Mikhail N. Shneider

Magnetohydrodynamic Control of Hypersonic Flows and Scramjet Inlets Using Electron Beam Ionization

The possibility of controlling scramjet inlets in off-design conditions by operating a near-surface magnetohydrodynamic (MHD) system upstream of the inlet is examined. The required electrical conductivity in air is supposed to be created by electron beams injected into the air from the vehicle along magnetic field lines. A simple model of a beam-generated ionization profile is developed and coupled with plasma kinetics, MHD equations, and two-dimensional inviscid flow equations. Calculations show that an MHD system with reasonable parameters could bring shocks back to the cowl lip when flying at Mach numbers higher than those for which the inlet was optimized. The MHD effect is not reduced to heating only because the work by j X B forces is a substantial part of the overall effect. Power requirements for ionizing electron beams could be lower than the electrical power extracted with MHD, so that a net power would be generated onboard. Problems associated with high Hall fields are discussed

Modeling of dielectric barrier discharge plasma actuator in air

A detailed physical model for asymmetric dielectric barrier discharge (DBD) in air at low voltages (1.5–2 kV) is developed. Modeling of DBD with an applied sinusoidal voltage is carried out in two dimensions. The leading role of charging the dielectric surface by electrons in the cathode phase is shown to be critical, acting as a harpoon that pulls positive ions forward and accelerates the gas in the anode phase. The positive ion motion back toward the exposed electrode is shown to be a major source of inefficiency in the sinusoidal or near-sinusoidal voltage cases. Based on understanding of the DBD physics, an optimal voltage waveform is proposed, consisting of high repetition rate, short (a few nanoseconds in duration), negative pulses combined with a positive dc bias applied to the exposed electrode.

Modeling of dielectric barrier discharge plasma actuators driven by repetitive nanosecond pulses

A detailed physical model for an asymmetric dielectric barrier discharge (DBD) in air driven by repetitive nanosecond voltage pulses is developed. In particular, modeling of DBD with high voltage repetitive negative and positive nanosecond pulses combined with positive dc bias is carried out. Operation at high voltage is compared with operation at low voltage, highlighting the advantage of high voltages, however the effect of backward-directed breakdown in the case of negative pulses results in a decrease of the integral momentum transferred to the gas. The use of positive repetitive pulses with dc bias is demonstrated to be promising for DBD performance improvement. The effects of the voltage waveform not only on force magnitude, but also on the spatial profile of the force, are shown. The crucial role of background photoionization in numerical modeling of ionization waves (streamers) in DBD plasmas is demonstrated.

Experimental investigation of dielectric barrier discharge plasma actuators driven by repetitive high-voltage nanosecond pulses with dc or low frequency sinusoidal bias

Experimental studies were conducted of a flow induced in an initially quiescent room air by a single asymmetric dielectric barrier discharge driven by voltage waveforms consisting of repetitive nanosecond high-voltage pulses superimposed on dc or alternating sinusoidal or square-wave bias voltage. To characterize the pulses and to optimize their matching to the plasma, a numerical code for short pulse calculations with an arbitrary impedance load was developed. A new approach for nonintrusive diagnostics of plasma actuator induced flows in quiescent gas was proposed, consisting of three elements coupled together: the schlieren technique, burst mode of plasma actuator operation, and two-dimensional numerical fluid modeling. The force and heating rate calculated by a plasma model was used as an input to two-dimensional viscous flow solver to predict the time-dependent dielectric barrier discharge induced flow field. This approach allowed us to restore the entire two-dimensional unsteady plasma induced flow ...

Electron-Beam-Generated Plasmas in Hypersonic Magnetohydrodynamic Channels

A novel concept is analyzed of hypersonic cold-air magnetohydrodynamic (MHD) power generators and accelerators with ionization by electron beams. Ionization processes are considered in detail. Strong coupling between hypersonic boundary layers and electrode sheaths is demonstrated, and anode voltage fall in hypersonic MHD channels is shown to be very high. A potential anode sheath instability and ways to suppress it are discussed. Electron beams are shown to be capable of generating an adequate conductivity in cold air, while allowing full control and stable operation of MHD channels. Example calculations of hypersonic accelerator and power generator performance appear to be promising.

Microwave diagnostics of small plasma objects

We suggest an approach for using microwave radiation in collisional, weakly ionized plasma diagnostics when plasma dimensions are relatively small compared with the microwave wavelength. We show that in this case the microwave diagnostics can be based on the measurement of the radiation scattered by an oscillating plasma dipole, similar to the Rayleigh scattering of an atom in light. Examples considered show possibilities of obtaining the decaying plasma parameters (time dependence of charge density and information about loss rates, for instance) from the measured scattered signal.

Magnetohydrodynamic Power Extraction from Cold Hypersonic Airflows with External Ionizers

A novel concept of hypersonic cold-air magnetohydrodynamics (MHD) power generators with ionization by electron beams is analyzed. Electron beams are shown to allow control and stable operation of MHD channels in cold high-speed flows. To avoid excessive energy cost of ionization and damage to beam-injection foils, electron beam current densities should be restricted to a few milliamperes per square centimeter. This reduces the conductivity in electron beam sustained MHD channels compared with that in conventional MHD generators, restricting performance and calling for very strong magnetic fields and high Hall parameters. The high Hall parameters cause ion slip and near-anode phenomena to become first-order issues. Example one-dimensional calculations of hypersonic power generator performance appear to be promising. Possible problems that could be caused by hypersonic boundary layers and electrode sheaths, including anode sheath instability and ways to avoid it, are also discussed.

Scramjet Inlet Control by Off-Body Energy Addition: A Virtual Cowl

The theory of energy addition to hypersonic airflow off the vehicle to increase air mass capture and reduce spillage in scramjet inlets at Mach numbers below the design value is explored. The heated region creates a virtual cowl and deflects flow streamlines into the inlet. Optimization studies are performed with a two-dimensional inviscid fluid code. The best location of the energy addition region is near the intersection of the nose shock of the vehicle with the continuation of the cowl line, and slightly below that line. In that case, the shock generated by the heating is close to the shock that is a reflection of the vehicle nose shock off the imaginary solid surface-extension of the cowl. Effects of the size and shape of the energy addition region on inlet performance are also studied.

Ionization in strong electric fields and dynamics of nanosecond-pulse plasmas

The paper describes experimental and computational studies of air plasmas sustained by high repetition rate high-voltage nanosecond pulses. Current and voltage measurements, together with earlier microwave diagnostics, allowed us to determine the efficiency of ionization. The energy cost per newly produced electron in these diffuse volumetric plasmas was found to be on the order of 100 eV, two orders of magnitude lower than in diffuse quasineutral DC and RF plasmas, and comparable with or even lower than in the cathode sheaths of glow discharges. A plasma kinetic model was developed and tested against the experimental Paschen breakdown curve in argon. The kinetic model was found to adequately describe the Paschen curve, and the important role of ionization by fast ions and atoms near the cathode, as well as the increase in secondary emission coefficient in strong fields described in the literature, was confirmed. Modeling of plasma dynamics in highvoltage nanosecond pulses yielded the energy cost of ionization, which was found to agree well with the experimental values. Both experiments and modeling revealed that the ionization cost per electron in these plasmas is relatively insensitive to the gas density. Detailed investigations of the plasma dynamics revealed a critical role of the cathode sheath that was found to take up most of the peak voltage applied to the electrodes. The extremely high E/N, much higher than the Stoletov’s field at the Paschen minimum point, results in a very high ionization cost in the sheath. In contrast, the E/N in the quasineutral plasma is closer to that associated with the Stoletov’s point, resulting in a near-optimal electron generation. This behavior (the reversal of ionization efficiencies in the sheath and in the plasma) is opposite to that in conventional glow discharges. The positive space charge in the sheath and its relatively slow relaxation due to the low ion mobility was also found to result in reversal of electric field direction in the plasma at the tail of the high-voltage pulse.

Cavity cooling of an optically trapped nanoparticle

We study the cooling of a dielectric nanoscale particle trapped in an optical cavity. We derive the frictional force for motion in the cavity field and show that the cooling rate is proportional to the square of oscillation amplitude and frequency. Both the radial and axial components of the center-of-mass motion of the trapped particle, which are coupled by the cavity field, are cooled. This motion is analogous to two coupled but damped pendulums. Our simulations show that the nanosphere can be cooled to