Directed Energy Past, Present, and Future Historical Overview of Directed-Energy Work at Dahlgren By Stuart Moran In 1962, the United States set off a megaton nuclear weapon 250 miles above the Pacific. The blast caused a large imbalance of electrons in the upper atmosphere that interacted with the Earth’s magnetic field to create oscillating electric fields over a large area of the Pacific. These fields were strong enough to damage electronics in Hawaii, a thousand miles away, and clearly demonstrated the effects of an electromagnetic pulse (EMP). It didn’t take long for the military to begin considering ways to create such pulses without using nuclear weapons. In the late 1960s, the Special Applications Branch at the Naval Weapons Laboratory at Dahlgren began studying ways to generate high-power oscillating electric fields that could be used as a weapon to damage enemy electronics. These devices were basically high-power versions of the old spark-gap transmitters used in the early days of radio. To construct a device that could produce nuclear EMP-like fields, stored electrical en- ergy was converted to radio-frequency (RF) energy that could be radiated from an an- tenna through the atmosphere to a target. These devices typically would store energy in a high-voltage capacitor and release the energy quickly using a spark-gap switch. This would then drive oscillating currents on an antenna, causing it to radiate. To achieve field strengths of thousands of volts per meter, typical of a nuclear EMP, devices operat- ing at hundreds of thousands of volts or more were needed. A number of radiating devices were studied in the early 1970s. Most belonged to a class of devices called Hertzian oscillators. A capacitor is charged to high voltage, the switch is closed, and current flows in the circuit, causing the stored energy to oscil- late between the electric field of the capacitor and the magnetic field of the inductor. To charge the capacitor to extremely high voltages, a step-up transformer of some type must be used. One of the fastest voltage multipliers, the Marx generator, was frequent- ly used. The losses from internal resistance and external radiation damp the oscillat- ing waveform, typically after a few cycles. The radiated pulses are, therefore, short in time and broad in frequency content.1 A simple diagram of the inductance-capacitance oscillator (L-C oscillator) is shown in Figure 1. Single-Pulse Burnout Devices Many types of Hertzian devices were designed, constructed, and tested at Dahlgren dur- ing the 1970s. The transmission-line oscillator, or cavity oscillator, used a quarter-wavelength 12 Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 3. DATES COVERED 2012 2. REPORT TYPE 00-00-2012 to 00-00-2012 4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER Historical Overview of Directed-Energy Work at Dahlgren 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION Naval Surface Warfare Center, Dahlgren Division,Corporate REPORT NUMBER Communication, C6,6149 Welsh Road, Suite 239,Dahlgren,VA,22448-5130 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited 13. SUPPLEMENTARY NOTES See also ADA556728. 14. ABSTRACT 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF ABSTRACT OF PAGES RESPONSIBLE PERSON a. REPORT b. ABSTRACT c. THIS PAGE Same as 14 unclassified unclassified unclassified Report (SAR) Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 Historical Overview of Directed-Energy Work at Dahlgren Figure 1. Inductance-Capacitance Oscillator (L-C Oscillator) Diagram coaxial pipe, which was switched at one end, to create Other types of devices to produce pulses were the oscillating waveform. A frozen wave generator, constructed, too. Vector inversion generators used a different type, had quarter-wave sections of cable spiral-wound capacitive plates to generate high that were charged plus and minus to create a two- voltages without transformers.2, 3 The Landecker cycle waveform “frozen” in the cable. All sections ring used a paddle-wheel arrangement of capac- were simultaneously switched, causing the wave to itors and inductors charged in parallel and dis- travel to an antenna. A special folded design was de- charged in series. The circular arrangement was veloped so one switch could be used, eliminating designed so the entire system would radiate as a the multiswitch synchronization problem. A Ross magnetic dipole, thus forming its own antenna.4 circuit used a square wave pulse, which traveled Switch timing was critical, and Dahlgren engineers down cable “tees,” creating reflections, which were attempted to verify reports that Landecker devel- timed to create several RF cycles. In the Travetron, oped a specific type that brought all capacitor leads the turn-on time of a series of spark-gap switch- into a single-center spark gap. es was incorporated as a designed delay, creating Scientists and engineers also looked at devices reflections through a series of gaps to produce the that used explosives to generate the electrical energy waveform. This design allowed higher frequencies. needed. These included explosive flux compressors All of these devices were designed, built, and test- of several types, which generated fields and then ex- ed to determine power and frequency capabilities, plosively squeezed the fields between conductors to as well as efficiency. amplify the peak power. In the early 1970s, a large Scientists and engineers at Dahlgren built and (70-ft clear zone) anechoic chamber was construct- tested versions of Hertzian oscillators operating up ed at Dahlgren with an explosive chamber in one to half a million volts. These devices powered rel- end. Explosives would be set off in the chamber to atively simple monopole or dipole antennas that drive various types of flux compressor schemes that could produce very high electric fields at hun- would generate electrical pulses fed into an oscillator dreds of meters. In the early 1970s, a special out- and antenna in the anechoic chamber. Pulse param- door field-measurement range was constructed. eters and field strengths could be measured. Imped- It housed high-voltage systems in underground ance-matching networks, matching transformers, trailers that fed antennas above ground on a spe- and methods of improving efficiency were studied. cially-built, 100-m-long ground plane that was Tests were performed at Dahlgren and at Los Ala- constructed for testing and field measurements. A mos using large antennas suspended from balloons.5 picture of the ground place in a fielded measure- In other schemes, piezoelectric devices were devel- ment range is shown in Figure 2. Field probes were oped, which could be compressed hydraulically and even carried aboard helicopters to make measure- then quickly released to produce high voltages. The ments above ground effects, as shown in Figure 3. concept was to use explosives to generate the high 13 Directed Energy Past, Present, and Future Figure 2. Field Measurement Range Figure 3. Airborne Electric Field Measurements 14 Historical Overview of Directed-Energy Work at Dahlgren pressures. Ferroelectric and ferromagnetic trans- Since many important target systems were not ducers driven by explosives were also tested.6 available for testing, much of the vulnerability in- formation was obtained from U.S. electronics, and Special Effects Warhead estimates were then made for foreign systems. In (SEW) Program addition to the tests done at Dahlgren, pulsers were In 1973, Dahlgren began the SEW Program to also constructed in mobile trailers that could be look at the feasibility of “burning out” enemy radar transported to other sites for testing against simu- and missile systems using single-shot, very high- lated or actual targets. The Mobile Oscillating Puls- peak-power EMPs. The program looked at the er System (MOPS) was an example that was carried feasibility of constructing an electromagnetic war- to test sites, such as China Lake, to perform tests head that could disable electronics beyond a nor- against radars and simulated foreign systems. mal hard-kill explosive range as far as a mile away. A key requirement for the SEW Program was The program was funded at several million dollars to demonstrate enforceable target vulnerability, a year through most of the 1970s. which means that a high percentage of the time a A major thrust of the SEW Program was to large percentage of the targets are affected. One im- better understand the effects of high fields on mil- portant finding was the broad difference between itary electronics. Little information was available an electromagnetic safety concern—where a 1 per- on the vulnerability of foreign or U.S. electronics, cent vulnerability was far too great—and a weap- particularly entire systems. A trailer-based RF im- on concern—where a 10 percent vulnerability was pulse system, employing a Marx-driven L-C oscil- not good enough. The field strengths between the lator charged at two million volts, was constructed safety requirements and weapon requirements of- at Dahlgren. This Transportable Oscillating Pulser ten were many orders of magnitude apart. System (TOPS) was connected to a large bounded- The SEW Program looked at many types of wave structure that produced uniform fields over a electronic component vulnerability, subsystem region large enough to place an entire radar or mis- vulnerability, and complete system vulnerability. sile system. The electric field emitted from the throat As a result, energy tables for burnout effects were of this system was so high that a special bag of high- developed. Subsequently, Dahlgren performed nu- voltage gas was needed until the radiating structure merous field tests against radar and communica- became large enough to transition to the normal at- tions systems between 1973 and 1978, and funded mosphere. A picture of TOPS is shown in Figure 4. component and subsystem testing on missiles. Figure 4. Transportable Oscillating Pulser System (TOPS) 15 Directed Energy Past, Present, and Future Repetitive Systems for teamed with the Naval Avionics Center (NAC) to Electronic Warfare build the systems. By 1980, China Lake fired the The electric fields required to damage military first air-launched prototypes at both low and high electronics in the 1970s often were very high, and altitudes. Devices, batteries, spark gaps, and anten- ranges typically were limited. As a spinoff of pro- nas continued to be developed, and new targets— grams trying to damage targets with a single pulse, such as spread-spectrum systems—were tested. some of these devices were reduced in size and Other delivery systems besides rockets were also power, and operated in a repetitive mode to gen- considered. erate noise pulses for the purpose of electronical- ly jamming target systems. In 1976, the Naval Air The Pulsed Power Systems Command (NAVAIR) began the Electro- Technology Program magnetic Countermeasures Program to study the Large directed-energy weapons (DEWs) of- application of high-repetition-rate Hertzian devic- ten required megawatts or gigawatts of peak power, es for use as noise jammers. The initial targets were so methods of supplying and modifying this pow- low-frequency radars. er were needed. As Dahlgren became involved in a In late 1976, Dahlgren performed effectiveness broad range of DEW systems, one attribute became tests against various radars using helicopter-mount- more and more obvious: the size, weight, and cost of ed Hertzian jammers. These devices were able to a directed-energy (DE) system were dominated by screen incoming target aircraft at useful ranges. The the pulsed-power technologies needed to drive the concept of a forward-launched rocket to deliver a system, not by the source device itself. Consequent- parachute-suspended Hertzian jammer also was in- ly, more effort began to be devoted to the power-de- vestigated. Dahlgren teamed with engineers at Chi- livery technologies needed for many of the weapon na Lake to study packaging concepts of utilizing an concepts. Pulsed-power components enabled ener- extended 5-inch Zuni rocket as a forward-fired de- gy to be stored over long periods of time (seconds) livery vehicle. A prototype is shown in Figure 5. and released very quickly (nanoseconds) to obtain Similar Hertzian devices were considered for a billion times increase in peak power. use as communications and data-link jammers. Dahlgren hosted a pulsed-power systems Several antenna deployment schemes were devel- symposium and workshop in 1976 and helped oped, and by fall 1978, successful ground launches initiate the International Pulsed Power Confer- had been performed in which the deployment se- ences, which began in 1977 and continues today quence and jammer operation were demonstrated. under the Institute of Electrical and Electron- The name Zuni Expendable Pulsed-Power Oscil- ics Engineers (IEEE). As Dahlgren’s involvement lator (ZEPPO) was given to the project. Dahlgren with systems design increased, it became apparent that new technologies were needed in the prime- power and pulsed-power area to support a vari- ety of new concepts. Dahlgren urged the Navy to initiate a Pulsed Power Technology Program to develop power sources, energy storage systems, high-power switches, and power conditioning sys- tems needed for a variety of future weapons. This program was initiated in 1978 and was originally funded by NAVAIR and then by the Directed En- ergy Program Office (PMS 405) in the early 1980s. In addition to the Pulsed Power Technology Pro- gram, PMS 405 also began funding free-electron lasers (FELs), chemical lasers, high-power mi- crowaves (HPMs), and charged-particle beams (CPBs). The Pulsed Power Technology Program at Dahlgren, in turn, funded many areas of research, both internal and external, over the next 10 years. Dahlgren served as the focal point for the Navy’s science and technology (S&T) in pulsed power and funded many universities, government lab- oratories, and commercial companies under the Figure 5. ZEPPO Payload Pulsed Power Technology Program. 16 Historical Overview of Directed-Energy Work at Dahlgren To provide large amounts of electrical prime high-voltage opening switch. Opening switches— power, new types of rotating machines were stud- which were needed for inductive energy store sys- ied, including flywheels, conventional alternators, tems—were studied, as well as magnetic switches, homopolar generators, rotary flux compressors, which used saturating magnetic material to sharp- and compensated pulsed alternators. These ma- en pulses. Magnetic switches operating at 10 kHz chines attempted to produce fast, high-power puls- were demonstrated by 1983.9 es using special materials to reduce losses, eddy In 1985, Dahlgren used internal funds to up- currents, and mechanical stresses. MHD genera- grade a facility to provide controls, diagnostics, and tors were developed using rocket-motor propellant 200 kW of average power at 50 kV to accommodate that could be started and stopped. In the mid- testing of new switches and water-based capacitors. 1980s, a full-scale hybrid (solid fuel/liquid oxidiz- This facility could control the power with a vacu- er) combustor was fabricated and tested at 10 MW, um-tube pulser and could generate over a million achieving world records for power-to-weight ra- volts with a rep-rated Marx generator. The facility tio and conductivity. By 1980, new types of energy was used to: storage systems were studied, including inductive • Develop water-dielectric energy storage, rep- storage and advanced capacitors using new types rated spark gaps, and pseudospark switches. of insulating materials and geometries. During the • Test a variety of switches developed by con- late 1980s, programs such as the Mile-Run Capac- tractors, such as back-lighted thyratrons.10, 11 itor Program reduced the capacitor size by a fac- A picture of one system being tested—a water pulse- tor of 10 through better synthesis of polymer films. forming line and spark-gap switch—is shown in Beginning with internal independent research Figure 6. funds, Dahlgren developed liquid dielectric mate- Dahlgren concentrated in-house switching ef- rials based on water/glycol mixtures at low tem- forts in spark gaps. New types of gases were stud- peratures. These water-capacitor devices could ied, as well as electrode materials, gas-flows, switch hold energy for orders-of-magnitude longer time geometries, and triggering techniques to produce periods than ever before, allowing pulseforming high-repetition-rate switches for electronic war- lines to be constructed that could be charged di- fare, as well as particle-beam weapons.12 Dahlgren rectly from rotating machines. Dahlgren scien- scientists and engineers demonstrated 100-µs re- tists developed a world-record high-voltage water covery of spark-gap switches after handling kilo- capacitor that could hold pulses for milliseconds joules of energy at hundreds of kilovolts, a world and became internationally recognized experts in record.13 The High Energy 2-Pulse System for fast water breakdown.7, 8 recovery experiment is shown in Figure 7. High-power fast switching was another impor- In 1986, Dahlgren ran a workshop on high- tant area of research. Dahlgren funded companies power switching for Navy tactical and Depart- to develop new types of multistage thyratrons that ment of Defense (DoD) strategic applications and could operate at very high voltages. By the early became involved with numerous DoD working 1980s, multistage thyratrons capable of operating groups on electromagnetic propulsion, high-pow- at over 200 kV, 40 kA with 20 nsec risetimes were er diagnostics, advanced energy conversion, pow- demonstrated. Vacuum switches, ignitrons, plas- er modulators, and pulsed power. Spark gaps were ma pinch switches, pseudospark switches, back- investigated to create underwater noise for subma- lighted thyratrons, and e-beam switches all were rines. Dahlgren also led four North Atlantic Treaty studied, as well as a variety of spark-gap switches. Organization (NATO) Advanced Study Institutes Higher power solid-state switches were developed, in Europe and the UK on various pulsed-pow- too, using new geometries and substrate materi- er topics. International assessments of key pulsed- al. Superconducting coils were considered, both power technologies were also performed. for energy storage and as opening switches. Dahl- gren engineers developed exploding-wire opening Particle-Beam Weapons switches, and several types of plasma pinch switch- Particle-beam weapons were a major focus of es were funded. They also worked on stacked cable DE work during the 1970s and 1980s. A CPB weap- pulsers. Additionally, concepts for electromag- on takes subatomic particles, generally electrons, netic armor were developed. These systems used and accelerates them to near the speed of light be- high-density capacitors to blunt penetrators. In- fore sending them toward a target. These fast elec- ductive energy storage—which could be far denser trons penetrate deeply into most materials, so they than capacitors—was studied, including methods are difficult to counter. The high-current electron of generating the seed current and the problematic beam was to be accelerated by an induction-type 17 Directed Energy Past, Present, and Future Figure 6. A Water Pulse-Forming Line and Spark-Gap Switch Test Figure 7. High Energy 2-Pulse System 18 Historical Overview of Directed-Energy Work at Dahlgren accelerator, repetitively pulsed. High electron- the current, voltage, and recovery requirements at beam currents (kiloamps) and a hole-boring series that time.15 The High-Voltage 5-Pulse System ex- of pulses were anticipated to create a stable, long- periment is shown in Figure 9. range beam. Since the beam was capable of pene- During these technology efforts, significant trating quickly and deeply into any target material, advances were achieved in all aspects of the pro- it had the potential to damage electronics and set gram. These included: off explosives before salvage fuzing could occur. • Generating high-current, high-energy beams The beam was predicted to be all-weather and es- (although still below weapons parameters) sentially countermeasure-proof. Even a near miss • Demonstrating a 360º turn in a high-current could cause substantial damage from high fields beam and X-rays produced by the deceleration of elec- • Propagating a single pulse through the air trons as they hit air molecules near the target. The • Demonstrating beam steering on a small scale CPB concept is shown in Figure 8. • Performing target interaction measurements Scientists and engineers from Dahlgren worked Multipulse, long-range propagation was never on the pulsed-power technologies needed to drive demonstrated. A comprehensive tri-service sum- these machines beginning in 1980, and it became mary called the Net Technical Assessment for CPB a major focus of the Pulsed Power Technology was sponsored by the Defense Advanced Research Program.14 The White Oak Laboratory developed Projects Agency (DARPA) in 1987 to describe the beam-steering concepts and looked at material in- accomplishments of the program. The report said teractions. By 1989, the program investigated: compact accelerators were the most pressing tech- • Propagation nology need. As a result, most funding was di- • Compact Recirculating Accelerators rected toward this topic. Funding was stopped in • Pointing and Tracking the early 1990s, however, due to the high expense, • Prime Power stretched timelines, and changes in the threat. • Material Interaction • Fratricide Pulsed Power and For a compact shipboard system, recirculating Electromagnetic Launchers accelerators were needed to make multiple passes During the 1980s, the Army and Air Force of the electron beam past the accelerating cavities. looked at short-range electromagnetic weapons This required a high-power, fast recovery switch, to penetrate stronger armor with higher veloci- which Dahlgren began working on in 1988. Using ties. The Navy worked on concepts for a weapon patented hydrogen switches and special triggering that could be mounted on ships to intercept missile techniques—efforts that had begun with internal systems at line-of-sight distances. The Navy—then research funds—Dahlgren demonstrated spark- the biggest user of space systems—was also inter- gap switches, the only technology that could meet ested in studies showing that small satellites could Figure 8. Charged-Particle Beam (CPB) Concept 19 Directed Energy Past, Present, and Future Figure 9. High-Voltage 5-Pulse System Experiment be electromagnetically launched into low Earth or- Early Dahlgren work on electromagnetic bit for the fraction of the cost for a normal launch. launchers—along with capacitor development and Through the 1980s, electric guns were funded switch advances from the Pulsed Power Technol- by independent research and independent explor- ogy Program—allowed Dahlgren to provide the atory development programs at Dahlgren, study- Navy with detailed conceptual designs in the late ing electric gun concepts for both rail guns and 1990s for near-term, long-range rail guns based on electrothermal (ET) guns. Kinetic energy weapons capacitor energy store. These efforts helped sup- were also investigated as part of the Pulsed Power port the decision to begin a long-range rail-gun Technology Program. Under these programs, pure program at Dahlgren that continues today, result- electric launchers were developed and tested at ing in world-record achievements. Capital invest- Dahlgren, including ones that self-formed projec- ment funds were used to construct a high-energy tiles.16–18 Also studied were ET guns that used the facility in 2005 to test pulsed-power components discharge of electrical energy at the gun breech to and module designs for use in electromagnet- generate a plasma jet. This plasma jet heated a low- ic launcher programs. An early electromagnetic molecular-weight working fluid, such as water, to launcher is shown in Figure 10. produce a heated gas that accelerated the projectile to higher velocities than conventional explosives. High-Energy Lasers (HELs) The Electrothermal-Chemical (ETC) Gun con- In general, megawatts of continuous laser cept augmented the electrical energy generating power are required to kill hard targets at long rang- the plasma jet with a chemical reaction. A 127mm es. Laser technologies that can produce this much ETC gun was investigated, and a 60mm ETC gun power are very limited. The Navy was a leader in was tested at Dahlgren, with the ability to fire short developing powerful chemical lasers in the 1970s bursts at a rate of 100 rounds per minute.19 and 80s. These lasers burned chemical reactants to 20