Recent advances in MPD thruster research at Princeton

A summary of last years anode, plasma, and cathode findings is presented. A summary of this years activities and findings is also presented. A brief discussion of previous and current understanding is given and covers the following topics: existence of microinstabilities; the scaling of Va with the Hall parameter; the scaling of anomalous resistivity with the Hall parameter; the relation between anomalous resistivity and the anode drop; the presence of turbulence in the anode region; numerical simulation with anomalous transport; the use of magnets to decrease dissipation; performance testing with the new anode; the mechanisms behind the ionization sink; and lithiated cathode research

Propulsive performance of a finite-temperature plasma flow in a magnetic nozzle with applied azimuthal current

The plasma flow in a finite-electron-temperature magnetic nozzle, under the influence of an applied azimuthal current at the throat, is modeled analytically to assess its propulsive performance. A correction to the nozzle throat boundary conditions is derived by modifying the radial equilibrium of a magnetized infinite two-population cylindrical plasma column with the insertion of an external azimuthal body force for the electrons. Inclusion of finite-temperature effects, which leads to a modification of the radial density profile, is necessary for calculating the propulsive performance, which is represented by nozzle divergence efficiency and thrust coefficient. The solutions show that the application of the azimuthal current enhances all the calculated performance parameters through the narrowing of the radial density profile at the throat, and that investing power in this beam focusing effect is more effective than using the same power to pre-heat the electrons. The results open the possibility for the design of a focusing stage between the plasma source and the nozzle that can significantly enhance the propulsive performance of electron-driven magnetic nozzles

Current sheet Formation in a Conical Theta Pinch Faraday Accelerator with Radio-Frequency Assisted Discharge

The inductive formation of current sheets in a conical theta pinch FARAD (Faraday Accelerator with Radio-frequency Assisted Discharge) thruster is investigated experimentally with time-integrated photography. The goal is to help in understanding the mechanisms and conditions controlling the strength and extent of the current sheet, which are two indices important for FARAD as a propulsion concept. The profiles of these two indices along the inside walls of the conical acceleration coil are assumed to be related to the profiles of the strength and extent of the luminosity pattern derived from photographs of the discharge. The variations of these profiles as a function of uniform back-fill neutral pressure (with no background magnetic field and all parameters held constant) provided the first clues on the nature and qualitative dependencies of current sheet formation. It was found that there is an optimal pressure for which both indices reach a maximum and that the rate of change in these indices with pressure differs on either side of this optimal pressure. This allowed the inference that current sheet formation follows a Townsend-like breakdown mechanism modified by the existence of a finite pressure-dependent radio-frequency-generated electron density background. The observation that the effective location of the luminosity pattern favors the exit-half of the conical coil is explained as the result of the tendency of the inductive discharge circuit to operate near its minimal self-inductance. Movement of the peak in the luminosity pattern towards the upstream side of the cone with increasing pressure is believed to result from the need of the circuit to compensate for the increase in background plasma resistivity due to increasing pressure

Experimental and Theoretical Studies of the Lithium-fed Multichannel and Single-channel Hollow Cathode

Cathode voltage and temperature profile measurements from lithium-fed single-channel hollow cathode (SCHC) and multichannel hollow cathode (MCHC) experiments are presented along with a the results of a theoretical model that includes the relevant phenomena. The lithium-fed SCHC experiments and the measurement of the plasma potential just downstream of the channel exit show that the plasma penetration length increases with current, and that the maximum temperature is independent of mass flow rate and weakly dependent on current (at high current). The model predicts important operating parameters including the cathode voltage, temperature profile, and ionization fraction as a function of current, lithium flow rate, and channel diameter. The SCHC model is also extended to a MCHC theory by including the thermal/radiative benefits of bundled channels. The theories capture experimental trends and predict cathode temperature to within 10% and voltage to within 4 V. One of the main insights provided by this study is that the arc penetrates to a location where the plasma density is great enough to supply the ion flux required to heat the cathode surface to the thermionic emission temperature. This has the effect of making the maximum temperature and voltage independent of mass flow rate and the penetration depth dependent on mass flow rate and current

Performance study of the ablative Z-pinch pulsed plasma thruster

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76691/1/AIAA-2001-3898-280.pd

Review of the EP activities of US academia

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76897/1/AIAA-2001-3227-398.pd

Space experiments with particle accelerators: SEPAC

The Space Experiments with Particle Accelarators (SEPAC), which flew on the ATLAS 1 mission, used new techniques to study natural phenomena in the Earth's upper atmosphere, ionosphere and magnetosphere by introducing energetic perturbations into the system from a high power electron beam with known characteristics. Properties of auroras were studied by directing the electron beam into the upper atmosphere while making measurements of optical emissions. Studies were also performed of the critical ionization velocity phenomenon.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31357/1/0000268.pd

Ablative Z-Pinch Pulsed Plasma Thruster

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77148/1/AIAA-4362-652.pd

A method for efficiently calculating head-related transfer functions directly from head scan point clouds

A method is developed for efficiently calculating head-related transfer functions (HRTFs) directly from head scan point clouds of a subject using a database of HRTFs, and corresponding head scans, of many subjects. Consumer applications require HRTFs be estimated accurately and efficiently, but existing methods do not simultaneously meet these requirements. The presented method uses efficient matrix multiplications to compute HRTFs from spherical harmonic representations of head scan point clouds that may be obtained from consumer-grade cameras. The method was applied to a database of only 23 subjects, and while calculated interaural time difference errors are found to be above estimated perceptual thresholds for some spatial directions, HRTF spectral distortions up to 6 kHz fall below perceptual thresholds for most directions

Fundamentals of a parametric method for virtual navigation within an array of ambisonics microphones

Fundamental aspects of a method for virtual navigation of a sound field within an array of ambisonics microphones, wherein the subset of microphones to be used for interpolation is determined parametrically, are presented. An existing, weighted-average-based navigational method serves as a benchmark due to its simplicity and its applicability to arbitrary sound fields but introduces comb-filtering and, for near-field sources, degrades localization. A critical review of existing methods is presented, through which a number of issues are identified. In the proposed method, those microphones that are nearer to the desired listening position than to any source are determined based on the known or inferred positions of sources. The signals from only those microphones are then interpolated using a regularized least-squares matrix of filters. Spectral distortions and source localization errors are characterized for the benchmark and proposed methods via numerical simulations of a two-microphone array, and an experimental validation of these simulations is presented. Results show that, for near-field sources, the proposed method significantly outperforms the benchmark in both spectral and localization accuracy due to the exclusion of the second microphone. For far-field sources, the proposed method achieves slightly decreased spectral distortions due to the flattened response of the interpolation filters