R. B. Cohen

Time-resolved imaging of flame kernels: Laser spark ignition of H2/O2/Ar mixtures

The shape and structure of developing flame kernels in laser-induced spark ignited hydrogen/air mixtures is investigated as a function of gas composition and time. Using planar laser-induced fluorescence (PLIF) to measure the spatial distribution of OH radicals produced inside the reaction zone, we have recorded the evolution of the nascent flame kernel in a series of images following the laser-induced spark. This series provides the rate of flame growth, the evolution of the flame shape, and the intensity of the PLIF signal as a function of time for both igniting flames and nonignition events. The reaction zones grow quickly at early times, but slowly decrease in propagation rate as the energy density within the flame kernel decreases. A distinct anisotropy is observed in the expanding spark and flame kernel. At short times (t < 100 μs), a toroidal shape is observed similar to that seen previously for electrode-spark ignitions and for laser ignitions in methane/air. There is also a tendency for the flame to grow back toward the ignition laser. Successful ignitions appear virtually identical to failed ignitions during the first 100 μs. Significant differences, notably in intensity, appear between 100 and 500 μs following the spark. These observations imply that early flame kernel growth is dominated by gas motion induced by the short-duration spark. The ultimate fate of an ignition lies with the chemistry of the reactions which determines whether the gas undergoes a transition from hot plasma to propagating flame.

Quenching of laser induced fluorescence of O2 (b1Σ+g) by O2 and N2

Abstract Molecular oxygen in the b 1 Σ + g electronic state was generated by direct excitation at 7628.225 A with a tunable dye laser. A new oxygen quenching rate constant of k = (4.0 ± 0.4) × 10 −17 cm 3 /molecule s was measured from the decay of fluorescence. A nitrogen quenching rate constant of k N 2 = (2.2 ± 0.1) × 10 −15 cm 3 /molecule s was obtained.

Electron‐impact ionization time‐of‐flight mass spectrometer for molecular beams

A method is described for performing electron‐impact ionization time‐of‐flight mass spectrometry in a molecular beam apparatus. It provides a convenient means for optimizing the performance of pulsed or continuous nozzle sources and can be used in conjunction with laser excitation. Mass spectra are produced either as analog waveforms or in a high repetition rate ion counting mode. The device can also be operated as a fast ionization gauge for time‐resolved detection of pulsed beams.

Spectroscopy and dynamics of jet‐cooled hydrazines and ammonia. I. Single‐photon absorption and ionization spectra

Electronic‐state properties of hydrazine (N2H4), monomethyl hydrazine (MMH), unsymmetrical dimethyl hydrazine (UDMH), and ammonia were investigated by single‐photon vacuum ultraviolet (vuv) absorption and photoionization spectroscopy in a molecular‐beam apparatus. The photoionization spectrum of NH3 shows a sharp threshold at a value of 10.16 eV in excellent agreement with previous measurements. The ionization thresholds of the hydrazines are very broad reflecting the large (nearly 2 eV) structural relaxation energy for conversion from a C2 pyramidal gauche neutral geometry to a D2h planar ion geometry. The measured threshold photoionization potentials are 8.32 eV (N2H4), 8.05 eV (MMH), and 7.87 eV (UDMH). The vuv absorption spectra of the hydrazines are also broad, which is to be contrasted with the well‐resolved spectrum of NH3. The band shapes can be attributed to planarization of the Rydberg excited states in analogy to the ion structure. A molecular‐orbital model is developed to provide understanding...

Ultrasensitive detection of atmospheric constituents by supersonic molecular beam, multiphoton ionization, mass spectroscopy

An ultrasensitive detection method for atmospheric monitoring has been developed based on the technique of supersonic molecular beam, resonance enhanced multiphoton ionization, and time-of-flight mass spectroscopy (MB/REMPI/TOFMS). Several organophosphonate and organosulfide compounds, representing simulants to a class of toxic compounds, were studied. Detection levels as low as 300 ppt (dimethyl sulfide) were obtained. Single-vibronic-level REMPI of the cooled molecules in conjunction with TOFMS provided selectivity of ~10(4) against chemically similar compounds in humid air expansions. The fragment ions formed by REMPI excitation are shown for diisopropyl methylphosphonate to depend strongly on the resonant intermediate state of the neutral molecule.

State‐selected reactive scattering. II. He+H+2→HeH++H

The endoergic reaction He+H+2→HeH++H is investigated in a molecular‐beam experiment as a function of H+2 vibrational energy at c.m. collision energies between 0.3 and 1.9 eV. Reactant ions generated by resonantly enhanced four‐photon ionization are impulsively accelerated to collide with a beam of He. Time‐of‐flight velocity distributions of HeH+, measured at one laboratory angle, yield the differential cross section at c.m. angles θ=0° and 180°. A shift from ‘‘He rebound’’ to ‘‘H+ stripping’’ behavior accompanies the enhancement in the cross section as the H+2 vibrational energy increases, which matches previous studies at higher collision energy. Small‐impact‐parameter events produce HeH+ with less recoil velocity (more internal energy) than those at large impact parameters. Within the limits of sensitivity and resolution (ΔE≊0.15 eV), definitive resonance features in the collision energy dependence of dσ/dω are not evident. Improvements in the technique to enable such observations are suggested.

State‐selected reactive scattering. I. H+2+H2→H+3+H

Cross sections for the reaction H+2+H2→H+3+H, differential in scattering angle and recoil energy, are measured in a molecular‐beam experiment at c.m. collision energies of 1.5, 2.3, 3.5, and 5.3 eV. Resonantly enhanced four‐photon ionization prepares H+2 in selected vibrational‐state distributions, allowing a systematic exploration of the effects of reactant energy on the product angular and energy distributions. Angular data are interpreted on the basis of competition between H+3 formation and collision‐induced dissociation. The nominal atom‐transfer (AT) and proton‐transfer (PT) processes are identified respectively with forward and backward scattered H+3. Effects of reactant energy on AT and PT cross sections in H+2+H2 are compared with previous observations on D+2+H2 and H+2+D2. The fraction of the available energy appearing as H+3+H recoil ranges from 26% to 39% depending on reactant conditions. Previous surface‐hopping trajectory calculations successfully predict most of the observed trends. Evidenc...

Microwave Electrothermal Thruster Performance

Thrust, specific impulse, thrust efficiency, and coupling efficiency were measured for a nominally 1 kW microwave electrothermal thruster operating on He, N 2 , and N 2 O, and for a 5 kW model operating on water. For He, N 2 , and N 2 O, thrust varied from approximately 100 to 700 mN at discharge pressures from 6.7 x 10 4 to 2.7 x 105 Pa and magnetron input powers from 900 to 1500 W. Thrust measurements agree well with calculations assuming one-dimensional, isentropic flow of a perfect gas with constant specific heats. Peak specific impulses for He, N 2 , and N 2 O were 418, 243, and 209 s, respectively. Water was run at magnetron input powers from 2.1 to 4.1 kW and discharge pressures from 4.0 x 10 4 to 1.5 x 10 5 Pa. Thrust varied from approximately 100 to 250 mN with a peak specific impulse of 428 s. Incorporating an impedance matching capability improved coupling efficiency with water to over 96%, but did not increase specific impulse. Measurements with a residual gas analyzer indicate that the water plume may be 50 to 70% dissociated, and that the lack of dependence of specific impulse on specific energy results from a tradeoff between dissociation losses at low specific energy and heating of the thruster body at high specific energy.

HIGH POWER MICROWAVE ELECTROTHERMAL THRU STER PERFORMANCE ON WATER

Thrust, specific impulse, thrust efficiency, and microwave coupling efficiency were measured for a nominally 1 kW microwave electrothermal thruster (MET) operating on He, N 2, and N 2O, and for a nominally 5 kW MET operating on water. For He, N 2, and N 2O thrust varied from approximately 100 to 700 mN at discharge pressures from 500 to 2000 Torr and magnetron input powers from 900 to 1500 W. Thrust measurements agree well with idealized calculations based on the assumptions o f one -dimensional, isentropic flow of a perfect gas with constant specific heats. Peak specific impulses for He, N 2, and N 2O were 418, 243, and 209 seconds respectively. Water was run at magnetron input powers from 2.1 to 4.1 kW and discharge pressures f rom 300 to 1100 Torr. Thrust varied from approximately 100 to 250 mN with a peak specific impulse of 428 seconds.

Time‐resolved mass and energy analysis by position‐sensitive time‐of‐flight detection

We describe a method for time‐resolved mass and kinetic energy analysis of ionic or neutral species (1–150 amu, 0.5–500 eV) in diagnostic measurements on spacecraft electric thrusters. Time‐of‐flight mass spectrometry is combined with position‐sensitive detection to measure energy spectra for multiple masses at sampling rates as high as 50 kHz. A rectangular microchannel plate detector with a 96‐element metal anode array is read out by fast analog‐to‐digital converters or by discriminators and scalers. The ion drift time varies as the square root of the mass‐to‐charge ratio, and the displacement along the detector varies as the square root of the energy‐to‐charge ratio. The energy resolution is enhanced by minimizing the field distortion near grid wires.