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

Isolation performance metrics for personal sound zone reproduction systems

Two isolation performance metrics, Inter-Zone Isolation (IZI) and Inter-Program Isolation (IPI), are introduced for evaluating Personal Sound Zone (PSZ) systems. Compared to the commonly-used Acoustic Contrast metric, IZI and IPI are generalized for multichannel audio, and quantify the isolation of sound zones and of audio programs, respectively. The two metrics are shown to be generally non-interchangeable and suitable for different scenarios, such as generating dark zones (IZI) or minimizing audio-on-audio interference (IPI). Furthermore, two examples with free-field simulations are presented and demonstrate the applications of IZI and IPI in evaluating PSZ performance in different rendering modes and PSZ robustness

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

MEMS Mega-pixel Micro-thruster Arrays for Small Satellite Stationkeeping

Small satellites flying in clusters require periodic “stationkeeping” to keep them in place. The required impulse is very small – the goal is not to keep the individual satellites in rigid formation, but only to keep them in well-defined orbitals with respect to one another. The necessary impulse, therefore, is only the amount needed to overcome the difference in drag between the most-affected and the least-affected satellites in the cluster. Estimates are that the differential drag can be overcome by providing ~1 mNsec (micro-Newton second) to ~1 mN sec (milli-Newton second) every 10 to 100 seconds throughout each satellite’s mission. The system we are developing will do that. The thrusters have very low power and energy thresholds for ignition (~10 mWatts, ~100 μJoules), and no moving parts so they are expected to be highly reliable. A single thruster array contains a quarter of a million separate thrusters

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