Motivation—The development of the scramjet propulsion system is an essential part of the development of hypersonic aircraft and long-range scramjet-powered air-to-surface missiles with Mach-8 cruise capability. Shown in Fig. 1 is a schematic of a conceptual Ajax vehicle. As shown at the bottom of the vehicle, this engine has a simple structure as required by the hypersonic aerodynamics. Basically, the combustor has the shape of a flat rectangular box with both sides open. Air in taking through the frontal opening mixes with fuel for combustion and the heated exhaust gas at the open end is ejected out through a MGD accelerator and a nozzle to produce the engine thrust. The hydrocarbon-fueled scramjet in a typical startup scenario, the fuel-air mixture will not auto-ignite. Moreover, the residence time of fuel through the combustion region is short, of order 1 ms. Thus some ignition aids are necessary to initiate and to hold main-duct combustion.
Objective—develop a plasma torch to be a viable Igniter/Fuel Injector in a Scramjet Engine; it requires that the torch can be operated at low flow rate and yet can deliver large amount of energy as well as penetrate deeply into the supersonic crossflow.
Progress—The torch module patented by Kuo et al.1 can run in dc or low frequency ac mode and can produce high power (a few kW in 60-Hz periodic mode2 or hundreds of kW in pulsed mode3) torch plasmas. Shown in Figs. 2a and b are two images of torch plumes, corresponding to (a) in a quiescent environment and (b) in a supersonic crossflow. As shown in the Fig. 2b, the supersonic crossflow causes significant deformation in the shape of plasma torch. A comparison of Fig. 2a to Fig. 2b indicates that the penetration height of the torch is reduced significantly as the plume is swept downstream by the high-speed flow.
The combustion efficiency in a supersonic combustor depends strongly on the plasma enthalpy as well as its spatial distribution (i.e., the penetration height of the torch). This torch module run in 60-Hz periodic mode has been evaluated as an igniter in a test engine. It was found that this igniter produced a substantial flame plume as illustrated in Fig. 3, which shows a single frame taken from video recordings of the plasma torch in operation 5 cm downstream of the ethylene-fueled single-hole injector.
A new microwave-augmented plasma torch that has enhanced plasma enthalpy and size (by the microwave) is being developed. It combines a new torch module with a rectangular microwave cavity in the form of a microwave adaptor. The new torch module is similar to the previous one, except it is longer and integrates the gas plenum chamber into it; moreover, a tungsten tube, which provides a port for fuel injection, replaces the original central electrode of a tungsten rod. The torch module is used not only to generate the arc plasma, but also to couple the microwave power from the cavity to the arc plasma for plasma enhancement. A photo of this torch device is presented in Fig. 4, in which a magnetron whose transmitting antenna inserts into the cavity from the other side (non-tapered section) is also shown.
The torch was tested to operate in the open air using compressed air as the feedstock. It could run stably over a very large flow rate range, with flow speed from subsonic to supersonic. The effect of microwave on the height and volume of the torch plasma varies with the gas flow rate. This effect is particularly significant at low gas flow rate.
This is demonstrated by the plume images, presented in Figs. 5a to c, of the torch plasmas generated before and after the magnetron is turned on, where the air supply pressure is 1.16 atm. A comparison of Fig. 5c to Fig. 5a clearly indicates that the microwave significantly increases the size and the volume of the torch plasma. Moreover, microwave also significantly enhances the luminosity of the plasma plume, which provides an indirect indication on the enhancement of the plasma enthalpy.
Participating Faculty: Professor Spencer Kuo (spkuo@rama.poly.edu)
Collaborators: Drs. Daniel Bivolaru, Skip Williams, and Cam Carter
Sponsor: Air Force Office of Scientific Research (AFOSR).
1S. P. Kuo, Daniel Bivolaru, Campbell D. Carter, Lance Jacobsen, and Skip Williams, “Operational Characteristics of a Periodic Plasma Torch”, IEEE Trans. Plasma Sci., 32(1), 262-268, 2004.
2S. P. Kuo, E. Koretzky, and L. Orlick, “Design and electrical characteristics of a modular plasma torch,” IEEE Trans. Plasma Sci., 27(3), 752-758, 1999; “Methods and Apparatus for Generating a Plasma Torch,” United States Patent No. US 6329628 B1, Dec. 11, 2001.
3S. P. Kuo and Daniel Bivolaru, “A pulsed torch Plasma in a Mach 2.5 supersonic crossflow,” submitted to IEEE Trans. Plasma Sci.
A Microwave-augmented Plasma Torch as an Igniter/Fuel Injector in a Scramjet Engine
Background—A major facility for conducting experiments related to basic radio science research as well as DoD missions is under development in Gakona, Alaska, as part of the High Frequency Active Auroral Research Program (HAARP). The present HAARP HF transmitting system is being expanded from a phased-array antenna of 48 elements to one with 180 elements. After completion of this upgrading, its maximum effective radiated power (ERP) will exceed 1 GW. A backscatter radar (450 MHz) will also be installed soon near the heating site to improve the remote sensing capability of the HAARP. A key objective of the program is to explore physical processes that can be initiated in the ionosphere and magnetosphere via interactions with high power radio waves. Shown in Fig. 1 is a photo of the HAARP HF transmitting system before being upgraded.Two DoD missions of the HAARP program are 1. an ionospheric virtual antenna for underwater communications and 2. an in-situ array of ELF/VLF transmitters for the population control of radiation belt electrons.
Motivation— Signals in communications with submerged submarines have to penetrate deeply into seawater, which is a conducting dielectric; the relative dielectric constant er@ 72 and the conductivity s@ 4 S/m. Thus the attenuation constant a for the high frequency wave is rather high. Fortunately, the attenuation constant decreases with the wave frequency. It has the dependence a = 4´10-3Öf Np/m for f << 1 GHz. Hence, the wave penetration problem can be resolved by adopting very low frequency carrier. For example, choosing f = 100 Hz leads to a = 4´10-2 Np/m and the penetration depth d = 25 m. However, the wavelength of 100Hz wave is 3000 km. To implement a high power and large size antenna (megawatts and hundreds of kilometers) on the ground is costly and has to face environmental impact problems. In the magnetosphere, very energetic electrons (in MeV level) in the radiation belts have strong impact on space systems, which are designed to survive certain amount of radiation(ionizing) dose accumulated during the lifetimes. Any unexpected radiation flux enhancement can cause satellites to accumulate radiation damage much faster than designed for, which leads to faster degradation of on-board active electronics.
Objective—to advance the understanding of wave-plasma interaction processes that help for the realization of the future Naval/DoD systems for the missions.
Progress—In the polar region, an electrojet current appears frequently in the lower ionosphere. A dc space charge field drives this current. Thus an amplitude-modulated powerful HF wave modulated at ELF/VLF frequency can be introduced to modulate the electron temperature, which results to the modulation of theelectron conductivityin a similar fashion. Consequently, the electrojet current driven by the background dc fields becomes oscillating in time to act virtually as an antenna. The ac part of the current becomes the source current of ELF/VLF radiation. A cartoon showing theantenna and its radiation is presented in Fig. 2. Our research effort is to continuously improve the antenna efficiency and the signal quality,[1-3] which are critical to practical applications.
Our recent work [4] showed that whistler waves could introduce chaotic scattering on energetic electrons; thus it can be an effective approach for controlling the population of energetic electrons in the radiation belts. This process is elaborated in Fig. 3. The threshold conditions are determined by the transition of the surface of section plots from regular to chaotic and by the decrease of the pitch angle to be less than the loss cone angle. This is demonstrated by the sequence of surface of section plots (a-c) and pitch angle scattering plots (d-f) presented in Fig. 4; the wave magnetic field B1 is normalized to the background magnetic field B0, i.e., B1/B0; six trajectories corresponding to W0/w = 3.65-3.9 with 0.05 increment are drawn in the same plot to show the frequency effect and to determine the optimal wave frequency. As shown, the trajectory of W0/w = 3.9 electron becomes the most chaotic at B1/B0~ 0.006 and its pitch angle is reduced to about 300 at B1/B0~ 0.015.
Participating Faculty: Spencer Kuo (spkuo@rama.poly.edu)
Collaborators: Drs. James T. Huynh, Paul Kossey, Steven Kuo, and Prof. M. C. Lee
Sponsors: the High Frequency Active Auroral Research Program (HAARP) and the Office of Naval Research (ONR)
[1] S. P. Kuo, M. C. Lee, P. Kossey, K. Groves, and J. Heckscher, Geophys. Res. Lett., 27, 85, 2000.
[2] S. P. Kuo, S. H. Lee, and P. Kossey, Phys. Plasmas, 9, 315, 2002.
[3] S. P. Kuo and S. H. Lee, Radio Sci., 39, RS1S32 (1-5), 2004.
[4] S. P. Kuo, P. Kossey, J. T. Huynh, and S. S. Kuo, IEEE Trans. Plasma Sci., 32(2), 362-369, 2004.
The Plasma Mitigation of the Shock Waves in Supersonic/Hypersonic Flights
Motivation—Shock waves have been a detriment for the development of supersonic/hypersonic aircrafts, which have to overcome high wave drag and surface heating from additional friction. The design for high-speed aircraft tends to choose slender shapes to reduce the drag and cooling requirements. While that profile is fine for fighter planes and missiles, it has long dampened dreams to build a wide-bodied airplane capable of carrying hundreds of people at speeds exceeding 760 mph. This is an engineering tradeoff between volumetric and fuel consumption efficiencies and this tradeoff significantly increases the operating cost of commercial supersonic aircraft. Moreover, shock wave produces notorious sonic boom on the ground. It occurs when flight conditions are changing to cause shock wave unstable. The faster the aircraft flies, the larger the boom. The noise issue raises environmental concerns, which have precluded for example, the Concorde supersonic jetliner from flying overland. A physical spike is used in the supersonic/hypersonic object to improve its body aspect ratio for reducing the wave drag. However, the additional frictional drag occurring on the spike structure and related cooling requirements limit the performance of a physical spike. Therefore, the development of new technologies for the attenuation or ideal elimination of shock wave formation around a supersonic/hypersonic vehicle is essentially needed.
Objective—to advance the understanding of plasma effects on the shock wave structure.
Experimental Progress—A cone-shaped model1-3 having a 600-cone angle was used as a shock wave generator in a Mach 2.5 wind tunnel. The tip and the body of the model were designed as two electrodes with the tip of the model designated as the cathode for gaseous discharge. 60 Hz/DC pulsed power supplies were used in the discharges for plasma generation. The produced plasmas acted as different types of spikes, which deflected the incoming flow before the flow reached the original (baseline) shock front location4. The modification effect of a plasma spike on the shock wave formed in front of the model was explored by examining shadowgraphs of the flow field taken during wind tunnel runs. Presented in Fig.1a is a baseline shadowgraph of the flow field in the absence of the plasma spike, which shows a usual attached conical shock over the model. The flow is from left to right.
The modification effect depends on the density and volume of the plasma spike produced by the discharge, which varies with time. This time varying spike is expected to cause the shock front position to also vary in time. This is demonstrated in Fig. 1, which includes a sequence of six shadowgraphs showing the responses of the shock wave to the growth and decay of the plasma spike in a discharge cycle. The growth and decay of the plasma spike are manifested by the variation of the background brightness in the shadowgraphs. First shadowgraph shown in Fig. 1a is dark, representing the baseline one obtained in the case that the discharge is off. As the plasma spike is intensified to reach the peak, its modification effect on the shock structure as shown in Fig. 1d also reaches the maximum. The shock front becomes very diffusive and spreads from the one shown in Fig. 1c to the further upstream region. The pronounced influence of plasma on the shock structure is clearly demonstrated. The diffused form of the shock front is an indication of shock wave being weakened by this plasma spike. This is an encouraging result, evidencing the effectiveness of this plasma scheme in reducing wave drag at supersonic speeds. It is noticed that as the shock front moves upstream its shock angle also increases.
Numerical Simulation¾We use a wedge5 of angle 150 to simulate the cone model used in the experiment. A uniform airflow from left to right with a velocity V0 = V0 encounters a two-dimensional plasma spike before noticing the presence of a symmetrical wedge on its way. Plasma is generated by the gaseous discharge in an imposed electric field. The results of two cases with z = 0.8, and 0.65 are presented in Fig. 2, in which the baseline shockfront (i.e., z = 1 case) is also presented for comparison, where z is related to the plasma density. As shown the shock front moves upstream as the plasma spike intensifies. It is found that shock becomes detached from the wedge as the strength of the plasma spike exceeds a critical level. In this example for a Mach 2.5 flow over a 150 wedge, the critical level is determined by z = 0.65, which converts to the peak electron density n0 = 6.5´1013 cm-3. This electron density is achievable by a diffusive arc discharge. The shock fronts presented in Fig. 2 correspond to different plasma spike’s intensities, which resembles the temporal variation of the discharge in one cycle. A good qualitative agreement between the theoretical results presented in Fig. 2 and the experimental results presented in Fig. 1 is shown.
Continuing Work¾Numerical simulation with three-dimensional cone model.
Participating Faculty: Spencer Kuo (spkuo@rama.poly.edu)
Collaborators: Drs. Daniel Bivolaru and Steven Kuo
Sponsor: Air Force Office of Scientific Research (AFOSR).
1S. P. Kuo, I. M. Kalkhoran, D. Bivolaru, and L. Orlick, “Observation of shock wave elimination by a plasma in a Mach-2.5 flow,” Phys. Plasmas, 7(5), 1345-1348, 2000.
2S. P. Kuo and Daniel Bivolaru, “Plasma effect on shock waves in a supersonic flow,” Phys. Plasmas, 8(7), 3258-3264, 2001.
3Daniel Bivolaru and S. P. Kuo, “Observation of supersonic wave mitigation by plasma aero-spike,” Phys. Plasmas, 9(2), 721-723, 2002.
4S. P. Kuo, “Conditions and a physical mechanism for plasma mitigation of shock wave in a supersonic flow,” Physica Scripta, 70, 161-165, 2004.
5S. P. Kuo and Steven S. Kuo, “A physical mechanism of non-thermal plasma effect on shock wave,” Phys. Plasmas, accepted for publication.
A Portable Arc-seeded Microwave Plasma Torch and Its Application for Decontamination of Biological Warfare Agents
Uniqueness¾Microwaves can provide electrodeless discharge to produce relatively large volume plasma with no need of gas flow through the discharge to stabilize it. With seeding, a low Q cavity having a relatively large exit hole to increase the diameter of the torch can be used. It turns out that a larger cavity hole also helps the evanescent microwave electric field to reach farther out of the hole. Therefore, the new type of arc/microwave hybrid plasma torch developed in this research effort does not need gas flow in its operation and yet can produce sizable plasma outside the cavity. A patened torch module1 used to generate the seeding plasma adds the flexibility to introduce gas flow in the operation. Gas flow can increase the size as well as the energy of the torch plasma. This microwave plasma torch (MPT) device is designed to be portable2. Other unique features of the design include 1) Rectangular cavity, rather than cylindrical, 2) Torch plasma streams out of the cavity from the sidewall, rather at the end wall, and 3) Torch plasma is energized by the TE mode electric field, rather than by the TM mode.
Motivation¾This torch runs in a 60 Hz periodic mode and can be operated in a wide range of airflow rate. Shown in Fig. 1 is a photo of the device and the plasma torch; the airflow rate is 1.133 l/s.The emission spectroscopy of the torch was examined2. As shown in Fig. 2, the spectral line of O I (777.194 nm) indicating relatively high atomic oxygen content in the torch was observed. This finding motivates the current research applying this plasma torch for the decontamination of biological warfare agents (BWA) such as bacterial spores. This is because the generated atomic oxygen is a reactive oxygen species (ROS). It reacts with nucleic acids, lipids, proteins and sugars of BWA. The oxidation of lipids, reducing sugars and amino acids leads to the formation of carbonyls and carbonyl adducts such as 4-hydroxy-2-nonenal (HNE). ROS are also responsible for deamidation, racemization and isomerization of protein residues. These chemical modifications result in protein cleavage, aggregation and loss of catalytic and structural function by distorting secondary and tertiary protein structures. These irreversibly oxidatively modified proteins cannot be repaired. This occurrence is known as protein degradation. Through these chemical reactions, most BWA are converted by ROS to carbon dioxide and water.
Objective¾to develop a decontamination tool that is capable of selectively and effectively destroying BWA for biodefense.
Progress¾In the decontamination experiments3, B. cereus ATCC 11778 spores were chosen as a stimulant substitute for B.anthracis (i.e., Anthrax). Three distances of 3, 4, and 5 cm were chosen to place the samples for exposure. After plasma treatment, the treated spores and debris were removed from the glass slides by means of extensive sonication using tissue culture water. The mixtures were plated onto Petri dishes with TSA media and incubated at 37oC for 16 hours. After the incubation, the resulting colony forming units (CFU) were counted. The counted results then compare with the control CFU (about 106 per sample before the treatment) provided by the supplier company to determine the decontamination efficacy. The efficacy of the plasma torch on decontamination of B. cereus is demonstrated in Fig. 3, which contains two images of the remaining CFU in the 10-1 diluted samples after 3-sec and 6-sec plasma treatments. The exposure distance was 3 cm.
The CFU counts N from the experimental results were normalized to the initial number N0. These data points are presented in Fig. 4 and are fitted by straight lines as the kill curves for dried B. cereus spores exposed to the plasma torch effluent at three exposure distances: 3, 4, and 5 cm. The x-axis is exposure times in seconds and the y-axis displays the log of the ratio of the number of viable spores remaining (N) to the CFU control number (N0). The time required to reduce the viable (BC) spore population by a factor of 10 by the MPT in this graph for 3, 4 and 5 cm distances are calculated to be 2.09, 2.84, 4.08 seconds. The results are compared with previously reported decontamination of Bacillus globigii (BG) spores using APPJ4 (dashed line) and hot gas (dotted lines), which require about 4.5 and 45 seconds.
Participating Faculty: Spencer Kuo (spkuo@rama.poly.edu)
Collaborators: Dr. Olga Tarasenko and Professor Kalle Levon
Sponsors: Air Force Office of Scientific Research (AFOSR) and the Othmer Institute at Polytechnic Institute of NYU.
1S. P. Kuo, E. Koretzky, and L. Orlick, “Design and electrical characteristics of a modular plasma torch,” IEEE Trans. Plasma Sci., 27(3), 752-758, 1999; “Methods and Apparatus for Generating a Plasma Torch,” United States Patent No. US 6329628 B1, Dec. 11, 2001.
2S. P. Kuo, Daniel Bivolaru, Henry Lai, Wilson Lai, S. Popovic, and P. Kessaratikoon “Characteristics of An Arc-seeded Microwave Plasma Torch,” IEEE Trans. Plasma Sci., 32(4), 1734-1741, 2004.
3Wilson Lai, Henry Lai, Spencer P. Kuo, Olga Tarasenko, and Kalle Levon, “Decontamination of biological warfare agents by a microwave plasma torch,” Accepted by Physics of Plasmas.
4H. W. Herrmann, I. Henins, J. Park, and G. S. Selwyn, “Decontamination of chemical and biological warfare (CBW) agents using an atmospheric pressure plasma jet (APPJ),” Phys. Plasma, vol. 6 no. 5, pp. 2284-2289, 1999.
Fields and Waves 
