Brett A. Cruden

Simultaneous Vacuum-Ultraviolet Through Near-IR Absolute Radiation Measurement with Spatiotemporal Resolution in An Electric Arc Shock Tube

We report on new characterization capabilities recently implemented in the NASA Ames Electric Arc Shock Tube (EAST) facility. A new optical configuration, completely enclosed within a high vacuum chamber and attached to the shock tube, has enabled observation of spatially (and hence, temporally) resolved radiation through the shock layer. These imaging optics are coupled with four spectrometers covering the complete wavelength range of 1201700 nm, allowing for simultaneous measurement, at the same axial location, of spectral features over a broad range at various spectral resolutions. Measurements in the new system have addressed several of the discrepancies between model and experiment in prior EAST testing. The presence of CN impurity emission has been nearly eliminated, while atomic C and H emissions have been reduced. The presence of background continuum radiation has been confirmed as a real effect in the shock tube. Stark broadening measurements have been performed on Hydrogen Balmer-α line and show the electron number densities in the shock to be somewhat higher than model predictions. Experimental artifacts in the old configuration have been discovered and explain disagreements in the measured absolute magnitude of radiance.

Absolute Radiation Measurement in Venus and Mars Entry Conditions

Comparisons of experimental characterization and model predictions of entry radiation relevant to Mars and Venus exploration are presented. Characterization is performed in the recently upgraded Ames Electric Arc Shock Tube (EAST) facility. Tests are performed in Mars (96% CO2, 4% N2) and Venus (96.5% CO2, 3.5% N2) simulant gases at downstream pressures and incident velocities spanning from 0.1-2.0 Torr and 3-12 km/s. Velocity and pressure conditions were chosen based on expected flight conditions (direct entry or aerocapture) in Mars and Venus atmospheres. The absolute radiance data are spatially and spectrally resolved and span the vacuum ultraviolet (VUV) through mid-IR (120-1650 nm, 35 µm). Resolved spectra of the CO 4 th positive band in the VUV are reported for the first time. Measurements of CO2 molecular vibrational radiation is also attempted at low velocity conditions. Radiation modeled under equilibrium assumption with NEQAIR code compares favorably to measured radiation under some, but not all, conditions.

Thermal Decomposition of Low Dielectric Constant Pulsed Plasma Fluorocarbon Films: I. Effect of Precursors and Substrate Temperature

Low dielectric constant (low k) fluorocarbon films have been deposited by pulsed plasma chemical vapor deposition with a variety of different precursors. Deposition rates and resulting film composition have been characterized as a function of pulse timing, deposition temperature, and substrate precursors. To examine the thermal decomposition process, we have constructed a novel appartus for observation of decomposition, utilizing laser interferometry to examine changes in film thickness/properties during the heating process. Using the technique in conjunction with X-ray photoelectron spectroscopy, we have identified at least two methods of decomposition. Loss of short chain side groups is seen to occur at temperatures as low as ∼100°C. At higher temperatures, bulk film decomposition is observed, and the rate is limited by mass-transport of the decomposition products. Additionally, plasma-deposited films are observed to incorporate oxygen on atmospheric exposure. Oxygen groups formed in the film are believed to contribute to the decomposition process.

Time resolved ultraviolet absorption spectroscopy of pulsed fluorocarbon plasmas

Ultraviolet absorption spectroscopy has been used to quantitatively measure CF2 transients in 1 Torr capacitively coupled pulsed plasmas. Time resolved concentrations were obtained for both tetrafluoroethylene (TFE) and hexafluoropropylene oxide (HFPO) feed gases. In the TFE plasma, the CF2 production kinetics follow a first order rise to concentrations of ∼1014/cm3. In the plasma afterglow, a net production of CF2 is observed for a few milliseconds before the transient becomes dominated by a second order recombination process. In the HFPO plasma, three distinct regimes are observed in the plasma on time. Two production regimes exist, one presumably due to HFPO dissociation and the second due to an unknown source, beginning about 5 ms into the on time. Finally, the CF2 concentration passes through a maximum and decreases to steady state. The afterglow processes are similar to those observed for TFE. The production observed in the off time is believed to be related to surface processes.

Neutral gas temperature measurements of high-power-density fluorocarbon plasmas by fitting swan bands of C2 molecules

The neutral gas temperature of fluorocarbon plasmas in a remote toroidal transformer-coupled source was measured to be greater than 5000K, under the conditions of a power density greater than 15W∕cm3 and pressures above 2torr. The rovibrational bands of C2 molecules (swan bands, dΠg3→aΠu3) were fitted to obtain the rotational temperature that was assumed to equal the translational temperature. This rotational-translational temperature equilibrium assumption was supported by the comparison with the rotational temperature of second positive system of added N2. For the same gas mixture, the neutral gas temperature is nearly a linear function of plasma power, since the conduction to chamber wall and convection are the major energy-loss processes, and they are both proportional to neutral gas temperature. The dependence of the neutral gas temperature on O2 flow rate and pressure can be well represented through the power dependence, under the condition of constant current operation. An Arrhenius type of depende...

Implications of catalyst control for carbon nanotube based thermal interface materials

We report on the characterization of a carbon nanotube (CNT) based thermal interface material grown by chemical vapor deposition on catalysts formed by a micelle templating method. The micelle templating method allows for controllable diameter and density in the CNT array that is not easily achieved by other techniques. In this work, we characterize the activity of the catalyst to be at least 10% by a root-counting method. This activity differs from that reported in other works, although the disparity may be largely explained by understanding the approximations in other characterizations. Characterization of thermal interface resistance shows nonmonotonic dependencies on length and catalyst/nanotube density, with optimum values of approximately 0.08 K cm2/W. This exceeds that characterized by others in the literature for a single CNT film interface and state of the art thermal greases. Dependencies on length and density are explained by considering how the compliancy of the CNT array is impacted by these ...

Detection of chamber conditioning by CF4 plasmas in an inductively coupled plasma reactor

During oxide etch processes, buildup of fluorocarbon residues on reactor sidewalls can cause run-to-run drift and will necessitate time for conditioning and cleaning of the reactor. Various measurements in CF4 and Ar plasmas are made in an attempt to identify a metric useable to indicate the chamber condition. Mass spectrometry and Langmuir probe data show that the buildup of fluorocarbon films on the reactor surface causes a decrease in plasma floating potential, plasma potential, and ion energy in argon plasmas. This change in floating potential is also observed in CF4 plasma operation, and occurs primarily during the first hour and a half of plasma operation. A slight rise in electron density is also observed in the argon plasmas. Because the change is seen in an argon plasma, it is indicative of altered physical, not chemical, plasma-surface interactions. Specifically, the insulating films deposited on metal surfaces alter the electromagnetic fields seen by the plasma, affecting various parameters inc...

Analysis of air radiation measurements obtained in the EAST and X2 shocktube facilities

This paper presents measurements of equilibrium radiation obtained in the NASA Ames Research Centers EAST facility and the University of Queenslands X2 facility. These experiments were aimed at measuring the level of radiation encountered during conditions relevant to Orion lunar return into Earths atmosphere. The facilities have targeted the same nominal test conditions of 10 km/s and 26.6 Pa (0.2 Torr). In addition, variations on the nominal shock speed have also been the focus of recent testing in the EAST facility. A comprehensive comparison between the EAST data and NEQAIR is presented in this paper with preliminary X2 comparisons where appropriate. Since the two facilities have different dimensions, and the tests have different shock speeds, NEQAIR simulations are used as a point of reference for the EAST and X2 comparison. Results obtained by independently reducing the data from both facilities are compared. The present analysis endeavors to provide a better understanding of the uncertainty in the measurements, as well as provide an initial comparison between EAST and X2. Furthermore, the present analysis explores various radiative mechanisms to determine if they are due to physical processes relevant to flight, or are just facility dependent phenomena. These phenomena include effects such as the magnitude of the background continuum.

Electron Density Measurement in Re-entry Shocks for Lunar Return

Results of electron density measurements in the Electric Arc Shock Tube (EAST) at NASA Ames Research Center are reported here. Measurements are made at conditions relevant for Lunar return of the Orion Command Module. Normal shocks are produced in the EAST with free-stream pressures of 0.1-1.0 Torr and velocities from 8-12 km/s. Nonintrusive electron density measurements are made by observing optical emission from various lines at high resolution and employing analysis of Stark broadening effects. The measurements show electron density in the shock to be up to several times larger than equilibrium density up to a few centimeters behind the shock front. The disagreement with equilibrium improves at higher velocities.

Ultraviolet absorption measurements of CF2 in the parallel plate pyrolytic chemical vapour deposition process

Polytetrafluoroethylene films have been deposited for use as low dielectric constant materials. Deposition is performed through pyrolysis of hexafluoropropylene oxide (HFPO) to produce CF2, which can then polymerize and deposit as a thin film. The variation of CF2 concentration as a function of reactor conditions has been characterized by ultraviolet absorption spectroscopy. CF2 concentration is observed to go through a maximum with respect to both pressure and pyrolysis temperature when it is present in large amounts (~1014 cm-3). A one-dimensional model including known kinetic reactions for HFPO decomposition and CF2 recombination and multi-component diffusive transport has been applied to the parallel plate system. The result is seen to overestimate the measured concentration and does not capture the maxima observed versus pressure and temperature. An additional mechanism of particle formation, by CF2 insertion into (CF2)n oligomers, has been introduced to produce a kinetic model that explains the CF2 concentration measurements.