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Michael D. Ryan, Jong-Uk Kim, James M. Kohel, Lester K. Su, Philip L. Varghese and Noel T. Clemens

A pulsed plasma source has been developed for use in fundamental studies of plasma-propellant interactions. Plasma injection has been proposed as a means to control and enhance combustion rates of various propellant materials, and thus such studies are of interest for applications in rocket propulsion, ETC (electrothermal chemical) launchers, and hypersonic mass acceleration technology. This work is performed with theoretical support from Prof. D. E. Wilson from the Department of Mechanical Engineering.

Formation of the plasma is initiated by the discharge of a 251 microfarad capacitor (rated 5 kV) through a fine (2.5 mil) copper fuse wire inside a cylindrical capillary section (30 mm by 3 mm i.d.), causing ablation and ionization of material from the polycarbonate (Lexan) capillary surface which then sustains the discharge. The resulting plasma rapidly expands from the open end of the capillary section to create a pulsed plasma jet. The electrothermal plasma is characterized by relatively low temperatures (~104 K, or 1 eV), high densities (n ~ 1032 cm-3) and high enthalpies. The peak current through the plasma is approximately 9 kA for a 3.1 kJ discharge energy, and the duration of the discharge is approximately 300 microseconds.

   

The time evolution of the plasma jet has been illustrated as an animated series of images (265 kB). (A single frame from the sequence is also shown here (above left).) These images were acquired using a gated, cooled, intensified CCD camera (578x384 pixel), and they reveal the visible emission from the rapidly expanding plasma. The regions of greatest luminosity correspond to regions of highest temperature in the dense plasma. The Mach disk is clearly evident in these images, along with a strong barrel shock structure.

These same features are evident in the laser schlieren image (above right). The time evolution of the precursor schock (blast) wave has been illustrated as an animated series of images. Shadowgraph and Schlieren images were acquired using the 532 nm output from an Nd:YAG laser, delayed relative to the firing of the plasma device, which is spatially filtered, expanded, and collimated to probe the region of the plasma jet. The parallel beam traverses the plasma and is imaged and recorded using a CCD camera. A 532 nm optical bandpass filter is used to view the transmitted laser signal while discriminating against the intense background radiation from the plasma. Gradients in the index of refraction due to shock fronts are revealed as dark structures in the camera image.

Emission spectra from the plasma source are also being obtained using a 1/4 m spectrometer coupled to an intensified gated linear diode array detector (512 pixel). The plasma source is imaged to a fiber optic which transmits the luminescence to the spectrometer. Monitoring the atomic and molecular emission from the plasma gives information on the plasma composition, temperature and density. An example of temporally- and spatially- resolved emission spectra obtained 90 microseconds after the initiation of the discharge (for 5.0 kV initial voltage) is shown here. In the figure "(I)" and "(II)" represent the atomic and ionic lines, respectively. Additional work is underway to incorporate planar laser-induced fluorescence (PLIF) imaging techniques to further characterize densities and velocities of atomic constituents of the dense plasma jet.

Publications:

• Experimental Study of an Underexpanded Pulsed Plasma-Jet, Jong-Uk Kim, N.T. Clemens and P.L. Varghese, AIAA-99-0452 37th Aerospace Sciences Meeting, Reno, NV, January 1999.

• Emission Spectroscopic Measurements and Analysis of a Pulsed Plasma Jet, J. M. Kohel, L. K. Su, N.T. Clemens and P.L. Varghese, IEEE Trans. Magnetics, Vol. 35, No. 1, 1999, pp. 201-206

• Characterization of a Pulsed Plasma Jet. Part 1: Experimental Studies, J.M. Kohel, L.K. Su, L.L. Raja, N.T. Clemens and P.L. Varghese, AIAA 98-0999 36th Aerospace Sciences Meeting, Reno, NV, January 1998.

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