P. Storm

Interior Plasma Diagnostics of Arcjet Thrusters

We review past experimental measurements of internal flow properties of arcjet thrusters. These measurements are generally classified as either intrusive, requiring design changes to prototype thrusters, or nonintrusive, and include measurements of cathode temperature, as well as static pressure, flow temperatures (vibrational, rotational, electronic, translational), electron density, and velocity throughout the interior region extending to the exit plane. Comparisons are made to available model predictions. These measurements, performed on a wide range of thrusters, and operating on variety of propellants, indicate that the nozzle plasma flow may be removed from local thermodynamic equilibrium.

Fluorescence velocity measurements in the interior of a hydrogen arcjet nozzle

The first use of laser-induced fluorescence for spatially resolved plasma velocity measurements within an arcjet nozzle are described.The arcjet was operated on a propellant of pure hydrogen.The Balmer-α transition of atomic hydrogen was scanned by the excitation laser source while simultaneously detecting the spectrally integrated resonance fluorescence.Velocity was determined from the Doppler shift in the peak of the laser excitation spectrum relative to a stationary reference.The arcjet operating conditions were chosen to facilitate a comparison between these measurements and arcjet modeling results of Butler et al.

Stark broadening corrections to laser-induced fluorescence temperature measurements in a hydrogen arcjet plume

Laser-induced fluorescence of the H(α) transition of atomic hydrogen has previously been performed in the plume of a hydrogen arcjet thruster. Measurements of plasma velocity and temperature, based on the Doppler shift and broadening of the H(α) line shape, were previously published [Appl. Opt. 32, 6117 (1993)]. In that paper the Stark broadening of the H(α) transition was estimated from static-ion calculations performed in the early 1970s and found to be negligible in comparison with the Doppler broadening. However, more recent dynamic-ion calculations have shown the Stark broadening to be considerably larger than was previously assumed, resulting in inaccurate temperature measurements. We present a reanalysis of the fluorescence data, taking into account the improved Stark broadening calculations. The correct atomic hydrogen translation temperature and electron number density are obtained from the Doppler and Stark broadening components of the measured line shape. The results indicate a substantial drop in temperature from those previously reported.

Arcjet nozzle flow-field characterization by laser-induced fluorescence.

Laser-induced fluorescence of the Balmer-alpha (H(alpha)) transition of atomic hydrogen was performed within the nozzle of a 1-kW class radiatively cooled arcjet thruster operating on hydrogen and synthesized-hydrazine propellants. Axial velocities were determined from the Doppler shift of the H(alpha) line center relative to a stationary reference, whereas translational temperatures and electron number densities were determined from a line-shape analysis of the H(alpha) transition. The results are compared with a numerical model and indicate excellent agreement with the velocities, as well as temperatures near the nozzle exit. There are discrepancies, however, in the temperatures far upstream of the exit and in the electron densities, suggesting needed improvements in the modeling of the recombination chemistry.

Axial emission measurements on a medium power hydrogen arcjet thruster

A numerical and experimental investigation of the axial emission of a 5 kW class radiatively-cooled hydrogen arcjet thruster is presented. The complete visible spectrum is modeled, taking into account graybody thermal emission from the cathode and plasma radiation from hydrogen atoms and free electrons. The cathode temperature and the arc electron number density are obtained from the measured emission spectrum. The cathode temperature is found to be in the neighbourhood of the melting point of tungsten and increases with arcjet power at a constant massflow rate, suggesting an increase in the current density at the arc attachment point. The measured near-cathode electron number density is also found to increase with power, suppofling the idea of an increasing arc current density with arcjet power. The measured and modeled arc electron densities are, however, in discrepancy by a factor of approximately three.

Doppler-free absorption measurements of Stark broadening in a flowing hydrogen plasma

Abstract This paper presents measurements of electron number density in a low-density hydrogen arcjet plasma flow. A spatially translated Langmuir probe is used to obtain these data, and comparisons are made to measurements of electron density using Doppler-free absorption spectroscopy (DFAS) of the Stark-broadened spectral line of the atomic hydrogen Balmer-alpha (H α ) transition. A lineshape model is developed for the DFAS spectra which includes crossover resonances, and collisional coupling between the electronic states within the n =2 atomic hydrogen level. The electron number densities extracted from the Langmuir probe traces are found to be significantly less than those determined from the DFAS measurements. This difference is attributed to the averaging of the collected current over the extended probe surface.

Measurements of electron density at the exit of an arcjet thruster

This paper presents measurements of electron number density and electron temperature at the exit of a low power 1-kW class radiatively-cooled arcjet thruster operating on hydrogen as a propellant. A spatially translated Langmuir probe is used to obtain these data, and comparisons are made to measurements of electron density using Doppler-free absorption spectroscopy (DFAS) of the Stark-broadened spectral line of the atomic hydrogen Balmer-alpha (Ha) transition. A lineshape model is developed for the DFAS spectra which include cross-over resonances, and collisional coupling between the electronic states within the n = 2 atomic hydrogen level. The electron number densities extracted from the Langmuir probe traces are found to be significantly less than those determined from the DFAS measurements. This difference is attributed to the averaging of the collected current over the extended probe surface.

Plasma and cathode emission from a high power hydrogen arcjet

Abstract : An experimental study of the measurement of cathode temperature, current distribution, and near-cathode electron number density in a high power hydrogen arcjet is presented. This study is motivated by the desire to better understand arc-electrode interactions in arcjet thrusters, which in many cases, is the main determinate of arcjet lifetime. Measurements such as these may also provide the needed boundary conditions for numerical arcjet simulations, presently under development. We describe in this paper the application of a non-intrusive in-situ measurement technique for on-axis, spectral imaging of the electrode region of arcjets, and the application of this technique to the measurement of the cathode and anode temperatures, cathode spot size, and current distribution in a 30kW hydrogen arcjet thruster. A relatively large field of view (twice the throat diameter) and high spatial resolution (9 micrometers) are achieved.