Superconducting Generators: Enabling Airborne Directed Energy Weapons

Abstract

Future military weapon systems will rely heavily on both lethal and non-lethal directed energy weapons (DEW), to expand the military’s capabilities to respond to a variety of situations. New systems could feature high power microwaves (HPM), high energy lasers (HEL), or other electromagnetic radiation sources. However, these innovative, complex systems will require significant amounts of reliable electrical power. An application important to the Air Force is that of an airborne high power system. Currently, no multimegawatt-class generator system is flight worthy, keeping possible airborne DEW applications grounded. On the horizon though is a new class of generators— small, compact, and light weight, using superconductor technology. Superconducting generators made of high temperature superconductors (HTS) will enable megawatt-class power systems to take flight. This paper will examine the current state of superconducting generator work in the Air Force and its relation to airborne DEW. Also discussed are new advances in flux pinning and ac losses in YBCO that have greatly increased the conductor’s performance in magnetic fields and will help further the development of HTS generator systems. INTRODUCTION Aircraft electrical generators have been developed and optimized over the years, but these conventional generators cannot provide the high power generation needs in the multimegawatt and 10’s of megawatts range without great size and weight liabilities. There are technologies which can push conventional generators to higher power without dramatic increases in size and weight, but at the same time, efficiency, thermal management, and fatigue life are all sacrificed. Because of this, the Air Force has been researching superconductors for years. A serious look at airborne applications began in the early 1970’s with a program that employed low temperature superconductors (LTS) for the rotor windings. There were many concerns with this design which required cryogenic cooling down to 4.2 K, among these, the integrity of the vacuum and cryogenic system as well as the thermal instability of the LTS material. These concerns led to the development of high-purity “hyperconducting” aluminum. This material is not superconducting, but its electrical resistance is so low at liquid hydrogen temperatures (20 K), it was seen as a possible solution to the problems with LTS at 4.2 K. A 1 MW allcryogenic generator was constructed with this highpurity aluminum that weighed only 100 kg. The next advance was the development of high temperature superconductors (HTS) for use in a generator application. This work was completed by American Superconductor who used Bi2Sr2Ca2Cu3Ox (BSCCO) conductor to create replacement windings for the cryogenic aluminum windings. These windings, when delivered to the prime contractor would create a machine capable of 1 MW of output power and weighed just 90 kg—a 10% savings. However, when the BSCCO windings were placed in the generator, 2 of the coils were destroyed during a welding process by a sub-contractor on the program. Future superconducting generators will be created using what some are calling the Second Generation HTS conductor. The material being developed is YBa2Cu3O7-δ (YBCO). YBCO has a higher current density, greater flux pinning, which yields better performance in magnetic fields, and has a distinct possibility to be up to 5 times cheaper than BSCCO since YBCO coated conductors do not require a silver matrix. Figure 1 below depicts a comparison between BSCCO and YBCO. However, there are still some issues such as the need for biaxially aligned superconducting layer and the low strain tolerance, which is a critical factor when making generator windings. However, possibilities exist to overcome these issues. Figure 1: Comparison of BSCCO and YBCO with respect to current density in applied magnetic field. 1st International Energy Conversion Engineering Conference 17 21 August 2003, Portsmouth, Virginia AIAA 2003-5917 This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. 2 American Institute of Aeronautics and Astronautics Recent advances in BSCCO have further increased its current carrying capability; however, due to its poor performance in magnetic fields, it is relegated to a relatively low operating temperatures to maintain high performace when compared with the HTS material of choice, YBCO. Like BSCCO, YBCO has a critical temperature above that of liquid nitrogen (77 K). BSCCO is a commercial conductor chiefly provided in the U.S. by American Superconductor corporation. It is available in long lengths, with high current density, as well as acceptable mechanical properties. All these factors indicate BSCCO could be used in generator windings in its current technological state, but only at lower temperatures—around 25-30K. The goal is to use YBCO that would only further decrease the size and weight of the system by operating at a higher temperature (60K and higher). An additional reduction would come from in the form of a smaller refrigeration system. AIRBORNE DIRECTED ENERGY WEAPONS The Air Force is considering a variety of energy sources as possible directed energy weapons. However, the power requirements for the new weapons systems far exceed that which is currently available on airborne or mobile platforms. Figure 2 below shows what a “standard” DEW system may look like. Currently, the electrical power generation and thermal management systems need development to meet the required capabilities. HTS generators get to the core of the power generation problem by providing a compact, lightweight, and flight worthy solution. For further discussion, it may be useful to break DEW into three classes which are currently under consideration/development. These classes are based on the electrical output requirements. First, there is the solid-state laser which has a high average radiation output power. An example of this type of weapon is a diode-pumped solid-state high energy laser (HEL). The diodes are the electrical load in the system and will emit infrared radiation which will excite the solid-state laser medium. Another two classes are subsets of high power microwave (HPM) weapons. The first of the HPM classes is one in which the HPM source is required to sustain a long duration burst of radiation. Two possible applications of this technology are large aircraft selfprotect and active denial technology. A final class of DEW is that of pulses of microwave radiation with a high peak power output while the average power remains relatively low. UCAV’s may prove to be a useful platform for these systems. A summary of the classes of DEW can be seen in Figure 3. One of the above technologies that is already in the demonstration phase is the active denial technology (ADT). ADT uses millimeter waves, as as opposed to true microwaves, to heat the skin, causing intense pain without damage. The ADT power sub-system closely resembles that in Figure 2. The goal of ADT is to provide field commanders with a non-lethal option where lethal force may be too excessive. The range of ADT exceeds that of current non-lethal technologies such as rubber bullets as well as small arms fire, keeping its operators out of harm’s way. Ground demonstrations are nearing fruition (Figure 4a) and an airborne system is in the works (Figure 4b) in which a HTS generator will be indispensable due to the confined space requirements, strict weight limitations, and high power needs of the ADT system. Figure 3: System block diagram for a generic electrically powered airborne DEW system. Figure 2: Three classes of electrically powered airborne DEW systems with examples of airborne DEW concepts.