The work presented in this paper explores the possibilities of increasing specific impulse of ALP by generating the ablative plasma in a strong magnetic field. A magnetic field of 4.5 Tesla was used to direct a laser (1064 nm YAG, 200-450 mJ/pulse) generated plasma in order to create a virtual nozzle and increase specific impulse. The work demonstrated that the magnetic field normal to the target surface serves to contain high pressure, high temperature plasma plume ejected from the surface. A pendulum was used to measure the thrust with and without magnetic field. It has been shown that for the conditions used in this experiment, magnetic field increased the thrust by a factor of 2.
Time-accurate velocity measurements in unseeded air are made by tagging nitrogen with a femtosecond-duration laser pulse and monitoring the displacement of the molecules with a time-delayed, fast-gated camera. Centimeter-long lines are written through the focal region of a ∼1 mJ, 810 nm laser and are produced by nonlinear excitation and dissociation of nitrogen. Negligible heating is associated with this interaction. The emission arises from recombining nitrogen atoms and lasts for tens of microseconds in natural air. It falls into the 560 to 660 nm spectral region and consists of multiple spectral lines associated with first positive nitrogen transitions. The feasibility of this concept is demonstrated with lines written across a free jet, yielding instantaneous and averaged velocity profiles. The use of high-intensity femtosecond pulses for flow tagging allows the accurate determination of velocity profiles with a single laser system and camera.
We examine several conducting spheres moving through a magnetic field gradient. An analytical approximation is derived and an experiment is conducted to verify the analytical solution. The experiment is simulated as well to produce a numerical result. Both the low and high magnetic Reynolds number regimes are studied. Deformation of the sphere is noted in the high Reynolds number case. It is suggested that this deformation effect could be useful for designing or enhancing present protection systems against space debris.
We present a novel design for an ultra-narrow spectral passband filter capable of image preservation. The filter is based on atomic mercury vapor absorption and refluorescence, and is capable of achieving a 0.1/cm passband, at 253.7 nm. The filter features a variable width passband, and temporal gating to improve out-of-band extinction. The refluorescence filter is designed to be used to observe light scattered from a flow field which is illuminated by an injection seeded, cavity-locked Ti:sapphire laser system. We briefly describe the laser system and its utility for diagnostics. We continue with a characterization of the refluorescence filter, detailing its operation, its spectral, temporal, and spatial discrimination, as well as its efficiency, and its application to nonintrusive flow diagnostics. (Author)
Unlike the conventional approach of using a laser sustained plasma to heat a propellant, molecular absorption of laser energy makes it possible to avoid the frozen flow losses associated with the high temperature and complex chemistry of a plasma. The molecular absorption concept is developed by exploring several thermodynamic pathways using a 1-D fluid theory for energy addition in the supersonic regime and different pathways are shown in H-K coordinates. The absorption physics of a promising molecular absorber, SF6, is described at arbitrary laser beam intensities using a two-temperature non-equilibrium model, which is then applied to calculate the nozzle length required to achieve a specific impulse of 250 sec through a 300 K isothermal expansion in the supersonic section. The results of this conversative example case for energy addition illustrate that over a length of less than 1 m laser power on the order of 20 kW can be absorbed in the supersonic region of a 10 g/sec H2 flow without creating a plasma.
Common old ordinary Raman scattering (COORS) is revisited to evaluate its applicability for hypersonic flow characterization. Due to its very low cross-section, Raman scattering is often considered unsuitable for measuring low-pressure gas properties that are found in ground test simulations of high-altitude hypersonic flights. Utilizing a recently developed one-dimensional (1D) light scattering technique with a volume Bragg grating filter and Stokes side band windowing, we demonstrate 1D rotational Raman measurements of temperature and neutral gas density across a bow shock in front of a blunt wedge model under Mach 6 hypersonic flow. The experiment was conducted in the Hypervelocity eXpansion Tunnel (HXT) at Texas A&M University. The measurements were successfully obtained during a single run of the tunnel operation, capturing the temperature and density distributions with dynamic ranges of 200 - 2000 K and 5x10²³ - 4x10²⁴ /m³ respectively, over both the freestream and post-shock regions, covering approximately 10 mm in length with a spatial resolution of < 0.5 mm. Time-resolved high-speed measurement capability at 100 kHz was also demonstrated, showcasing the robustness of 1D COORS for gas diagnostics.
Precise wind velocity measurements are of interest for a variety of applications including weather forecasting and detection of clean air turbulence and wind shear. This work investigates a concept known as spectral hole burning to create an actively tunable atomic vapor filter in order to improve the resolution of LIDAR wind velocity profiling. By adjusting the frequency and intensity of a pump beam in an alkali vapor cell, steep slopes in the transmission profile may be used to monitor shifts in frequency of a probe beam. A three-level atomic model to simulate the spectral hole is described and validated experimentally through pump-probe spectroscopy. The model then optimizes parameters including cell length, cell temperature, pump intensity and pump frequency to find the steepest transmission slope obtainable. At this location, frequency resolutions of 0.69 MHz are predicted per 1% change in transmission, corresponding to a 0.27 m/s velocity sensitivity. From this information, the feasibility and limitations of using spectral hole burning as a controllable atomic filter for LIDAR velocimetry measurements are discussed.
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