This program seeks to advance the field of high temperature materials, space propulsion and electro-energetic physics research. With the discovery of a new class of electromagnetic pulsed plasma propulsion devices such as the Electrodeless Lorentz Force (ELF) Thruster, it is critical to characterize current, and develop and new materials for these systems that exhibit superior performance under the demanding conditions of space propulsion or directed energy. The aim is to discover and develop fundamental and integrated science among materials science, electric propulsion, and electro energetic physics that advances not only pulsed plasma thruster development but any future space power and propulsion concepts based on high energy density plasmas. Through a combination of experiment, surface analysis, numerical modeling and analytical study, the behavior of the plasma-material boundary under transient thermal convective and radiative loads will be investigated. Specifically, transient non-equilibrium plasma flows will be studied over a broad range of energy densities (1 kJ/m2 – 10 MJ/m2), on time scales of microseconds to milliseconds, using atomic and molecular propellants with masses from hydrogen to xenon. The chemical and physical effects of these flows on a range of insulative, conducting and composite materials will be quantified through detailed surface analysis on the micro, meso and macro scale. Both theoretical and numerical analysis of the particle dynamics that result from this research will be undertaken to understand and predict processes at the different time and length-scales corresponding to non-equilibrium conditions that will be relevant to not only space propulsion applications but to fields as diverse as plasma processing to nuclear fusion.
The 2½ year research effort will accomplish the following three major objectives, each with several associated tasks as described below.
Plasma Energetics Objective:
(1) Using existing facilities at the Plasma Dynamics Laboratory, redesign, modify, and make operational an experimental test bed for the specific purpose of obtaining a plasma source and sample chamber for transient plasma/materials interaction studies. The plasma source for these studies will be the Field Reversed Configuration (FRC) similar to that used in the pulsed plasma ELF thruster. (2) Assemble and equip this new plasma source facility with the baseline diagnostic set and determine the basic plasma flow parameters as well as focus on the energy coupling mechanisms and the transfer of plasma flows to the target during dynamic operation. (3) Produce a wide range of transient plasma conditions for study. These will include pulse durations from a few microseconds to several milliseconds with a plasma jet having a kinetic energy fluence on target ranging from 1 kJ/m2 to 10MJ/m2 making possible a transient materials exposure spanning the range from the smallest thruster to the largest fusion energy or directed energy devices. (4) Create high energy density plasmas in atomic and molecular gasses from hydrogen through xenon and examine both chemical and material effects with the largest variety of fuels possible.
Using the experimental setup described above, study the effect of plasma exposure on the surface chemistry of chosen materials/material systems. With the incorporation of chemically active feed gases, a wide range of surface chemical phenomena is expected to occur. These studies will consist of the following tasks: (1) examine the alteration on the surface chemistry due to the following mechanisms: (a) Chemical adsorption of fuel gases (b) Chemical sputtering/erosion (c) Preferential physical sputtering, and (d) Deposition from neighboring walls due to sputtering. (2) Study the evolution of materials surfaces with plasma exposure. This includes variation in pulse length per exposure as well as the number of exposures. (3) Explore thin-film deposition-based wall conditioning techniques, and (4) Examine alternative plasma-facing materials. This would include both insulators such as silica, boron nitride, silicon carbide, berylia, etc. and conductors e.g. titanium, tungsten, stainless steel, molybdenum, etc., as well as composites.
Through both analytical analysis and numerical modeling a basic understanding of the behavior of the plasma–wall interface under non-equilibrium, transient exposure will be formulated. The initial effort will include (1) a feasibility study to investigate the best course for both the modeling and experimental route that would identify and answer critical wall materials issues. It will include investigation into current MHD codes, plasma surface modeling codes, and theoretical analysis appropriate for the plasma-wall materials interaction observed.. (2) A zero-dimensional model will be implemented on a standard surface such as fused silica to identify the critical parameters and material properties. (3) A primary task will be a numerical study of the FRC plasma flows produced in the tests using existing codes at the PSI Center at the University of Washington such as the 3D, extended MHD code NIMROD that was employed in modeling the behavior of the FRC formed with rotating magnetic fields in the ELF thruster. With data from the wide range in experimental conditions, the plasma models will be baselined, modified and fine-tuned to provide predictive capabilities for the optimal design of a wide range of future spacecraft propulsion systems.
A new facility for the study of Non-Equilibrium Materials Studies (NEMS) has recently been completed at the Plasma Dynamics Laboratory of the University of Washington. It has been constructed as the primary experimental test bed for obtaining transient plasmas with pulse durations from a few microseconds to several hundred microseconds (see Fig. 1). The device was constructed so that the Field Reversed Configuration (FRC) plasmoid can be generated from Rotating Magnetic Fields (RMF). With this plasma formation and sustainment technique alone, a plasma jet having a kinetic energy density fluence on target ranging from 8 to 400 kJ/m2 can be realized. The device is also equipped with a set of 3-turn theta pinch coils energized with a 132 kJ, 25 kV pulse power capacitor bank. With this bank the RMF plasma can be rapidly compressed to increase the plasma energy, and hence target energy fluence, by up to a factor of 30 times that of the RMF system alone. The facility is currently being equipped with the baseline diagnostic set to determine the basic plasma flow parameters as well as a translatable sample chamber/holder. This holder is capable of positioning samples all the way into the central formation region. It can be removed while maintaining the sample under vacuum to avoid oxidation and/or contamination.
In order to begin the determination of the materials’ behavior to transient plasmas, as well as training students in the art of materials analysis, a special experimental setup was installed on a operating plasma source. The initial sample exposures were then carried out in parallel to the facility construction (see Fig. 2). In this ad hoc device a range of transient plasma conditions could be achieved that were relevant for study. High energy density plasmas in argon and oxygen were produced and exposed to metal samples of consisting of copper and aluminum. The results from the copper samples will be discussed in the next section.
The materials study was initially focused on the differences in material properties when exposed to a chemically inert gas (argon) and a chemically very active gas (oxygen). Using the experimental setup described in Fig. 2 a study on the effect of plasma exposure with regard to the surface chemistry and morphology was conducted. As this was an initial foray for the NEMS study, a limited range exposure (1000 transient pulses) was employed. With the incorporation of chemically active feed gases, significant surface chemical phenomena were expected to occur and did. A more detailed materials analysis from these studies is now underway and will be reported at the NEMS review meeting at NASA Glenn. Preliminary results for the copper samples exposed to argon and oxygen are shown in Fig. 3. It was observed that in argon there is no significant change or erosion for the sample positioned at the source wall. This sample was ostensibly protected and isolated from plasma contact by the axial magnetic field during most of the FRC plasmoid formation and ejection. The downstream probe did show significant wear as surface scratches show clear signs of erosion as scan be seen in Fig. 3. The results in oxygen were quite different. There was significant chemical activity observed at specific sites on both the front and back of the downstream sample. Given the substantial weight gain seen on the downstream sample, the formation of oxides must be rapidly occurring there. It is not clear at this point as to whether significant erosion might be occurring as well.
The primary task initially undertaken was a numerical study of the range of FRC plasma flows that will be produced in the new NEMS test bed facility. This was done with 2D resistive MHD code Moqui, which has been the workhorse code for understanding FRC behavior. It has been observed in the past that the code was accurate enough to be employed in a predictive mode for new experimental configurations, and that was how it was employed here. The typical RMF equilibria was obtained with an ionic mass of 16 (oxygen), but the radiation package was not employed as this has little influence at these low plasma densities and temperatures. The results of the calculations and implications for future experiments are presented in Fig. 4. Based on these results, RMF generated FRCs will provide for multi-species operation over large range of energy and energy density, as well as ratio of Ek/Eth. By adding a compression field with the theta pinch coils, a much higher energy and energy fluence in the jet can be obtained (30X). In this case the increased energy is due to the much higher ion temperatures as indicated in Fig. 5.