Session BM23: Workshop III: Plasma Discharges for Aerospace

Advanced Electric Prolusion System Enabling a Sustainable Return to the Lunar Surface through NASA Gateway

NASA continues to evolve a human exploration approach for beyond low-Earth orbit. The center of this approach is NASA’s Gateway that is envisioned to provide a maneuverable outpost in lunar orbit to extend human presence in deep space and expand on NASA exploration goals. NASA announced at the May 2020 NASA Advisory Council’s Human Explorations and Operations Committee a new plan that calls for launching the first two elements of Gateway as a co-manifested mission in the late 2023 timeframe.   Launching the Power and Propulsion Element (PPE) and the Habitation and Logistics Outpost (HALO) together reduces mission risk, utilizes the PPE high-powered Electric Propulsion (EP) system to transport both elements to the lunar orbit, and reduces overall cost. The PPE is baselined to include three 12.5-kW Advanced Electric Propulsion Systems (AEPS) and four 6-kW Hall thrusters, currently under development by Maxar, for a total beginning of life propulsion power of over 48-kW. High-power solar electric propulsion is one of the key technologies that has been prioritized because of its significant exploration benefits, specifically, for missions beyond low Earth orbit. Spacecraft size and mass are currently dominated by onboard chemical propulsion systems and propellants that may constitute more than 50 percent of spacecraft mass. This impact can be substantially reduced through the utilization of SEP, due to its higher specific impulse and lower propellant load required to meet the equivalent mission delta-V. Studies performed for NASA’s HEOMD and Science Mission Directorate (SMD) have demonstrated that 40-kW-class SEP provides the necessary capabilities that would enable near term and future architectures, and science missions.  Accordingly, NASA has been developing a 12 kW Hall thruster electric propulsion thruster that can serve as the building block for a 40-kW-class SEP capability.   

Nanosecond Plasma: Peculiarities of Kinetics and Diagnostics

In the last 20 years pulsed nanosecond discharges have been widely studied for the problems of flow control, ignition and stabilization of combustion, initiation of detonation. The parameters of the resulting plasma are usually determined by various aspects of the experiment: the composition of the gas mixture, the amplitude, duration and rise time of the high voltage pulse, the pulse repetition rate divided by the flow rate, the geometry of the electrode system.

We propose to consider the plasma produced by nanosecond discharges, paying attention to two key parameters: the specific energy deposited in the gas and the magnitude of the reduced electric field. We apply this approach to analyzing the electron density, densities of main radicals, as well as heat release and hydrodynamic perturbations. Volumetric and surface discharges in the pressure range from units of mbar to tens of bar will be considered. It will be shown that the use of the same pulse allows one to obtain dramatically different plasma parameters: specific energy in the range from 0.001 to 10 eV per particle, electron density from 10¹² to 10¹⁹ cm-3. The obtained plasma can vary from "classical" low-temperature to pulsed "arc" plasma, depending on the request of the application.

A special attention will be given to check of validity of diagnostics like E-field measurements by electrical probes, optical emission spectroscopy and laser E-FISH technique, actinometry and TALIF measurements of density of atoms. A fast, sub-nanosecond transition from streamer to filamentary nSDBD at high pressures will be analyzed by OES combined with a microimaging of high spatial (a few um) and temporal (0.5 ns) resolution.   

Coffee Break

Coffee Break   

Controlling a Multistage Ignition Assisted by a High-Frequency Corona Discharge

Within the frame of the modern strategy of low-temperature combustion (LTC), it is proposed to use lean fuel-air mixtures that are sensitive to the equivalence ratio. Such mixtures display a multistage ignition behavior. Nanosecond discharge impact on a multistage ignition provides the key to controlling oxidation and combustion processes in systems for practical applications. Using propane-air mixture it is shown by modeling that a nanosecond discharge does not suppress, but stimulates the development of cool flame and increases its intensity; and also reduces the non-monotonic oxidation of mixture with increasing the initial temperature, up to suppression of the negative temperature coefficient of oxidation rate. Based on these results, the method is proposed for organizing the combustion of lean mixture in the HCCI engine by changing the reaction path of LTC stage by activating the mixture with a high-frequency corona discharge (5 MHz) and stimulating the auto-ignition due to compression. This method allows us to improve the exhaust composition for CO, UHC, NO_x.  The possibility of controlling the propagation mode of combustion wave with the transition to propagation of auto-ignition waves using a high-frequency corona discharge is shown. The discharge does not directly ignite the mixture due to a small specific energy input. These results can be applied to a hybrid engine, which in stable conditions operates as HCCI engine, and in unstable conditions, a high-frequency corona discharge could be an initiator of combustion.   

Plasma-based control for laminar-turbulent transition: past experience and future directions

Atmospheric plasma discharges have been proposed as a robust and efficient technique for flow control. Dielectric Bartrier Discharge (DBD) plasma actuators boast a wealth of attractive features such as ease and simplicity of construction, directionality of forcing and fast response time. These particular features render plasma actuators extremely attractive for controlling receptive and naturally unstable flows, typically found in transitional regimes. Precise and deterministic plasma-based control of shear layer instabilities is emerging as a viable route to skin friction drag reduction, separation control and acoustic emission mitigation. In this talk, a summary of recent work on the use of plasma actuators in transition control efforts is presented. Special emphasis is given in a partiular class of transitional flows, namely swept wing boundary layer and the ensuing development of crossflow instabilities. Theoretical and experimental approaches are reviewed and salient features of the interaction are discussed. Particular emphasis is given to the receptivity of these flows to unsteady plasma actuation. Finally future directions in this ongoing field ar ebriefly outlined.   

Lunch Break

Lunch Break   

Plasma-chemical kinetic study for low-temperature oxidation of hydrogen

Plasma assisted combustion has been extensively investigated for the last couple of decades. Non-thermal plasmas, in particular, have been found to improve ignition characteristics and promote detonation. However, due to a lack of fundamental understanding of the plasma-chemistry involving hydrocarbons, modeling of the plasma kinetics as predictive tool development is running behind as compared to physical modeling of the plasma. Here, we present a kinetic study for H₂/O₂/(Ar) mixtures to establish a foundation for plasma-assisted hydrocarbon combustion systems. We used a temperature-controlled dielectric barrier discharge (DBD) reactor and a gas chromatography to obtain experimental data; we developed a chemical kinetic platform to simultaneously calculate electron-induced chemistry (ZDPlaskin) and thermally induced chemistry (ChemKin). Through systematically varying the reaction temperature, the equivalence ratio of the mixture, and the discharge power, we discovered somewhat negative temperature behavior for the H₂ conversion in a temperature range of 600–750 K. However, this behavior was found to be influenced not only by the gas temperature but also by the reduced field intensity, indicating the importance of electron-induced chemistry. Additionally, we found the inevitable and undesirable chemical effect of a balance gas (Ar, He, N₂) on the plasma-chemical kinetics, showing significant role of excited (metastable) states on the dissociation of H₂ and O₂ in a highly diluted system. This clearly shows that future kinetic studies for plasma assisted combustion should be conducted using a practical balance gas.

 

Acknowledgements: The research reported in this presentation was funded by King Abdullah University of Science and Technology (KAUST), under award number BAS/1/1384-01-01.   

High-Speed Aerodynamic Flow Control using Nanosecond Repetitively Pulsed (NRP) Plasmas

Nearly all high-speed flight vehicles are plagued by significant aerodynamic challenges posed by high-speed turbulent boundary layers and shock wave-boundary layer interaction (SWBLI).  Such aerodynamic challenges must be addressed for efficient and accessible supersonic/hypersonic flight and economical access to space. One potential solution is plasma-based flow control.  The ability of surface plasma to effect low-speed aerodynamic flows has been well-established, and recent work has demonstrated significant promise for high-speed compressible flow control using pulsed filamentary plasma discharges. Plasma actuators have many advantages: no moving parts, active control at frequencies up to >100 kHz with very fast time response, and flush integration with surfaces. In addition, when compared to more traditional DC or AC-driven plasmas, using ultra-short duration (~10-100 ns) pulses at high frequencies can significantly reduce the required power budget. Interest in such nanosecond repetitively pulsed (NRP) plasmas for flow and combustion control has increased significantly in the past decade, and several recent studies have demonstrated control authority of high-speed, compressible flows using NRP plasma-based actuators. This presentation will provide a brief introduction to NRP plasmas and review recent work on using such plasmas as actuators for high-speed aerodynamic flow control.  Critical knowledge gaps and directions for future research will also be discussed.   

Coffee Break

Coffee Break   

Modeling and Simulation of Coupled Plasma-Flow Interactions for Aerodynamic Actuation and Sensing

This talk presents an overview of modeling and simulation of plasma actuation and sensing of aerodynamic flow.  The physical attributes, e.g. momentum transfer requirements, of plasma actuators for different flow regimes ranging from low speed flows to hypersonic flow will be discussed and appropriate modeling approaches for flow, air plasma chemistry, and plasma dynamics will be presented.  The talk will then discuss computational challenges associated with the temporal/spatial stiffness of plasma-aerodynamic flow interactions and approaches to overcome these numerical issues.  Finally, the complementary problem of aerodynamic flow sensing will be introduced and a novel microwave plasma flow sensor concept will be developed using high-fidelity modeling.       

Electrospray Facility Effects and Their Impact on Life-Limiting Mechanisms

Electrospray, a non-plasma-based form of electric propulsion is not immune to the facility effects seen with plasma-based thrusters. While electrospray facility effects may be inconsequential with regards to performance, they can be substantial enough to dominate life-limiting mechanisms. Busek produces multiple types of electrospray thrusters that operate from droplet-dominated emission to ion-dominated emission and through similar phenomena, both suffer from facility effects that can dictate thruster life.  For droplet mode thrusters, where electrospray beam interception on electrostatic grids is typically <0.5%, grid currents measured has been determined to be ~50% accounted for through back-streaming of ion-induced secondary electrons and reflected droplets. Back-streaming propellant contaminates grids and can result in thruster failure. With high specific impulse thruster variants, back-streaming of droplet mass is less of an issue; however, sustained glow discharges are commonly observed at the extraction grid plane and even coupling to the target if placed at short distance. Busek has developed electrospray beam targets tailored to suppression of the properties of the incoming beam species and performed detailed measurement of ion-induced secondary electron yield from thruster grid and target materials. Data collected is intended to better inform life models that aim to predict thruster life when direct testing of required mission life plus the typical 50% margin, 6 years of continuous operation, is problematic and cost prohibitive.   

Exploiting the richness of dynamical behavior for system state estimation

Understanding of Hall Effect Thruster (HET) physics has moved in lockstep with the application of experimental diagnostics to interrogate the plasma.  Historically, the electric propulsion (EP) community has focused strongly on velocity-space and spatially-resolved plasma diagnostics to study HET behavior and perform model calibration.  This presentation will focus on the recent efforts in the EP community to explore the development of suitable dynamical representations of plasma signals to provide inexpensive, accessible, high quality validation metrics suitable for system state estimation and model calibration.  In addition, it will briefly explore intriguing new applications of nonlinear dynamical systems theory enabled by ready access to high speed signals from modern data acquisition systems.