NASA Glenn Research Center (GRC) is currently leading the development of multiple electric propulsion systems to flight readiness. The Advanced Electric Propulsion System is a 12.5 kW Hall thruster system that is being developed by the Solar Electric Propulsion Technology Demonstration Mission (SEP TDM) project, under the sponsorship of the Space Technology Mission Directorate. NASA's Evolutionary Xenon Thruster-Commercial (NEXT-C) is 7 kW class gridded ion thruster system that being developed under the sponsorship of the Science Mission Directorate. NASA GRC is also providing electric propulsion discipline support to the Power and Propulsion Element and the Double Asteroid Redirection Test (DART) missions, which will be the first applications for these technologies, respectively. Lower technology readiness level (TRL) projects are underway for applications including CubeSats, small spacecraft and Mars exploration vehicles. Under the sponsorship of the Small Spacecraft Technology Program, NASA GRC has performed numerous independent verification and validation tests of CubeSat class electric propulsion systems in support of a growing number of small US businesses that are developing these systems. Lastly, three technology development efforts focused on 100 kW EP strings led by Aerojet Rocketdyne, Ad Astra and MSNW were recently completed.

We present a novel technique to measure time-resolved laser-induced fluorescence signals in plasma sources that have a relatively constant Fourier spectrum of oscillations in steady-state operation, but are not periodically pulsed, e.g., Hall thrusters. The technique uses laser modulation of the order of MHz and recovers signal via a combination of band-pass filtering, phase-sensitive detection, and averaging over estimated transfer functions calculated for many different cycles of the oscillation. Periodic discharge current oscillations were imposed on a hollow cathode. Measurements were validated by comparison with independent measurements from a lock-in amplifier and by comparing the results of the transfer function average to an independent analysis technique triggering averaging over many oscillation cycles in the time domain. The performance of the new technique is analyzed and compared to prior techniques, and it is shown that this new technique has a niche in measurements where the analog photomultiplier signal has a nonwhite noise spectral density and cycles of oscillation are not sufficiently repeatable to allow for reliable triggering or a meaningful average waveform in the time domain.

Undergraduate students of six universities participated in a design and build outreach program sponsored by the US Air Force during the 2011–2012 academic year. The goal was to design and build a thrust vectoring system for a small jet engine (about 20 pounds of thrust). Student and professor exit surveys were taken with almost all participants contributing to these surveys. Based on the survey results and the professors’ insights, learning outcomes and student impact are assessed. In addition, any other lessons learned during this extensive project-based learning activity are described.

This paper presents the results of a two-axis laser-induced fluorescence velocimetry study of singly charged xenon in the interior and near the channel walls of the H6 Hall thruster. The thruster has a nominal operating discharge power of 6 kW and has been operated for similar to 330 h before testing. It is operated under seven conditions spanning discharge voltages of 150-600 V and anode mass flow rates of 10-30 m g/s. The mean velocity of the near-wall ion populations varies from 0.5 to 25 k m /s with an angle of incidence relative to the wall surface that varies from 30 to 80 deg. In general, the mean velocity and angle of incidence are strongly correlated with the discharge voltage and weakly correlated with the anode mass flow rate. There is strong correlation between the axial locations where high energy (greater than a few tens of electron volts) ions exist and where erosion is discernible in the channel profile. This evidence supports the notion that erosion is negligible below a certain energy threshold. Potential application of this velocimetry technique for qualitative analysis of Hall thruster channel wall lifetime is also discussed.

We present a novel technique to measure time-resolved laser-induced fluorescence (TRLIF) signals in plasma sources that have a relatively constant Fourier spectrum of oscillations in steady-state operation, but are not necessarily periodically pulsed or repeatable; e.g. Hall thrusters. The technique uses laser modulation on the order of MHz and recovers signal via a combination of band-pass filtering, phase-sensitive detection, and averaging over estimated transfer functions calculated for many different cycles of the oscillation. Periodic discharge current oscillations were imposed on a hollow cathode and the resulting oscillations in the LIF profile are observed. Measurements were validated by comparison with independent measurements of the average velocity distribution from a lock-in amplifier and by comparison with an independent time-resolved analysis technique. This new technique has a niche in measurements where the analog photomultiplier signal has a strong background signal with a nonwhite spectral density and the cycles of the oscillation are not sufficiently repeatable to allow for reliable triggering or a meaningful average waveform in the time domain. Hall thruster oscillations are one example of such conditions, and this technique will be used to interrogate unperturbed Hall thruster breathing and rotating spoke modes in future work.

An undergraduate student design and build project has been established by the US Air Force, Air Force Research Laboratory as part of an outreach program. During the 2011–2012 academic year, undergraduate students of six universities participated in designing a thrust vectoring system for a small (20 pound-thrust) jet engine. A description of the project parameters and student designs is given in this paper. It proved to be an extremely successful project, and other professors and students can learn from the different approaches taken by the six different teams and the project itself. Industry will also be interested in the depth and breadth of an undergraduate project that is being used to educate their future engineering workforce.

The STMD GCD ISP project is tasked with developing, maturing, and testing enabling human exploration propulsion requirements and potential designs for advanced high-energy, in-space propulsion systems to support deep-space human exploration and reduce travel time between Earth's orbit and future destinations for human activity. High-power Hall propulsion systems have been identified as enabling technologies and have been the focus of the activities at NASA Glenn-In-house effort to evaluate performance and interrogate operation of NASA designed and manufactured Hall thrusters. Evaluate existing high TRL EP devices that may be suitable for implementation in SEP TDM.

The NASA Office of the Chief Technologist Game Changing Division is sponsoring the development and testing of enabling technologies to achieve efficient and reliable human space exploration. High-power solar electric propulsion has been proposed by NASA's Human Exploration Framework Team as an option to achieve these ambitious missions to near Earth objects. NASA Glenn Research Center (NASA Glenn) is leading the development of mission concepts for a solar electric propulsion Technical Demonstration Mission. The mission concepts are highlighted in this paper but are detailed in a companion paper. There are also multiple projects that are developing technologies to support a demonstration mission and are also extensible to NASA's goals of human space exploration. Specifically, the In-Space Propulsion technology development project at NASA Glenn has a number of tasks related to high-power Hall thrusters including performance evaluation of existing Hall thrusters; performing detailed internal discharge chamber, near-field, and far-field plasma measurements; performing detailed physics-based modeling with the NASA Jet Propulsion Laboratory's Hall2De code; performing thermal and structural modeling; and developing high-power efficient discharge modules for power processing. This paper summarizes the various technology development tasks and progress made to date

We present the initial results from a cavity ring-down sensor for measuring the density of sputtered boron atoms originating from the discharge channel of a 6-kW Hall thruster. The sensor traps 250 nm light in a high-finesse cavity to greatly increase its sensitivity to boron atoms. Measurements were obtained with the thruster operating at seven conditions spanning discharge voltages from 150 to 600 V, and anode mass flow rates from 10 to 30 mg/s. Power level at these operating conditions ranged from 1.5 to 10 kW. Boron density was found to vary from <1×10 14 to ~1×10 15 m -3 with the higher density measurements being found near the two channel walls. We apply a simple two-dimensional velocity model to obtain boron flux. The boron flux is then correlated to the boron nitride sputter rate, which is found to vary from 4×10 -4 to 6×10 -3 mm 3 /s. A simple power law equation is formulated to correlate the relative sputter rate for the 6-kW Hall thruster as a function of the tested operating conditions. Although the uncertainty in this initial result is high, the result clearly demonstrates previously untapped potential for the use of cavity ring-down spectroscopy to study the problem of Hall thruster channel wall erosion.

We present the initial results from a cavity ring-down sensor for measuring the density of sputtered boron atoms originating from the discharge channel of a 6-kW Hall thruster. The sensor traps 250 nm light in a high-finesse cavity to greatly increase its sensitivity to boron atoms. Measurements were obtained with the thruster operating at seven conditions spanning discharge voltages from 150 to 600 V, and anode mass flow rates from 10 to 30 mg/s. Power level at these operating conditions ranged from 1.5 to 10 kW. Boron density was found to vary from <1×10 14 to ~1×10 15 m -3 with the higher density measurements being found near the two channel walls. We apply a simple two-dimensional velocity model to obtain boron flux. The boron flux is then correlated to the boron nitride sputter rate, which is found to vary from 4×10 -4 to 6×10 -3 mm 3 /s. This paper then goes on to describe a number of technical challenges encountered in the course of developing the sensor. These challenges include cavity alignment maintenance in the presence of mechanical vibration and mirror degradation. The paper presents the solutions used during this experiment to address the said challenges as well as suggests more permanent solutions for future research. The experiment presented in this paper clearly demonstrates the potential for the use of cavity ring-down spectroscopy to study the problem of Hall thruster channel wall erosion, as well as provide a roadmap for future research in this area.

Sputter monitoring system using continuous-wave cavity ring-down spectroscopy (cw-CRDS) was built for both lifetime assessment and contamination effects in Hall thrusters. First, we have performed proof of principle measurements of sputtered boron atoms from BN targets by argon ions using pulsed CRDS. The measurement strategy is based upon detection of boron atoms via an absorption line from ground state at a wavelength of 249.773 nm. The path-integrated number density is 1.8 × 1013 m-2 at ion beam current of 45 mA and argon ion energy of 1,000 eV. The number density is proportional to the ion beam current, as expected. These results show the validity of the boron sensor for detecting sputtered boron atoms. Next, in order to achieve the required detection sensitivity and time response, we implement CRDS with a continuous-wave (cw) laser for enhanced sensitivity. The target was changed to manganese (λ=403.07 nm) instead of boron. The results show that a detection limit of per-pass absorbance of 0.6 ppm for a 1-s measurement time. Scaling the experimental results and accounting for changes in laser system and mirror reflectivity indicate that the BN detection system should have sufficient signal-to noise for expected Hall thruster conditions.

We present and validate a method for extracting velocity distribution functions from laser-induced fluorescence measurements obtained using a xenon ion line with unknown hyperfine constants. The method involves a direct deconvolution of intermodulated experimental results from the laser-induced fluorescence measurements. The intermodulated experiments were done on the 5d[4]7/2 → 6p[3]5/2 transition for singlycharged xenon at 834.7 nm. The hyperfine constants for this line are large enough to distort the results but small enough to have eluded accurate measurement. This approach is validated using both simulated results and a set of measurements taken in a Hall thruster environment. The validity of the method is scrutinized through the uncertainty analysis that follows. The method was determined to contribute at most ±0.1% to the absolute uncertainty of the velocities and at most a 4% decrease to the full-width-at-half-maximum of the velocity distribution functions measured in the accelerating plasma of the Hall thruster.

The purpose of this research is to develop a plasma system capable of reproducing plasma densities found during atmospheric re-entry of a capsule. We developed a 150 mm diameter helicon source at the University of Michigan Plasmadynamics and Electric Propulsion Laboratory (PEPL) and used a Langmuir probe to characterize plasma properties downstream. The helicon source was operated with argon gas at a background pressure of 0.6 mTorr. We used a commercial RF-compensated single Langmuir probe to measure ion number density and electron temperature in the region downstream of the helicon source where we want to create conditions similar to those found during hypersonic flight within the atmosphere. We measured these values with and without the presence of a large 450 mm wide by 550 mm long surface downstream in the horizontal plane to simulate a vehicle surrounded by plasma in order to determine how the downstream body affects plasma properties. We found that the presence of a surface downstream of the helicon source lowers the downstream plasma density range from between 1.7 x 10(17) and 3.3 x 10(17) m(-3) down to 0.55 x 10(17) and 1.3 x 10(17) m(-3). In addition, the peak plasma potential decreases from 65 to 55 V, but the electron temperature remains unchanged ranging between 1.5 and 6.5 eV.

As a vehicle reenters the atmosphere or travels at hypersonic speeds within it, a bow shock forms around the leading edge of the vehicle. The air is superheated as it passes through the shock wave and becomes ionized. This plasma layer prevents the transmission of radio frequency communications to or from the vehicle, causing what is know as communications blackout. In this paper, we present results from experiments performed to evaluate the use of crossed electric and magnetic fields to lower the plasma density in a region surrounding an antenna. Plasma number density, plasma frequency, and signal attenuation measurements were made with a Langmuir probe, hairpin resonance probe, and S2-1 probe, respectively. The hairpin resonance probe and the S2-1 probe measured frequency responses for input frequencies ranging from 200 up to 4000 MHz. Results show that this approach is a viable method for communications blackout amelioration. We found that the plasma number density decreases by as much as 70% with the operating conditions used in this work, and the plasma frequency dropped by as much as 75%. The increased reduction in the plasma frequency, as compared to the plasma number density, was due to the addition of greater magnetic field strength when the frequency measurements were made. In addition, frequencies that were previously attenuated by more than 10 dB have almost no attenuation after the application of the electric and magnetic fields.

We present a numerical method for simulating neutral xenon absorption spectra from diode-laser spectroscopy of the Zeeman-split 6S′[1/2]→6P′[1/2] line at 834.682 nm-air in a galvatron’s plasma. To simulate the spectrum, we apply a Voigt profile to a spectrum of σ-transition lines of even- and odd-numbered isotopes computed from anomalous Zeeman and nonlinear Zeeman hyperfine structure theories, respectively. Simulated spectra agree well with Zeeman-split spectra measured from 30 to 300 G. A commercial nonlinear least-squares solver (LSQNONLIN) returns field strengths and translational plasma kinetic temperatures that minimize the error between simulated and experimental spectra. This work is a preamble to computing magnetic field topology and the speed distribution of neutral xenon particles in the plume of a Hall thruster from diode laser-induced fluorescence.

Sputter erosion of boron nitride (BN) plays a key life-limiting role in many Hall thrusters. We report on the development of cavity ring-down spectroscopy (CRDS) as a sensitive and accelerated diagnostic for sputter erosion . The measurement approach is based upon probing sputtered boron atoms in the region of 250 nm. We present quantitative detection of sputtered BN using CRDS with a pulsed laser source. The pulsed measurement results are compared with modeled signal levels. We update the development of a continuous-wave CRDS system using the frequency-quadrupled output of an external cavity diode laser as the light source. Characterizations of the measurement sensitivity show that with integration times of several minutes, the current performance would allow high signal-to-noise erosion measurements for expected thruster conditions. Approaches to increase sensitivity (reduce measurement time) and to integrate with vacuum chambers for thruster testing are summarized.

Relative erosion rates of the discharge cathode assembly of a 30-cm ion engine are measured using laser-induced fluorescence. Molybdenum and tungsten erosion products are interrogated downstream of the discharge cathode assembly during beam extraction. Erosion of the discharge cathode assembly is characterized for both keepered and unkeepered configurations. The erosion increases with both discharge current and voltage, and spatially resolved measurements agree with observed erosion patterns. Erosion rates are calculated using data from previous wear tests. Magnitudes and trends in the rates are correlated with both previous and subsequent wear tests. Laser-induced fluorescence is demonstrated to be a technique to measure relative internal erosion rates, and a path is identified for measuring absolute rates.