1414
IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 4, AUGUST 2008
Images From the Development of a High-Power Microwave System Thomas A. Holt, Andrew J. Young, Mohamed A. Elsayed, John W. Walter, Andreas A. Neuber, and M. Kristiansen
Abstract—A recently developed self-contained compact singleshot high-power microwave (HPM) system was tested and characterized. The explosive-driven system utilizes a reflex triode virtual cathode oscillator (vircator) as the HPM source. An openshutter image acquired with a digital single-lens reflex camera during operation was used to show plasma development extending beyond the anode–cathode gap of the vircator. The plasma’s self-emission is due to ionized material eroded and desorbed from both the cathode and the anode. Index Terms—Explosively driven pulsed power, helical flux compression generator (HFCG), power conditioning, vircator.
T
RADITIONAL high-power microwave (HPM) systems occupy large volumes and require expensive prime power, power conditioning, and support systems (vacuum, gas, thermal management, etc.). Consequently, traditional HPM systems are not well suited for use as remote and/or single shot systems. HPM diodes typically require high-voltage (>100 kV), fast rise time (tens of nanoseconds) pulses for operation and operate for several tens to hundreds of nanoseconds. The aforementioned operational parameters indicate that, while peak instantaneous powers are large, the energy required by the HPM source is moderate (typically several kilojoules can yield hundreds of megawatts of HPM output power). Therefore, a prime power source of modest size coupled to an energy amplifier could provide the required energy to the HPM diode. Significant development of energy amplification and power conditioning systems by the explosive pulsed power community over the past half century has enabled the creation of inexpensive singleshot systems capable of driving HPM loads that have been successfully tested on numerous occasions, for instance [1]. The desire to create an inexpensive single-shot HPM system has directly led to the selection of readily available technologies as the prime power and power conditioning components. The prime power is a lead-acid battery-based system conditioned using a dc–dc converter to charge a capacitor. The stored energy in the capacitor is switched into the energy amplifier using a solid-state switching scheme. About half of the capacitively stored energy is transferred to the seed current coil of the energy amplifier. This efficiency is primarily limited by losses in the solid-state switching scheme, the 2-ft transmission line used to couple the capacitor and the seed coil, and the resistive loss
Manuscript received November 30, 2007; revised March 27, 2008. The authors are with the Center for Pulsed Power and Power Electronics, Department of Electrical and Computer Engineering, Texas Tech University, Lubbock, TX 79409 USA (e-mail:
[email protected]). Digital Object Identifier 10.1109/TPS.2008.924487
Fig. 1. Isometric three-quarter section view of the power conditioning system and reflex triode vircator.
of the seed current coil itself. Shortening the transmission line connection would result in higher energy transfer at the cost of destruction of the seed source with each shot. The energy amplifier is a helical flux compression generator (HFCG) that uses flux trapping and two cascaded stages to drive the ∼2-µH inductive energy store (IES). The energy amplification occurs because chemical energy stored in high explosives (400 g) is used to accelerate a metallic liner against an established magnetic field. Electrical energy gain is observed as a consequence of the compression of the magnetic flux; however, the overall energy efficiency of this process is low. The dual-stage HFCG used in this system has been tested 39 times into various inductive loads (1–3 µH) and is capable of amplifying an input energy of 150 J by a factor of 20, storing ∼3 kJ of magnetic energy in the IES. The energy amplification process completely destroys the HFCG, and as a consequence, all components of the system with the exception of the HFCG are placed outside of a detonation containment chamber and survive for reuse in subsequent shots. An IES, an exploding wire array (EWA), and a peaking gap comprise the power conditioning system (see Fig. 1). The EWA is designed to open at a current magnitude of 40 kA and is made of 20 parallel-coupled wires that are 127 µm thick and 160 mm long. Current from the HFCG flows through the IES and is diverted away from the HPM load through the EWA for most of the run time of the HFCG until the EWA opens. The fast interruption of current induces a large voltage (>100 kV) that causes the peaking gap to close thereby applying a voltage pulse across the HPM diode.
0093-3813/$25.00 © 2008 IEEE
HOLT et al.: IMAGES FROM THE DEVELOPMENT OF A HIGH-POWER MICROWAVE SYSTEM
1415
Fig. 2. (a) Time-integrated picture of reflex-triode vircator during operation with field shapers in place. Open-shutter image acquired with a Canon-10 D (APS-C sensor) using a 2x focal length extender with an f/2.8 70–200 mm telephoto zoom lens. The 10-s exposure was captured at an effective focal length of 448 mm at ISO 200, f/38 at a distance of 62 ft away from the subject. The spectral response of the camera system approximately covers the range from ∼300 to 800 nm. (b) Reference image.
The HPM diode, a reflex triode virtual cathode oscillator (vircator), used in the experiment was designed for an operational frequency of ∼3.8 GHz, and was made with a velvet cathode and stainless steel screen anode with a transparency of 70%. The output of the IES is coupled to the anode of the vircator through the insulator via the peaking gap electrode. The chamber housing the vircator was evacuated to a pressure of ∼7.5 × 10−6 torr, and the load voltage was approximately 100 kV for the experiments that have been conducted to date. A time-integrated picture of the vircator during operation is shown in Fig. 2. At the beginning of operation, electrons are explosively emitted from the velvet cathode surface. The explosive emission of electrons is accompanied by emission of cathode material and is later followed by the emission of anode material due to electron bombardment (erosion of the vircator screen is typically visible after repeated shots). Further secondary emission from the cathode and the chamber wall due to positive ion impact is also possible (a similar description of plasma formation is given in [2]). The observed light emission in the time integrated picture of Fig. 2 is believed to be due to cathode and anode material excited by electron impact and transported into the vacuum space primarily in the form of ions accelerated under the applied 100-kV potential difference. The electron and ion energy distribution is unknown; however, it is believed that
the diluted plasma carries ions with a large spread of energies. Some of the plasma particles obviously undergo optical transitions in the few electronvolt range as corresponding with the observed light emission in Fig. 2. The exact nature of the transition (continuum, high or low lying levels, etc.) is unknown. Note that the cathode and chamber wall are at the same potential (anode is at ∼100 kV), such that the charged and excited particles eventually occupy the space to the left and right of the anode. A peak output power of ∼4 MW was measured from an HPM signal that was approximately 110-ns wide (full-width at halfmaximum) for the aforementioned shot. It is estimated that the vircator was operating at an efficiency of ∼1%. A previous shot (fielded without either field shaping rings or a peaking gap) fired with a similar peak load voltage and load pressure yielded approximately the same output power from a 70-ns wide pulse. Additional testing and further system development is underway. R EFERENCES [1] G. Gorbachev, S. Korovin et al., “High-power microwave pulses generated by a resonance relativistic backward wave oscillator with a power supply system based on explosive magnetocumulative generators,” Tech. Phys. Lett., vol. 31, no. 9, pp. 775–778, Sep. 2005. [2] M. Yatsuzuka, M. Nakayama et al., “Plasma effects on electron beam focusing and microwave emission in a virtual cathode oscillator,” IEEE Trans. Plasma Sci., vol. 26, no. 4, pp. 1314–1321, Aug. 1998.
