Vadim Brovkin

BASICS IN BEAMED MW ENERGY DEPOSITION FO R FLOW/FLIGHT CONTRO L

Present investigation is focused on formulation of valid and realistic physical basis in exploration of the challenging phenomenon in plasma aerodynamics - interference of MW discharge with gas dy namic structures in supersonic flow. We now distinguish three basic phenomena, reflecting the main features of such interference. These phenomena - drag reduction via discharge -induced vortex creation in a shock layer, regular -Mach discharge -induced transi tion in intersecting shocks and radical change in flow separation in streamlining of spike -tipped bodies through discharge -affecting of viscous interaction - cover the most impressive areas of plasma aerodynamics and at the same time form its basis. Each o f these examples demonstrates wide abilities of MW energy deposition method and presents the complicated physics, which is not fully understood yet. Also discharge structure plays a key role in effective interaction with gas dynamic discontinuities. The pa rameters of MW plasmoid internal structure are quantified, the principle of information extraction from spectra of strongly inhomogeneous plasma objects is proposed. Discharge performance requirements are formulated. Beamed energy deposition as a principle tool for plasma aerodynamic phenomena realization seems both claimed and inherent and in general demands application combined MW and laser techniques.

Interaction of Microwave-Generated Plasma with a Hemisphere Cylinder at Mach 2.1

Microwave energy deposition is a novel method for flow control in high-speed flows. Experiments have demonstrated its capability for beneficial flowfield modification in supersonic flow including, for example, drag reduction for blunt bodies. A fully three-dimensional, time-accurate gas dynamic code has been developed for simulating microwave energy deposition in air and the interaction of the microwave-generated plasma with the supersonic flow past a blunt body. The thermochemistry model includes 23 species and 238 reactions. The code is applied to the simulation of microwave energy deposition in supersonic flow past a hemisphere cylinder. The computed centerline surface pressure is compared with the experiment. The interaction of the microwave-generated plasma with the flowfield structure is examined.

Plasma formation during solid-body irradiation by microwaves and its application for localizing the energy input

Problems associated with the thresholds for plasma production (and connected with it, the nonlinearity of microwave energy release) during irradiation of complex metal - dielectric targets by microwaves in a deep vacuum or in a high-pressure gas were studied. The discovered effect of the low threshold for plasma formation, for which no adequate model exists, may find a number of applications, in particular, in quasi-stationary current generation, gas-discharge rocket engines and microwave soldering.

Effect of heterogeneous discharge plasma on shock wave structure and propagation

New experimental results on shock wave (SW) propagatiofi’and SW structure in cold non-equilibrium weakly ionized plasma are presented, and are correlated with those previously published. The present work is also devoted to development of a theoretical model of phenomena. We purpose that there are hot filaments, inside of the plasma volume, which have extremely high plasma parameters. Simulation of the SW propagation in this structural plasma was conducted and is presented in this work. It was revealed that-the SW structure in this structural medium is very close to the experimental results. Under this condition the temperature of hot filaments rather than the average plasma temperature controls SW velocity. Description of plasma volume as nonhomogeneous formation is a general idea that explains peculiarities of SW propagation through it. Introduction. Experimental studies and simulations have proved the possibility of decreasing SW and acoustic wave (AW) intensity by a factor of up to 10 and an increase of sound speed up to about 2000 ms in non-equilibrium plasma. Significantly, the thermal effects in a plasma do not explain the dispersion and dissipation of AW and SW in a plasma area. So, the study of AW (SW) dispersion and dissipation in Low Temperature Nonequilibrium Plasma (LNp) is very important for development of plasma aerodynamics. An idea of significant non-uniformity in distribution of plasma parameters has allowed approaching to explanation of the most effects. Recently, several theoretic groups have tried to explain the effects by means of spatial non-homogeneity of plasma parameters [l-5]. Experimental studies and simulations have also revealed very significant dispersion and dissipation of SW are obtained in non-equilibrium cold (in average) pulse discharge. The important role of non-uniformities in the plasma formation in pulse discharges is one of the preliminary results of this work. A new approach to the plasma formation process, namely, the large influence of nonuniformity of discharge parameters distribution on SW propagation, forces us to repeat measurements of ,SW .velocity in plasma and visualization at specific condition of non-steady heterogeneous discharge. ,,

High Speed Flow Control Using Microwave Energy Deposition

In recent years a variety of beamed energy deposition techniques have been investigated for flow control in high speed flows. Among these, microwave energy deposition has been demonstrated experimentally to achieve significant drag reduction for blunt body flows. A gas dynamic model for microwave energy deposition in air is developed using 23 species and 238 reactions. The model is applied to the simulation of microwave energy deposition in supersonic flow past a cylinder.

Effect of gasdynamic processes on structure and threshold of laser spark initiated microwave discharge

The process of the laser-spark-assisted initiation of microwave (MW) discharge in the free space in air has been experimentally studied. It is established that, at a preset MW field intensity, the maximum time for which the laser spark retains its initiating ability increases with the laser pulse energy. In the interval of air pressures of 150–750 Torr, a significant decrease in the MW discharge initiation threshold and the period of retained initiating ability of laser spark are determined by laser-spark-induced gasdynamic perturbations.

Microwave Discharge Control by Magnetic Field

Results of experimental and theoretical investigations of free-localized and directed microwave discharges (MWD) are presented. The control over plasma parameters is achieved by changing of MW power level, pulse duration and by application of transverse magnetic field (0 200mT) and gas flows (V ~ 0.7 1M). The flexible scheme for MW energy supply (F=13GHz, W~10-50 kW/cm, τ~5-40μs) of plasma region and for changing the interaction conditions (MWD magnetic field, MWD gas flow) is used in this study. Such general features of discharges as its structure, velocity and size, the temperature and conductivity in plasma channels and their evolution in external magnetic field and airflow are explored. The obvious changing of separate discharge channels behavior and MWD structure modifying, the formation of circular plasma meshes and vortexes and plasma swirls are registered in our experiments. Comparative study of MW and RF ball discharges is performed. The similarity of ball RF and MW discharges is shown.

Microwave Energy Deposition in Supersonic Flows on Laser-Initiated Dipole Structures

The results of feasibility study of organization axially-elongated MW discharges in high density supersonic flow (up to atmospheric static pressure) are reported. The laser-ignited structures in test flow are creating with using of double pulse laser. The test flow has Mach number 1,35, static pressure 173 Torr., static temperature 215 K and stagnation temperature300 K. Experimental setup contains the double – pulse YAG laser (2×145 mJ, 10 ns, 532 nm) and microwave system. The last based on impulse high-power magnetron, (3,125 sm, 1.5 mks, peak power 250 KW) and quasi-optical focusing system with axial orientation of electric field. The maximal MW electric field strength on the focus area of focusing system (quasi – optical cylindrical paraboloid) is about 4,6-4,7 kV/sm. This level allows to realize MW discharge in gas flows with static pressure diapason less than 110 Torr. The focusing laser lens has a focus length 23,3 mm and permits to achieve in the focus area the electric field strength (optical diapason) about 7,3*10 6 V/sm. This level is sufficient for laser spark creation in gas with static pressure diapason 760-70 Torr. The first and the second laser pulses could have the tuning time shift. Results of our experiments clearly show that laser initiation technique permits to realize the stable MW energy deposition in high-density supersonic flows (up to atmospheric static pressure) under comparatively weak electrical MW field (substantially less than threshold level for tested flow). This kind of Laser-MW discharge could be realized as to single laser initiation, as for double (dipole, or multipoint) laser initiation. At the last case, the elongation of plasmoid in tested flow could be essentially increased.

Absorption and Scattering of Electromagnetic Waves by Microwave Streamer

The wide utilization of microwave discharges in plasma aerodynamics experimental studies makes the interaction of mw streamers with electromagnetic waves extremely important. This paper presents new results of experimental and theoretical investigation of scattering dynamics of linearly polarized electromagnetic waves on a thin (in comparison with wavelength  ) plasma channel. The results of experiments are analyzed in the frames of integral model. Introduction A characteristic feature of a nonequilibrium high-pressure microwave discharge (,   is the transport electron collision frequency and   is the circular frequency of the field) is its spatial inhomogeneity. It is in this range of parameters that the diversity of diffusion “unsmeared” plasma configurations shows up most vividly. By now, dipole type plasma structures, which align in the field of linearly polarized electromagnetic waves and consist of thin plasma channels (plasmoids) oriented parallel to the external electric field, have been most extensively studied experimentally and theoretically (see references in [1]). One channel, several channels, or a spatially regular multiplasmoid structure is formed, depending on the discharge ignition conditions. The plasma dipole, which is the main element of a plasma structure and affects noticeably the interference pattern of the electromagnetic field in the discharge region, results from the development of a microwave streamer. The latter comprises two ionization waves traveling in two mutually opposite directions along the external electric field. The uncompensated space charges of opposite signs that emerge during the oscillatory motion of electrons relative to a stationary (in a period 1 2    T ) ion background are located symmetrically relative to the streamer center. These charges are concentrated mainly at the ionization wave fronts in regions of enhanced field and maximum plasma density gradients. In this work, we present an analytic model based on the integrated approach [1], which makes it possible to describe the electromagnetic wave scattering on a thin plasma channel. The dynamics of a signal scattered by a single plasmoid is studied experimentally in air in pressure range P = (1.÷ 1.4)×10 4 Pa. In terms of this model, we explain the peculiarities in behavior of the scattered signal, which is recorded at various distances from the plasmoid. On the basis of the experimental results, we carried out the correction of the model. 50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 09 12 January 2012, Nashville, Tennessee AIAA 2012-0790 Copyright

Regimes of Laser Plasmas - MW Field Interaction

The possibility of laser spark initiation of microwave discharge in quiescent air in wide range of air pressure has been investigated. For the first time the stable electrodeless MW discharge initiated by laser spark produced on the second harmonics of Nd:YAG laser with 15ns-pulse duration in air under the atmospheric pressure is realized. The MW breakdown thresholds under the variation of radiated laser energy and air pressure are measured. Stable breakdown at reduced MW field intensity is registered under a laser beam action. The effect of MW breakdown onset/stabilization in the presence of sub-breakdown intensity of laser beam is eliminated. The initiation ability of laser spark over MW pulse delay depends substantially upon air pressure and changes from 220μs under the normal conditions to more than 100ms under 70Torr. MW discharge development in the direction of sub-breakdown laser beam, but across MW beam is observed. This effect is analogues to laser triggering of DC spark. The possibility of Ruby laser spark initiation of 1μs pulse duration MW discharge in quiescent air under the atmospheric pressure was also demonstrated. The critical delays for MW radiation coupling with decaying laser plasma, exceeding 300μs were determined. The additional energy input due to MW energy deposition was recorded. Experiments with 400ps laser pulse duration on the first and second harmonics of Nd:YAG laser have shown that energy for laser spark creation is more than order of value less in comparison with nano-second pulse duration. The breakdown levels were attained under substantially lower levels of the applied energy – less than 1mJ for 532nm radiation and about 3mJ for 1064nm radiation. The shock waves, arising after picosecond-laser breakdown in air under the normal conditions, are registered by means of the Shlieren system. The effectiveness of transformations of laser pulse energy in an energy, inputted in the gas can be estimated as 0.3±0.1. Numerical investigation of laser plasma decay is carried out and regimes of MW filed interaction with decaying laser plasma are analyzed. Copyright