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Hyperthermal atomic oxygen source for near-space simulation experiments
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Image of FIG. 1.
FIG. 1.

Cutaway diagram of MIDJet 2.4 GHz microwave resonance cavity. precursor gas (flowing at 12 SLM) is introduced through three 20-mil-diameter injectors, spaced equally around the outside of the cavity. The microwave radiation is coupled from the 5 kW magnetron to the antenna using WR-340 waveguide. The microwave antenna extends through an o-ring-sealed (cavity and antenna) window into the resonant cavity. The discharge flow is exhausted through the 2-mm-diameter AlN nozzle. A small portion of the flow passes through the water-cooled SiC skimmer, typically positioned 28 mm downstream.

Image of FIG. 2.
FIG. 2.

AO beam facility layout, including (from left) the magnetron, WR-340 waveguide, microwave source, skimmer and differential pumping chambers, reaction chamber, and TOF-MS detection apparatus. For the AO-alkene experiments the counterflow was leaked directly into the TOF-MS source region in the center of the reaction chamber. Either a laser pulse or the electron gun was introduced into the opposite side to permit ionization. The TOF-MS and second differential chamber were removed for the IR emission experiments. The pumping speeds indicated for the two diffusion pumps are nominal; actual pumping speeds were determined to be 510 and for the small and large diffusion pumps, respectively.

Image of FIG. 3.
FIG. 3.

(a) REMPI scan of the transition, with a model showing contributions from the three spin sublevels at 300 K. (b) Log-log plot showing the dependence of the REMPI ion signal on laser fluence.

Image of FIG. 4.
FIG. 4.

Mass spectra from MIDJet source with 12 SLM neat gas flow, as a function of nominal microwave power. The neutral AO and species were ionized using electron impact at 70 eV. The ion signals were corrected as described in the text to yield the ratios present in the beam. Note that the signal at zero applied power arises from fragmentation.

Image of FIG. 5.
FIG. 5.

Equilibrium model calculations of the AO and mole fractions as a function of the source temperature. The calculated equilibrium constants for AO are a function of temperature only, and are independent of pressure. The AO and fractions are shown for the 20 psi “hot firing” pressure observed for the MIDJet source. The highest observed 25% dissociation fraction corresponds to a source temperature of 3460 K as shown by the finer lines.

Image of FIG. 6.
FIG. 6.

Left: photograph of 1-mil-thick Kapton-H film erosion within the AO beam reaction chamber, with the AO beam emanating from the skimmer chamber on the left hand side. Right: Derived AO beam profile for 16.4 cm downstream of the skimmer orifice, determined through photogrammetric analysis of Kapton-H erosion. The radius vs time profile was fit to a Gaussian function, and the function integrated over the two spatial coordinates to obtain the profile shown.

Image of FIG. 7.
FIG. 7.

Arrhenius plot of the best-fit, integrated AO flux (units of ) vs the measured Kapton-H temperature (units of kelvin). The derived activation energy is equal to 0.126 eV as shown.

Image of FIG. 8.
FIG. 8.

(a) REMPI scan of the transition detecting the first excited state at . (b) Comparison of REMPI signals for a calibrated density (offset in the figure) with the signal from the AO beam under the same detection conditions. See text for discussion.

Image of FIG. 9.
FIG. 9.

TOF mass spectra obtained for 0.36 eV collisions of AO beam with leaked counterflow. (a) EI ionization, and flowing, microwave off, (b) same conditions, but with 4.5 kW microwave power, (c) 118 nm PI, and flowing, microwave off, (d) same conditions, but with 4.5 kW microwave power. Peak assignments are shown and described in the text.

Image of FIG. 10.
FIG. 10.

TOF mass spectra obtained for 0.36–0.41 eV collisions of AO beam with alkene counterflow, using 10.48 V, 118 nm PI. (a) counterflow, (b) counterflow, (c) counterflow, and (d) counterflow. Selected peak assignments are shown and described in the text. Impurities are marked with an asterisk, and consist of acetone and its decomposition product for the data, and butadiene impurity in the isotopically substituted propene, .

Image of FIG. 11.
FIG. 11.

Color-coded contour plots of IR emission data, on a logarithmic intensity scale, for collisions of the AO beam with (a) , (b) , and (c) , as a function of wave number and delay time relative to the opening of the pulsed value. The emission is dominated by emission from highly excited CO and near , and to a lesser extent from excited near and in the region.

Image of FIG. 12.
FIG. 12.

Slices through the DSMC flow field, showing [AO], , [DMMP], the total gas number density, and the total number density with the DSMC grid overlaid on the flow field. Units are .

Image of FIG. 13.
FIG. 13.

Top: Slices through the flow field showing volumetric photon decay rates and radiation maps of (left) and (right) formed on collision of the crossed beams. Units are . Bottom: Radiation maps obtained by integrating the volumetric decay rates along the line of sight of the detector, perpendicular to the crossing beams. Units are .


Generic image for table
Table I.

Properties of AO beam under different levels of applied microwave power to 12 SLM of feedstock. The source temperature was inferred from the equilibrium model described in the text, using the ratios determined in the TOF-MS measurement. The beam velocity and AO energy were calculated from the source temperature and other constants using Eq. (5).

Generic image for table
Table II.

Measured AO beam profile parameters as a function of distance between the SiC skimmer orifice and the Kapton-H sheet, uncorrected for Kapton temperature. See text for discussion concerning the temperature-corrected AO beam intensities.

Generic image for table
Table III.

Parameters for the determination of the concentration in the AO beam, comparing the signal against that from photodissociation. The was detected through two-photon absorption in both cases. The detailed conditions are described in the text.

Generic image for table
Table IV.

Adiabatic IEs for select species relevant to the crossed-beam experiments, ordered by species mass. For comparison, the PI data shown in Figs. 9 and 10 were obtained using 118 nm (10.48 eV) single-photon ionization. Some peaks, notably and the other parent alkenes, are detected even though their IEs slightly exceed the photon energy, due to internal energy effects. The IEs were taken from the NIST Chemistry Webbook (Ref. 76) except where noted.

Generic image for table
Table V.

Chemistry and reactions assumed for DSMC modeling of the reaction of AO with DMMP in the crossed beam experiment.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Hyperthermal atomic oxygen source for near-space simulation experiments