C. Enloe

Force Production Mechanisms of a Dielectric-Barrier Discharge Plasma Actuator

*† ‡ § This paper details the principles behind the time-averaged force production by a dielectric-barrier discharge plasma actuator. A theoretical derivation shows that the force produced is due to the acceleration of ions through the applied electric field, and subsequent collisions with neutral particles. This work shows that the force production is independent of the density of the neutral particles, but is governed by ion density, volume of the plasma, and the applied electric field. Force production of the plasma actuator was experimentally measured in a large vacuum chamber used to control air pressure. Power dissipation, applied voltage, and plasma intensity were also measured in these experiments. These results indicate a linear relationship between force production and air pressure, with the force going to zero at vacuum conditions. Dependencies of electric field strength and number of ions in the plasma as a function of pressure are also measured. These two nonlinear relationships are determined to be the only factors affecting the amount of force produced by the plasma actuator.

Momentum Transfer for an Aerodynamic Plasma Actuator with an Imposed Boundary Layer

In previous work at the United States Air Force Academy, the phenomenology and behavior of the aerodynamic plasma actuator (a dielectric barrier discharge (DBD)) has been investigated. In order to provide additional insight into the phenomenology associated with the transfer of momentum to air by a plasma flow actuator, the velocity distributions upstream and downstream of a plasma actuator with an induced boundary layer were measured using freestream velocities of approximately 4.6 and 6.8 m/s for a range of frequencies (5-20 kHz) and voltages (7.5-10 kV amplitude). The body forces on the air were calculated using a control volume momentum balance. The results show that the body force acts in the sub-boundary layer region. For constant voltage, the body force is proportional to frequency producing a constant impulse per cycle, and the energy dissipation per cycle and efficiency are independent of frequency. The body forces are not affected by the freestream velocity.

Plasma-induced force and self-induced drag in the dielectric barrier discharge aerodynamic plasma actuator

§Through high-time-resolution laser interferometry, we observe the motion of a test article under the influence of a dielectric barrier discharge aerodynamic plasma actuator, and from this motion we deduce the time history of the force produced by the actuator to a resolution substantially smaller than the period of the actuator’s AC cycle. We find that the negative- and positive-going half cycles of the plasma discharge produce a force on the surrounding neutral air in the same direction, but that only the negative-going half cycle produces a force sufficient to substantially overcome the drag induced by accelerating the air in the immediate vicinity of the aerodynamic surface (within the boundary layer under normal circumstances).

Acoustic testing of the dielectric barrier discharge (DBD) plasma actuator

The dielectric barrier discharge (DBD) plasma actuator has been shown to be effective for flow control. Much remains unanswered, however, as to how the actuator couples momentum into air. A better understanding of the coupling mechanism is crucial to determining the performance limitation of the actuator and the breadth of applications to which it can be applied. The small physical volume and transient nature of the actuator plasma make it difficult to make direct measurements. In previous work we have investigated the plasma actuator’s optical emission signature extensively. In this work, we measure and analyze the acoustic emissions, both directional characteristics and waveform, from an actuator in an attempt to shed light on the coupling process. Two sets of measurements were made, each using a different apparatus. Both sets of tests reveal that the actuator adds a larger amount of momentum into the air during the negative-going half of the AC voltage cycle and a smaller amount on the other half of the cycle. It was observed that the acoustic pattern produced is a radiation pattern characteristic of a coherentlydriven system. The results suggest that compressibility effects may play a role in the momentum coupling.

Miniaturized electrostatic analyzer manufactured using photolithographic etching

A design for a new type of electrostatic analyzer, comprised of an arrangement of thin, patterned, conducting, and insulating plates and suitable for extreme miniaturization, is presented. The prototype, destined for use on a microsatellite-based experiment, is fabricated from photolithographically etched stainless steel plates with Teflon insulating layers. Results from tests on the prototype using a highly monoenergetic electron beam show that, as predicted, the device operates as a bandpass energy filter. The energy resolution and angular field of view of this design depend on the relative geometries of the apertures in the conducting plates and can be tailored to specific applications by varying these parameters.

Effect of Volumetric Momentum Addition on the Total Force Production of a Plasma Actuator

Dielectric barrier discharge (DBD) plasma actuators are examined experimentally and computationally. Experimental temporal force measurements have shown that while the plasma is present the actuator experiences an accelerating force and when the plasma is extinguished, a decelerating force appears. This occurs twice during each AC bias cycle. In addition, while the accelerating force is approximately equal in magnitude and direction during each half of the AC cycle, the decelerating force is not. Navier-Stokes simulations of the neutral air flow with a prescribed plasma force reveal that the variation in the decelerating actuator force is consistent with structural changes in the plasma itself. These plasma structural changes alter the volume over which the plasma force is imparted to the air which, in turn, change the amount of air drag incurred by the wall jet created by the plasma. During each actuator bias cycle, 70-90% of the momentum supplied by the plasma actuator is destroyed by drag with the wall.

Frequency Effects on the Efficiency of the Aerodynamic Plasma Actuator

We present the results of temporally- and spatially-resolved neutral density measurements and direct measurements of force produced by the aerodynamic plasma actuator (a dielectric barrier discharge plasma in which an asymmetric arrangement of electrodes leads to momentum coupling into neutral air). Specifically, we make measurements of low-duty-cycle plasma discharges that, although not practical for operational use of the device, yield insight into the mechanism of the actuators operation and the relationship between the charged/neutral particle interactions and the overall inertia of the neutral fluid. Our measurements indicate that the momentum coupling between the charged particles in the plasma and the neutral particles in the air occurs on timescales much shorter than that for the bulk fluid motion. As a consequence, we conclude that in terms of modeling the fluid flow, it is sufficient to treat the actuator as a heat and momentum input into a small control volume, or even as an input on a boundary, depending on the scale size of the model in question. Nonetheless, our measurements of force production on different waveforms indicate that the amount of momentum coupled to the fluid is a strong function of the details of the plasma discharge.

COMPACT THERMAL ION DETECTOR FOR SPACE AND LABORATORY APPLICATIONS

We have developed a new compact thermal ion sensor suitable for spaceflight or laboratory applications. The device is highly sensitive and has a wide dynamic range due to its use of a microchannel plate charge multiplier. It allows spatial variations in velocity and anisotropic temperatures to be measured in a flowing plasma stream using a segmented anode. It has a high rejection to solar UV due to the trajectories of the particles within the device. The device has been flight tested as part of the Charging Hazards and Wake Studies flight experiment and sample data from the flight are presented. We discuss interpretation of the output of the device including extracting the parallel and perpendicular temperatures and the flow energy of a plasma stream.

Bootstrap Surface Charging at GEO: Modeling and On-Orbit Observations From the DSCS-III B7 Satellite

We present an analysis of the charging interactivity between surrounding surface materials aboard a spacecraft at geosynchronous altitudes. In particular, bootstrap charging of a small surface may occur if is surrounded by a large negatively charged surface. Here, a negative potential barrier forms above the small surface, resulting in suppression of photo- and secondary electron emission from that surface. Additionally, the small surface experiences an enhancement of the collection of the photo- and secondary electrons emitted from the surrounding surface. This mechanism results in the charging of the small surface to higher levels than that of the patch in isolation, and in many cases the final potential will reach that of the potential of the larger surrounding surface. With this study we examine bootstrap charging behavior with model data and with data collected on orbit. We have modeled the DSCS-III B7 geosynchronous satellite with realistic geometry and spacecraft materials. Additionally, a previous study has shown that bootstrap charging has been observed on the DSCS-III B7 geosynchronous spacecraft. Both Astroquartz and Kapton cloth patches charged up to the frame potential of the satellite during periods of severe frame charging. The results of modeling bootstrap charging of a small Kapton patch floating relative to the DSCS-III frame fixed at a potential of -1,000 V show that the patch will indeed charge up negatively to match the frame potential, with the temporal increase in negative potential following an exponential time characteristic.

Novel bandpass electrostatic analyzer

A novel electrostatic analyzer for energetic charged particles has been developed that has a simple geometry and has demonstrated a wide field of view (90°×4° in the prototype device). The bandpass of the device can be adjusted by changing the size of the entrance and exit apertures, and the design intrinsically retains information about the spatial distribution of the incoming particles. Comparisons between numerical simulations of the device and laboratory tests are presented.