junction temperatures are raised above 600° Kelvin. Since thermal energy diffuses through the semiconductor, failure mechanisms depend on the microwave pulse duration. If the pulse duration is short compared with thermal diffusion times, then the temperature increases in proportion to the deposited energy. Pulse durations (t) shorter than about 100 nanoseconds fall into this regime, and the threshold power for damage varies as 1/t. Experimental testing has shown that for pulse durations between 100 nanoseconds and 10 microseconds, the power required for damage scales as 1/t112 • And for pulses longer than 10 microseconds, a steady state in which the thermal diffusion rate equals the rate of energy deposition and temperature is proportional to power, resulting in a constant power requirement for damage. In this case, the power requirement scales as t. The consequence of these scaling factors is that short pulses require very high power but little energy, while very long pulses require large amounts of energy but little power. This analysis results in a vast range of HPM sources capable of damaging semiconductor devices. The above applies to single-shot pulse durations, but if UNCLASSIFIED//F9R 8FFI&il>I. 11&5 AN! X vii UNCLASSIFIED/,'FOlt err1e1At YSI!! 8,.LY a PRR is applied such that there is insufficient time for thermal diffusion between pulses (about 1 millisecond), then there will be an overall constant rise in temperature. For this reason, the PRR capability is of extreme importance for any HPM source. As a matter of course, assets to be tested include those of friend and foe alike, the goal being to find vulnerabilities in both and correcting those found in our own assets. Several techniques are employed to mitigate vulnerabilities found in assets, including filtering of conductor lines, using metallic enclosures, eliminating any unnecessary openings in the outer enclosure, and using ferrite or other magnetic materials. Any electronics inside a completely sealed metallic container (Faraday cage) would have no vulnerability to HPM of any variety; however, such a scenario is also of little or no use, since there could be no communication to or from the enclosure. Assets therefore must include some openings for communications, instruments, air flow, and sensors, sometimes as a matter of fulfilling their function. From the HPM source perspective, care must be taken to prevent fratricide and harm to friendly assets. To prevent fratricide, all connections to the source must be filtered to prevent fast transients from returning to the control unit. Some signals can be transmitted using fiber-optic cable; however, there is usually a piece of equipment at the source end that must be filtered. Some connections, such as the high-voltage power supplies, can make good use of high inductance filtering to eliminate fast transients, while others, such as trigger lines, must make use of other filtering means, such as transformer coupling, lightning arrestors, transorbs, and fast-acting, high-voltage diodes. Preventing harm to friendly assets is difficult and is a major reason why HPM has rarely been employed in actual battlefield settings. To ensure there is no harm to friendly assets, all assets would have to be tested for vulnerabilities, something that is not done at present. The antenna is a major factor in this matter. Unfocused antennas radiate a pattern that spreads as it progresses outward and, thus, the area subjected to the EM fields increases with distance. It is then harder to separate one's own assets from the radiated fields. In addition, it is very difficult to detect these sort of pulsed sources, since the pulses are very short (typically 1-500 nanoseconds), and even in burst mode, the bursts are usually less than 10 seconds. The short burst mode operation is necessary because of the high peak powers and subsequent heating of key components such as switches. The UWB sources would be the most difficult to detect, since they have nearly zero energy at any one frequency and so would not be detected at all by instruments such as spectrum analyzers. UNCLASSIFIED/,creR 8FFl@IAL U:!I! ()fl[ I viii UNCLASSIFIED//FOtt erPIEIAL USE Ol'J[f Critical Technologies Several technology areas are critical to the design and fabrication of a working HPM source. First, because the pulsed-power section of the source must operate at high voltages, it must contain insulating materials capable of withstanding the required voltage. The insulating scheme chosen is critical to the success of the project; therefore, several insulation techniques will be discussed, along with the merits and drawbacks of each. Another pulsed-power technology that is critical to the success of any source is high-voltage switching; various types of switches will be discussed, along with applications. Finally, cathode materials are a technology area that has been thoroughly researched and is critical to narrowband HPM generation; several cathode materials will be discussed, along with needs for future cathodes. INSULATION Electrical insulating materials or dielectrics are essential not only for pulsed power and HPM generation but for the proper functioning of all electrical and electronic equipment. In fact, usually the size and operating limitations of a piece of equipment are determined by the choice of insulating material. In the past, all manner of varnishes, tars, petroleum asphalts, natural resins, gums, saps, and minerals were used for electrical insulation. Now there is an almost endless list of possible insulating materials. The question now is, which material is the most appropriate for the task at hand? In pulsed power, and even more so in HPM applications, the choice is critical. All properties of a material must be weighed against one another to make the proper choice. Such properties as voltage breakdown, dielectric loss, dielectric constant, cure temperatures, hardness, tensile strength, flow modulus, and the variation of all these with frequency, voltage, and temperature must be considered before an appropriate insulating material can be selected. Many of these properties have never been published for most materials, and even when they have been published, they are typically known only at one or two frequencies. Designing insulation for challenging applications is at best a compromise between evils. Most of the material studies are carried out for the power industry, making them valid only at 50 or 60 hertz. Measurements at these low frequencies usually provide little or no clue about the values at much higher frequencies. Therefore, it is often up to diligent engineers to obtain materials data on their own. One recent research area of interest is in developing what are termed artificial dielectrics in an effort to decrease insulation weight. This material is made by suspending hollow glass microspheres in a lightweight dielectric medium. Depending on the concentration of these spheres, the dielectric constant can be lowered and tailored for the application, and the loss tangent of the media can also be reduced . Coating the spheres with a conductor such as aluminum can also raise the dielectric constant. Thus far, as might be expected, the dielectric strength of such materials is much lower than that of many thermoplastics, but they have been useful for applications such as radomes. Insulation generally falls into one of three categories: (1) homogeneous insulation, where the entire insulating volume is filled with the same media, be it solid, liquid, or gas; (2) laminated insulation, where the insulating volume is filled with some manner of UNCLASSIFIED//FOR OEEJCJOP 1!iii QDIL¥ 1 UNCLASSIFIED//Pelt OfFIEJIAk Wlilii ,u1L¥ layering of different materials; and (3) insulation by other means, such as magnetic insulation-these are usually used only where special conditions apply. UNIFORM HOMOGENEOUS Uniform homogeneous insulation implies that the insulating material is consistent throughout the volume. However, in some cases, such as that of epoxies, there is a uniform loading of some other material, usually to increase some desired characteristic of the final product. Examples are loading with silica to increase dielectric strength and loading with glass fibers to increase mechanical strength. The loaded material is typically of such small dimensions that it has only a very small effect on other material parameters from the truly homogeneous case. The use of uniform homogeneous insulation also results in a more easily modeled design. SOLID Solid insulation is often the easiest and typically the most desirable form of insulation since it does not require maintaining or replacing a liquid level or containing or monitoring pressure. This fact is often critical to a source project if maintenance or long shelf lives are important factors. Plastics The true title for this section should be "Thermoplastic Polymers (Plastics)," as they comprise one of the largest groups of insulating materials used in pulsed power and HPM generation. The term "plastics" includes acetals, acrylics, amides, imides polyarylate, polybutylene, polycarbonate, polypropylene, styrene, and sulfone polymers. Plastics were first used as insulation in the 1930s, and it is hard to conceive of constructing a high-voltage pulse source without them. Plastic materials have been tailored to suit a wide variety of applications. In the early 1980s, plastics manufacturers soliciting Sandia National Labs stated that they could engineer plastics to meet any set of material properties desired. It later became apparent that this was not the case and that, as usually occurs in nature, when one parameter was made more desirable, others were made less desirable. In spite of this fact, some well-engineered plastics are now available for some very demanding applications, such as switch housings and transmission lines. Nevertheless, virtually no new plastics are being introduced today. For the past 20 years, engineers have worked with essentially the same plastic materials, although some improvements have been made in the quality of resins and extruding and casting methods. In spite of this, there is still much more variation in specifications (especially mechanical specifications, such as tensile strength) for plastics from batch to batch than there is for metals. For this reason, the most demanding plastics applications where the limits of some specification will be approached require purchasing and independently testing a specific batch to assure confidence. One interesting and well-documented phenomenon associated with plastics is the nonlinearity of electrical breakdown strength with thickness. In very thin layers, some plastics display extremely high breakdown strength. For instance, polypropylene in half­ mil (1 mil = 1/1000 inch) layers yields 7,000-volts-per-mil breakdown strength, while in one-eighth-inch thickness, this figure drops off to 900 volts per mil. One theory to explain this is that the proximity of imperfections in the material across the thickness reduces the dielectric strength in thicker samples. This fact can be used to advantage by layering thin sheets of insulation together to form thicker insulating regions (see UNCLASSIFIED/fFOA 8FFI@IAL 1:181!! OHLY 2 UNCLASSIFIED/ /FOlil OFFl&IAl WSE 8,.lV section on laminated insulation). Many plastics come in a wide variety of shapes, forms, and grades, including bulk volumes, a variety of sheet thicknesses, and various rod diameters. The subject of using plastics as insulation fills volumes in reference books and has yet to be exhausted . Table 1 shows selected dielectric properties, collected over several years, on some of the most common plastics for high-voltage use. Table 1. Dielectric Properties of Some HPM Plastics Material Trade Name Breakdown Voltage (kV/mil) Acetal Delrin 4.0 Polypropylene 6.0 Polyetherim ide Ultem 7.0 Polysulfone Ultrason S 7.5 Polyethersulfone Ultrason E 5.8 Polycarbonate Lexan 6.3 Polvohenylene Ether Noryl 0.6 Polyphenylene Sulfide Ryton 0.4 Polyethylene 5.0 Polyvinylch loride 1.8 Epoxies One of the greatest advantages of casting epoxies is that a high dielectric strength can be attained with low maintenance, a long shelf life, and ease of transportation compared with liquid or laminated insulation schemes. Some of the best epoxies ever used for high-voltage insulation have only recently become available. These advancements are due mainly to efforts by the automotive industry to miniaturize the ignition coil to the point where a separate coil could be incorporated into the spark plug cap at each cylinder. Technologies have been devised for casting several varieties of epoxy to allow larger volume castings. The goals are to minimize voids and bubbles, deal with any exothermal effects, and reduce shrinkage. In addition, a good candidate material for high-voltage casting must have a high dielectric strength at the frequencies required, a long pot life, good adhesion, and an unlimited cure depth at a low temperature. With many epoxies, shrinkage and the glass transition point are functions of the cure temperature. New, state-of-the-art epoxies have several desirable characteristics never before available in a single product that make them ideal for high­ voltage applications. Two such characteristics are a low viscosity at room temperature and a long pot life. This means the epoxy can be mixed (resin and hardener) and the unit to be insulated can be filled under vacuum to eliminate voids and bubbles. Some of these epoxies have the viscosity of milk at about 100 degrees Fahrenheit and a pot life of several hours. A third desirable characteristic is a very low, almost imperceptible exotherm. This allows insulation of items sensitive to heat, such as thin plastics, paper, and electronic components or integrated circuits. A fourth desirable characteristic is low shrinkage, even in large castings. This allows insulation of regions where dimensional stability is important, such as at distances from high-voltage sections and resonant structures. A fifth desirable characteristic is good adhesion, both to itself and to components to be insulated. This is important because any separation from a component creates a void region where the dielectric strength will be compromised. UNCLASSIFIED//5OB OFFICIO ls. U&'li QPlls.¥ 3 UNCLASSIFIED//POI\ OPPICIAL l:191!! er~LY Adhesion to itself allows casting in several stages without fear of voids or mechanically weakened areas. A final desirable characteristic-one that is of obvious importance-is a very high dielectric strength. With attention to detail and diligence in the casting procedures, dielectric strengths of more then 4 kV/mil on 0.125-inch thickness have been achieved. All these advantages have allowed operation of high-voltage pulse systems at increased power levels and at half the volume of those previously insulated with mineral oil. Urethanes and Silicones These materials are used for casting solid high-voltage equipment, as well as for coating components to reduce the effects of shrinkage or shock. Typically these materials are very hard to use with vacuum casting techniques and, thus, have a much lower dielectric strength than do the best epoxies, especially in larger volumes. Another drawback is that many urethanes and silicones require either moisture or volatile ingredients in the curing process, both of which cause problems with high-voltage systems. Nonetheless, a wide variety of these materials are used in the fabrication of high-voltage pulse systems for applications that require their characteristics. LIQUIDS Liquid insulation has been the primary type of insulation for high-voltage systems since the beginning of the field . Over the years, mineral oils, vegetable oils, hydrocarbons, and even tars and saps have been used as insulation. Dielectric liquids have long served as electrical insulation in power transformers, capacitors, cables, and switching equipment. Several once commonly used fluids are no longer available because of their toxicity and environmental impact. As a result, liquids for insulation that do not have these problems have now been developed for certain applications, including mineral oils, silicon oils, fluoropolymers, and high-molecular-weight paraffin oils. Most of the dielectric fluids made are tailored to the power industry, which accounts for about 99 percent of the demand for these liquids. As a result, many such liquids contain additives that, while necessary for the power industry, are detrimental to high-voltage applications. These include low-vapor-pressure additives for controlling viscosity and antioxidants for improved aging. In addition, most insulating liquids also contain moisture and dissolved gases, which are only weakly bound to the liquid molecules and are easily freed when high electric field stresses are present. Scientists have for years worked to extend the usefulness of transformer oils, fuorinert, and castor oil. They have also developed corona-processing equipment for improving the high-voltage characteristics of insulating oils. This has allowed state-of-the art insulation design using insulating oils and oil-impregnated systems. The corona processing involves flowing the liquid insulation media through a high-field-stress region while under vacuum to remove dissolved gases and low-vapor-pressure constituents from the oil. The liquid is then filtered to remove particles larger than 5 microns. This process improves the corona initiation voltage limit for the liquid and greatly extends the life of components insulated with the media. It has also allowed a significant reduction in the size and, thus, energy density of pulsed transformer systems. GASEOUS Insulating gases are used in many high-voltage applications where weight is a primary issue. Typically the use of gases as an insulating media requires pressurization and, UNCLASSIFIED//FOR 0FFI€1Ak Uili ,u1k¥ 4 UNCLASSIFIED//FOR OFFl@IAL l!ISl!!! 9Ht5i" thus, implies heightened safety concerns. Even low-volume vessels can contain hundreds of joules of energy in the compressed gas, and a housing failure can hurl fragments at deadly velocities. Highly compressed gases are used only in cases where some prized benefit is worth the increased cost and design trials to be exacted. One example of this is extremely fast switching where the electrode spacing is proportional to the added inductance during conduction-the smaller electrode spacing requires higher gas pressure for insulation. Almost every common gas has been used as insulation, and many have attributes making them desirable for certain applications. Sulfur hexafluoride, nitrogen, air, argon, helium, oxygen, and hydrogen are commonly used. Of these, only sulfur hexafluoride is an electronegative gas, meaning it has the ability to remove electrons from the volume through the formation of negative ions and thereby increase the dielectric strength. Electrical discharges in sulfur hexafluoride result in foul-smelling sulfur compounds that also deposit on the switch housing and electrodes and require frequent cleaning. These discharge compounds also tend to be highly corrosive, especially in the presence of water. Other gases with electronegative species, typically other halogens such as chlorine, also make good insulators. These gases are usually much denser than air, and breakdown voltage is roughly proportional to density, thus higher voltages can be supported even at low pressures. The halogenated hydrocarbon refrigerants, such as CCl4, CCl2F2, CCbF, and C2Cl2f4, are also popular for insulation. The breakdown of air has been thoroughly researched, and in fact the breakdown voltage of a calibrated gap can be used to determine the magnitude of high voltages. Table 2 shows the breakdown voltages of several insulating gases relative to that of air. Table 2. Relative Spark Breakdown Strength of Gases Gaseous insulation as a switch medium limits the pulse repetition rate (PRR) to 500­ 600 pulses per second because of the creation of numerous metastable states and elevated energy levels by the previous pulses. This is true for all gases listed here except hydrogen. Hydrogen can be used at a much higher PRR; however, its dielectric strength is only 65 percent that of air and, thus, almost twice as much pressure is required for the same operating voltage. When the pressure is doubled, the energy content increases by a factor of four, leading to elevated safety concerns. Using hydrogen for switch insulation poses no explosive danger provided the oxygen content in the gas is kept below about 5 percent. Other handling problems associated with hydrogen include hydrogen embrittlement-it will leak through even tiny holes, including the pores in metal tanks, eventually causing the metal to become brittle and fail. In addition, hydrogen is flammable when mixed with oxygen. A hydrogen flame is colorless but very hot, which can be dangerous if leaks develop in pressurized switches or gas lines. One class of hydrogen switches, hydrogen thyratrons, makes use of the low-pressure characteristics of gases to eliminate the safety concerns associated with high pressures (these are discussed in a later section dedicated exclusively to gas switches). UNCLASSIFIED//FOR OFFICIO! 1 !SE ON! X 5 UNCLASSIFIED//POlt OFFl@IAL lel§li 8PUs:lf LAMINATED Laminated insulation has been used in some very demanding applications in which size is of primary importance and very small repeating structures are required, including high-energy-density capacitors, high-voltage transformers, and high-voltage transmission lines or pulse-forming lines. Laminated insulation schemes make use of the nonlinearity of electrical breakdown strength with thickness mentioned earlier for plastics. Such schemes offer the possibility of significant improvements in state-of-the­ art insulation, including a reduction in the size and an increase in the energy density of high-voltage pulsed systems. As new and improved materials become available, the possibilities for such improvements will become more substantial. PLASTIC- PAPER-OIL With appropriate attention to process details, very high dielectric strength is routinely achieved using this lamination scheme. This is in fact the insulation method used in most high-voltage and high-energy-density capacitors, with the addition of foil layers on either side of the plastic (usually biaxially oriented polypropylene) to form the capacitor. Use of corona-processed oil dramatically improves the utility of this insulating scheme. The plastic is frosted on at least one side and, together with the very thin ( 1 mil or less) paper layer, allows the oil to penetrate throughout the volume during the impregnation process. Without the oil, the tightly wound plastic layers can become sealed around small volumes of air that will not be filled with oil, and breakdowns will occur. Once the paper is impregnated with the oil, tests have shown that it attains essentially the same dielectric strength as the oil. Often, vacuum and pressure are alternately applied to ensure full penetration of the oil into the full volume. It is vitally important that no bubbles or voids be left in the insulation volume. For this reason, once the insulating volume is ready for impregnation, it should be left under vacuum at slightly elevated temperature for at least 24 hours. This not only ensures air pockets are removed but also allows the removal of surface moisture from the plastic and paper, which will also contribute to voltage breakdown. The paper not only aids impregnation but also serves as a path for residual charge to dissipate between voltage applications. The plastic has a very high surface resistivity, and some residual charge can become trapped on the surface after each discharge, resulting in charged regions of different magnitudes and even polarities, which can eventually lead to dielectric failure. Using this insulation scheme with biaxially oriented polypropylene as the plastic and Shell Diala AX as the impregnating oil, average dielectric strength of more than 2.1 kV/mil and operating voltages higher than 1.3 MV have been attained in large volumes. PLASTIC- PAPER- EPOXY Since the oil is the weakest dielectric medium in the preceding insulation scheme, it is reasonable to assume that replacing it with a stronger dielectric medium can improve the overall dielectric strength. Another advantage of this scheme is that in the end we would have a solid insulated volume with the advantages mentioned earlier. Thus far, only smaller volumes (1-2 gallons) have been successfully insulated with this scheme. The problem is that the increase in viscosity over the oil, although small, makes it more difficult to ensure that fu ll impregnation is achieved. Meanwhile, the programs for which this scheme is desired insist on nearly 100-percent certainty of success. The most successful process to date involves using quarter-inch sections of 1-mil paper followed by quarter-inch open sections for each layer. This is a tedious task in large volumes but UNCLASSIFIED/fFOA OFFICiIAk Wlilii 0PUs:¥ 6 UNCLASSIFIED//FOR OFFI@IAl W&liii 9PU.Y has resulted in an average dielectric strength of more than 3 kV/mil for the volumes mentioned. Adapting this scheme to the manufacture of high-voltage capacitors could result in significant improvement over the current state of the art of 1 joule per cubic centimeter. DIELECTRIC TAPERING This insulation scheme is little known but has been used with much success in many high-voltage systems, especially where compact high voltage is required. The basic scheme is to first design the system while minimizing the peak electric field stress. This involves hours of small but well-chosen changes to a design in order to shape the field lines and achieve the least range from minimum to maximum field stress. We then find the surfaces, which have the highest electric field stresses and therefore the highest probability of breakdown. In evaluating these parameters, it must be remembered that dielectric media are much less likely to initiate breakdown than are conducting surfaces under the same electric field stress. The conductor surfaces under highest field stress are then layered with high-voltage coatings (usually acrylics, polyurethanes, silicones, or engineered coatings) with dielectric constants chosen to reduce the electric field strength at the conductor surface. This technique works because the conductor is the source of electrons, without which breakdown will not occur. Since the electric field is excluded from regions of relatively higher dielectric constant, if the insulating volume is filled with mineral oil (relative dielectric constant of 2.2), then a conducting surface coated with 10 mils of polyurethane (relative dielectric constant of 3.6) will have a lower electric field stress than it would without the coating, and the increase in field stress in the mineral oil will be minimal. Dialectic tapering can be applied using several layers of coatings with progressively lower relative dielectric constant from the conducting surface and dramatically reduces the conducting surface electric field stress. Using finite element electric field solving codes and several hours of iteration, this technique can often reduce peak electric field stress for a system by 50 percent. The technique works best when the volume dielectric fluid has a low relative dielectric constant, such as mineral oil has (Er =2.2), since coatings are readily available for Er = ~3 to 5. In practice, care must be taken in choosing and applying the coatings to ensure that no voids or bubbles are introduced at the conductor surface. Careful inspection and repair of any flaws is relatively simple with this technique. Another, more recent use of this concept is what is termed continually varying dielectrics in ultrawideband (UWB) guiding structures, such as transmission lines with greatly reduced dispersion at bends. CATHODE MATERIALS This area of research is vitally important to any HPM source requiring electron beam generation. All high-power microwave tubes, including virtual cathode oscillators and cavity resonators, rely on a bunched flow of free electrons to set up oscillating electric fields and thereby generate a radiofrequency (RF) output. The electron flow is usually initiated by applying a high-voltage pulse to a vacuum diode. For high -power operation, the cathode must be capable of emitting a very high electron current density using one of several emission mechanisms. UNCLASSIFIED//FOR OFFI@IAL Y!JIE OHL¥ 7 UNCLASSIFIED//FOR orr1e1At 1:1§1: QPIL¥ These mechanisms include: • Thermionic emission (apply heat - 1,000 °Celsius) • Secondary emission ( electron bombardment; > 100 eV) • Field emission (apply a very strong electric field; 107 V/cm) • Explosive emission (form a plasma on the surface; i;.,= 0) The emission mechanisms of major importance for HPM at present are thermionic emission and explosive electron emission; however, field emission shows some hope with the advancements in nanostructures. Explosive emission, creating a dense plasma at the cathode surface, is of primary importance at this point. A review of pure metals reveals a direct correlation between the work function (ea,) and melting temperatures. When cathodes are made from metals with low work functions, there are problems with metal deposition onto other components. Most cathodes of use in HPM tubes depend on a surface flashover at a dielectric-metal interface. The surface flashover generates plasma, typically at tens-of-kilovolts-per-centimeter electric fields. The threshold and nature of the plasma depend greatly on the cathode materials. Therefore, the choice of cathode material is of critical importance in the design and operation of any HPM tube. No discussion of HPM diodes could be complete without mentioning space charge limited current flow. This stems from the fact that at some magnitude of current density, the density of electrons in the anode-cathode gap begins to shield the cathode from further emission owing to their cumulative effect on the electric field at the cathode surface. The current density at which this happens is given by the Child­ Langmuir law : Jsc(kA/cm 2) = 2.33 x 10-6 (V(MV)312 /d(cm)2) and is dependent on the diode voltage and the anode-cathode spacing. So, if we could have the ideal cathode material, what would its characteristics be? The response has not changed much in more than 60 years, as can be seen in the following extraction from a textbook on the subject. Primary Characteristics of an Ideal Cathode (J. R. Pierce, 1946) : • Emits electrons freely, without any form of persuasion such as heating or bombardment (electrons would leak off from it into vacuum as easily as they pass from one metal to another). • Emits copiously, supplying an unlimited current density. • Lasts forever, its electron emission continuing unimpaired as long as it is needed. • Emits electrons uniformly, traveling at practically zero velocity. Efforts are still under way to increase the output power, pulsed emission duration, repetition rate, and emission uniformity by investigating new and existing cathode materials in an effort to draw closer to the ideal cathode. Some of the materials currently being investigated are ceramic cloth and felt, carbon structures including nanotubes and microfibers, and carbon structures coated with cesium iodide. UNCLASSIFIED/ /EOA: OlililCil.\k W&li 8HLY 8 UNCLASSIFIED//FOR 8ffl@IJllt l:191!! l>flt I VELVET Velvet has been an explosive electron emission standard for more than 20 years. This material works by means of the dielectric-metal surface flashover mechanism mentioned earlier. Five steps are involved in the explosive emission process for dielectric fibers: • Surface flashover generates a cold, dense plasma/gas column. • The applied electric field extracts a space-charge-limited current flow. • The flow of current resistively heats the gas column. • The gas columns expand at a rate determined by the gas temperature. • The gas continues to expand into the anode-cathode gap. Velvet has several desirable properties that have endeared it to the pulsed power community and kept it a useful material for all this time. First, it emits at relatively low field strengths (~10 kV/cm), allowing a wider range of use than do many other materials. Second, it has a fast turn-on time. Third, the insulating nature of the velvet fibers provides a sort of built-in ballast during operation. Velvet also has a wide range of vacuum compatibility (pressures from 10-3 to 10-s Torr), easing the expense of vacuum hardware. Finally, velvet is inexpensive and readily available. All these factors combined have made velvet cathodes common for the past two decades. However, velvet cathodes also have drawbacks. First, velvet outgases heavily, especially during and after explosive emission. Significant amounts of material are released from the velvet during this process. The increased pressure inside the HPM device then leads to gap closure (conductive bridging of the anode-cathode region) and early termination of the RF output from the device. The closure rate can be estimated from: Velocity of closure (m/s) = 100 (d* /d) 213 Vd 1/ 2 where: d is the diode gap, d* is the velvet tuft density, and Vd is the diode voltage. Second, velvet has a very limited lifetime, partly owing to the material lost during each shot. Some material lasts for only about 100 shots in single-shot mode. Third, because of the increase in pressure after each shot, the repetition rate is very limited. Finally, lack of control over the manufacturing process resu lts in a wide variation in performance. Results are not reproducible, even between one roll and the next from the same manufacturer. CARBON Carbon cathodes have been used in diodes for more than three decades and have some appealing characteristics. The outgassing characteristics of carbon cathodes are much better than those of velvet, although the threshold voltage for emission is generally much higher. The primary material given off during outgassing from carbon cathodes UNCLASSIFIED/,CfOlil OFFIQIAk Uili QPlk¥ 9 UNCLASSIFIED// FOR OPPl@IAL Ylili 011! X appears to be hydrogen ions, which, because of their low mass, contribute to the problem of gap closure. The gap closure velocity using carbon cathodes quoted by most reports researched is 2-2.5 cm/µsec, and diode gaps from the same reports were 1-4 centimeters. Carbon cathodes also have much longer lifetime than velvet. However, one of the greatest advantages of carbon is the ever-expanding ability to form both macro­ and nanostructures using it as a base or substrate. Structures formed using carbon include pyramids, fibers, microfibers, nanotubes, and tufts. Many possible carbon structures still have yet to be formed and tested. Thus far, structures with the most surface area appear to perform best. CERAMICS Ceramics, much like carbon, can be formed into at least microstructures and have some features that have attracted interest in them for a couple of decades. These include virtually unlimited lifetimes and extremely low outgassing. A problem with ceramics, however, is that very high threshold fields are required for diode operation. Coatings to improve the performance of ceramic cathode structures may exist, and research is continuing in this area. CESIUM IODIDE COATED One of the most recent and impressive materials to be used in cathodes for HPM tubes is cesium iodide. Cesium is a pure metal that has a work function of only 1.9 ev and a melting temperature of 28 °Celsius; thus, it is liquid at only slightly above room temperature. Cesium ions are quite heavy, and that is why this coating was used initially. It was believed that the gap closure rate would be slowed since the heavy cesium ions would progress much more slowly across the anode-cathode gap than would other ion species, given the same electric field. This has proved to be the case, and closure velocities that are about one-fourth those for velvet or bare carbon (0.4 cmh1sec) have been attained. As a result, the HPM emission times have been extended. Typically, the cesium salt is dissolved in water as a saturated solution and then the carbon cathodes are dipped several times. Subsequently, the cathodes must be baked under vacuum for several hours to remove the water from the surface and leave the hardened cesium salt. Once completed, the cathodes have a very long lifetime unless contaminated by back splatter of material from the anode. At present, HPM programs investigating the performance of cathodes having some form of cesium coating over carbon nanostructures show the most potential for progress in the state of the art. The goal of these programs is hundreds of kiloamps for tens of microseconds, resulting in gigawatt narrow-band HPM sources running at repetition rates of possibly 100 hertz and thus capable of 100-megajoule energy output per burst. Also of great interest at present are cathodes termed "hybrids," which utilize multiple emission mechanisms in beam generation. The cathodes developed by these programs are to be used with the magnetically insulated line oscillator (Sandia National Laboratories [SNL]), the relativistic klystron oscillator (Kyle Hendricks, Air Force Research Laboratory [AFRL]), the reltron (Bruce Miller, SNL), and the super reltron, among others. UNCLASSIFIED/,,roA &FFI@IAL U91!! f)flt I 10 UNCLASSIFIED/} P'Olt OP'P'l@IAL l:ISE &P•LY Table 3 shows findings of cathode material studies at SNL and at the AFRL. Table 3. Cathode Study Findings Material Emission Threshold (kV) Lifetime (# of Shots) Outgassing (Neutrals/Electron) Csl-Carbon Micro.fibers < 3 kV/cm > 72,000 4 ­ 6.5 (substrate) "Sandia Red" Velvet 8 kV/cm - 8,000 10 "MILO Green" Velvet 10 kV/cm - 4,000 10-14 Velveteen Low Low <600 > 12 F-Velvet Low < 100 > 12 Ceramic Cloth > 120 kV/cm --­ --­ Ceramic Felt > 100 kV/cm --­ --­ Carbon Pyramids > 80 kV/cm --­ --­ Carbon Nanotubes 20-50 kV/cm Arc rate of ­ 2% -4 Bare Carbon Microfiber (packing density) 15-40 kV/cm > 36,000 4.3 ­ 6.5 (substrate) Csl-Carbon Fiber Tufts < 3 kV/cm > 200,000 -4 Metal I Ceramic 95 kV/cm (diode collapse unless> 150 kV/cm) --­ 8 HIGH-VOLTAGE SWITCHING High-voltage switching is among the most challenging of technologies for HPM sources. Although high-voltage switches have been used for several decades, and thousands of experiments have been performed on the mechanisms involved in liquid and gaseous breakdown, there are still many aspects of the phenomenon that defy explanation. This is especially true as the time required to reach the fully conducting state becomes extremely short. The usual explanation for this process involves Townsend avalanching, whereby electron streamers begin at the cathode in an average electric field of only 20­ 25 kV/cm and, by virtue of an enhanced electric field at their tip, progress in an orderly fashion to the anode. At this time, a heating phase begins, and an increasing amount of current is passed through the streamer until the switch finally reaches the fully conducting state. The problem with this explanation is that it is most likely incorrect and relies on exaggerated ion densities to explain how switches can reach full conduction in less than a billionth of a second. Alternative explanations involving runaway electron generation provide a better match to observations. Very fast switching is critically important to the concept of UWB HPM. The basic concept is to generate a square pulse with the fastest rise time possible. A Fourier transform of this waveform results in a frequency spectrum containing frequencies determined by the width and rise time of the square pulse. The period of the lowest frequency is twice the pulse width, and the rise time is about one-quarter the period of the highest frequency. UNCLASSIFIED/ ,<FQA OFFICIO I. Uili OPII.V 11 UNCLASSIFIED//POI\ OPPICIJllt l!l!H! e ..tv An oft-forgotten aspect of the spectral content is that the spectrum also contains only the odd harmonics of the lowest frequency by the definition of a Fourier transform . Thus, the faster the rise time and the wider the pulse, the broader the spectral content. Therefore, switching speed is the most important parameter of a UWB HPM source. The types of switches used for HPM are essentially the same as the types of insulation discussed earlier: gaseous, liquid, and solid state at lower voltages. The single most important attribute of gases and liquids in switching is their self-healing ability, which implies some measure of PRR capability. GASEOUS SWITCHING Gas switches are commonly used for HPM sources as both a prime power and a high­ speed or peaking switch. As mentioned in the discussion of insulation earlier, when used in a high-speed switch, gas pressure can pose substantial safety concerns. In fact, all of the discussion regarding gaseous insulation also applies to gas switching, since a gas switch is simply a gas-insulated region that we wish to fail in a timely fashion. The higher the voltage impressed across a gas switch, the greater the pressure required to prevent the switch from conducting until the peak voltage is reached. This is why when a gas switch is used as a final-stage peaking switch, very high pressures are often required. A fast-rising pulse is crucial to source design since the rise time determines the upper frequency content. This is why UWB HPM sources usually contain a peaking switch at the output to decrease the rise time and increase the spectral content. If the peaking switch is charged past the DC breakdown level faster than streamers can form conduction channels, then the final breakdown occurs in an overvolted (compared with the DC breakdown voltage) switching state. The higher electric field strength between the switch electrodes results in shortened breakdown times since breakdown develops in an elevated electric field. All switches exhibit some capacitance to an applied pulse because of their electrode spacing, resulting in a displacement current as this switch capacitance charges. This is seen on the other side of the switch as a pre-pulse. The magnitude of the pre-pulse depends on the rate of change of the charging voltage as well as the electrode cross-sectional area and spacing. Sometimes efforts to reduce this pre-pulse are required if it causes problems at the load or undesired spectral content from the antenna. The pre-pulse phase of breakdown occurs at the speed of light in the media since it is essentially a field phenomenon. Because of the added inductance and design of the switch components, pre-pulse has a distinct charging profile. The next phase of breakdown is a resistive phase as the weakly conducting streamer channel heats to the final arc or inductive phase and the switch is fully conductive. Since the final phase is inductive, very low switch inductance and very short gaps are required for fast rise times. Both the resistive and inductive phase periods contribute to the rise time as: 2 1 3 4 3 where: -rr = (88ns x p 1 f (Z 1 x E 1 ) / ) with p being the gas density as a multiple of that for sea level air, Z is the circuit impedance in ohms, and E is the electric field between the electrodes in kV/cm. Also, ,r and TL are known as the resistive and inductive rise times, respectively. The resistive UNCLASSIFIED//FOR OFFICIO! 1 !SE ON! Y 12 UNCLASSIFIED//FOA OFFI€1Ak Wlilii ONkY rise time is the time required to heat the gas channel to full conductivity, and the inductive rise time is the delay caused by the addition of the switch into the circuit . There are two contributions to the inductive rise time, with Le being the spark channel inductance and Lh the housing inductance. A shorter switch gap reduces the inductive time by lowering the channel inductance but also increases the electric field in the gap, reducing the resistive time and resulting in a faster rise time. Even though the switch electrodes are usually designed for minimal cross-sectional area at a given current, the very short electrode separation required can still result in high interelectrode switch capacitance. As mentioned earlier, it is also preferable to charge the switch very quickly to achieve an overvolted switching condition, and, therefore, very fast switches always have some level of pre-pulse. Because the PRR is also of great importance, hydrogen has been chosen most often for high-speed gas switching in UWB HPM sources. Switches of this type have achieved rise times of just over 100 picoseconds (ps) and PRRs of 1,500 pulses per second. Another type of gas switch meriting mention for its utility and indispensability in the HPM pulsed-power driver circuits is the hydrogen thyratron. The thyratron is a partial vacuum switch . Figure 1 shows what is known as the Paschen curve for air; however, all gases exhibit the same curve characteristics. At some product of pressure and electrode spacing, a minimum value of breakdown voltage is reached. While high­ pressure gas switches operate in the region on the right side of the Paschen minimum, the hydrogen thyratron operates on the left side, beyond the Paschen minimum. The physics of voltage breakdown in this region results in smaller electrode spacing holding off higher voltages and reduced pressure at the same spacing enabling greater voltage holdoff. The single-stage thyratron operates at only tens of kilovolts, while high­ pressure gas switches may operate at several hundreds of kilovolts. When coupled with a good pulse transformer, a properly chosen thyratron forms the heart of an excellent driver for HPM sources. The thyratron also has the capability to initiate breakdown using modest trigger levels ( ~1 kilovolt) and with nanosecond timing, allowing the use of multiple switches to share current. Breakdown Voltage vs. Pressure x Gap (Air) 100000 Ill 0- > I Q) 10000 C') «J- 0 > C 3: 1000 0 "C ..:.:: «J Q)..