Sergey O. Macheret

Numerical modeling of DBD plasma actuators and the induced air flow.

An earlier developed detailed physical model of an asymmetric dielectric barrier discharge (DBD) plasma actuator in air driven by repetitive nanosecond voltage pulses and ac/dc bias is used to model realistic experimental conditions, such as nonideal pulses with multiple reflections. The force and heating rate calculated by the plasma model is used as an input to two-dimensional viscous flow solver to predict the time-dependent DBD-induced flowfield. The calculations reproduce the wall jet and vortices observed in experiments and enable determination of the induced flow velocity at the plasma edge and the body force magnitude from the schlieren imaging. A three-electrodes DBD system is theoretically explored, and its advantages are shown to only exist during a very short time following the voltage pulse.

I t SHOCK WAVE PROPAGATION AND STRUCTURE i IN NON-UNIFORM GASES AND PLASMAS

Theoretical estimates and computational analysis of the role of temperature gradients, non-uniform heating, and wall effects in determining structure of propagating shocks in glow discharges and of bow shocks around moving spheres is performkd. The multi-peak structure of density-gradient shock profiles observed in glow discharges is shown to be consistent with gas dynamic effects of shock distortion and complex flow structure caused by transverse temperature gradients. A possible/role of finite geometry combined with non-uniform temperature distribution in ballistic range studies is analyzed. Vibratio’ P al relaxation phenomena are indicated as potentially important in determining bow shock structure around blunt bodies moving through molecular plasmas.

MODELING OF PLASMA GENERATION IN REPETITIVE ULTRA-SHORT DC, MICROWAVE, AND LASER PULSES

Reduction of the power budget for sustaining a prescribed electron density in plasmas generated by DC or oscillating electric fields can be achieved if high-energy electrons are generated in these plasmas, which requires the ratio E/N in the DC case, or the ratio E/co in low-pressure microwave or laser plasmas, to be very high. Applying the strong field for only a short time, and then allowing the plasma to decay is the essence of the concept of ultra-short repetitive pulse ionization explored in this paper. Modeling of repetitive pulses show that there is an optimum electric field that minimizes the power budget. This minimum power budget is much lower than that in a continuous DC or RF plasma, but still substantially higher than in plasmas sustained by electron beams. Modeling of spatio-tempor al dynamics of plasmas in nanosecond pulses shows strong coupling between non-local dynamics of high-energy electrons, ionization and electric field. At high gas pressures, cathode sheath phenomena and ionization non-uniformity are suppressed. With an appropriate choice of frequency and amplitude, the efficient repetitive-pulse ionization method can be extended to electromagnetic waves, i.e., to electrodeless systems. At low gas density, highpower nanosecond microwave pulses at frequency 10-100 GHz can accelerate electrons to ~ 1 keV energy, resulting in much more efficient ionization than that in conventional low-power systems. Moreover, most of the ionization would occur after the pulse, as the high-energy electrons produced during the pulse lose their energy on ionization. In principle, the method of plasma generation by ultrashort repetitive highpower electromagnetic pulses can be extended to high gas densities, and the requirements for the laser source are estimated in the paper.

Plasma-Enabled Tuning of a Resonant RF Circuit

A concept of plasma-enabled tuning of radio frequency systems is introduced and experimentally verified in this paper. Specifically, a commercial gas discharge tube (GDT) is employed as a plasma cell integrated in an inductor-capacitor (LC) resonator. The principal factor affecting the characteristics of the LC circuit with the embedded GDT, studied for the first time in this paper, is the decrease in the cathode sheath thickness and thus increase in the capacitance of the plasma cell with increasing current in the abnormal glow discharge regime. The reduction of plasma cell resistance is an additional factor. The sample LC resonator demonstrates frequency tunability by 55% in the range of 240-372 MHz, while the discharge current increases up to 90 mA, at the expense of reduction in quality factor from 62 to 5 as a result of ohmic losses. Theoretical estimates based on the fundamental principles of glow discharges supplemented by circuit simulations show good agreement with measurements. The local minimum in frequency behavior of the resonators quality factor is shown to be related to frequency-dependent plasma parameters. The results of this paper indicate that plasma tuning techniques could be a viable alternative for emerging applications.

Observation of MHD Effects with Non-Equilibrium Ionization in Cold Supersonic Air Flows

An earlier experimental investigation has been performed using short duration, high repetition rate, high voltage (2 ns, 100 kHz, ~10 kV/cm) pulses to ionize a Mach 3 air flow. The wind tunnel was placed in a 5 Tesla magnetic field. In this paper these results, are further interpreted with the help of modeling. A cold, supersonic, non-equilibrium plasma was created. Peak electron densities between 5×10 11 and 10 12 per cm 3 were reached in the experiments. The electrical properties of the resultant MHD channel were investigated. Power extraction via MHD effects was demonstrated in the cold, supersonic, unseeded air flow. The Hall parameter was estimated from the electrical properties of the MHD channel. Modeling predictions were found to be in very good agreement with experimentally measured Faraday current between the pulses. Modeling showed that between pulses the electrons have low energy, electron and ion currents are comparable, and the cathode fall is very small.

Modeling weakly-ionized plasmas in magnetic field

Despite its success at simulating accurately both non-neutral and quasi-neutral weakly-ionized plasmas, the drift-diffusion model has been observed to be a particularly stiff set of equations. Recently, it was demonstrated that the stiffness of the system could be relieved by rewriting the equations such that the potential is obtained from Ohms law rather than Gausss law while adding some source terms to the ion transport equation to ensure that Gausss law is satisfied in non-neutral regions. Although the latter was applicable to multicomponent and multidimensional plasmas, it could not be used for plasmas in which the magnetic field was significant. This paper hence proposes a new computationally-efficient set of electron and ion transport equations that can be used not only for a plasma with multiple types of positive and negative ions, but also for a plasma in magnetic field. Because the proposed set of equations is obtained from the same physical model as the conventional drift-diffusion equations without introducing new assumptions or simplifications, it results in the same exact solution when the grid is refined sufficiently while being more computationally efficient: not only is the proposed approach considerably less stiff and hence requires fewer iterations to reach convergence but it yields a converged solution that exhibits a significantly higher resolution. The combined faster convergence and higher resolution is shown to result in a hundredfold increase in computational efficiency for some typical steady and unsteady plasma problems including non-neutral cathode and anode sheaths as well as quasi-neutral regions.

Electrode design for magnetohydrodynamic power panels on reentering space vehicles

Power can be extracted from the ionized uid surrounding a re-entering space vehicle using magnetohydrodynamic (MHD) principles, provided an appropriate system of magnetic elds, thermal protection, and current collection electrodes can be designed and manufactured. Given computational uid dynamics results for a re-entering spacecraft presented elsewhere (1), this paper examines possible electrode congurations and the implications that these will have for power generation. For practical electrode designs and realistic magnetic elds it is shown that current recirculation, which is detrimental to power generation and to thermal management, will generally arise. The necessity for fully-coupled CFD-MHD calculations is also demonstrated.

Control of sub-critical microwave filamentary plasma in dense gases

Summary form only given. In the present paper, we describe successful initiation and guiding of microwave plasma filaments with a low-energy pulsed laser. A 50 kW, 2.45 GHz microwave power with 1 ms pulse length was directed into a waveguide-based resonant cavity filled with room air. The electric field in the cavity was kept below that needed for self-initiated breakdown. When ArF laser beam (15 ns pulse, 50 mJ/pulse) was focused at a location inside the waveguide, the streamer started and continued to grow long after the laser pulse, reached the waveguide wall and persisted to the end of the microwave pulse. The streamer followed the track of the laser beam rather than the electric field. The paper describes the observed dynamics of the streamer and the theoretical interpretation in detail. The paper also describes the development of a laboratory-scale supersonic wind tunnel with microwave plasma. In the tunnel, a 2D dielectric nozzle is mounted inside the microwave waveguide, so that the flow direction coincides with that of the microwave propagation.

A tunable VHF gas discharge tube resonator

A plasma-based tuning technique based on a gas discharge tube (GDT) integrated in a resonant structure is introduced in this paper. Specifically, the GDT is implementing a variable capacitor in a lumped LC resonator. The electron number density and consequently the electromagnetic properties of plasma are controlled by varying the magnitude of the excitation field. In this work we also theoretically and experimentally investigate the tunability-RF loss trade-off. The fabricated GDT LC resonator at 305 MHz shows 26% of tunability while the discharge current increases up to 75 mA. The resonators quality factor varies between 43.7-2.0 for the same range. However, a quality factor above 10 can be maintained if the tuning range is reduced to 4%. The results reveal that this technique could be a suitable tuning scheme for applications where other technologies fail due to high-power or harsh-environment requirements.

Low temperature plasma for tunable resonant attenuation

A cold-plasma-based technique for tuning an evanescent-mode cavity resonator is introduced and studied experimentally for the first time in this paper. The technique involves a plasma jet that constitutes a variable resistance integrated in the cavity. The electron density and consequently the electromagnetic properties of plasma, including its resistivity, are controlled by varying the magnitude of the sinusoidal excitation voltage. The transmission coefficient of the two-port fabricated resonator at 2.735 GHz exhibits 11 dB tunability when the magnitude of the 20-kHz plasma-excitation voltage increases from zero to 5.26 kV (peak-to-peak). The resonators quality factor varies in the acceptable range of 684-342 for these conditions. The measured and simulated results reveal that this approach may become a promising tuning technology particularly in demanding applications where conventional solid-state techniques are ineffective due to temperature, power, or linearity limitations.